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Lighting By Design 2nd Edition Christopher Cuttle Ma Fcibse Fiesanz Fiesna Lc

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Lighting by Design
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Lighting by Design 2nd edition Christopher Cuttle AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD • PARIS SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Butterworth-Heinemann is an imprint of Elsevier
Architectural Press is an imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP, UK The Boulevard, Lanford Lane, Kidlington, Oxford OX5 1GB, UK 84 Theobald’s Road, London WC1X 8RR, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA First edition 2003 Second edition 2008 Copyright © 2003, 2008, Christopher Cuttle. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (44) (0) 1865 843830; fax (44) (0) 1865 853333; email: permissions@elsevier.com. Alternatively you can submit your request online by visiting the Elsevier web site at http:/ /elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data Cuttle, Christopher Lighting by design. – 2nd ed. 1. Lighting, Architectural and decorative 2. Lighting, Architectural and decorative – Design I. Title 729.28 Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress Library of Congress Catalog Number: 2008931250 ISBN: 978-0-7506-8768-3 For information on all Architectural Press publications visit our web site at http://elsevierdirect.com Typeset by Charon Tec Ltd., A Macmillan Company. (www.macmillansolutions.com) Printed and bound in Slovenia 08 09 10 11 12 10 9 8 7 6 5 4 3 2 1
Contents Preface to first edition vii Preface to second edition ix Acknowledgements xi Introduction xiii Part One: Observation 1 1 Visible characteristics of objects 3 1.1 Visual constancy and modes of appearance 4 1.2 Visible properties of materials 19 1.3 Object characteristics and perceived attributes 31 2 Visible characteristics of lighting 37 2.1 Ambient illumination 37 2.2 Visual discrimination 51 2.3 Illumination hierarchy 74 2.4 The ‘flow of light’ 80 2.5 The ‘sharpness’ of lighting 99 2.6 Luminous elements 113 3 Measurable characteristics of lighting 125 3.1 Illuminance-based measurements 125 3.2 Luminance-based measurements 129 Part Two: Visualization 133 4 Envisioning the concept 135 4.1 Seeing lighting clearly 135 4.2 Allusion and illusion 142 4.3 Lighting concepts 155 5 Concept development 161 5.1 Getting the picture 161 5.2 The Design Features Report 165
Contents Part Three: Realization 169 6 Delivering the lumens 171 6.1 Indirect flux 173 6.2 Flux distribution 183 6.3 Direct flux 195 6.4 The light field 209 7 Getting the lighting you want 223 7.1 Lighting specification 223 7.2 Contractual agreements 226 Appendices A1 Technical concepts, terms and symbols 229 A2 Terms and symbols used in the text 234 A3 Summary of lighting concepts, design criteria, and associated metrics 235 A4 Summary of calculations 236 References 240 Further reading 242 Index 245 vi
Preface to first edition The need for this book arises from the fact that many archi- tects and interior designers do not envision electric lighting as part of their design philosophies. Generally, architects recognize Le Corbusier’s dictum that ‘Architecture is the masterly, correct and magnificent play of masses brought together in light’. As they create space, architects position apertures with care, admitting daylight to reveal forms and their textures, and so define the space, and as Corbusier had observed, this involve- ment with light lies at the heart of architecture. But then a strange thing can happen. The design is handed over to a building services engineer, whose range of responsibilities includes ventilation, heating and air conditioning; sound sys- tems; sprinklers; and electric lighting. For all of these services, the engineer’s overriding concern is to achieve uniform distri- butions, and in the case of lighting, this typically means that a prescribed illuminance is provided uniformly over a horizontal work plane 700mm above floor level. The result brings untold dismay to architects. By day, their building has light and shade, with forms and textures interacting with the flow of light induced by the thoughtfully located fenestration. By night, all of this recedes into the dull blandness of consistent, invariant illumination. The first group that this book is intended for is architects and interior designers who seek to achieve their design objectives both by day and by night. However, that does not mean pro- viding a daylit appearance around the clock. Electric lighting has its own aesthetic, and a prime aim of the book is to get designers to appreciate the different ways in which daylight and electric lighting interact with buildings. This considera- tion may bring the designer into contact with specialist light- ing designers, which may include building services engineers who have developed a passion for lighting, and these people are the second group for whom the book is intended. Overall, the book is intended for designers seeking to bring in-depth understanding of electric lighting into the architectural design process.
Preface to first edition The Artist One evening there came into his soul the desire to fashion an image of The Pleasure that abideth for a Moment. And he went forth into the world to look for bronze. For he could only think in bronze. But all the bronze in the whole world had disappeared, nor anywhere in the whole world was there any bronze to be found, save only the bronze of the image of The Sorrow that endureth for Ever. Now this image he had himself, and with his own hands, fash- ioned, and had set it on the tomb of the one thing he had loved in his life. On the tomb of the dead thing he had most loved had he set this image of his own fashioning, that it might serve as a sign of the love of man that dieth not, and a symbol of the sorrow of man that endureth for ever. And in the whole world there was no other bronze save the bronze of this image. And he took the image he had fashioned, and set it in a great furnace, and he gave it to the fire. And out of the bronze of the image of The Sorrow that endureth for Ever he fashioned an image of The Pleasure that abideth for a Moment. Oscar Wilde (Source: Small, I. (ed.) Oscar Wilde: Complete Short Fiction, Penguin Classics, 1994). viii
Preface to second edition Since the first edition was published in 2003, I have had a good number of opportunities to teach its content. These have caused me again and again to think through ways of explain- ing the concepts that are the basis of this approach to lighting design. I have satisfied myself that I have a simpler and clearer way of introducing the ‘sharpness’ of lighting concept, and this has led me to rewrite (and shorten) Section 2.5. For the example of an applied lighting calculation that I had given in Section 6.1, I followed the procedure of manual calcu- lations using data from lighting manufacturers’ catalogues. For this edition I have shown how a computer-based lighting pro- gram, in this case DIALux, can be applied for the calculations that I make use of. As well as making the calculations quick and easy (providing we keep our objectives clearly in mind), a great advantage of using this type of software is that it enables on-line searching for luminaires with suitable photometric per- formance. This is demonstrated in the revised Section 6.1. There are other additions to the text, such as discussion of opponent colours theory and the colour mismatch vector method of illustrating colour rendering properties in Section 2.2, but the most obvious difference is that this edition is in full colour. I wanted to take the opportunity to make this discus- sion of lighting much more visual, and while there is no short- age of books that present pictures of ‘good’ lighting, I wanted instead to illustrate ways in which the appearance of architec- tural spaces may be affected by lighting. This has led me to add groups of my own colour photographs to introduce each chap- ter, and generally the aim has been to show comparisons of interior spaces influenced by changes of lighting. I did not take these photographs specifically for this purpose, but rather I use a camera to record my observations of lighting. I do this to develop my own observation-based experience of lighting, and I recommend this practice to anyone who shares this interest.
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Acknowledgments The most wonderful thing about working in lighting is the people that you encounter. Scientists and artists; engineers and designers; architects and psychologists; optometrists and ergonomists; are all concerned about how people interact with light. It is a topic that is virtually without boundaries, and it has brought me into contact with an extraordinary variety of people from whom I have gathered so much that I know that I cannot properly acknowledge all of them. However, some people have changed the way I think, and these people I particularly want to acknowledge. David Pritchard pulled me out of the commercial stream of a London luminaire manufacturer and into the technical depart- ment. They were a lively bunch and I learned a lot from them, and also I joined the Illuminating Engineering Society. At the London monthly meetings I was encountered speakers of the stature of J.M. Waldram, R.G. Hopkinson, and W.R. Stevens, and lighting became an interest rather than a job. After five years in London I joined Derek Phillips, a young archi- tect who had taken on the challenge of establishing Britain’s first independent lighting consultancy practice. I met clients rather than customers. I learned how to visualize lighting, and what it was to feel responsible for one’s own work. My next move was to join the Daylight Advisory Service of Pilkington Glass at St. Helens, Lancashire. Under the leadership of J.A. (Joe) Lynes, the DAS was developing a quite remark- able reputation for its contributions to daylighting design, and I became increasingly involved in giving seminars on the DAS’s design tools at schools of architecture. It was Professor James Bell who encouraged me to study for my Masters degree at the University of Manchester, and at about this time, Harry Hewitt invited me to join the IES Lighting Design Panel. This group of experts had the task of looking ahead to guide the society’s work. The panel’s meetings were always stimulating, and never more so than when Peter Jay took over the leader- ship. While I had the good fortune to engage with some out- standing intellects at this time, I have to make special mention
of Joe. He literally drew my understanding of lighting into the third dimension, and although we worked together for only two years, I have benefited ever since from the friendship that we have maintained. In 1976 I emigrated with my young family to join a brand new school of architecture in Wellington, New Zealand. It was a young faculty that developed a collegiate bond that drove all of us. The lack of a lighting community came as a shock, but fairly soon we had the IESNZ up and going, and soon after that New Zealand joined the International Commission on Illumination (CIE). Things seemed to be well on track when Mark Rea invited me to join the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, New York, in order to set up the world’s first Master of Science in Lighting degree program. I went to Rensselaer on a three-year contract and stayed for nine years. Once again I was with a newly established outfit where the adrenalin was flowing and my learning curve was as steep as ever. The students were challenging and the faculty was out- standing. Peter Boyce, Howard Brandston, Naomi Miller and Jan Moyer remain firm friends. I returned to New Zealand and once again I was working with architecture students and getting them to visualize their design concepts in light. Some of the ideas that I make use of have appeared in published papers, and I am grateful to Lighting Research and Technology for having given me opportunities to offer my thoughts for peer scrutiny. Also, I want to thank Lighting Design Application, who between 1995 and 1999 published 34 articles of mine in a monthly column titled “Cuttle on Calculations”. Opportunities of this sort are enormously valuable for developing one’s own ideas, and I need to make particular mention of the writings of Dr. J.A. Worthey, whose studies of light source size have provided the basis for the sec- tion on the ‘sharpness’ of lighting. I retired from the School of Architecture at the University of Auckland, New Zealand in 2007, and I acknowledge the sup- port that enabled me to write this book. Special thanks are due to the faculty photographer, Lynne Logan, who did all the studio photography for the illustrations. Other illustrations are either acknowledged with due gratitude in the captions, or they are my own. Kit Cuttle Auckland, 2008 Acknowledgments xii
Introduction This book is concerned with devising electric lighting instal- lations for architectural spaces that will contribute towards achieving architectural design objectives. It is written for archi- tects, interior designers and specialist lighting designers. It presumes a basic knowledge of lighting technology, although a brief summary is given in the Appendices for the benefit of those who might need an occasional reminder. The book comprises three parts. Part One is titled Observation, and the thesis is that the aspects of lighting that concern a designer are those that can be seen to make a difference. The problem is that we all take lighting for granted, and we sim- ply do not notice what lighting can do until we direct our- selves to look for it. If people enjoy the visual experience of a space or the objects it contains, the lighting must have been working well for them. That they remember the architec- ture or the beautiful art, and they don’t remember the first thing about the lighting, is not the issue. To become a light- ing designer it is necessary to understand the role of light- ing in revealing that experience. This is done by objectively examining interactions of light and matter and develop- ing an extensive range of observation-based experience of lighting. Part Two is titled Visualization. A lighting design concept develops in the designer’s mind, and its strength depends on the designer’s ability to visualize three-dimensional space and to bring to that vision observation-based experience of light- ing. This use of the term visualization should not be confused with computer-generated renderings. The process described involves mentally applying lighting design criteria to build up a visualization of the design situation in light, and devel- oping the skill to communicate and discuss that concept with a client and other professional designers working on the project. Part Three is titled Realization. Unlike stage and studio lighting designers, the architectural lighting designer realizes the design
concept through the medium of a technical specification. This leap from the cerebral to the technical involves calculations and understanding the performance characteristics of lighting equipment, but the designer must never lose sight of the prin- ciple that what matters is what can be seen to make a differ- ence. It is intended that a reader who follows all three parts will become good at seeing small differences of lighting. Introduction xiv
Part One: Observation All discovery starts with observation. Whether we think of Aristotle leaping out of his bath and startling the Athenian townsfolk with his cries of ‘Eureka!’, or Newton wondering what caused the apple to fall on his head, or Einstein imagining himself to be sitting on a photon, or Sherlock Holmes’ admon- ishions to Watson: it is all a matter of observation. The process of visual perception operates throughout our wak- ing hours, continually seeking to make sense of the flow of information being delivered to the brain through the sense of vision. It is obvious that lighting is necessary for vision to oper- ate, and there is a substantial amount of knowledge on ways in which lighting may influence how well the visual process is able to operate. However, this book is more concerned with how lighting may influence our perceptions of our surround- ings. There is far less reliable knowledge, and it takes careful observation to identify the aspects of appearance that we rely on to form our perceptions, and how they may be affected by lighting. While this may seem to be a daunting task, it should be obvi- ous that the essential components of lighting design are there for all to see. The first vital step towards becoming a lighting designer is to develop confidence in the evidence of your own eyes.
1 At first, it seems obvious that we provide lighting to enable people to see, so that all lighting can be assessed in terms of how well it enables people to see. Lighting that maximizes the luminance contrast of visual detail enables very small detail to be accurately detected, and this is the basis of many light- ing recommendations and standards. However, observation of our surroundings shows a much larger range of ways in which objects can differ in appearance. Consider for a moment the judgements that we commonly make in deciding whether a surface is clean and dry; whether fresh fruit is good to eat; or whether a colleague looks tired. These judgements are based on observation of appearance, but what are the differences of appearance that are critical in making these judgements? Any of these everyday assessments of appearance can be influenced by subtle aspects of lighting, and so too can our more complex assessments of the appearance of architectural spaces. A basis of theory enables designers to examine their own observations of the things that surround them. Differences of object appearance have their origin in the physical processes by which light is reflected, refracted, dispersed and scattered by matter. But human vision did not evolve to enable us to observe these processes: it evolved to enable us to recognize our surroundings. Understanding of the roles of these processes requires directed observation, and when we apply observation analytically, we find that the number of physical processes that is responsible for all of the differences that we can discriminate is quite limited. With this insight, we start to gain knowledge of how to control light to achieve a visible effect that we have in mind. It is, in fact, quite remarkable how the astounding range of human visual sensations is governed by so few processes. Lighting is both the medium that makes things visible, and it is a visible medium. At one level, it reveals the identifying attributes that enable us to recognize the objects that surround us, and at another level it creates patterns of colour, and light and shade, which add other dimensions to the visual scene. Visible characteristics of objects Facing page: Union Station, Washington DC. The 1988 renovation of architect Daniel Burnham’s Union Station (opened in 1907) included new lighting designed by William Lam Associates. The uplighting in the Main Concourse is by two-to-one combinations of metal halide and high pressure sodium lamps, the sodium lamps having been added to cause the gold leaf decoration to gleam. This is effective both by day and by night, despite the vastly different overall appearance of the terminal
Observation 4 This chapter examines the role of lighting at the former level, that is to say, its role in making visible the aspects of appear- ance that enable us to perceive our surroundings. We start by considering what we need to know about the processes of vision and visual perception. 1.1 Visual constancy and modes of appearance The underlying aim of lighting design is to control the luminous environment in order to influence the perceived environment. Figure 1.1 provides a simple model of visual perception, which shows that several stages are involved in making this connection. THE LUMINOUS ENVIRONMENT generates THE RETINAL IMAGE which is the stimulus for THE PROCESS OF VISION which provides information to enable THE VISUAL PERCEPTION PROCESS THE PERCEIVED ENVIRONMENT to recognize the objects and surfaces that form the visual basis for Figure 1.1: A simple model of the human visual perception process. Lighting designers exercise operational control in the luminous environment, with the aim of influencing an observer’s perceived environment. A complex series of processes occur between the designer’s input and its effect
Visible characteristics of objects 5 The luminous environment This is the physical environment made luminous by light. It is here that the lighting designer exercises control. The retinal image The optical system of the human eye focuses an inverted image onto the retina, shown in Figure 1.2. This image is constantly changing with movements of the head and the scanning move- ments of the eyes. It is often said that the eye is like a camera, but the only similarity is that it forms a focused image in which, for every pixel, there is a corresponding element in the lumi- nous environment. The main difference is that the eye operates as an instrument of search. Unlike photographic film, the struc- ture of the retina is far from uniform. High-resolution detection occurs only at the fovea, a small area of tightly packed photore- ceptors, and except at very low light levels, resolution declines progressively to the periphery of the retina. While the relatively slow movements of the human body occur, more rapid move- ments of the head enable attention to focus onto things that have been noticed, while still more rapid movements of the eyes within their sockets cause objects of interest to be scanned for detail. The eye is not a picture-making device: it is the opti- cal instrument of search that is actively involved in the process of seeking information of the surrounding environment. The distribution of luminance and colour that comprises the retinal image is modified by light losses that occur in the opti- cal media of the eye, and these losses are not constant as they increase significantly with age. Here we encounter an interesting conundrum. Because the retinal image is the stimulus for vision, we have no way of examining it. So, we are forced to accept measures of the luminous environment as practical indicators Near vision Distant vision Iris contracted Pupil Iris opened Retina Ciliary muscle Optic nerve Sclera Fovea Lens flattened Lens rounded Blind spot Figure 1.2: Sectional diagram of the human eye showing lens curvatures for near and distant focus. (Source: Coaton, J.R. and Marsden, A.M. (eds) Lamps and Lighting, Arnold, 1997)
Observation 6 of the stimulus for vision, which means that we are by default assuming a notion of ‘normal vision’. This notion presumes that those who need optical correction to achieve a sharply focused image will have it, and while allowance may be made for reduc- ing image brightness with age, this is often overlooked in prac- tice. This latter point is discussed in Section 2.2. The process of vision The purpose of the visual process is to provide an ever-chang- ing flow of information to the visual cortex of the brain. The retinal image stimulates photoreceptors embedded in the retina, causing a series of minute electrical impulses to flow along the optic nerve pathways to the brain (Figure 1.3). It might seem more appropriate to compare the eye with a television camera than with the more familiar picture-making camera, but even here the comparison falls short. There are millions of photore- ceptors in the retina, and processing of their responses occurs at several stages along the route to the brain. The first level of processing occurs actually within the retina, ena- bling the optic nerve to transmit the visual information with far fewer nerve fibres than the number of photoreceptors. Further modification of the signals from the two retinas occur in the chi- asma, where responses from both left-hand sides of the retinas are channelled to the left-hand lobe of the visual cortex, and the right-hand channel is similarly directed. Further processing occurs in the lateral geniculate bodies before the signals reach the cortex. While there is still plenty that is not understood about the work- ing of these processes, much information on the performance of human vision has been gathered in recent years. The prime Lateral geniculate body Retina Optic chiasma Optic nerve Visual area of cortex Figure 1.3: Schematic diagram of the binocular nerve pathways (adapted from Boyce, P.R. Human Factors in Lighting, Applied Science Publishers, 1981)
Visible characteristics of objects 7 source of this information is studies involving measurements of the ability of an observer to detect small differences of luminance or colour, and this aspect of visual ability is discussed in Section 1.2. Its relevance to this discussion is that if an item of detail is to be part of the perceived environment, then its presence must be indicated by a visually detectable stimulus. The visual perception process The perception of a surrounding environment may be influ- enced by input from any of the senses, together with memory cues. Although vision is usually the dominant source of sensory information, the perception may be significantly influenced by inputs from other senses, such as auditory, olfactory, and tac- tile senses, together with memories derived from these senses. This simple model should not be taken too literally, as just how a perception of an environment is assembled from the signals that flow through the optic nerve pathways is much less well understood than the process of vision. The perceived environment This is the construct within the brain that serves as a model for the physical environment, and it has two distinct roles. It is within this mental construct that a person orientates and makes operational decisions, such as how to navigate through the space without colliding with furniture or other objects. Also, the mental construct represents the person’s assessment of their environment. If one person finds a space pleasant and another does not, then we can assume that the perceived environments that each of them has formed are different. While some inter- personal differences are inevitable, it is evident that there are broad similarities which enable designers to satisfy both select and random groups of people. Luminous environments can be created that lead to a majority of sighted people generating perceived environments that both enable satisfactory levels of operational decision-making, and which also provide for posi- tive evaluations of their surroundings. Referring back to Figure 1.1, we can use this model to set light- ing design into context. The designer’s objective is to bring to life a perceived environment that exists as a mental image in the designer’s brain. The image comprises more than a view. Depending on the designer’s philosophy, it is likely to incorpo- rate subjective concepts which relate to evaluative responses to the luminous environment, which is to be achieved by applying
Observation 8 lighting to a physical environment. This is the essential function that the lighting designer controls. The link between the lumi- nous environment and the perceived environment is the chain of functions indicated in the basic model of visual perception. Added to this, it is inevitable that past experience will influ- ence an individual’s visual perception of their environment, and this gives one more reason why we need to recognize that the luminous environment and the perceived environment are not the same thing. That we have incomplete understanding of how the visual perception functions operate is not an overrid- ing deficiency, as we can employ observation to explore ways in which variations in the luminous environment influence the perceived environment. This is a vital aspect of any design process. At the same time, we should seek theory that confirms observation as this enables us to organize knowledge. It is with this purpose in mind that observation is the central theme of Part One of this book. Aspects of appearance Consider this hypothesis: architectural lighting should provide for reliable recognition of the surfaces and objects that form the environment. The basis of this premise is that every object that is represented within the perceived environment is asso- ciated with certain attributes, some of which are essential for recognition of the object, and some of which affect assessment of the object’s qualities. A designer can be expected to look for more than lighting that simply makes everything visible. Much design effort may have been expended on selecting materials and specifying colours and textures, and it is important that these selected qualities are effectively revealed. Examine the four views shown in Figure 1.4. In every case, the objects are instantly recognized, but being able to cor- rectly name an apple, a peach, and a pineapple does not tell us much about these objects. Are they ripe? Are they wholesome? Would they be good to eat? What different impressions do we gain from the various views of the colour and texture of each of these objects? These are the judgements that determine our attitudes towards these familiar objects, but what are the aspects of appearance that influence our assessments? We have expectations of what good fruit should look like, and we inevitably compare the different views of the objects with our expectations. The perceived objects are more than images: they
Visible characteristics of objects 9 are entities in our minds that are ‘coloured’ by our expecta- tions. If the fruit appears unattractive in one view, those per- ceived attributes of the object that do not meet expectations will stand out in the mind of the viewer. The perceived object is not a simple transposition of the retinal image: it carries the viewer’s evaluation of the perceived object. A fruit vendor who seeks to meet the viewer’s expectations will polish the apples, but not the peaches. However, the apples will not shine unless the lighting has the propensity to reveal that attribute. There is, of course, no such thing as ‘shiny lighting’, and lighting alone cannot make the peach appear shiny. However, lighting that can produce a pattern of light and shade on the smooth, velvet surface of the peach that differentiates it from both the Figure 1.4: (a) The objects are both visible and recognizable, but while the perceived attributes enable recognition they do not necessarily engender favourable assessments of the objects. (b) The spatial distribution of the lighting is the same as for (a) but the spectral distribution is different, and gives more favourable assessment of the chromatic attributes of the objects. (c) The spatial distribution of light contrasts the matt and glossy surfaces of the peach and the apple, and their smooth forms from the rough surface of the pineapple. However, the spectral distribution is as for (a) and does not favour the chromatic attributes. (d) The peach and the apple look ripe, and the foliage of the pineapple appears fresh. Both the spatial and spectral distributions of light reveal differences of object attributes and support favourable assessments of them (a) (b) (c) (d)
Observation 10 jagged surface of the pineapple and shiny surface of the apple has properties that meet the expectations of the vendor and his customers. If the lighting also aids discrimination of colours that are associated with fruit that is healthy and ripe, it will gain the customer’s approval. The evaluative aspects of perception are primarily concerned with discrimination, and this process is served by lighting that provides for discrimination of object attributes, that is to say, lighting that maximizes differences of object appearance. Whenever the retinal image stimulates the perception of an object, that object is inevitably perceived to have certain attributes. The apple has the attribute of gloss, and the peach does not. If we doctored the surface of the peach with a clear varnish a viewer might perceive a nectarine, but not a glossy peach. Not all things can be perceived to have all attributes. If the image of the apple appeared to be flickering, this would be perceived to be an attribute of the lighting. We can not per- ceive an apple that is cyclically altering its surface lightness. If subsequent observation revealed that the flicker was somehow emanating from the object, we might decide that we are look- ing at a plastic model of an apple with a lamp inside, but we would now have a quite different understanding of the object. In our perceived environment, it would not be an apple. Visual constancy Visual constancy may be described as the process by which per- ceived objects maintain more or less stable attributes despite changes in the retinal images by which they are recognized. An understanding of how we develop perceptions of our environ- ments and the role that perceived attributes play in enabling us to come to terms with surroundings is crucial to understanding the roles that lighting can play in influencing people’s perceptions of their environments. For all of our lives we are surrounded by objects, and while indoors, our environments are bounded by surfaces. For the moment, we will treat all of these surfaces as objects. The volume of the space is filled with air, but unless it is dusty or misty, we have no visual awareness of the air. It is, however, necessary for us to recognize the objects that surround us. We need to under- stand why we are in this place, and what is our relationship to these objects. We need to be able to navigate through our environment, and for this we need to have a perception of a stable world, or at least one in which the movements of objects are understandable and reasonably predictable.
Visible characteristics of objects 11 The perceptual process works so well that we do not con- sciously distinguish between the perceived environment and the physical environment, so that ‘I saw it with my own eyes’ seems to the speaker to be irrefutable proof of an event. Psychologists have developed a number of visual illusions to enable them to study the perceptual process. These are images that reliably confuse the perceptual process, and these confu- sions can give insight into the workings of the process. A famous illusion is shown in Figure 1.5. The figure shows two vertical lines. Disregarding their chevron endings, do they seem to you to be the same length? If you need to, use a measure to confirm that they are in fact identical in length. So, why does the one on the left appear to be longer? Could it be that one pair of chevrons is stretching the line by applying tension, while the other is squashing it in compression? That cannot be right, as it is the reverse of the perceived difference. The accepted explanation is rather engaging. It is that the line on the left appears as a receding corner, as if looking into a corner of a room, and the line on the right appears as an advancing corner, as if the external corner of a building. As you perceive the line on the left to be more distant, and its retinal image is of the same size, you perceive it to be larger. Does this explanation seem convincing to you? Try Figure 1.6. Do the black bars seem to you to match in size? You can check that they are identical, but it is almost impossible to see them as equal without obscur- ing the surrounding lines. Consider something rather closer to everyday life. You meet a couple of friends, and as you walk towards your friends to greet them, their images on your retina enlarge. Why would you not see your friends to be enlarging like a pair of inflating balloons? The answer is that in order for you to be able to navigate your way among people, furniture and other hazards, your brain is continually interpreting your changing retinal images, and updating the model of your environment and your location and movement within it. Your decreasing distance from your friends is an aspect of that perception which is inseparable from your recognition of your friends. Even though the setting in which you meet may be quite unfamiliar, you have developed the skill to orientate yourself within that environment and to navigate your way through it without difficulty. You may have encountered many people since you have developed that skill, and while some of them may have enlarged somewhat during your acquaintanceship, you know that it takes more than a few seconds to achieve this transformation. Figure 1.5: The Müller–Lyer figure: the vertical lines are the same length Figure 1.6: Distance–size illusion: in this case, the black bars are the same length
Observation 12 This discussion has been concerned with the phenomenon of size constancy, which, as we can see from Figures 1.5 and 1.6, is easily demonstrated. This is one aspect of the visual constancies, which may be described as perceptual phenom- ena which enable us to ascribe stable attributes to visually per- ceived objects. Another of the constancies is lightness constancy which is not as easily demonstrated on the pages of a book. According to Peter Jay (1973), the German physicist Hermann von Helmholtz (1821–94) posed the question, ‘Why does a lump of coal in sunlight look black even if it has higher lumi- nance than a sheet of white paper that is in the shade?’ You can readily, and quite comfortably, confirm Helmholtz’s obser- vation. Lumps of coal are less commonplace household items than in Helmholtz’s day, but on a sunny day, place a suitably black object (such clothing is fashionable at the time of writing) in the full sun and settle yourself close by in the shade while continuing to read this book. The sunlit object will not lose its blackness nor will this page loose its whiteness, even if the light level difference is such that a luminance meter would show that the reflected luminous flux density is greater from the coal (or black clothing) than from the paper. What is the explanation? It is easily demonstrated that simultaneous contrast can affect perceived lightness (Figure 1.7), but this is not to be confused with Helmholtz’s question. He is asking why it is that recognized objects retain their different identifying visual characteristics even when the effect of lighting would seem to be to cause them to reverse. Of course, our lives would become chaotic if objects changed from black to grey to white when carried from shade to full light. You could walk out of your house in the morning and find your- self unable to recognize it when you return in the evening. Visual constancy is an essential fact of life. Glancing back to Figure 1.1, the retinal image of the lump of coal may have higher luminance than the image of the paper, but the perceptual process did not Figure 1.7: Simultaneous contrast: the grey squares are identical. This could be confirmed by superimposing a mask with five cut-outs coinciding with the grey squares, so they are all seen against the same background
Visible characteristics of objects 13 evolve to inform us of this photometric fact. It evolved to enable us to develop a mental construct that provides us with a reliable representation of our environment, and that means that objects are perceived to retain their intrinsic characteristics, even where large differences of illuminance occur. The appearance of the lump of coal will not be identical whether it is in sunlight or shade, but it remains unmistakably black. How do we make sense of this situation? In particular, if your purpose for reading this book is to learn how to plan distribu- tions of illumination, how are you able to cope with the notion that visual constancy operates so that the appearances of objects are more or less unaffected by lighting? Modes of appearance As has been stated, any ‘thing’ that is recognized is perceived to have certain attributes. The ‘modes of appearance’ concept explains that the perceived attributes that may be associated with a particular ‘thing’ depend upon the ‘mode’ in which it is perceived. This concept provides a theoretical framework for analysing observations of illuminated objects, and it provides a useful concept for examining the roles that lighting can play in influencing the appearance of an illuminated space and the objects it contains. The originator of the ‘modes’ concept was David Katz (1935), whose concern was the ways in which the appearances of col- ours are influenced by the ways in which the stimulus is experi- enced. He drew distinctions between surface colours and volume colours, and between colours that are perceived to be revealed by illumination and those perceived to be self-luminous. His explorations of the role of colour constancy and the influ- ence of illumination, as well as the work of other contributors, have been reviewed elsewhere (Cuttle, 2004). In Figure 1.8(a), red and green coloured cards are placed side- by-side on a table top, and a strip of transparent yellow film is laid across them. A colorimeter would indicate four distinct col- ours, but an observer would perceive three: red and green in surface mode, and yellow in film mode. These modes could be changed. In Figure 1.8(b), a card with four cut-outs is placed on top, and the observer perceives four colours in surface mode. While the names of these modes may seem to describe a physi- cal viewing condition, it is important to recognize that the mode of appearance of an element is determined by how it is perceived by an observer. ‘Modes of appearance’ may be referred to as a
Observation 14 classification of assumptions that underlie perception. A change of mode may cause the perceived attributes of an object to change from those associated with its physical properties, and this may be described as a ‘constancy breakdown’. It is one of this book’s underlying themes that it is a prime pur- pose of a lighting designer to identify chosen attributes of archi- tectural spaces and the objects they house, and to reveal these attributes. It is, therefore, of high relevance that the ‘modes’ theory explains that certain attributes may be associated only with objects perceived in certain modes. Perhaps the most remarkable difference concerns the attributes of brightness and lightness. If I shine a spotlight onto a wall, I create a zone that is perceived to have the attribute of brightness. It might be possible for someone to create an identical luminance pattern by selectively spray painting the wall, but it is unlikely that any- one would be convinced that they were looking at a spotlight- ing effect. The appearance of a spotlight being directed onto a wall is instantly recognized, and the spotlit area is perceived in located illumination mode. It has the attribute of brightness, which may be graded on a bright–dim scale. Meanwhile, the appearance of the wall is essentially unchanged. It is perceived in object surface mode and retains its appearance of uniform lightness, which may be graded on a light–dark scale. This is an important distinction for understanding which are the perceived attributes that may be affected by lighting, and which are the attributes that are not likely to be affected. The brightness of an element perceived in either located illu- mination or illuminant modes is largely determined by its lumi- nance relative to the adaptation condition determined by the overall field, and this is discussed further in Section 2.1. The lightness of an element’s perceived object surface mode is related to reflectance, and the relationship is fairly well estab- lished. The Munsell colour system incorporates a scale of value, which is a subjective scale of lightness. To establish such a (a) Figure 1.8: (a) A red and a green card are overlaid with a yellow film. The card colours are perceived in surface mode, and the yellow in film mode. A mask with apertures is overlaid in (b), and four colours are perceived in surface mode (b)
Visible characteristics of objects 15 scale, a subject might be presented with a black and a white colour card, and be required to select from a large number of slightly different grey cards a card that appears to be mid-way between the black and white cards. Then the subject would find cards to fit into the two gaps, and so would proceed to produce a black–grey–white scale of equal perceived intervals. If the black card has a reflectance close to zero and the white card close to one, it might be expected that the mid-way card would have a reflectance around 0.5. This is not so. Munsell’s value scale of zero to ten is shown related to reflectance in Figure 1.9, and it can be seen that value 5, which is the subjec- tive mid-point, corresponds to a reflectance of approximately 0.2. In other words, what is perceived to be a mid-grey sur- face absorbs 80% of incident light. If a designer wants to make use of surfaces that reflect half of the incident light, they will need to have a Munsell value of 7.5 and will be distinctly light in appearance. The point of this discussion is that lightness is a subjective assessment of surface appearance, and while it is related to surface reflectance, it is not a linear relationship. Brightness is also a subjective assessment, but it relates to the emission light from the object rather than to an intrinsic prop- erty of an object’s surface. Various authors have proposed their own sets of ‘modes’ to suit their own purpose, and most have chosen to drop Katz’s film mode. This is not because it is wrong, but because it is unlikely 0 1 2 3 4 5 6 7 8 9 10 0.1 0 Reflectance Munsell value 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 Figure 1.9: Munsell value and reflectance
Observation 16 to be relevant in design applications. In the ‘modes’ model used for this text there are six modes of perception, if we consider the two object modes separately, and each has its own set of perceived attributes (Cuttle, 1999). Any ‘thing’ that is recog- nized will be perceived in one of these modes, and the mode of perception determines which perceived attributes may be asso- ciated with the ‘thing’. The modes are listed in Table 1.1, with examples of phenomena that are likely to be perceived in each of the modes. Generally, the examples assume that visual con- stancy applies. This table should be read in conjunction with Table 1.2, which shows the modes and associated perceived attributes. A blank indicates that the attribute is not associated with the designated mode of perception, and a cross indicates that there may be an association. This model makes a major distinction between located (perceived to have dimension) and non-located modes. Objects perceived in surface or volume modes are always Table 1.1 Six modes of appearance Mode Examples Non-located Illuminant mode Sky, ambient fog (Note 1). An illuminated surface viewed through an aperture (Note 2). Integrating sphere. Illumination mode Ambient illumination, such as the general lighting within a room. Located Illuminant mode A lamp or luminaire; a self-luminous object. Illumination mode A patch or a pattern of light focused onto a surface or object (Note 3). Object modes Surface An opaque surface; an object seen by reflected light (Note 4). Volume A cloud. A plume of smoke. A transparent or translucent medium. Note 1 While we know intellectually that fog is not self-luminous, ambient or pervading fog is likely to be perceived as a luminous body rather than as an illuminated medium. Note 2 Aperture is considered not to be a mode of appearance: it is a viewing condition that causes a patch of a surface to be perceived in a non-located mode although the aperture itself may be perceived in located object mode. Again, an object does not have to be self-luminous to be perceived in an illuminant mode. Note 3 The located illumination mode differs from the non-located illumination mode in that the illumination is perceived to have size, and perhaps outline or pattern. It is not perceived to have three-dimensional form as it takes on the form of whatever it is focused onto. Note 4 It is important to distinguish between the perception of the surface and the perception of the incident light. Consider a surface illuminated by a flickering source: the flicker will be perceived as an attribute of the illumination, not the surface.
Visible characteristics of objects 17 located, whereas illuminant and illumination modes may be located or non-located. If, upon entering a room, you have an impression of a brightly lit space, this is a non-located illumina- tion mode perception. Alternatively, if you notice that an artwork hanging on the wall is brightly lit, that is a located illumination mode perception. The attributes of brightness and lightness have special roles. Anything that enters conscious perception has either the attribute of brightness or lightness, so that one or the other of these attributes is always associated with the perception. These two attributes are mutually exclusive. Where visual constancy holds, the objects and surrounding sur- faces that comprise a situation to be illuminated are usually per- ceived in either the surface or volume object modes, although both modes may apply simultaneously. The body colour of a glass vase may be perceived in volume mode while its form is perceived in surface mode. For all visible objects, incident illumination inter- acts with the physical properties of their materials, providing the visual stimuli for perceptions of their distinctive attributes. It has been stated that anything that is perceived through the process of vision has either the attribute of brightness or light- ness, but not both. It requires careful observation to confirm that this is so. Consider Figure 1.10: what does it show? Of course it shows a suspended matt white sphere. You perceived the sphere instantly, and furthermore you perceived it to be uniformly white. However, this photograph of the sphere is not uniformly white. Instead it shows a shading pattern from light grey to darker grey. Take a sheet of paper and punch a small hole in it. Slide the paper across the photograph, and you will Table 1.2 Matrix of modes of appearance and perceived attributes Perceived attributes Modes of appearance Non-located Located Illuminant Illumination Illuminant Illumination Surface Volume Brightness X X X X Lightness X X Hue X X X X X X Saturation X X X X X X Flicker X X X X Pattern X X X X Texture X Gloss X Clearness X
Observation 18 see how the aperture changes from near white to dark grey. Why did you perceive it to be uniformly white? The answer is that even in this two-dimensional representation, visual constancy is at work. The object depicted in this photograph is simply a Christmas tree decoration that had been sprayed with matt white paint, but just conceivably, the ball could have been cunningly sprayed in shades of grey and photographed in totally diffused illumination to produce an identical image. What you have perceived is the more probable explanation. In terms of modes of appearance, you perceived the sphere in located, object, surface mode. We will not concern ourselves with how it is that a two-dimensional image causes a three-dimensional object to be perceived. You perceived this object to have the attribute of lightness, and you perceived the lightness to be uni- form. If you had the actual object in your hand, you could dem- onstrate that it retains its appearance of whiteness over a wide range of viewing conditions. It would be possible to confuse a viewer as to the surface lightness, but it takes a contrived viewing condition to do it. If an aperture viewing condition is created, so that the viewer is shown only part of the surface through a hole in another material, such as the sheet of paper that you prepared for viewing Figure 1.10, the viewer is unlikely to be able to make any assessment of surface lightness. In fact, if the surface form- ing the aperture has a much higher luminance, the visible surface could appear to be black. The point is that such a restricted view- ing condition has changed the mode of appearance. It is now perceived in non-located illuminant mode, and it has the attribute of brightness, not lightness. Now that your attention has been drawn to the object-mode perception, what of the shading pat- tern that is visible in Figure 1.10? This is perceived in the located Figure 1.10: A simple object in a complex light field
Visible characteristics of objects 19 illumination mode. It has the attribute of brightness, and may have other attributes such as the chromatic attributes of hue and saturation. It is very worthwhile to make yourself one of these devices. Observe it carefully in a variety of lighting conditions and confirm these findings. There is more reference to viewing simple devices of this sort in following chapters. The concepts of visual constancy and modes of appearance are enormously instructive for lighting designers. Once we have viewing conditions sufficient to enable objects to be recognized, these objects will be perceived to retain their identifying attributes over a large range of lighting conditions. There is limited scope to modify the perceptions of object characteristics while visual con- stancy applies. For example, the perceived hue and saturation of an object may be influenced by the colour rendering properties of the lamps. If a nominally white light source is used, this will have a quite subtle effect on the appearance of the object, which may nonetheless be appreciated. However, if a distinctly coloured effect is produced, it is likely to be perceived as an attribute of the illumination rather than of the object or surface. It is an important observation to distinguish between aspects of appear- ance that are perceived in an object mode, that is to say, which are perceived to be attributes of a recognized object, and aspects that are perceived in illumination mode, which means that they are recognized as attributes of the lighting. The outcome of these observations is quite profound. To think of lighting solely as the medium by which objects and surfaces are made visible is to ignore creative opportunities for influenc- ing users’ perceptions. Think of lighting also as a visible medium that may be perceived in illuminant or illumination modes, and which may be located or non-located. It has the attribute of brightness, not lightness, and while the range of attributes is more restricted than for the surface and volume modes, the perception of these attributes is not directed towards recogni- tion of stable, physical characteristics. Herein lies a wealth of opportunities for lighting designers. In the words of Marshall McLuhan, ‘The medium is the message.’ 1.2 Visible properties of materials When an unfamiliar object is introduced to an infant, it is explored with all the senses. It is handled, and it is held close to the face where young eyes can accommodate the image in fine detail. It is sniffed, shaken and sucked. All of the resulting sensory inputs contribute to the perception of the object. As the infant matures, each new encounter can be referred back to a mental library of
Observation 20 sensory experience, and as this develops, so the sense of vision becomes the dominant source of the perceived environment. For every visible element in the perceived environment, there is a corresponding element in the luminous environment that is either self-luminous, or it is the result of an interaction between light and matter. It is the light and matter interactions that pro- vide the bulk of the information that enables us to recognize the vast array of materials that comprise our environments. To understand the interactions that we initiate when we illuminate an object, we need to take a look at the nature of light. The spectrum of light It can be physically demonstrated that light is a stream of pho- tons, where a photon is an elementary energy particle. In vac- uum, all photons travel at the same velocity, this being the great universal constant, the speed of light. It is equal to approxi- mately 300,000 kilometres per second, or in scientific notation, 3 108 m/s. Photons differ only in their energy levels, where the photon energy level e (Joules) is given by the expression: e h ν Joules where h Planck’s constant and ν frequency (Hz). Life would be more simple if every observed property of light could be explained by this simple model, but such is not the case. It is, in fact, slightly embarrassing to have to admit that this book, entirely devoted to lighting, will not attempt to pro- vide a comprehensive explanation of the physical nature of light, although some suggestions for further reading are offered in the Bibliography. It is sufficient to say that some of the com- monly encountered properties of light are more conveniently explained by treating light as waves of radiant energy rather than as a flow of particles, and it is for this reason that the vis- ible spectrum is usually defined in terms of the wavelength of light. As has been shown, photon energy and frequency are directly proportional. Frequency and wavelength are inversely proportional, and are related by the expression: ν c/λ where c is the velocity of light (m/s), and λ is wavelength (m). In this way, the visible spectrum is conventionally defined as extending from 380 to 770 nanometres (nm), where 1nm 109 m or one billionth of a metre, although it would be equally
Visible characteristics of objects 21 valid to define the spectrum in terms of photon energy levels or frequency, as is shown in Figure 1.11. Light meets matter: the gaseous state Light travels in vacuum without loss of energy, and (as far as we need be concerned) it obeys the law of rectilinear propa- gation, which means that it travels in straight lines. Things change when light encounters matter. The first state of matter that we will consider is the gaseous state, in which the atoms or molecules are free to move, subject only to very small inter- molecular forces. Some scattering of photons occurs as they travel through such a medium. Where the gas molecules are small in relation to the wavelength of light, diffraction scatter- ing occurs as photons interact with these particles. Each particle acts as a centre of radiation and scatters light in all directions. The degree of diffraction scattering is proportional to the fourth power of the frequency of the light (ν4 ), so that the shorter vis- ible wavelengths are scattered much more strongly than the longer wavelengths. Outside the earth’s atmosphere sunlight has a colour temperature of 5,800K, but down at ground level, sunlight has a yellowish appearance and a colour temperature of around 3,000K. The difference is due to the scattering of shorter wavelengths which occurs in the upper atmosphere, the effect of which is evident as the blue sky. Larger particles are encountered in the lower atmosphere, such as water droplets, dust particles, and atmospheric pollutants. These may cause reflection scattering, where the particles act as tiny mirrors, and having random orientations, they produce randomly distributed reflections. Reflection scattering occurs 300 400 500 600 700 800 1 1015 8 1014 6 1014 5 1014 4 1014 Frequency (Hz) Wavelength (nm) IR Visible spectrum UV Violet Blue Green Yellow Orange Red Figure 1.11: The range of the visible spectrum is 380–770 nanometres, or approximately 4 1014 to 8 1014 hertz. It is therefore an octave of wavelength or frequency
Observation 22 also in liquid-state and solid-state matter, and is dealt with more thoroughly later in this section. Alternatively, the effect of interactions with larger particles may be absorption, where the particles convert the photon energy into some other form of energy. Usually this is heat, but other forms may occur as in photochemical reactions. The loss in inten- sity of a parallel beam of light in a homogeneous medium (not necessarily gaseous) follows an exponential decay function: I I 0 exp( ) αχ where I0 is the initial beam intensity, I is the intensity after trav- elling a distance χ in the medium, and α is the linear absorp- tion coefficient, which usually varies with wavelength. For the moment, we may note that while scattering and absorption in the atmosphere have much to do with both nat- ural and artificial outdoor lighting, it is generally disregarded from considerations of indoor lighting. Over the short distances involved, and with the expectation of a clean atmosphere, it is usually practical to assume that photons travel indoors as they do in vacuum, that is to say, without visible effect. Where some visible effect does occur, as when artificial smoke is added to the atmosphere, this is generally the result of reflection scatter- ing and absorption. Liquid-state matter The next state of matter to consider is the liquid state, in which the freedom of molecules to move with respect to each other is more restricted by cohesive forces. Liquids have fixed volumes: they assume the shape of the vessel containing them; and in the absence of other forces, the surface to the atmosphere forms a planar boundary. With the exception of metals in the liquid state, liquids are generally transparent, but differ from the previously considered gaseous-state media by having much higher densities. There are some materials that do not have def- inite fusion temperatures as they cool from the liquid state, and they become more viscous until they assume the rigid cohesion of a solid-state material without losing the molecular structure of a liquid. Glass and the transparent plastics are examples, and these materials may be described as either amorphous solids or supercooled liquids. Optically they behave as liquids, although we describe them as transparent solids. Both diffraction and reflection scattering occur, and there is a marked reduction in
Visible characteristics of objects 23 the velocity of light. To examine the effect of velocity change, we employ the light wave model. Figure 1.12 shows wavefronts of light radiating outwards from source S, and the direction of any ray from S is normal to the wavefront. Two rays are shown, a and b, and as they pass through the rare-to-dense medium boundary, the reduced veloc- ity of light in the dense medium causes closer spacing of the wavefronts. Ray a is incident normal to boundary, and passes through without deviation, but b is refracted towards the nor- mal. The direction of the ray is still normal to the wavefronts, but now the origin of the waves is the apparent source Sap. The angles of incidence and refraction are related by Snell’s law. The velocity of light in air is not significantly different from its velocity in vacuum, and in practice, it is so common for the rare medium to be air that the difference in velocity may be consid- ered due only to the effect of the dense medium. This enables the refracting power of a transparent medium to be expressed by its refractive index μ, and for Snell’s law to be expressed as: sin sin i r μ This expression assumes that the angle i is measured relative to the normal in air, and the angle r occurs in the dense medium. Sap S a a b b Reflected ray Refracted ray Dense medium e.g. water Rare medium e.g. air i i r Figure 1.12: Two rays from S passing from a rare to dense medium. The rate of propagation is lower in the dense medium, causing the wave fronts to close up, and for the oblique ray to change direction
Observation 24 Values of μ for several dense transparent media are given in Table 1.3. Refraction at a medium boundary is accompanied by reflection (Figure 1.12), and where the boundary is optically smooth, this is regular reflection which is governed by two laws: ● the angle of incidence equals the angle of reflection ● the incident ray, the normal and the reflected ray all lie in the same plane. The proportion of the incident light that undergoes regular reflec- tion is defined by Fresnel’s equations, and depends upon the angle of incidence and, for a ray incident in air, the refractive index of the dense medium. Figure 1.13 shows the directional dependence of reflectance for a typical glass surface, so that as the angle of incidence increases, the proportion of reflected light increases only gradually at first, and then sharply. Regular reflec- tion is sometimes called specular reflection, and where both regu- lar and diffuse reflection occur, the portion of reflected light that is due to regular reflection may be called the specular component. In Figure 1.14(a), a ray encounters a boundary to a dense medium and is refracted towards from the normal. It is a princi- ple of optics that every ray is reversible, so if the arrows were to be reversed, the figure would show a ray incident in the dense medium and being refracted away from the normal. Case (b) shows a ray incident in the dense medium at an increased angle r, and in this case the refracted ray is coincident with the boundary. This is the critical angle for the dense medium, which is equal to sin1 (1/μ). Some values of critical angle are given in Table 1.3. What happens if we further increase the angle r? This is shown in case (c), where total internal reflection occurs. This is regular reflection, and is ‘total’ because it occurs without loss of energy. This is the principle of the fibre optic, so that a ray that enters the end of the fibre not too far out of parallel with the axis of the fibre may undergo repeated reflections from the internal surface. Table 1.3 Refractive index values and critical angles for some transparent materials Material Refractive index μ Critical angle (degrees) Water 1.33 49 Acrylic 1.49 42 Soda (common) glass 1.52 41 Polystyrene 1.59 39 Flint glass 1.62 38
Visible characteristics of objects 25 Solid-state matter The third state of matter is the solid state, in which atoms and molecules are not free to move, but vibrate about fixed posi- tions. The model for inorganic materials has these particles geo- metrically arranged in a crystalline lattice. Such solid materials are opaque, but as has been explained, some materials that are described as amorphous possess the transparency property of liquids while having other physical characteristics of solids. Except in rather unusual situations, the great majority of the objects and surfaces that form architectural interior spaces comprise 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Incident angle (degrees) Reflectance Unpolarized Perpendicular polarized Parallel polarized 90 80 70 60 50 40 30 20 10 0 Figure 1.13: Fresnel reflection at an air/glass boundary Normal Normal Normal i (a) (b) (c) r2 r1 r r Figure 1.14: Refraction and total internal reflection at a dense/rare boundary (Source: Bean, A.R. and Simons, R.H. Lighting Engineering: Applied Calculations, Architectural Press, 2001)
Observation 26 opaque solid materials. Whereas these materials appear to us to have distinct boundaries, if we could reduce our scale of dimen- sions to that of an arriving photon, the molecular structures would present a view of an open lattice comprising an ordered array of molecules with an abundance of clear space between them. Arriving photons would be likely to penetrate some dis- tance into the lattice before interacting with particles. These par- ticles are large in relation to the wavelength of light, and while some photons will be absorbed, others will undergo reflection scattering whereby the particles act as tiny mirrors. In this way, some of the photons that have entered the surface layer of the crystalline lattice are re-emitted by back-scattering. Because the alignments of the mirrors are effectively random, the re-emitted light is totally diffused and independent of the direction of the incident photons and for this reason the process is known as iso- tropic re-emission. Such a surface is a uniform diffuse reflector, and is said to obey Lambert’s law. Figure 1.15 shows a small element of uniform diffuse reflector. The reflected light from this element has a cosine distribution, that is to say, the luminous intensity varies as the cosine of the angle measured from the normal. Also, the projected area of the surface from any viewing direction varies as the cosine of the same angle, so that element has the same luminance from all directions. A uniform diffuse reflector that reflects all of the inci- dent light would be a perfect diffuse reflector. Although no real materials achieve this, some come close to it. Reference white surfaces used in photometry laboratories may reflect 99% of inci- dent light at all visible wavelengths, and fresh white paint may be Light may be incident from any direction Reflected light always has a cosine distribution Perfect diffuse reflector Figure 1.15: The perfect diffuse reflector
Visible characteristics of objects 27 as high as 95%. It is often assumed that a surface must be shiny to have high reflectance, but that is not so. Matt white paint reflects a higher proportion of incident light than does a silvered glass mirror or a polished metal surface, but of course the distri- bution of reflected light is quite different. More importantly, the diffuse reflector does not form a reflected image. If the reflected images formed by shiny surfaces include images of bright light sources, this can cause those surfaces to appear much brighter than adjacent diffuse reflectors and give rise to the misconcep- tion that they are reflecting more of the incident light. Photons incident on diffuse reflectors are either back-scat- tered (diffusely reflected) or absorbed. The reflectance of the surface is the ratio reflected to incident light, and this may be strongly wavelength-selective. Low-lightness surfaces absorb high proportions of the incident photons, and coloured surfaces may be described as wavelength-selective absorbers. This con- cept will be discussed further in the following section, but we should not lose sight of the fact that although we may describe some saturated surface colours as ‘bright’, particularly when referring to primary hues such as red or green, this ‘brightness’ is not achieved by red or green light being added to the sum of reflected light. It is due to the surface layer of the material strongly absorbing the complementary hues from the incident light spectrum. Our sensations of brightness are not simply determined by the amount of light arriving at the eye. The crystalline lattice structure applies generally to inorganic materials, but organic solid materials may take up different forms of structure with more scope for randomness. For example, white paper viewed through a microscope is seen to consist of a mass of fine fibres, which individually may be almost trans- parent. Although the molecular structure is quite different from the crystalline lattice, reflection occurs by isotropic re-emission and the matt surface of high-grade unglazed white paper is another close approximation of the perfect diffuse reflector. The effect of applying glaze to paper, or polish to flooring or furniture, is to add regular reflection to isotropic re-emission. The glaze and the polish are amorphous substances overlaying the structure of the solid material, and incident photons undergo a rare-to-dense medium transition, with some reflection loss, before undergoing the back-scattering and absorption that char- acterize the attributes of the perceived object. There are many examples of this combination of reflection characteristics. Paint comprises particles of pigments, which are wavelength-selective light absorbers, suspended in a clear vehicle, which traditionally
Observation 28 was an oil-based varnish and now is more often a clear plastic coating material. The difference between gloss and matt paint is the vehicle. For a gloss paint, the vehicle dries out to a smooth, hard surface where regular reflection occurs. Figure 1.16 illustrates the processes at work when a beam of white light (W, comprising a combination of red, green and blue (R, G and B) components) is incident on a glossy red painted surface. The paint layer comprises particles of pigment suspended in a transparent amorphous vehicle that cures to form a smooth, non-electro-conductive surface. Both regular reflection and isotropic re-emission occur, giving differences of appearance to the observers A, B and C. For the regular reflection: ● direction is in accordance with laws of regular reflection ● quantity is in accordance with Fresnel’s reflection laws ● colour is the colour of the source ● luminance source luminance directional specular reflect- ance determined by Fresnel’s laws (Figure 1.13). Electric non-conductive material Glossy surface e.g. red paint A B C Reflection scattering 2) Isotropic re-emission Absorption W W B G R R 1) Regular reflection Figure 1.16: Reflection from a shiny dielectric surface, in this case gloss red paint (after Hebbelynck, 1987)
Visible characteristics of objects 29 Note that the luminance of the reflected source image is inde- pendent of the distance of the source, and that this image will be seen by observer A but not observers B and C. For the isotropic re-emission: ● direction is in accordance with Lambert’s law ● quantity is determined by reflectance ρ, where ρ 1 α ● colour is determined by wavelength selective absorption ● luminance illuminance reflectance/π. Note that luminance is proportional to illuminance, and so is dependent on distance from the source. Observers B and C see the full saturation of the red pigment, but for observer A the redness is diluted, or even obliterated, by the regular reflection, depending on the relative luminances. A large diffuse light source would dilute the colour saturation equally for all observers. As shown in Figure 1.13, the proportion of incident light reflected from the surface of an amorphous material varies strongly with the angle of incidence. If the surface is smooth, regular reflection occurs and an image is formed, but generally this effect is apparent only for oblique viewing angles. Greatly increased levels of regular reflection can be achieved when an electro-conductive material is polished to an optically smooth surface, to the point where the high reflectance due to Fresnel reflection at high angles of incidence is achieved for all incident directions, so that variation with incidence angle is effectively eliminated. Examples of this have been referred to: polished metals such as silver, aluminium and chromium provide regular reflection that is independent of wavelength, while gold, brass and copper have wavelength dependent characteristics. Figure 1.17 illustrates the processes for an electro-conductive surface, in this case polished brass. There is no isotropic re-emis- sion, and all reflection is regular. Nonetheless, some absorption occurs, selectively at the shorter wavelengths, and this accounts for the characteristic metallic yellow colour of brass. In this case: ● direction is in accordance with laws of regular reflection ● colour is the colour of the source less the absorption losses ● luminance source luminance reflectance, where ρ 1 α.
Observation 30 Note that in this case the source image luminance is largely independent of direction, as well as being totally independent of source distance. Roughened or textured surfaces may par- tially or totally eliminate the reflected image, giving rise to vari- ous impressions of surface quality ranging from shiny, through sheen, to matt. Even so, the reflection process is quite different from isotropic re-emission, and so is the impression of colour. The metallic colours cannot be achieved by mixing pigments. Their appearance depends on modifying the source image, rather than the incident illumination. Interaction processes of light and matter To summarize the foregoing, the basic purpose of visual per- ception is to enable recognition of object attributes. The source of the information flowing through from the visual process is the interactions that occur when light encounters matter. The foregoing paragraphs describe the interaction processes that concern us, and in fact, they are quite limited in number. Wecanreducethenumberfurther.Althoughdiffractionscattering was mentioned, it involves interactions with very small particles Electric conductive material Glossy surface e.g. polished brass A B C 3) Regular reflection Absorption W B G R G R Y W 1) Regular reflection Figure 1.17: Reflection for an electro-conductive surface, in this case polished brass (after Hebbelynck, 1987)
Visible characteristics of objects 31 and is not of concern for indoor lighting. This leaves the follow- ing processes: ● Absorption is almost inevitable; in fact, the only proc- ess mentioned that does not involve some degree of light absorption is total internal reflection. It is easy to dismiss absorption as an unfortunate source of inefficiency, but it should be recognized that this is the basis of surface light- ness, and selective absorption is the origin of surface colour. ● Regular reflection: – from the surface of liquid-state (including amorphous) materials, the Fresnel reflection enables us to distinguish glossy from matt surfaces – from electro-conductive surfaces, which give us reflected sparkle and the metallic colours. ● Refraction and dispersion within liquid-state materials, which give shape clues and may reveal spectral colours. ● Reflection scattering: – from particles in gaseous-state or liquid-state materials, giving cloudiness or translucency – isotropic re-emission in surface layer of solid-state materi- als, which reveal lightness, hue and saturation attributes. This is not a complete list of ways in which light may interact with matter, but it covers the interaction processes that con- cern us. Lighting is the source of energy that stimulates these optical phenomena, and provides much of the information that enables the perception process to discriminate differences of opaque, transparent and translucent materials. Ways in which lighting may be controlled to selectively promote or suppress these processes will be discussed in the following chapter, and that discussion will include recognition of object attributes such as form and texture. In the meantime, it should be noted that phenomena such as diffraction and polarization can be demonstrated in an optical laboratory, but it would be unusual for these to be of concern to an architectural lighting designer. 1.3 Object characteristics and perceived attributes The processes of interaction of light and matter described in the previous section cause the events in the luminous environment
Observation 32 that are the stimulus for vision (Figure 1.1). The visual process is the source of information for the perceptual process, which gives rise to the sensation of a perceived environment in the viewer’s brain, and comprises recognized objects with distinct characteristics. Whereas a single pixel in the luminous envi- ronment can be specified completely in terms of luminance and chromaticity, the object that contains this pixel may be perceived to have characteristics of substance, utility, beauty, value, affection, and so forth. These interpretations which occur during every moment of our waking hours derive from recogni- tion of perceived attributes. The perceived attributes that may be associated with any ‘thing’ that is seen depend on the mode of appearance in which it is perceived (Tables 1.1, 1.2). We are so dependent on vision for understanding our sur- roundings that it is difficult, or even impossible, to imagine the world as perceived by less vision-dependent species. A bat is, quite simply, ‘as blind as a bat’, and yet it can navigate at speed through forests and within caves. We know that it employs a sonar system similar in principle to that used by ships to locate submarines or shoals of fish, but what does the world ‘look’ like to a bat? Obviously they cannot experience colour, but we are able to recognize black and white images. However, can we even imagine a three-dimensional world with no light or shade? What is the image in a bat’s brain as it swoops between obstacles to intercept an insect in mid-air? Although this is not a lighting issue, it may cause us to think about how we perceive our environments, and the role of lighting in that process. It is important to appreciate that something that has the physi- cal properties of an object is not necessarily perceived in one of the ‘object modes’. Figure 1.18 shows two views of a lumi- naire. In case (a), the surface form, texture and lightness of the glass shade are all clearly visible. We could assess its lightness on a scale of zero to ten. We could make a reasonable guess of its reflectance. We perceive this shade in object surface mode. However, our perception of the shade is different in case (b). We recognize that it is still the same shade, but now that it glows we really have no idea of its texture, and it is quite meaningless to discuss its lightness. We could certainly discuss its brightness, and this lies at the heart of our changed percep- tion of the shade. We are now perceiving it in illuminant mode, and as indicated in Table 1.2, the range of associated attributes is different, and furthermore, the number of attributes is
Visible characteristics of objects 33 reduced. Of course we know that the glass shade is not self- luminous, but nonetheless, the ‘thing’ that our intellect informs us is a trans-illuminated object is perceived as if it is the source of light. Furthermore, an object may be perceived simultaneously in more than one mode. The glass object shown in Figure 1.19 is perceived to have both surface and volume attributes. The two cases show how a change of background can give a dif- ferent balance of the perceived attributes. Without disturbing the light sources, the object can be presented to give emphasis (a) (b) Figure 1.18: In view (a) the glass shade is perceived in object mode and has attributes of lightness and texture. In view (b) the shade is perceived in illuminant mode and has the attribute of brightness (a) (b) Figure 1.19: (a) and (b) How do we perceive transparent media? This glass vase is perceived in both surface and volume modes, and it can be seen in (a) that the directional lighting reveals the surface attribute of gloss while hue is perceived in volume mode. For view (b) only the background has been changed. The surface highlights are still evident, but it can be seen that the chromatic attributes visible in (a) are revealed not only by transmitted light but also by internally reflected light
Observation 34 to its internal colour or to the smooth glossiness of its sur- face. Differences of this sort are explored in the following chapters, but we should note that throughout these changes our understanding of the object’s fundamental nature remains intact. The differences of appearance may influence our sense of appreciation of the object, but basically it remains a coloured glass vase. Such is the power of the perceptual system to rec- ognize object attributes that, providing there is sufficient light to enable the visual process to operate effectively, viewing con- ditions have to be severely constrained for viewers to be con- fused over object recognition. Consider for a moment; if we could present the glass vase in Figure 1.19 so that the surface attributes were completely invisible, what would a viewer per- ceive? Is it possible to imagine the volume attributes without a bounding surface? Fortunately the perceptual process very rarely presents us with such confusion. The basic purpose of visual perception is to enable recognition of object attributes. Each attribute is associated with certain optical properties of the object, and is recognized by a charac- teristic interaction with light. Generally, the prime purpose of indoor lighting is to enable recognition of stable environments comprising recognized objects within which people can orien- tate themselves and navigate with confidence. However, the perceptual process is very adept at doing this, and copes well over a vast range of visual conditions. On one hand, this per- mits many lighting solutions that provide for no more than suf- ficiency of illumination to be found acceptable. On the other hand, it offers opportunities for designers to apply imagination to selecting object attributes for emphasis without compro- mising the basic requirements that lighting for occupied space must fulfil. It needs to be noted that the perceptual process involves plac- ing interpretations upon the visible effects of optical inter- actions. Every ‘thing’ that we perceive in our surroundings is recognized to have certain attributes, and the range of attributes that may be associated with a ‘thing’ depends upon the mode in which it is perceived. Objects perceived in the sur- face and volume modes have the greatest range of associated attributes, and this is where lighting designers often look to for opportunities to influence the appearance of surroundings. In Figure 1.19, the light that is reflected towards the viewer from the surface has undergone a different reflection process from the light that has been reflected or refracted within the volume of the object. While some lighting designers work on the basis
Visible characteristics of objects 35 of an intuitive understanding of this difference, a designer who understands the optical nature of this difference is in a stronger position to control the processes, and to select attributes for emphasis. To explore how this is done, we move on from characteristics of objects to characteristics of lighting, which opens up more opportunities for influencing the appearance of surroundings.
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rare, probably unique. On taking it up to answer H. J.'s Query, I found some matter relating to the German academies of the seventeenth century, which I think may be interesting. Mr. Hallam (Literature of Europe, IV. v. 9.) says: The Arcadians determined to assume every one a pastoral name and a Greek birthplace; to hold their meetings in some verdant meadow, and to mingle with all their own compositions, as far as possible, images from pastoral life; images always agreeable, because they recall the times of primitive innocence. The poetical tribe adopted as their device the pipe of seven reeds bound with laurel, and their president, or director, was denominated General Shepherd or Keeper—Custode Generale. He slightly mentions the German academics of the sixteenth century (III. ix. 30.), and says: It is probable that religious animosities stood in the way of such institutions, or they may have flourished without obtaining much celebrity. The academy of Pegnitz-shepherds (Pegnitzshäfer-orden) took its name from the little river Pegnitz which runs through Nuremberg. Herr Sigmond von Birken was elected a member in 1645. He chose Floridan as his pastoral name, and the amaranth as his flower. In 1658 he was admitted to the Palm Academy (Palmen-orden), choosing the name Der Erwacsene (the adult?), and the snowdrop. In 1659, a vacancy having occurred in the Pegnitz-Herdsmen (Pegnitz-Hirten) he was thought worthy to fill it, and in 1679 he received the diploma of the Venetian order of the Recuperati. He died in 1681. This, and what can be hung upon it, is Die Betrübte Pegnitz, a dialogue of 406 pages. It opens with a meeting of shepherds and shepherdesses, who go in and out of their cottages on the banks of the Pegnitz, and tell one another, what all seem equally well acquainted with, the entire life of their deceased friend.
It would not be easy to find a work more clumsy in conception and tasteless in execution. Herr von Birken seems to have been a prosperous man, and to have enjoyed a high pastoral reputation. His works are enumerated, but the catalogue looks ephemeral. There is, however, one with a promising title: Die Trockene Trunkenheit, oder die Gebrauch und Missbrauch des Tabacks. His portrait, as Der Erwachsene, is prefixed. It has not a shepherd-like look. He seems about fifty, with a fat face, laced cravat, and large flowing wig. There are twenty-four emblematical plates, rather below the average of their time. As so secondary a town as Nuremberg had at least three academies, we may infer that such institutions were abundant in Germany, in the seventeenth century: that of the Pegnitz shepherds lasted at least till the beginning of the eighteenth. In Der Thörichte Pritschmeister, a comedy printed at Coblenz, 1704, one of the characters is Phantasirende, ein Pegnitz Schäffer, who talks fustian and is made ridiculous throughout. The comedy is von Menantes. I have another work by the same author: Galante, Verliebte, und Satyrische Gedichte, Hamburg, 1704. I shall be very glad to be told who he was, as his versification is often very good, and his jokes, though not graceful, and not very laughable, are real. H. B. C. U. U. Club. MARRIAGES EN CHEMISE.—MANTELKINDER.— LEGITIMATION. (Vol. vi., pp. 485. 561.) The popular error on the legal effect of marriage en chemise is, I think, noticed among other vulgar errors in law in a little book published some twenty years ago under the name of Westminster Hall, to which a deceased lawyer of eminence, then young at the
bar, was a contributor. I believe the opinion to be still extensively prevalent, and to be probably founded, not exactly in total ignorance, but in a misconception, of the law. The text writers inform us that the husband is liable for the wife's debts, because he acquires an absolute interest in the personal estate of the wife, c. (Bacon's Abridgment, tit. Baron and Feme.) Now an unlearned person, who hears this doctrine, might reasonably conclude, that if his bride has no estate at all, he will incur no liability; and the future husband, more prudent than refined, might think it as well to notify to his neighbours, by an unequivocal symbol, that he took no pecuniary benefit with his wife, and therefore expected to be free from her pecuniary burdens. In this, as in most other popular errors, there is found a substratum of reason. With regard to the other vulgar error, noticed at the foot of Mr. Brooks' communication (p. 561.), that all children under the girdle at the time of marriage are legitimate, the origin of it is more obvious. Every one knows of the legitimatio per subsequens matrimonium of the canonists, and how the barons assembled in parliament at Merton refused to engraft this law of the Church on the jurisprudence of England. But it is not perhaps so well known that, upon such a marriage the premature offspring of the bride and bridegroom sometimes used to perform a part in the ceremony, and received the nuptial benediction under the veil or mantle of the bride or the pallium of the altar. Hence the children so legitimated are said to have been called by the Germans Mantelkinder. The learning on this head is to be found in Hommel's Jurisprudentia Numismatibus Illustrata (Lipsiæ, 1763), pp. 214-218., where the reader will also find a pictorial illustration of the ceremony from a codex of the Novellæ in the library of Christian Schwarz. The practice seems to have been borrowed from the form of adopting children, noticed in the same work and in Ducange, verb. Pallium, Pallio cooperire; and in Grimm's Deut. Rechts Alterth., p. 465. Let me add a word on the famous negative given to the demand of the clergy at Merton. No reason was assigned, or, at least, has been
recorded, but a general unwillingness to change the laws of England. As the same barons did in fact consent to change them in other particulars, this can hardly have been the reason. Sir W. Blackstone speaks of the consequent uncertainty of heirship and discouragement of matrimony as among the causes of rejection,— arguments of very questionable weight. Others (as Bishop Hurd, in his Dialogues) have attributed the rejection to the constitutional repugnance of the barons to the general principles of the canon and imperial law, which the proposed change might have tended to introduce,—a degree of forethought and a range of political vision for which I can hardly give them credit, especially as the great legal authority of that day, Bracton, has borrowed the best part of his celebrated Treatise from the Corpus Juris. The most plausible motive which I have yet heard assigned for this famous parliamentary negative on the bishops' bill at Merton, is suggested (quod minimè reris!) in an Assistant Poor-Law Commissioner's Report (vol. vi. of the 8vo. printed series), viz. that bastardy multiplied the escheats which accrued to medieval lords of manors. E. Smirke. A venerable person whose mind is richly stored with shreds and patches of folk-lore and local antiquities, on seeing the curious marriage entry (p. 485.), has furnished me with the following explanation. It is the popular belief at Kirton in Lindsey that if a woman, who has contracted debts previous to her marriage, leave her residence in a state of nudity, and go to that of her future husband, he the husband will not be liable for any such debts. A case of this kind actually occurred in that highly civilised town within my informant's memory; the woman leaving her house from a bedroom window, and putting on some clothes as she stood on the top of the ladder by which she accomplished her descent. K. P. D. E.
In that amusing work, Burn's History of the Fleet Marriages, p. 77., occurs the following entry:—The woman ran across Ludgate Hill in her shift; to which the editor has added this note:—The Daily Journal of 8th November, 1725, mentions a similar exhibition at Ulcomb in Kent. It was a vulgar error that a man was not liable to the bride's debts, if he took her in no other apparel than her shift. J. Y. Saffron Walden. EDITIONS OF THE PRAYER-BOOK PRIOR TO 1662. (Vol. vi., pp. 435. 564.) As Mr. Sparrow Simpson invites additions to his list from all quarters, I send him my contribution: and as I see that he has included translations of our Liturgy into other languages, I do the same: 1552. Worcester. Jo. Oswen. Folio. 1560. London. Jugge and Cawood. 4to. 1565. London. Jugge and Cawood. 8vo. 1607. London. Folio. 1629. London. Folio. 1629. Cambridge. Folio. 1632. London. 4to. 1633. London. 4to. 1634. London. Folio. 1635. London. 4to. 1638. Cambridge. 4to. 1639. London. Folio. 1641. London. 4to. 1660. Cambridge. Folio. 1644. The Scotch, by Laud and the Scotch bishops. Printed by John Jones. 8vo.
1551. Latine versa, per Alex. Absium. Lipsiæ. 4to. 1594. London. 8vo. S. A. by Reginald Wolfe. London. 4to. 1638. In Greek. London. 8vo. 1616. In French. London. 4to. 1608. In Irish. Dublin. Folio. 1612. In Spanish. London. 4to. 1621. In Welsh. London. 4to. All the foregoing editions are in the Bodleian Library. I may add to them the following three: 1.—1551. Dublin, by Humfrey Powell. Folio 2.—1617(?). Dublin. Company of Stationers. 4to 3.—1637. Dublin. The first of these, which is the first book printed in Ireland, is extremely rare. I believe only two copies are certainly known to exist; one of which is in the library of Trinity College, Dublin; and the other in that of Emmanuel College, Cambridge. Both are in very fine condition. The second is in my possession. The book is quite perfect; but some wiseacre has carefully erased the date. The Almanac for xxvi Yeares tells nothing, being for the years 1603 to 1628. But the book contains a prayer for Frederick, the Prince Elector Palatine, and the Lady Elizabeth, his wife, with their hopeful issue. He married the princess in 1613; and in 1619 he was elected King of Bohemia, and thenceforward would be prayed for under his higher title. If the Sunday letter in the calendar is to be trusted, the book was printed (according to De Morgan's Book of Almanacs) in 1617. The Dublin Society of Stationers was established in that year; and it is not unlikely that they commenced their issues with a Prayer-Book. I have never seen nor heard of another copy, with which I might compare mine, and thus ascertain its date.
The third, of 1637, is reported; but I have never met with it. H. Cotton. Thurles. ETYMOLOGY OF PEARL. (Vol. vi., p. 578.) The inquiry of your correspondent Ifigfowl respecting the etymology of the word pearl does not admit of a simple answer. The word occurs in all the modern languages, both Romance and Teutonic: perla, Ital. and Span.; perle, French and German, whence the English pearl. Adelung in v. believes the word to be of Teutonic origin, and considers it as the diminutive of beere, a berry. Others derive it from perna, the Latin name of a shell-fish (see Ducange in perlæ; Diez, Grammatik der Romanischen Sprachen, vol. i. p. 235.). Neither of these derivations is probable: it is not shown that beere had a diminutive form, and perna was a local and obscure name: see Pliny, N. H. xxxii. ad fin. Salmasius (Exercit. Plin., p. 40. ed. 1689) thinks that perla is formed from perula, for sperula, the diminutive of sphæra. A more probable origin is that the word is formed from the Latin pirum, as suggested by Diez, in allusion to the pear-shaped form of the pearl. Ducange in v. says that the extremity of the nose was called pirula nasi, from its resemblance to the form of a pear. But pirus was used to denote a boundary-stone, made in a pyramidal shape (Ducange in v.); and this seems to have been the origin of the singular expression pirula nasi, as being something at the extremity. Another supposition is, that the word perla is derived from the Latin perula, the diminutive of pera, a wallet. A wallet was a small bag hung round the neck; and the word perula, in the sense of a small bag, occurs in Seneca and Apuleius. The analogy of shape and mode of wearing is sufficiently close to suggest the transfer of the name. Perula and perulus are used in Low Latin in the sense of pearl. Ducange cites a passage from a hagiographer, where perula
means the white of the eye, evidently alluding to the colour of the pearl. The choice seems to lie between perula as the diminutive of pera or of pirum. Neither derivation is improbable. It is to be observed that the modern Italian form of pirum, the fruit of the pear, is pera; the modern feminine noun being, as in numerous other cases, formed from the plural of the Latin neuter noun (see Diez, ib. vol. ii. p. 19.). The analogy of unio (to which I shall advert presently) supports the derivation from the fruit; the derivation from pera, a wallet, is, on merely linguistical grounds, preferable. The Greek name of pearl is μαργαρίτης, originally applied to a precious stone, and apparently moulded out of some oriental name, into a form suited to the Greek pronunciation. Scott and Liddell in v. derive it from the Persian murwari. Pliny, H. N. ix. 56., speaking of the pearl, says: Apud Græcos non est, ne apud barbaros quidem inventores ejus, aliud quam margaritæ. The Greek name Margarita was used by the Romans, but the proper Latin name for the pearl was unio. Pliny (ibid.) explains this word by saying that each pearl is unique, and unlike every other pearl. Ammianus Marcellinus (lib. xxiii. ad fin.) thinks that pearls were called uniones, because the best were found single in the shell; Solinus (c. 53.) because they were always found single. The more homely explanation of Salmasius seems, however, to be the true one; namely, that the common word for an onion, growing in a single bulb, was transferred to the pearl (Exercit. Plin., pp. 822-4.; Columella de R. R. xii. 10.). The ancient meaning of unio is still preserved in the French ognon. L. Your correspondent asks the etymon of our English word pearl. It would not be uninteresting to learn, at the same time, at what period pearl came into general use as an English word? Burton, who wrote his Anatomy in the reign of James I., uses the word union (from the Latin unio) instead of pearl (Anat. Melanc., vol. ii. part 2. sec. 3. mem. 3., and ib., p. 2. sec. 4. mem. 1. subs. 4.). In the latter
passage he says Those smaller unions which are found in shells, amongst the Persians and Indians, are very cordial, and most part avail to the exhilaration of the heart. The Latin term unio differs from margarita, in so far as it seems to have been applied by Pliny to distinguish the small and ill-shaped pearls, from the large round and perfect, which he calls margaritæ. And in his ninth book, c. 59., he defines the difference philologically, as well as philosophically. Philemon Holland, who published his translation of Pliny in 1634, about thirteen years after Burton published the first edition of his Anatomy, uses the word pearl indifferently as the equivalent both of margarita and unio. Query: Was the word union generally received in England instead of pearl in Burton's time, and when did it give place to it? J. Emerson Tennant. MARTIN DRUNK. (Vol. v., p. 587.) Has not the following song something to do with the expression Martin drunk? It is certainly cotemporary with Thomas Nash the Elizabethan satirist, and was long a favourite three man's song. It is copied from Deuteromelia, or the Second Part of Musick's Melodie, 4to., 1609: MARTIN SAID TO HIS MAN. Martin said to his man, Fie! man, fie! O Martin said to his man, Who's the foole now? Martin said to his man, Fill thou the cup, and I the can;
Thou hast well drunken, man, Who's the foole now? I see a sheepe shering corne, Fie! man, fie! I see a sheepe shering corne, Who's the foole now? I see a sheepe shering corne, And a cuckold blow his horne; Thou hast well drunken, man, Who's the foole now? I see a man in the moone, Fie! man, fie! I see a man in the moone; Who's the foole now? I see a man in the moone, Clowting of St. Peter's shoone; Thou hast well drunken, man, Who's the foole now? I see a hare chase a hound, Fie! man, fie! I see a hare chase a hound, Who's the foole now? I see a hare chase a hound, Twenty mile above the ground; Thou hast well drunken, man, Who's the foole now? I see a goose ring a hog, Fie! man, fie! I see a goose ring a hog, Who's the foole now? I see a goose ring a hog, And a snayle that did bite a dog;
Thou hast well drunken, man, Who's the foole now? I see a mouse catch the cat, Fie! man, fie! I see a mouse catch the cat, Who's the foole now? I see a mouse catch the cat, And the cheese to eate the rat; Thou hast well drunken, man, Who's the foole now? Edward F. Rimbault. GÖTHE'S REPLY TO NICOLAI. (Vol. vi., p. 434.). Had M. M. E. gone to the fountain-head, and consulted Göthe's own statement in his autobiography, he would have seen in the Werke, vol. xxvi. p. 229., that Mr. Hayward's note was not written with that writer's usual care. Göthe does not say that his reply to Nicolai's Joys of Werter, though circulated only in MS., destroyed N.'s literary reputation: on the contrary, he says that his squib (for it was no more) consisted of an epigram, not fit for communication, and a dialogue between Charlotte and Werter, which was never copied, and long lost; but that this dialogue, exposing N.'s impertinence, was written with a foreboding of his sad habit, afterwards developed, of treating of subjects out of his depth, which habit, notwithstanding his indisputable merits of another kind, utterly destroyed his reputation. This was most true: and yet all such assertions must be taken in a qualified sense. Nearly thirty years after this was written I partook of the hospitality of N. at Berlin. It was in 1803, when he was at the head, not of the Berlin literati, but of the book-manufactory of Prussia. He was then what, afterwards and elsewhere, the Longmans, Murrays, Constables, Cottas, and
Brockhauses were,—the great publisher of his age and country. The entrepreneur of the Neue Deutsche Bibliothek may be compared with the publishers of our and the French great Cyclopædias, and our Quarterly Reviews. It was unfortunate for the posthumous reputation of the great bibliopolist that he, patronising a school that was dying out, made war on the athletes of the rising school. He assailed nearly every great man, philosopher or poet, from Kant and Göthe downwards, especially of the schools of Saxony, Swabia, and the free imperial cities. No wonder that he became afterwards what Macfleckno and Colly Cibber had been to Dryden and Pope. In some dozen of the Xenien of Göthe and Schiller, in 1797, he was treated as the Arch- Philistine. M. M. E. characterises him as the friend and fellow-labourer of Lessing. Now Lessing was incomparably the most eminent littérateur of the earlier part of that age,—the man who was the forerunner of the philosophers, and whose criticisms supplied the place of poetry. The satirists of the Xenien affect to compassionate Lessing, in having to endure a companion so forced on him as Nicolai was, whom they speak of as a thorn in the crown of the martyr. The few who care for the literary controversies of the age of Göthe in Germany will be greatly assisted by an edition of the Xenien, with notes, published at Dantzig, 1833. H. C. R. PHOTOGRAPHIC CORRESPONDENCE. Processes upon Paper.—The favourable manner in which the account I have given of the Collodion process has been received, not only by your readers in general, as has been evinced by many private letters, but also by the numerous correspondents it has drawn forth, induces me, after some little delay, to request space for a description
of the following processes upon paper. In giving these I wish it to be understood that I may offer but little that is original, my object being to describe, as plainly as I possibly can, these easy methods, and to make no observation but what I have found to be successful in my own hands. I have had the good fortune to obtain the friendship of some of the most successful photographers of the day; and taking three very eminent ones, I find they have each some peculiarities in his mode of manipulation, varying with each other in the strength of the solutions employed, and producing results the most agreeable to their respective tastes. Reviewing these different processes in my own mind, and trying with patience the various results, I conclude that the following quantities are calculated to produce an adequate degree of sensibility in the paper, and yet to allow it to be prepared for the action of light for many hours previous to its use, and yet with more certainty than any other I am acquainted with. I think I may always depend upon it for twenty-four to thirty-six hours after excitement, and I have seen good pictures produced upon the third day. I believe it is a rule which admits of no contradiction, that the more you dilute your solution, the longer the excited paper will keep; but in proportion to its diminished sensibility, the time of exposure must be prolonged, and therefore I am, from this waste of time and other reasons, disposed to place much less value upon the wax-paper process than many do. The process I am about to describe is so simple, and I hope to make it so intelligible to your non-photographic readers, that a perfect novice, using ordinary care, must meet with success; but should I fail doing so upon all points, any information sought through the medium of N. Q. shall meet with explanation from myself, if not from other of your experienced correspondents, whose indulgence I must beg should the communication be deemed too elementary, it being my earnest desire to point out to archæologists who are desirous of acquiring this knowledge, how easily they themselves may practise this beautiful art, and possess those objects they would desire to preserve, in a far more truthful state than could be otherwise accomplished.
I have not myself met that uniform success with any other paper that I have with Turner's photographic of Chafford Mills: a sheet of this divided into two portions forms at the same time a useful and also a very easily-managed size, one adapted for most cameras, forming a picture of nine inches by seven, which is adequate for nearly every purpose. Each sheet being marked in its opposite corners with a plain pencil-mark on its smooth side (vide antè, p. 372.), the surface for all future operations is in all lights easily discerned. In my instructions for printing from collodion negatives, a form of iodized paper was given, which, although very good, is not, I think, equal to the following, which is more easily and quickly prepared, exhibits a saving of the iodide of potassium, and is upon the whole a neater mode. Take sixty grains of nitrate of silver and sixty grains of iodide of potassium; dissolve each separately in an ounce of distilled water; mix together and stir with a glass rod. The precipitate settling, the fluid is to be poured away; then add distilled water to the precipitate up to four ounces, and add to it 650 grains of iodide of potassium, which should re-dissolve the precipitated iodide of silver, and form a perfectly clear solution; but if not, a little more must be carefully added, for this salt varies much, and I have found it to require 720 grains to accomplish the desired object. The fluid being put into a porcelain or glass dish, the paper should be laid down upon its surface and immediately removed, and being laid upon a piece of blotting-paper with the wet surface uppermost, a glass rod then passed over it to and fro ensures the total expulsion of all particles of air, which will frequently remain when the mere dipping is resorted to. When dry, this paper should be soaked in common water for three hours, changing the water twice or thrice, so as to remove all the soluble salts. It should then be pinned up to dry, and, when so, kept in a folio for use. I have in this manner prepared from sixty to eighty sheets in an evening with the greatest ease. It keeps good for an indefinite time, and, as all experienced photographers are aware, unless you possess good iodized paper,
which should be of a primrose colour, you cannot meet with success in your after-operations. Iodized paper becomes sometimes of a bright brimstone colour when first made; it is then very apt to brown in its use, but tones down and improves by a little keeping. To excite this paper, dissolve thirty grains of nitrate of silver in one ounce of distilled water, and add a drachm and a half of glacial acetic acid; of this solution take one drachm, and one drachm of saturated solution of gallic acid, and add to it two ounces and a half of distilled water. The iodized surface of the paper may then be either floated on the surface of the aceto-nitrate of silver or exciting fluid, and afterwards a rod passed over, as was formerly done in the iodizing, or the aceto-nitrate may be applied evenly with a brush; but in either instance the surface should be immediately blotted off; and the same blotting-paper never used a second time for this, although it may be kept to develop on and for other purposes. It will be scarcely needful to observe that this process of exciting must be performed by the light of a candle or feeble yellow light, as must the subsequent development. The excited paper may be now placed for use between sheets of blotting-paper; it seems to act equally well either when damp or when kept for many hours, and I have found it good for more than a week. The time for exposure must entirely depend upon the degree of light. In two minutes and a half a good picture may be produced; but if left exposed for twenty minutes or more, little harm will arise; the paper does not solarize, but upon the degree of image visible upon the paper depends the means of developing. When long exposed, a saturated solution of gallic acid only applied to the exposed surfaces will be sufficient; but if there is little appearance of an image, then a free undiluted solution of aceto-nitrate may be used, in conjunction with the gallic acid, the former never being in proportion more than one-third. If that quantity is exceeded, either a brownish or an unpleasant reddish tint is often obtained. These negatives should be fixed by immersing them in a solution of hyposulphite of soda, which may be of the strength of one ounce of
salt to eight ounces of water—the sufficiency of immersion being known by the disappearance of the yellow colour, and when they have been once immersed they may be taken to the daylight to ascertain this. The hyposulphite must now be perfectly removed by soaking in water, which may extend to several hours; but this may be always ascertained by the tongue, for, if tasteless, it has been accomplished. If it is deemed advisable—which I think is only required in very dark over-done pictures—to wax the negative, it is easily managed by holding a piece of white wax or candle in front of a clean iron rather hot, and passing it frequently over the surface. The superabundant wax being again removed by passing it between some clean pieces of blotting-paper. Although the minuter details can never be acquired by this mode which are obtained by the collodion process, it has the advantage of extreme simplicity, and by the operator providing himself with a bag or square of yellow calico, which he can loosely peg down to the ground when no other shade is near, to contain spare prepared papers, he can at any future time obtain a sufficient number of views, which afterwards he can develop at his leisure. It requires no liquids to be carried about with you, nor is that nice manipulation required which attends the collodion process. The wax-paper process has been extolled by many, and very successful results have been obtained: the paper has the undoubted advantage of keeping after being excited much longer than any other; but, from my own experience, just so much the weaker it is made, and so as to safely rely upon its long remaining useful, so it is proportionally slower in its action. And I have rarely seen from wax negatives positives so satisfactory in depth of tone, as from those which have been waxed after being taken on ordinary paper. It is all very well for gentlemen to advocate a sort of photographic tour, upon which you are to go on taking views day after day, and when you return home at leisure to develop your past proceedings: I never yet knew one so lukewarm in this pursuit as not to desire to know, at his earliest possible opportunity, the result of his labours; indeed,
were not this the case, I fear disappointment would more often result than at present, for I scarcely think any one can exactly decide upon the power of the light of any given day, without having made some little trial to guide him. I have myself, especially with collodion, found the action very rapid upon some apparently dull day; whilst, from an unexplained cause, a comparatively brighter day has been less active in its photographic results. As in the previous process, I would strongly advise Turner's paper to be used, and not the thin French papers generally adopted, because I find all the high lights so much better preserved in the English paper. It may be purchased ready waxed nearly as cheap as it may be done by one's self; but as many operators like to possess that which is entirely their own production, the following mode will be found a ready way of waxing:—Procure a piece of thick smooth slate, a trifle larger than the paper to be used; waste pieces of this description are always occurring at the slate works, and are of a trifling value. This should be made very hot by laying it close before a fire; then, covered with one layer of thick blotting-paper, it will form a most admirable surface upon which to use the iron. Taking a piece of wax in the left hand, an iron well heated being pressed against it, it may rapidly be made to flow over the whole surface with much evenness, the surplus wax being afterwards removed by ironing between blotting- paper. When good, it should be colourless, free from gloss, and having the beautiful semi-transparent appearance of the Chinese rice-paper. To iodize the paper completely, immerse it in the following solution: Iodide of potash 200 grains. Mannite 6 drachms. Cyanide of potash 5 grains. Distilled water 20 ounces. Allow it to remain three hours, taking care that air-particles are perfectly excluded, and once during the time turning over each sheet of paper, as many being inserted as the fluid will conveniently cover, as it is not injured by after keeping. It should be then removed from
the iodide bath, pinned up, and dried, ready for use. When required to be excited, the paper should, by the light of a candle, be immersed in the following solution, where it should remain for five minutes: Nitrate of silver 4 drachms. Glacial acetic acid 4 drachms. Distilled water 8 ounces. Being removed from the aceto-nitrate bath, immerse it into a pan of distilled water, where let it remain about a quarter of an hour. In order to make this paper keep a week or two, it must be immersed in a second water, which in point of fact is a mere reduction of the strength of the solutions already used; but for ordinary purposes, and when the paper is to be used within three or four days, one immersion is quite sufficient, especially as it does not reduce its sensitiveness in a needless way. It may now be preserved between blotting-paper, free from light, for future use. The time of exposure requisite for this paper will exceed that of the ordinary unwaxed, given in the previous directions. The picture may be developed by a complete immersion also in a saturated solution of gallic acid; but should it not have been exposed a sufficient time in the camera, a few drops of the aceto-nitrate solution added to the gallic acid greatly accelerates it. An excess of aceto-nitrate often produces an unpleasant red tint, which is to be avoided. Instead of complete immersion, the paper may be laid upon some waste blotting-paper, and the surface only wetted by means of the glass rod or brush. The picture may now be fixed by the use of the hyposulphite of soda, as in the preceding process. It is not actually necessary that this should be a wax-paper process, because ordinary paper treated in this way acts very beautifully, although it does not allow of so long keeping for use after excitement; yet it has then the advantage, that a negative may either be waxed or not, as shall be deemed advisable by its apparent depth of action.
Hugh W. Diamond. Exhibition of recent Specimens of Photography at the Society of Arts. —This exhibition, to which all interested in the art have been invited to contribute, was inaugurated by a conversazione at the Society's rooms, on the evening of Wednesday, the 22nd of December: the public have since been admitted at a charge of sixpence each, and it will continue open until the 8th of January. We strongly recommend all our friends to pay a visit to this most delightful collection. By our visit at the crowded conversazione, and another hasty view since, we do not feel justified to enter into a review and criticism of the specimens so fully as the subject requires; but in the mean time we can assure our archæological readers that they will find there such interesting records of architectural detail, together with views of antiquities from Egypt and Nubia, as will perfectly convince them of the value of this art with reference to their own immediate pursuits. Those who feel less delight in mere antiquity will be gratified to see, for the first time, that there are here shown photographs which aim at more than the bare copying of any particular spot; for many of the pictures here exhibited may rank as fine works of art. We feel much delicacy and hesitation in mentioning any particular artist, where so many are entitled to praise, especially in some particular departments. We could point out pictures having all the minute truthfulness of nature, combined with the beautiful effects of some of the greatest painters. We must, however, direct especial attention to the landscapes of Mr. Turner, the views in the Pyrenees by Mr. Stewart, and one splendid one of the same locality by Le Gray. Mr. Buckle's views in paper also exhibit a sharpness and detail almost equal to collodion; as do the various productions of Mr. Fenton in wax paper. The effects obtained also by Mr. Owen of Bristol appear to be very satisfactory: why they are, with so much excellence, called experimental, we cannot tell. In collodion Mr. Berger has exhibited some effective portraits; and we think the success of Mr. De la Motte has been so great, that in some of his productions little remains to be desired. We cannot conclude
this brief notice without directing attention to the minuteness and pleasing effect of the views in Rome by M. Eugène Constant, which are also from collodion; as also the specimens from albumen negatives of M. Ferrier; and, lastly, to the pleasant fact that lady amateurs are now practising this art,—very nice specimens being here exhibited by the Ladies Nevill, whose example we shall hope to see followed.
Replies to Minor Queries. Quotation in Locke (Vol. vi., p. 386.).—The words Si non vis intelligi non debes legi were, I believe, the exclamation of St. Jerome, as he threw his copy of Persius into the fire in a fit of testiness at being unable to construe some tough lines of that tough author. I set down this reply from memory, and am unable to give the authority for it. W. Fraser. Pic-nic (Vol. vi., pp. 152. 518.).—The Query of A. F. S. (p. 152.) as to the etymology of pic-nic still remains unanswered. The Note of W. W. (p. 518.) merely refers to the time (1802) when pic-nic suppers first became fashionable in England. Under a French form, the word appears in a speech of Robespierre's, quoted in the British and Foreign Review for July, 1844, p. 620.: C'est ici qu'il doit m'accuser, et non dans les piques-niques, dans les sociétés particulières. An earlier instance occurs in one of Lord Chesterfield's letters (No. 167.), dated October 1748. Jaydee. Discovery at Nuneham Regis (Vol. vi., pp. 386. 488. 558.).— Nuneham Regis was granted to John Dudley, Duke of Northumberland, in the seventh year of King Edward VI.; but as it was forfeited on his attainder, in the first year of Queen Mary, and immediately granted by her to Sir Rowland Hill, knight, and citizen of London, from whom Sir Thomas Leigh, knight, and alderman of London, almost immediately acquired it; and as he exercised the right of presentation to the vicarage in the first year of the reign of Queen Elizabeth, there is no probability of the body of John, Duke of Northumberland, being removed from the Tower of London to Newnham.
The letters T. B. on the clothes on the body at Nuneham are distinctly worked in Roman capitals, like those on a common sampler. I have seen them. J. S.s. Door-head Inscriptions (Vol. vi., p. 543.).—
Sit mihi nec glis servus nec hospes hirudo. From servant lazy as dormouse, Or leeching guest, God keep my house. Mr. Woodward tells us that he quotes this inscription from memory: it is so very pertinent that it seems a pity even to hint a correction, but, as I read it, it seemed partly familiar to me, and I find something so like the latter part of it in two ancient authors, that I am tempted to inquire whether he may not have omitted one letter, which alters the sense as given above, and yet gives a sense as good. Among the Symbols of Pythagoras, I read the following: Ομωροφιους χελιδωνας μη εχειν. Domesticas hirundines ne habeto. To the same effect (but, strange to say, without any reference to Pythagoras' dictum), we find it in the proverbia of Polydore Virgil (A.D. 1498): Hirundo suscipienda non est. and the exposition is the same in both: Hirundo garrula semper, i.e. garruli et tumigeri homines recipiendi non sunt. I find no original for the former part of the inscription. Probably Mr. Woodward will agree with me, that it is difficult to decide whether a greedy or a gossipping guest would be the worst household infliction; but as a careful householder might well deprecate either, as matter of curiosity perhaps he would refer to the original
inscription again, and decide whether he has or has not omitted an n. A. B. R. Belmont. Stratford Parsonage, Wilts: Parva sed apta Domino. 1675. Montacute House, Somerset: Through this wide opening gate None come too soon, none go too late. And yours. Sudbury House, Derbyshire: Omne Bonum Dei Donum. At Verona: Patet Janua, Cor magis. The next I have seen somewhere: Detur digniori. H. T. Ellacombe. Clyst St. George. Cross and Pile (Vol. vi., pp. 386. 513.).—The pile is invariably on the obverse or head side of a coin; and pile or poll both mean the head, from whence the poll tax and poll groat—a tax paid by the head, or a personal tax, of which we have an historical example of its collector in the case of Wat Tyler.
Ruding, in Annals of the Coinage, vol. ii. p. 119., 8vo., edit. 1819, states that Ed. I. A.D. 1304, in the delivering out the stamps for the coinage, orders that three piles and six crosses shall be given. It is well known to all numismatists that all, or most early coins, both Saxon and English, had a head on the obverse and a cross on the reverse—the latter being placed on the coins as symbolical of Christianity. Pile also means the hair, or any filament: as the pile of velvet, the nap of woollen cloth, c. And Jamieson, in his Scotch Dictionary, says: Pile. The soft hair which first appears on the chins of young men. Coles, Ashe, Webster, and others give the same meaning. The superstitious effect of the cross as a charm or amulet is well known; from whence the saying: I have never a cross in my purse to keep the Devil away. Again: Priests were coin-proof against the Devil, they never being without money; of course, always had a cross in their pocket.— Gilpin's Beehive of the Romish Church, 1636, p. 251. And Nash, in the Supplication of Pierce Penniless to the Devil, makes Pierce to say: Whereas your impious excellence hath had the poore tenement of my purse anytime this half year for your dancing schole, and he, notwithstanding, hath received no penye nor crosse for farme, c. And the poet Skelton says:
. . . . . . . . and in his pouche, The Devil might dance therein for any crouche. P. 71. Trusting the above will be satisfactory to D. W. S., I beg to conclude, thinking, you will say I have already made much ado about nothing. Goddard Johnson. Rhymes upon Places (Vol. vi. p. 281.).—Perhaps you will think the following rhymes upon places worth insertion: I stood upon Eyemouth Fort, And guess ye what I saw? Fairmiside and Furmintong, Neuhouses and Cocklaw, The fairy fouk o' Fosterland, The witches o' Edincran, The bly-rigs o' Reston; But Dunse dings a'. Near the seaside village of Eyemouth, in Berwickshire, is a promontory marked with a succession of grassy mounds, the remains of a fort built there in the regency of Mary of Lorraine. A number of places are represented as visible from the fort: but here fact is not strictly adhered to. Fosterland once existed in the parish of Bunkle as a small village; but even its vestiges are not now visible on the brown moor where it once stood. Edincran, properly Auchinchran, is an estate in the vicinity of Fosterland, as is Reston also. There is a variation as follows: The fairy fouk o' Fosterland, The witches o' Edincran, And the rye-kail o' Reston Gar'd a' the dogs die.
The rye-kail alluded to must have been a broth chiefly made from rye, which grain, it is well known, is sometimes so much tainted as to be poisonous. C. Benson. Birmingham. Ἀρνί ο ν (Vol. vi., p. 509.).—Probably your correspondent is aware of the explanation given by Dr. Wordsworth in his book on the Apocalypse, but does not think it satisfactory. Still, as he does not allude to it, I venture to transcribe it: The Apocalypse abounds in contrasts. For example, the LAMB, who is always called Ἀμνὸς, never Ἀρνίον, in St. John's Gospel, is called Ἀρνίον, never Ἀμνὸς, in St. John's Apocalypse, in which Ἀρνίον occurs twenty-nine times. And why does ὁ Αμνος here become τὸ Ἀρνίον? To contrast Him more strongly with τὸ Θηρίον, that is, to mark the opposition between the LAMB and the Beast. To this a note is appended: This contrast is even more striking in the original, where it is aided by an exact correspondence of syllables and accents. On one side are— 'Ἡ πόρνη καὶ τὸ Θηρίον:' On the other— 'Ἡ Νύμφη καὶ τὸ Ἀρνίον.' See Rev. xxi. 2. 9., xxii. 17.—Is the Church of Rome Babylon? p. 58.: London, 1851. A. A. D.
Ἀρνίον and ἀμνὸς both denote a lamb. In John i. 29. 36., the latter is applied to Jesus by John the Baptist. In Acts viii. 32., and 1 Pet. i. 19., the term is manifestly derived from Isa. liii. 7., the Septuagint translation. But, in the Revelation, the word selected by the apostle is simply to be viewed as characteristic of his style. Taken in connexion with John i. 29. 36., the difference presents one of those points which so strikingly attest the authenticity of the Scripture. If the writer had drawn upon his imagination, in all likelihood he would have used the word ἀρνίον in the Gospel; but he employed another, because the Baptist actually made use of a different one, i. e. one different from that which he was in the habit of employing. B. H. Cowper. Who was the greatest General (Vol. vi., p. 509.).—In reply to the following Query, Who was the greatest general, and why and wherefore did the Duke of Wellington give the palm to Hannibal? I think the following note appended to the eloquent sermon of Dr. Croly, preached on the death of the Duke, Sept. 19th, not only shows the humility of the Duke in giving preference to Hannibal over himself, but it contains so just a comparison between the two generals, that it deserves recording in the valuable and useful pages of the N. Q. as well as being a perfect and true answer to C. T.: It has been usual, the note says, to compare Wellington with Hannibal. But those who make the comparison seem to forget the facts:— Hannibal, descending from the Alps with a disciplined force of 26,000 men, met the brave Roman Militia, commanded by brave blockheads, and beat them accordingly. But, as soon as he was met by a man of common sense, Fabius, he could do nothing with him; when he met a manœuvring officer, the Consul Nero, he was outmanœuvred, and lost his brother Asdrubal's army, which was equivalent to his losing Italy; and when he met an active officer, Scipio, he was beaten on his own ground. Finally,
forced to take refuge with a foreign power, he was there a prisoner, and there he died. His administrative qualities seem to have been of the humblest, or of the most indolent, order. For fourteen years he was in possession of, or in influence with, all the powers of southern Italy, then the richest portion of the peninsula. Yet this possession was wrested from him without an effort; and where he might have been a monarch, he was only a pensioner. His punic faith, his flight, his refuge, and his death in captivity, might find a more complete resemblance in the history of Napoleon. The following, concluding sentence of Dr. Croly's note conveys a truer and far more just comparison with another great general: The life of the first Cæsar forms a much fairer comparison with that of Wellington. Both nobly born; both forcing their way up through the gradations of service, outstripping all their age; forming their characters by warfare in foreign countries; always commanding small armies, yet always invincible (Cæsar won the World at Pharsalia with only 25,000 men): both alike courageous and clement, unfailing in resources, and indefatigable in their objects; receiving the highest rewards, and arising to the highest rank of their times; never beaten: both of first-rate ability in council. The difference being in their objects; one to serve himself, the other to serve his country; one impelled by ambition, the other by duty; one destroying the constitution of his country, the other sustaining it. Wellington, too, has given the soldier and statesman his 'Commentaries,' one of the noblest transcripts of a great administrative mind. J. M. G. Worcester. Beech-trees struck by Lightning (Vol. vi., p. 129.).—On Thinnigrove Common, near Nettlebed, Oxon, a beech-tree, one of three or four
growing round a pit, was shattered by lightning about thirteen or fourteen years ago. A gentleman who has lived sixty years in the neighbourhood of the beech woods near Henly, tells me that he remembers three or four similar cases. Single beech-trees, which are very ornamental, generally grow very low and wide-spreading, which may be the reason why they often escape. On the other hand, in the woods where they run up close and very high, they present so many points of attraction to the electric fluid, that probably for that cause it is not often the case that one tree in particular is struck. Corylus. Portsmouth. Passage in Tennyson (Vol. vi., p. 272.).—It appears to me that Tennyson has fallen into the error of a Latin construction. I call it an error, because in that language the varied terminations of the cases and numbers make that plain which we have no means of evidencing in English. I should translate it Numenii strepitus volantis—The call of the curlew dreary (drearily) gleams about the moorland, as he flies o'er Locksley Hall. The summer note of the curlew is a shrill clear whistle, but in winter they sometimes indulge in a wild melancholy scream. Corylus. Portsmouth. Inscriptions in Churches (Vol. vi., p. 510.).—I differ from your reply to Norwood's Query, in which you refer to the colloquy between Queen Elizabeth and Dean Nowell as the origin of these inscriptions. No doubt they were derived from the custom of our ante- Reformation ancestors, of painting figures and legends of saints upon the walls of churches; but the following instance will suffice to prove that they originated in the reign of Edward VI., and not in Queen Elizabeth's. In the interesting paper by the Rev. E. Venables in the Transactions of the Cambridge Camden Society, on The Church of St. Mary the
Great, Cambridge, he gives, under the year 1550, the following extracts from the churchwardens' accounts: For makyng of the wall where Saynt George stood in the chyrche vjd It. payd for wythynge ye chyrch xxs iiijd It. payd for wryghtynge of ye chyrch walls with Scriptures iiijlib iijs iiijd . Shortly after the accession of Queen Mary in 1553, the following entry occurs: Payd to Barnes for mendyng over the rode and over the altar in the chapell, and for washing oute the Scriptures 4s 4d . They do not appear to have been restored after this, for in the year 1840 some of the plaister between two of the windows of the south aisle peeling off, discovered traces of wryghtynge beneath; and I and another member of the Cambridge Camden Society spent some time in laying it bare, and after much difficulty made out that it was the Lord's Prayer in English, headed, The Lord's Prayer, called the Paternoster, and written in the church text of the period, the whole enclosed in a sort of arabesque border; it was not merely whited over, but had evidently been partially effaced, or partly washed oute, before being concealed under its dreary shroud of whitewash. On examination there were traces of more of this writing between the other windows, but we had not time to make any further investigation, for the church was then being cleaned, and in a few days all that we had laid bare was again concealed under a veil of whitewash. Thus, I think, we may assign to the reign of Edward VI., not merely the obliteration of the numerous frescoes of St. Christopher, the great dome, c., which are now so constantly coming to light, but also the origin of wryghtynge of ye chyrch walls with scriptures in
their stead, some ten or twelve years earlier than the remarkable colloquy between Queen Elizabeth and the worthy Dean of St. Paul's. Norris Deck. Cambridge. Dutensiana (Vol. vi., p. 376.).—Lowndes gives a list of Dutens' works, which does not include Correspondence interceptée, of which he was the author; and I have seen a presentation copy of it proving this. W. C. Trevelyan. Early Phonography (Vol. vi., p. 424.).—Have the modern phonographists ever owned their debt of gratitude to their predecessors in the phonetic art? The subjoined advertisement may perhaps be considered an answer to this Query: Hart's Orthography, 1569; or, 'An Orthographie conteyning the due order and reason, howe to write or paint thimage of manne's voice, most like to the life or nature. Composed by J. H. [John Hart], Chester Heralt;' reprinted from a copy in the British Museum. Cloth, 2s. An unanswerable defence of Phonetic Spelling, and one of the earliest schemes of Phonetic Orthography. A considerable portion of the book being printed in the author's Phonetic Alphabet (given in the present edition in Phonetic Longhand), we have thus exhibited the pronunciation of the age of Shakspeare. W. C. Trevelyan. Kentish Local Names; Dray (Vol. vi., p. 410.).—In the low embanked land in the west of Somersetshire, between Bristol and Taunton, the
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Lighting By Design 2nd Edition Christopher Cuttle Ma Fcibse Fiesanz Fiesna Lc

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    Lighting by Design 2ndedition Christopher Cuttle AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD • PARIS SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Butterworth-Heinemann is an imprint of Elsevier
  • 9.
    Architectural Press isan imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP, UK The Boulevard, Lanford Lane, Kidlington, Oxford OX5 1GB, UK 84 Theobald’s Road, London WC1X 8RR, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA First edition 2003 Second edition 2008 Copyright © 2003, 2008, Christopher Cuttle. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (44) (0) 1865 843830; fax (44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http:/ /elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data Cuttle, Christopher Lighting by design. – 2nd ed. 1. Lighting, Architectural and decorative 2. Lighting, Architectural and decorative – Design I. Title 729.28 Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress Library of Congress Catalog Number: 2008931250 ISBN: 978-0-7506-8768-3 For information on all Architectural Press publications visit our web site at http://elsevierdirect.com Typeset by Charon Tec Ltd., A Macmillan Company. (www.macmillansolutions.com) Printed and bound in Slovenia 08 09 10 11 12 10 9 8 7 6 5 4 3 2 1
  • 10.
    Contents Preface to firstedition vii Preface to second edition ix Acknowledgements xi Introduction xiii Part One: Observation 1 1 Visible characteristics of objects 3 1.1 Visual constancy and modes of appearance 4 1.2 Visible properties of materials 19 1.3 Object characteristics and perceived attributes 31 2 Visible characteristics of lighting 37 2.1 Ambient illumination 37 2.2 Visual discrimination 51 2.3 Illumination hierarchy 74 2.4 The ‘flow of light’ 80 2.5 The ‘sharpness’ of lighting 99 2.6 Luminous elements 113 3 Measurable characteristics of lighting 125 3.1 Illuminance-based measurements 125 3.2 Luminance-based measurements 129 Part Two: Visualization 133 4 Envisioning the concept 135 4.1 Seeing lighting clearly 135 4.2 Allusion and illusion 142 4.3 Lighting concepts 155 5 Concept development 161 5.1 Getting the picture 161 5.2 The Design Features Report 165
  • 11.
    Contents Part Three: Realization169 6 Delivering the lumens 171 6.1 Indirect flux 173 6.2 Flux distribution 183 6.3 Direct flux 195 6.4 The light field 209 7 Getting the lighting you want 223 7.1 Lighting specification 223 7.2 Contractual agreements 226 Appendices A1 Technical concepts, terms and symbols 229 A2 Terms and symbols used in the text 234 A3 Summary of lighting concepts, design criteria, and associated metrics 235 A4 Summary of calculations 236 References 240 Further reading 242 Index 245 vi
  • 12.
    Preface to firstedition The need for this book arises from the fact that many archi- tects and interior designers do not envision electric lighting as part of their design philosophies. Generally, architects recognize Le Corbusier’s dictum that ‘Architecture is the masterly, correct and magnificent play of masses brought together in light’. As they create space, architects position apertures with care, admitting daylight to reveal forms and their textures, and so define the space, and as Corbusier had observed, this involve- ment with light lies at the heart of architecture. But then a strange thing can happen. The design is handed over to a building services engineer, whose range of responsibilities includes ventilation, heating and air conditioning; sound sys- tems; sprinklers; and electric lighting. For all of these services, the engineer’s overriding concern is to achieve uniform distri- butions, and in the case of lighting, this typically means that a prescribed illuminance is provided uniformly over a horizontal work plane 700mm above floor level. The result brings untold dismay to architects. By day, their building has light and shade, with forms and textures interacting with the flow of light induced by the thoughtfully located fenestration. By night, all of this recedes into the dull blandness of consistent, invariant illumination. The first group that this book is intended for is architects and interior designers who seek to achieve their design objectives both by day and by night. However, that does not mean pro- viding a daylit appearance around the clock. Electric lighting has its own aesthetic, and a prime aim of the book is to get designers to appreciate the different ways in which daylight and electric lighting interact with buildings. This considera- tion may bring the designer into contact with specialist light- ing designers, which may include building services engineers who have developed a passion for lighting, and these people are the second group for whom the book is intended. Overall, the book is intended for designers seeking to bring in-depth understanding of electric lighting into the architectural design process.
  • 13.
    Preface to firstedition The Artist One evening there came into his soul the desire to fashion an image of The Pleasure that abideth for a Moment. And he went forth into the world to look for bronze. For he could only think in bronze. But all the bronze in the whole world had disappeared, nor anywhere in the whole world was there any bronze to be found, save only the bronze of the image of The Sorrow that endureth for Ever. Now this image he had himself, and with his own hands, fash- ioned, and had set it on the tomb of the one thing he had loved in his life. On the tomb of the dead thing he had most loved had he set this image of his own fashioning, that it might serve as a sign of the love of man that dieth not, and a symbol of the sorrow of man that endureth for ever. And in the whole world there was no other bronze save the bronze of this image. And he took the image he had fashioned, and set it in a great furnace, and he gave it to the fire. And out of the bronze of the image of The Sorrow that endureth for Ever he fashioned an image of The Pleasure that abideth for a Moment. Oscar Wilde (Source: Small, I. (ed.) Oscar Wilde: Complete Short Fiction, Penguin Classics, 1994). viii
  • 14.
    Preface to secondedition Since the first edition was published in 2003, I have had a good number of opportunities to teach its content. These have caused me again and again to think through ways of explain- ing the concepts that are the basis of this approach to lighting design. I have satisfied myself that I have a simpler and clearer way of introducing the ‘sharpness’ of lighting concept, and this has led me to rewrite (and shorten) Section 2.5. For the example of an applied lighting calculation that I had given in Section 6.1, I followed the procedure of manual calcu- lations using data from lighting manufacturers’ catalogues. For this edition I have shown how a computer-based lighting pro- gram, in this case DIALux, can be applied for the calculations that I make use of. As well as making the calculations quick and easy (providing we keep our objectives clearly in mind), a great advantage of using this type of software is that it enables on-line searching for luminaires with suitable photometric per- formance. This is demonstrated in the revised Section 6.1. There are other additions to the text, such as discussion of opponent colours theory and the colour mismatch vector method of illustrating colour rendering properties in Section 2.2, but the most obvious difference is that this edition is in full colour. I wanted to take the opportunity to make this discus- sion of lighting much more visual, and while there is no short- age of books that present pictures of ‘good’ lighting, I wanted instead to illustrate ways in which the appearance of architec- tural spaces may be affected by lighting. This has led me to add groups of my own colour photographs to introduce each chap- ter, and generally the aim has been to show comparisons of interior spaces influenced by changes of lighting. I did not take these photographs specifically for this purpose, but rather I use a camera to record my observations of lighting. I do this to develop my own observation-based experience of lighting, and I recommend this practice to anyone who shares this interest.
  • 15.
  • 16.
    Acknowledgments The most wonderfulthing about working in lighting is the people that you encounter. Scientists and artists; engineers and designers; architects and psychologists; optometrists and ergonomists; are all concerned about how people interact with light. It is a topic that is virtually without boundaries, and it has brought me into contact with an extraordinary variety of people from whom I have gathered so much that I know that I cannot properly acknowledge all of them. However, some people have changed the way I think, and these people I particularly want to acknowledge. David Pritchard pulled me out of the commercial stream of a London luminaire manufacturer and into the technical depart- ment. They were a lively bunch and I learned a lot from them, and also I joined the Illuminating Engineering Society. At the London monthly meetings I was encountered speakers of the stature of J.M. Waldram, R.G. Hopkinson, and W.R. Stevens, and lighting became an interest rather than a job. After five years in London I joined Derek Phillips, a young archi- tect who had taken on the challenge of establishing Britain’s first independent lighting consultancy practice. I met clients rather than customers. I learned how to visualize lighting, and what it was to feel responsible for one’s own work. My next move was to join the Daylight Advisory Service of Pilkington Glass at St. Helens, Lancashire. Under the leadership of J.A. (Joe) Lynes, the DAS was developing a quite remark- able reputation for its contributions to daylighting design, and I became increasingly involved in giving seminars on the DAS’s design tools at schools of architecture. It was Professor James Bell who encouraged me to study for my Masters degree at the University of Manchester, and at about this time, Harry Hewitt invited me to join the IES Lighting Design Panel. This group of experts had the task of looking ahead to guide the society’s work. The panel’s meetings were always stimulating, and never more so than when Peter Jay took over the leader- ship. While I had the good fortune to engage with some out- standing intellects at this time, I have to make special mention
  • 17.
    of Joe. Heliterally drew my understanding of lighting into the third dimension, and although we worked together for only two years, I have benefited ever since from the friendship that we have maintained. In 1976 I emigrated with my young family to join a brand new school of architecture in Wellington, New Zealand. It was a young faculty that developed a collegiate bond that drove all of us. The lack of a lighting community came as a shock, but fairly soon we had the IESNZ up and going, and soon after that New Zealand joined the International Commission on Illumination (CIE). Things seemed to be well on track when Mark Rea invited me to join the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, New York, in order to set up the world’s first Master of Science in Lighting degree program. I went to Rensselaer on a three-year contract and stayed for nine years. Once again I was with a newly established outfit where the adrenalin was flowing and my learning curve was as steep as ever. The students were challenging and the faculty was out- standing. Peter Boyce, Howard Brandston, Naomi Miller and Jan Moyer remain firm friends. I returned to New Zealand and once again I was working with architecture students and getting them to visualize their design concepts in light. Some of the ideas that I make use of have appeared in published papers, and I am grateful to Lighting Research and Technology for having given me opportunities to offer my thoughts for peer scrutiny. Also, I want to thank Lighting Design Application, who between 1995 and 1999 published 34 articles of mine in a monthly column titled “Cuttle on Calculations”. Opportunities of this sort are enormously valuable for developing one’s own ideas, and I need to make particular mention of the writings of Dr. J.A. Worthey, whose studies of light source size have provided the basis for the sec- tion on the ‘sharpness’ of lighting. I retired from the School of Architecture at the University of Auckland, New Zealand in 2007, and I acknowledge the sup- port that enabled me to write this book. Special thanks are due to the faculty photographer, Lynne Logan, who did all the studio photography for the illustrations. Other illustrations are either acknowledged with due gratitude in the captions, or they are my own. Kit Cuttle Auckland, 2008 Acknowledgments xii
  • 18.
    Introduction This book isconcerned with devising electric lighting instal- lations for architectural spaces that will contribute towards achieving architectural design objectives. It is written for archi- tects, interior designers and specialist lighting designers. It presumes a basic knowledge of lighting technology, although a brief summary is given in the Appendices for the benefit of those who might need an occasional reminder. The book comprises three parts. Part One is titled Observation, and the thesis is that the aspects of lighting that concern a designer are those that can be seen to make a difference. The problem is that we all take lighting for granted, and we sim- ply do not notice what lighting can do until we direct our- selves to look for it. If people enjoy the visual experience of a space or the objects it contains, the lighting must have been working well for them. That they remember the architec- ture or the beautiful art, and they don’t remember the first thing about the lighting, is not the issue. To become a light- ing designer it is necessary to understand the role of light- ing in revealing that experience. This is done by objectively examining interactions of light and matter and develop- ing an extensive range of observation-based experience of lighting. Part Two is titled Visualization. A lighting design concept develops in the designer’s mind, and its strength depends on the designer’s ability to visualize three-dimensional space and to bring to that vision observation-based experience of light- ing. This use of the term visualization should not be confused with computer-generated renderings. The process described involves mentally applying lighting design criteria to build up a visualization of the design situation in light, and devel- oping the skill to communicate and discuss that concept with a client and other professional designers working on the project. Part Three is titled Realization. Unlike stage and studio lighting designers, the architectural lighting designer realizes the design
  • 19.
    concept through themedium of a technical specification. This leap from the cerebral to the technical involves calculations and understanding the performance characteristics of lighting equipment, but the designer must never lose sight of the prin- ciple that what matters is what can be seen to make a differ- ence. It is intended that a reader who follows all three parts will become good at seeing small differences of lighting. Introduction xiv
  • 20.
    Part One: Observation Alldiscovery starts with observation. Whether we think of Aristotle leaping out of his bath and startling the Athenian townsfolk with his cries of ‘Eureka!’, or Newton wondering what caused the apple to fall on his head, or Einstein imagining himself to be sitting on a photon, or Sherlock Holmes’ admon- ishions to Watson: it is all a matter of observation. The process of visual perception operates throughout our wak- ing hours, continually seeking to make sense of the flow of information being delivered to the brain through the sense of vision. It is obvious that lighting is necessary for vision to oper- ate, and there is a substantial amount of knowledge on ways in which lighting may influence how well the visual process is able to operate. However, this book is more concerned with how lighting may influence our perceptions of our surround- ings. There is far less reliable knowledge, and it takes careful observation to identify the aspects of appearance that we rely on to form our perceptions, and how they may be affected by lighting. While this may seem to be a daunting task, it should be obvi- ous that the essential components of lighting design are there for all to see. The first vital step towards becoming a lighting designer is to develop confidence in the evidence of your own eyes.
  • 22.
    1 At first, itseems obvious that we provide lighting to enable people to see, so that all lighting can be assessed in terms of how well it enables people to see. Lighting that maximizes the luminance contrast of visual detail enables very small detail to be accurately detected, and this is the basis of many light- ing recommendations and standards. However, observation of our surroundings shows a much larger range of ways in which objects can differ in appearance. Consider for a moment the judgements that we commonly make in deciding whether a surface is clean and dry; whether fresh fruit is good to eat; or whether a colleague looks tired. These judgements are based on observation of appearance, but what are the differences of appearance that are critical in making these judgements? Any of these everyday assessments of appearance can be influenced by subtle aspects of lighting, and so too can our more complex assessments of the appearance of architectural spaces. A basis of theory enables designers to examine their own observations of the things that surround them. Differences of object appearance have their origin in the physical processes by which light is reflected, refracted, dispersed and scattered by matter. But human vision did not evolve to enable us to observe these processes: it evolved to enable us to recognize our surroundings. Understanding of the roles of these processes requires directed observation, and when we apply observation analytically, we find that the number of physical processes that is responsible for all of the differences that we can discriminate is quite limited. With this insight, we start to gain knowledge of how to control light to achieve a visible effect that we have in mind. It is, in fact, quite remarkable how the astounding range of human visual sensations is governed by so few processes. Lighting is both the medium that makes things visible, and it is a visible medium. At one level, it reveals the identifying attributes that enable us to recognize the objects that surround us, and at another level it creates patterns of colour, and light and shade, which add other dimensions to the visual scene. Visible characteristics of objects Facing page: Union Station, Washington DC. The 1988 renovation of architect Daniel Burnham’s Union Station (opened in 1907) included new lighting designed by William Lam Associates. The uplighting in the Main Concourse is by two-to-one combinations of metal halide and high pressure sodium lamps, the sodium lamps having been added to cause the gold leaf decoration to gleam. This is effective both by day and by night, despite the vastly different overall appearance of the terminal
  • 23.
    Observation 4 This chapter examinesthe role of lighting at the former level, that is to say, its role in making visible the aspects of appear- ance that enable us to perceive our surroundings. We start by considering what we need to know about the processes of vision and visual perception. 1.1 Visual constancy and modes of appearance The underlying aim of lighting design is to control the luminous environment in order to influence the perceived environment. Figure 1.1 provides a simple model of visual perception, which shows that several stages are involved in making this connection. THE LUMINOUS ENVIRONMENT generates THE RETINAL IMAGE which is the stimulus for THE PROCESS OF VISION which provides information to enable THE VISUAL PERCEPTION PROCESS THE PERCEIVED ENVIRONMENT to recognize the objects and surfaces that form the visual basis for Figure 1.1: A simple model of the human visual perception process. Lighting designers exercise operational control in the luminous environment, with the aim of influencing an observer’s perceived environment. A complex series of processes occur between the designer’s input and its effect
  • 24.
    Visible characteristics ofobjects 5 The luminous environment This is the physical environment made luminous by light. It is here that the lighting designer exercises control. The retinal image The optical system of the human eye focuses an inverted image onto the retina, shown in Figure 1.2. This image is constantly changing with movements of the head and the scanning move- ments of the eyes. It is often said that the eye is like a camera, but the only similarity is that it forms a focused image in which, for every pixel, there is a corresponding element in the lumi- nous environment. The main difference is that the eye operates as an instrument of search. Unlike photographic film, the struc- ture of the retina is far from uniform. High-resolution detection occurs only at the fovea, a small area of tightly packed photore- ceptors, and except at very low light levels, resolution declines progressively to the periphery of the retina. While the relatively slow movements of the human body occur, more rapid move- ments of the head enable attention to focus onto things that have been noticed, while still more rapid movements of the eyes within their sockets cause objects of interest to be scanned for detail. The eye is not a picture-making device: it is the opti- cal instrument of search that is actively involved in the process of seeking information of the surrounding environment. The distribution of luminance and colour that comprises the retinal image is modified by light losses that occur in the opti- cal media of the eye, and these losses are not constant as they increase significantly with age. Here we encounter an interesting conundrum. Because the retinal image is the stimulus for vision, we have no way of examining it. So, we are forced to accept measures of the luminous environment as practical indicators Near vision Distant vision Iris contracted Pupil Iris opened Retina Ciliary muscle Optic nerve Sclera Fovea Lens flattened Lens rounded Blind spot Figure 1.2: Sectional diagram of the human eye showing lens curvatures for near and distant focus. (Source: Coaton, J.R. and Marsden, A.M. (eds) Lamps and Lighting, Arnold, 1997)
  • 25.
    Observation 6 of the stimulusfor vision, which means that we are by default assuming a notion of ‘normal vision’. This notion presumes that those who need optical correction to achieve a sharply focused image will have it, and while allowance may be made for reduc- ing image brightness with age, this is often overlooked in prac- tice. This latter point is discussed in Section 2.2. The process of vision The purpose of the visual process is to provide an ever-chang- ing flow of information to the visual cortex of the brain. The retinal image stimulates photoreceptors embedded in the retina, causing a series of minute electrical impulses to flow along the optic nerve pathways to the brain (Figure 1.3). It might seem more appropriate to compare the eye with a television camera than with the more familiar picture-making camera, but even here the comparison falls short. There are millions of photore- ceptors in the retina, and processing of their responses occurs at several stages along the route to the brain. The first level of processing occurs actually within the retina, ena- bling the optic nerve to transmit the visual information with far fewer nerve fibres than the number of photoreceptors. Further modification of the signals from the two retinas occur in the chi- asma, where responses from both left-hand sides of the retinas are channelled to the left-hand lobe of the visual cortex, and the right-hand channel is similarly directed. Further processing occurs in the lateral geniculate bodies before the signals reach the cortex. While there is still plenty that is not understood about the work- ing of these processes, much information on the performance of human vision has been gathered in recent years. The prime Lateral geniculate body Retina Optic chiasma Optic nerve Visual area of cortex Figure 1.3: Schematic diagram of the binocular nerve pathways (adapted from Boyce, P.R. Human Factors in Lighting, Applied Science Publishers, 1981)
  • 26.
    Visible characteristics ofobjects 7 source of this information is studies involving measurements of the ability of an observer to detect small differences of luminance or colour, and this aspect of visual ability is discussed in Section 1.2. Its relevance to this discussion is that if an item of detail is to be part of the perceived environment, then its presence must be indicated by a visually detectable stimulus. The visual perception process The perception of a surrounding environment may be influ- enced by input from any of the senses, together with memory cues. Although vision is usually the dominant source of sensory information, the perception may be significantly influenced by inputs from other senses, such as auditory, olfactory, and tac- tile senses, together with memories derived from these senses. This simple model should not be taken too literally, as just how a perception of an environment is assembled from the signals that flow through the optic nerve pathways is much less well understood than the process of vision. The perceived environment This is the construct within the brain that serves as a model for the physical environment, and it has two distinct roles. It is within this mental construct that a person orientates and makes operational decisions, such as how to navigate through the space without colliding with furniture or other objects. Also, the mental construct represents the person’s assessment of their environment. If one person finds a space pleasant and another does not, then we can assume that the perceived environments that each of them has formed are different. While some inter- personal differences are inevitable, it is evident that there are broad similarities which enable designers to satisfy both select and random groups of people. Luminous environments can be created that lead to a majority of sighted people generating perceived environments that both enable satisfactory levels of operational decision-making, and which also provide for posi- tive evaluations of their surroundings. Referring back to Figure 1.1, we can use this model to set light- ing design into context. The designer’s objective is to bring to life a perceived environment that exists as a mental image in the designer’s brain. The image comprises more than a view. Depending on the designer’s philosophy, it is likely to incorpo- rate subjective concepts which relate to evaluative responses to the luminous environment, which is to be achieved by applying
  • 27.
    Observation 8 lighting to aphysical environment. This is the essential function that the lighting designer controls. The link between the lumi- nous environment and the perceived environment is the chain of functions indicated in the basic model of visual perception. Added to this, it is inevitable that past experience will influ- ence an individual’s visual perception of their environment, and this gives one more reason why we need to recognize that the luminous environment and the perceived environment are not the same thing. That we have incomplete understanding of how the visual perception functions operate is not an overrid- ing deficiency, as we can employ observation to explore ways in which variations in the luminous environment influence the perceived environment. This is a vital aspect of any design process. At the same time, we should seek theory that confirms observation as this enables us to organize knowledge. It is with this purpose in mind that observation is the central theme of Part One of this book. Aspects of appearance Consider this hypothesis: architectural lighting should provide for reliable recognition of the surfaces and objects that form the environment. The basis of this premise is that every object that is represented within the perceived environment is asso- ciated with certain attributes, some of which are essential for recognition of the object, and some of which affect assessment of the object’s qualities. A designer can be expected to look for more than lighting that simply makes everything visible. Much design effort may have been expended on selecting materials and specifying colours and textures, and it is important that these selected qualities are effectively revealed. Examine the four views shown in Figure 1.4. In every case, the objects are instantly recognized, but being able to cor- rectly name an apple, a peach, and a pineapple does not tell us much about these objects. Are they ripe? Are they wholesome? Would they be good to eat? What different impressions do we gain from the various views of the colour and texture of each of these objects? These are the judgements that determine our attitudes towards these familiar objects, but what are the aspects of appearance that influence our assessments? We have expectations of what good fruit should look like, and we inevitably compare the different views of the objects with our expectations. The perceived objects are more than images: they
  • 28.
    Visible characteristics ofobjects 9 are entities in our minds that are ‘coloured’ by our expecta- tions. If the fruit appears unattractive in one view, those per- ceived attributes of the object that do not meet expectations will stand out in the mind of the viewer. The perceived object is not a simple transposition of the retinal image: it carries the viewer’s evaluation of the perceived object. A fruit vendor who seeks to meet the viewer’s expectations will polish the apples, but not the peaches. However, the apples will not shine unless the lighting has the propensity to reveal that attribute. There is, of course, no such thing as ‘shiny lighting’, and lighting alone cannot make the peach appear shiny. However, lighting that can produce a pattern of light and shade on the smooth, velvet surface of the peach that differentiates it from both the Figure 1.4: (a) The objects are both visible and recognizable, but while the perceived attributes enable recognition they do not necessarily engender favourable assessments of the objects. (b) The spatial distribution of the lighting is the same as for (a) but the spectral distribution is different, and gives more favourable assessment of the chromatic attributes of the objects. (c) The spatial distribution of light contrasts the matt and glossy surfaces of the peach and the apple, and their smooth forms from the rough surface of the pineapple. However, the spectral distribution is as for (a) and does not favour the chromatic attributes. (d) The peach and the apple look ripe, and the foliage of the pineapple appears fresh. Both the spatial and spectral distributions of light reveal differences of object attributes and support favourable assessments of them (a) (b) (c) (d)
  • 29.
    Observation 10 jagged surface ofthe pineapple and shiny surface of the apple has properties that meet the expectations of the vendor and his customers. If the lighting also aids discrimination of colours that are associated with fruit that is healthy and ripe, it will gain the customer’s approval. The evaluative aspects of perception are primarily concerned with discrimination, and this process is served by lighting that provides for discrimination of object attributes, that is to say, lighting that maximizes differences of object appearance. Whenever the retinal image stimulates the perception of an object, that object is inevitably perceived to have certain attributes. The apple has the attribute of gloss, and the peach does not. If we doctored the surface of the peach with a clear varnish a viewer might perceive a nectarine, but not a glossy peach. Not all things can be perceived to have all attributes. If the image of the apple appeared to be flickering, this would be perceived to be an attribute of the lighting. We can not per- ceive an apple that is cyclically altering its surface lightness. If subsequent observation revealed that the flicker was somehow emanating from the object, we might decide that we are look- ing at a plastic model of an apple with a lamp inside, but we would now have a quite different understanding of the object. In our perceived environment, it would not be an apple. Visual constancy Visual constancy may be described as the process by which per- ceived objects maintain more or less stable attributes despite changes in the retinal images by which they are recognized. An understanding of how we develop perceptions of our environ- ments and the role that perceived attributes play in enabling us to come to terms with surroundings is crucial to understanding the roles that lighting can play in influencing people’s perceptions of their environments. For all of our lives we are surrounded by objects, and while indoors, our environments are bounded by surfaces. For the moment, we will treat all of these surfaces as objects. The volume of the space is filled with air, but unless it is dusty or misty, we have no visual awareness of the air. It is, however, necessary for us to recognize the objects that surround us. We need to under- stand why we are in this place, and what is our relationship to these objects. We need to be able to navigate through our environment, and for this we need to have a perception of a stable world, or at least one in which the movements of objects are understandable and reasonably predictable.
  • 30.
    Visible characteristics ofobjects 11 The perceptual process works so well that we do not con- sciously distinguish between the perceived environment and the physical environment, so that ‘I saw it with my own eyes’ seems to the speaker to be irrefutable proof of an event. Psychologists have developed a number of visual illusions to enable them to study the perceptual process. These are images that reliably confuse the perceptual process, and these confu- sions can give insight into the workings of the process. A famous illusion is shown in Figure 1.5. The figure shows two vertical lines. Disregarding their chevron endings, do they seem to you to be the same length? If you need to, use a measure to confirm that they are in fact identical in length. So, why does the one on the left appear to be longer? Could it be that one pair of chevrons is stretching the line by applying tension, while the other is squashing it in compression? That cannot be right, as it is the reverse of the perceived difference. The accepted explanation is rather engaging. It is that the line on the left appears as a receding corner, as if looking into a corner of a room, and the line on the right appears as an advancing corner, as if the external corner of a building. As you perceive the line on the left to be more distant, and its retinal image is of the same size, you perceive it to be larger. Does this explanation seem convincing to you? Try Figure 1.6. Do the black bars seem to you to match in size? You can check that they are identical, but it is almost impossible to see them as equal without obscur- ing the surrounding lines. Consider something rather closer to everyday life. You meet a couple of friends, and as you walk towards your friends to greet them, their images on your retina enlarge. Why would you not see your friends to be enlarging like a pair of inflating balloons? The answer is that in order for you to be able to navigate your way among people, furniture and other hazards, your brain is continually interpreting your changing retinal images, and updating the model of your environment and your location and movement within it. Your decreasing distance from your friends is an aspect of that perception which is inseparable from your recognition of your friends. Even though the setting in which you meet may be quite unfamiliar, you have developed the skill to orientate yourself within that environment and to navigate your way through it without difficulty. You may have encountered many people since you have developed that skill, and while some of them may have enlarged somewhat during your acquaintanceship, you know that it takes more than a few seconds to achieve this transformation. Figure 1.5: The Müller–Lyer figure: the vertical lines are the same length Figure 1.6: Distance–size illusion: in this case, the black bars are the same length
  • 31.
    Observation 12 This discussion hasbeen concerned with the phenomenon of size constancy, which, as we can see from Figures 1.5 and 1.6, is easily demonstrated. This is one aspect of the visual constancies, which may be described as perceptual phenom- ena which enable us to ascribe stable attributes to visually per- ceived objects. Another of the constancies is lightness constancy which is not as easily demonstrated on the pages of a book. According to Peter Jay (1973), the German physicist Hermann von Helmholtz (1821–94) posed the question, ‘Why does a lump of coal in sunlight look black even if it has higher lumi- nance than a sheet of white paper that is in the shade?’ You can readily, and quite comfortably, confirm Helmholtz’s obser- vation. Lumps of coal are less commonplace household items than in Helmholtz’s day, but on a sunny day, place a suitably black object (such clothing is fashionable at the time of writing) in the full sun and settle yourself close by in the shade while continuing to read this book. The sunlit object will not lose its blackness nor will this page loose its whiteness, even if the light level difference is such that a luminance meter would show that the reflected luminous flux density is greater from the coal (or black clothing) than from the paper. What is the explanation? It is easily demonstrated that simultaneous contrast can affect perceived lightness (Figure 1.7), but this is not to be confused with Helmholtz’s question. He is asking why it is that recognized objects retain their different identifying visual characteristics even when the effect of lighting would seem to be to cause them to reverse. Of course, our lives would become chaotic if objects changed from black to grey to white when carried from shade to full light. You could walk out of your house in the morning and find your- self unable to recognize it when you return in the evening. Visual constancy is an essential fact of life. Glancing back to Figure 1.1, the retinal image of the lump of coal may have higher luminance than the image of the paper, but the perceptual process did not Figure 1.7: Simultaneous contrast: the grey squares are identical. This could be confirmed by superimposing a mask with five cut-outs coinciding with the grey squares, so they are all seen against the same background
  • 32.
    Visible characteristics ofobjects 13 evolve to inform us of this photometric fact. It evolved to enable us to develop a mental construct that provides us with a reliable representation of our environment, and that means that objects are perceived to retain their intrinsic characteristics, even where large differences of illuminance occur. The appearance of the lump of coal will not be identical whether it is in sunlight or shade, but it remains unmistakably black. How do we make sense of this situation? In particular, if your purpose for reading this book is to learn how to plan distribu- tions of illumination, how are you able to cope with the notion that visual constancy operates so that the appearances of objects are more or less unaffected by lighting? Modes of appearance As has been stated, any ‘thing’ that is recognized is perceived to have certain attributes. The ‘modes of appearance’ concept explains that the perceived attributes that may be associated with a particular ‘thing’ depend upon the ‘mode’ in which it is perceived. This concept provides a theoretical framework for analysing observations of illuminated objects, and it provides a useful concept for examining the roles that lighting can play in influencing the appearance of an illuminated space and the objects it contains. The originator of the ‘modes’ concept was David Katz (1935), whose concern was the ways in which the appearances of col- ours are influenced by the ways in which the stimulus is experi- enced. He drew distinctions between surface colours and volume colours, and between colours that are perceived to be revealed by illumination and those perceived to be self-luminous. His explorations of the role of colour constancy and the influ- ence of illumination, as well as the work of other contributors, have been reviewed elsewhere (Cuttle, 2004). In Figure 1.8(a), red and green coloured cards are placed side- by-side on a table top, and a strip of transparent yellow film is laid across them. A colorimeter would indicate four distinct col- ours, but an observer would perceive three: red and green in surface mode, and yellow in film mode. These modes could be changed. In Figure 1.8(b), a card with four cut-outs is placed on top, and the observer perceives four colours in surface mode. While the names of these modes may seem to describe a physi- cal viewing condition, it is important to recognize that the mode of appearance of an element is determined by how it is perceived by an observer. ‘Modes of appearance’ may be referred to as a
  • 33.
    Observation 14 classification of assumptionsthat underlie perception. A change of mode may cause the perceived attributes of an object to change from those associated with its physical properties, and this may be described as a ‘constancy breakdown’. It is one of this book’s underlying themes that it is a prime pur- pose of a lighting designer to identify chosen attributes of archi- tectural spaces and the objects they house, and to reveal these attributes. It is, therefore, of high relevance that the ‘modes’ theory explains that certain attributes may be associated only with objects perceived in certain modes. Perhaps the most remarkable difference concerns the attributes of brightness and lightness. If I shine a spotlight onto a wall, I create a zone that is perceived to have the attribute of brightness. It might be possible for someone to create an identical luminance pattern by selectively spray painting the wall, but it is unlikely that any- one would be convinced that they were looking at a spotlight- ing effect. The appearance of a spotlight being directed onto a wall is instantly recognized, and the spotlit area is perceived in located illumination mode. It has the attribute of brightness, which may be graded on a bright–dim scale. Meanwhile, the appearance of the wall is essentially unchanged. It is perceived in object surface mode and retains its appearance of uniform lightness, which may be graded on a light–dark scale. This is an important distinction for understanding which are the perceived attributes that may be affected by lighting, and which are the attributes that are not likely to be affected. The brightness of an element perceived in either located illu- mination or illuminant modes is largely determined by its lumi- nance relative to the adaptation condition determined by the overall field, and this is discussed further in Section 2.1. The lightness of an element’s perceived object surface mode is related to reflectance, and the relationship is fairly well estab- lished. The Munsell colour system incorporates a scale of value, which is a subjective scale of lightness. To establish such a (a) Figure 1.8: (a) A red and a green card are overlaid with a yellow film. The card colours are perceived in surface mode, and the yellow in film mode. A mask with apertures is overlaid in (b), and four colours are perceived in surface mode (b)
  • 34.
    Visible characteristics ofobjects 15 scale, a subject might be presented with a black and a white colour card, and be required to select from a large number of slightly different grey cards a card that appears to be mid-way between the black and white cards. Then the subject would find cards to fit into the two gaps, and so would proceed to produce a black–grey–white scale of equal perceived intervals. If the black card has a reflectance close to zero and the white card close to one, it might be expected that the mid-way card would have a reflectance around 0.5. This is not so. Munsell’s value scale of zero to ten is shown related to reflectance in Figure 1.9, and it can be seen that value 5, which is the subjec- tive mid-point, corresponds to a reflectance of approximately 0.2. In other words, what is perceived to be a mid-grey sur- face absorbs 80% of incident light. If a designer wants to make use of surfaces that reflect half of the incident light, they will need to have a Munsell value of 7.5 and will be distinctly light in appearance. The point of this discussion is that lightness is a subjective assessment of surface appearance, and while it is related to surface reflectance, it is not a linear relationship. Brightness is also a subjective assessment, but it relates to the emission light from the object rather than to an intrinsic prop- erty of an object’s surface. Various authors have proposed their own sets of ‘modes’ to suit their own purpose, and most have chosen to drop Katz’s film mode. This is not because it is wrong, but because it is unlikely 0 1 2 3 4 5 6 7 8 9 10 0.1 0 Reflectance Munsell value 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 Figure 1.9: Munsell value and reflectance
  • 35.
    Observation 16 to be relevantin design applications. In the ‘modes’ model used for this text there are six modes of perception, if we consider the two object modes separately, and each has its own set of perceived attributes (Cuttle, 1999). Any ‘thing’ that is recog- nized will be perceived in one of these modes, and the mode of perception determines which perceived attributes may be asso- ciated with the ‘thing’. The modes are listed in Table 1.1, with examples of phenomena that are likely to be perceived in each of the modes. Generally, the examples assume that visual con- stancy applies. This table should be read in conjunction with Table 1.2, which shows the modes and associated perceived attributes. A blank indicates that the attribute is not associated with the designated mode of perception, and a cross indicates that there may be an association. This model makes a major distinction between located (perceived to have dimension) and non-located modes. Objects perceived in surface or volume modes are always Table 1.1 Six modes of appearance Mode Examples Non-located Illuminant mode Sky, ambient fog (Note 1). An illuminated surface viewed through an aperture (Note 2). Integrating sphere. Illumination mode Ambient illumination, such as the general lighting within a room. Located Illuminant mode A lamp or luminaire; a self-luminous object. Illumination mode A patch or a pattern of light focused onto a surface or object (Note 3). Object modes Surface An opaque surface; an object seen by reflected light (Note 4). Volume A cloud. A plume of smoke. A transparent or translucent medium. Note 1 While we know intellectually that fog is not self-luminous, ambient or pervading fog is likely to be perceived as a luminous body rather than as an illuminated medium. Note 2 Aperture is considered not to be a mode of appearance: it is a viewing condition that causes a patch of a surface to be perceived in a non-located mode although the aperture itself may be perceived in located object mode. Again, an object does not have to be self-luminous to be perceived in an illuminant mode. Note 3 The located illumination mode differs from the non-located illumination mode in that the illumination is perceived to have size, and perhaps outline or pattern. It is not perceived to have three-dimensional form as it takes on the form of whatever it is focused onto. Note 4 It is important to distinguish between the perception of the surface and the perception of the incident light. Consider a surface illuminated by a flickering source: the flicker will be perceived as an attribute of the illumination, not the surface.
  • 36.
    Visible characteristics ofobjects 17 located, whereas illuminant and illumination modes may be located or non-located. If, upon entering a room, you have an impression of a brightly lit space, this is a non-located illumina- tion mode perception. Alternatively, if you notice that an artwork hanging on the wall is brightly lit, that is a located illumination mode perception. The attributes of brightness and lightness have special roles. Anything that enters conscious perception has either the attribute of brightness or lightness, so that one or the other of these attributes is always associated with the perception. These two attributes are mutually exclusive. Where visual constancy holds, the objects and surrounding sur- faces that comprise a situation to be illuminated are usually per- ceived in either the surface or volume object modes, although both modes may apply simultaneously. The body colour of a glass vase may be perceived in volume mode while its form is perceived in surface mode. For all visible objects, incident illumination inter- acts with the physical properties of their materials, providing the visual stimuli for perceptions of their distinctive attributes. It has been stated that anything that is perceived through the process of vision has either the attribute of brightness or light- ness, but not both. It requires careful observation to confirm that this is so. Consider Figure 1.10: what does it show? Of course it shows a suspended matt white sphere. You perceived the sphere instantly, and furthermore you perceived it to be uniformly white. However, this photograph of the sphere is not uniformly white. Instead it shows a shading pattern from light grey to darker grey. Take a sheet of paper and punch a small hole in it. Slide the paper across the photograph, and you will Table 1.2 Matrix of modes of appearance and perceived attributes Perceived attributes Modes of appearance Non-located Located Illuminant Illumination Illuminant Illumination Surface Volume Brightness X X X X Lightness X X Hue X X X X X X Saturation X X X X X X Flicker X X X X Pattern X X X X Texture X Gloss X Clearness X
  • 37.
    Observation 18 see how theaperture changes from near white to dark grey. Why did you perceive it to be uniformly white? The answer is that even in this two-dimensional representation, visual constancy is at work. The object depicted in this photograph is simply a Christmas tree decoration that had been sprayed with matt white paint, but just conceivably, the ball could have been cunningly sprayed in shades of grey and photographed in totally diffused illumination to produce an identical image. What you have perceived is the more probable explanation. In terms of modes of appearance, you perceived the sphere in located, object, surface mode. We will not concern ourselves with how it is that a two-dimensional image causes a three-dimensional object to be perceived. You perceived this object to have the attribute of lightness, and you perceived the lightness to be uni- form. If you had the actual object in your hand, you could dem- onstrate that it retains its appearance of whiteness over a wide range of viewing conditions. It would be possible to confuse a viewer as to the surface lightness, but it takes a contrived viewing condition to do it. If an aperture viewing condition is created, so that the viewer is shown only part of the surface through a hole in another material, such as the sheet of paper that you prepared for viewing Figure 1.10, the viewer is unlikely to be able to make any assessment of surface lightness. In fact, if the surface form- ing the aperture has a much higher luminance, the visible surface could appear to be black. The point is that such a restricted view- ing condition has changed the mode of appearance. It is now perceived in non-located illuminant mode, and it has the attribute of brightness, not lightness. Now that your attention has been drawn to the object-mode perception, what of the shading pat- tern that is visible in Figure 1.10? This is perceived in the located Figure 1.10: A simple object in a complex light field
  • 38.
    Visible characteristics ofobjects 19 illumination mode. It has the attribute of brightness, and may have other attributes such as the chromatic attributes of hue and saturation. It is very worthwhile to make yourself one of these devices. Observe it carefully in a variety of lighting conditions and confirm these findings. There is more reference to viewing simple devices of this sort in following chapters. The concepts of visual constancy and modes of appearance are enormously instructive for lighting designers. Once we have viewing conditions sufficient to enable objects to be recognized, these objects will be perceived to retain their identifying attributes over a large range of lighting conditions. There is limited scope to modify the perceptions of object characteristics while visual con- stancy applies. For example, the perceived hue and saturation of an object may be influenced by the colour rendering properties of the lamps. If a nominally white light source is used, this will have a quite subtle effect on the appearance of the object, which may nonetheless be appreciated. However, if a distinctly coloured effect is produced, it is likely to be perceived as an attribute of the illumination rather than of the object or surface. It is an important observation to distinguish between aspects of appear- ance that are perceived in an object mode, that is to say, which are perceived to be attributes of a recognized object, and aspects that are perceived in illumination mode, which means that they are recognized as attributes of the lighting. The outcome of these observations is quite profound. To think of lighting solely as the medium by which objects and surfaces are made visible is to ignore creative opportunities for influenc- ing users’ perceptions. Think of lighting also as a visible medium that may be perceived in illuminant or illumination modes, and which may be located or non-located. It has the attribute of brightness, not lightness, and while the range of attributes is more restricted than for the surface and volume modes, the perception of these attributes is not directed towards recogni- tion of stable, physical characteristics. Herein lies a wealth of opportunities for lighting designers. In the words of Marshall McLuhan, ‘The medium is the message.’ 1.2 Visible properties of materials When an unfamiliar object is introduced to an infant, it is explored with all the senses. It is handled, and it is held close to the face where young eyes can accommodate the image in fine detail. It is sniffed, shaken and sucked. All of the resulting sensory inputs contribute to the perception of the object. As the infant matures, each new encounter can be referred back to a mental library of
  • 39.
    Observation 20 sensory experience, andas this develops, so the sense of vision becomes the dominant source of the perceived environment. For every visible element in the perceived environment, there is a corresponding element in the luminous environment that is either self-luminous, or it is the result of an interaction between light and matter. It is the light and matter interactions that pro- vide the bulk of the information that enables us to recognize the vast array of materials that comprise our environments. To understand the interactions that we initiate when we illuminate an object, we need to take a look at the nature of light. The spectrum of light It can be physically demonstrated that light is a stream of pho- tons, where a photon is an elementary energy particle. In vac- uum, all photons travel at the same velocity, this being the great universal constant, the speed of light. It is equal to approxi- mately 300,000 kilometres per second, or in scientific notation, 3 108 m/s. Photons differ only in their energy levels, where the photon energy level e (Joules) is given by the expression: e h ν Joules where h Planck’s constant and ν frequency (Hz). Life would be more simple if every observed property of light could be explained by this simple model, but such is not the case. It is, in fact, slightly embarrassing to have to admit that this book, entirely devoted to lighting, will not attempt to pro- vide a comprehensive explanation of the physical nature of light, although some suggestions for further reading are offered in the Bibliography. It is sufficient to say that some of the com- monly encountered properties of light are more conveniently explained by treating light as waves of radiant energy rather than as a flow of particles, and it is for this reason that the vis- ible spectrum is usually defined in terms of the wavelength of light. As has been shown, photon energy and frequency are directly proportional. Frequency and wavelength are inversely proportional, and are related by the expression: ν c/λ where c is the velocity of light (m/s), and λ is wavelength (m). In this way, the visible spectrum is conventionally defined as extending from 380 to 770 nanometres (nm), where 1nm 109 m or one billionth of a metre, although it would be equally
  • 40.
    Visible characteristics ofobjects 21 valid to define the spectrum in terms of photon energy levels or frequency, as is shown in Figure 1.11. Light meets matter: the gaseous state Light travels in vacuum without loss of energy, and (as far as we need be concerned) it obeys the law of rectilinear propa- gation, which means that it travels in straight lines. Things change when light encounters matter. The first state of matter that we will consider is the gaseous state, in which the atoms or molecules are free to move, subject only to very small inter- molecular forces. Some scattering of photons occurs as they travel through such a medium. Where the gas molecules are small in relation to the wavelength of light, diffraction scatter- ing occurs as photons interact with these particles. Each particle acts as a centre of radiation and scatters light in all directions. The degree of diffraction scattering is proportional to the fourth power of the frequency of the light (ν4 ), so that the shorter vis- ible wavelengths are scattered much more strongly than the longer wavelengths. Outside the earth’s atmosphere sunlight has a colour temperature of 5,800K, but down at ground level, sunlight has a yellowish appearance and a colour temperature of around 3,000K. The difference is due to the scattering of shorter wavelengths which occurs in the upper atmosphere, the effect of which is evident as the blue sky. Larger particles are encountered in the lower atmosphere, such as water droplets, dust particles, and atmospheric pollutants. These may cause reflection scattering, where the particles act as tiny mirrors, and having random orientations, they produce randomly distributed reflections. Reflection scattering occurs 300 400 500 600 700 800 1 1015 8 1014 6 1014 5 1014 4 1014 Frequency (Hz) Wavelength (nm) IR Visible spectrum UV Violet Blue Green Yellow Orange Red Figure 1.11: The range of the visible spectrum is 380–770 nanometres, or approximately 4 1014 to 8 1014 hertz. It is therefore an octave of wavelength or frequency
  • 41.
    Observation 22 also in liquid-stateand solid-state matter, and is dealt with more thoroughly later in this section. Alternatively, the effect of interactions with larger particles may be absorption, where the particles convert the photon energy into some other form of energy. Usually this is heat, but other forms may occur as in photochemical reactions. The loss in inten- sity of a parallel beam of light in a homogeneous medium (not necessarily gaseous) follows an exponential decay function: I I 0 exp( ) αχ where I0 is the initial beam intensity, I is the intensity after trav- elling a distance χ in the medium, and α is the linear absorp- tion coefficient, which usually varies with wavelength. For the moment, we may note that while scattering and absorption in the atmosphere have much to do with both nat- ural and artificial outdoor lighting, it is generally disregarded from considerations of indoor lighting. Over the short distances involved, and with the expectation of a clean atmosphere, it is usually practical to assume that photons travel indoors as they do in vacuum, that is to say, without visible effect. Where some visible effect does occur, as when artificial smoke is added to the atmosphere, this is generally the result of reflection scatter- ing and absorption. Liquid-state matter The next state of matter to consider is the liquid state, in which the freedom of molecules to move with respect to each other is more restricted by cohesive forces. Liquids have fixed volumes: they assume the shape of the vessel containing them; and in the absence of other forces, the surface to the atmosphere forms a planar boundary. With the exception of metals in the liquid state, liquids are generally transparent, but differ from the previously considered gaseous-state media by having much higher densities. There are some materials that do not have def- inite fusion temperatures as they cool from the liquid state, and they become more viscous until they assume the rigid cohesion of a solid-state material without losing the molecular structure of a liquid. Glass and the transparent plastics are examples, and these materials may be described as either amorphous solids or supercooled liquids. Optically they behave as liquids, although we describe them as transparent solids. Both diffraction and reflection scattering occur, and there is a marked reduction in
  • 42.
    Visible characteristics ofobjects 23 the velocity of light. To examine the effect of velocity change, we employ the light wave model. Figure 1.12 shows wavefronts of light radiating outwards from source S, and the direction of any ray from S is normal to the wavefront. Two rays are shown, a and b, and as they pass through the rare-to-dense medium boundary, the reduced veloc- ity of light in the dense medium causes closer spacing of the wavefronts. Ray a is incident normal to boundary, and passes through without deviation, but b is refracted towards the nor- mal. The direction of the ray is still normal to the wavefronts, but now the origin of the waves is the apparent source Sap. The angles of incidence and refraction are related by Snell’s law. The velocity of light in air is not significantly different from its velocity in vacuum, and in practice, it is so common for the rare medium to be air that the difference in velocity may be consid- ered due only to the effect of the dense medium. This enables the refracting power of a transparent medium to be expressed by its refractive index μ, and for Snell’s law to be expressed as: sin sin i r μ This expression assumes that the angle i is measured relative to the normal in air, and the angle r occurs in the dense medium. Sap S a a b b Reflected ray Refracted ray Dense medium e.g. water Rare medium e.g. air i i r Figure 1.12: Two rays from S passing from a rare to dense medium. The rate of propagation is lower in the dense medium, causing the wave fronts to close up, and for the oblique ray to change direction
  • 43.
    Observation 24 Values of μfor several dense transparent media are given in Table 1.3. Refraction at a medium boundary is accompanied by reflection (Figure 1.12), and where the boundary is optically smooth, this is regular reflection which is governed by two laws: ● the angle of incidence equals the angle of reflection ● the incident ray, the normal and the reflected ray all lie in the same plane. The proportion of the incident light that undergoes regular reflec- tion is defined by Fresnel’s equations, and depends upon the angle of incidence and, for a ray incident in air, the refractive index of the dense medium. Figure 1.13 shows the directional dependence of reflectance for a typical glass surface, so that as the angle of incidence increases, the proportion of reflected light increases only gradually at first, and then sharply. Regular reflec- tion is sometimes called specular reflection, and where both regu- lar and diffuse reflection occur, the portion of reflected light that is due to regular reflection may be called the specular component. In Figure 1.14(a), a ray encounters a boundary to a dense medium and is refracted towards from the normal. It is a princi- ple of optics that every ray is reversible, so if the arrows were to be reversed, the figure would show a ray incident in the dense medium and being refracted away from the normal. Case (b) shows a ray incident in the dense medium at an increased angle r, and in this case the refracted ray is coincident with the boundary. This is the critical angle for the dense medium, which is equal to sin1 (1/μ). Some values of critical angle are given in Table 1.3. What happens if we further increase the angle r? This is shown in case (c), where total internal reflection occurs. This is regular reflection, and is ‘total’ because it occurs without loss of energy. This is the principle of the fibre optic, so that a ray that enters the end of the fibre not too far out of parallel with the axis of the fibre may undergo repeated reflections from the internal surface. Table 1.3 Refractive index values and critical angles for some transparent materials Material Refractive index μ Critical angle (degrees) Water 1.33 49 Acrylic 1.49 42 Soda (common) glass 1.52 41 Polystyrene 1.59 39 Flint glass 1.62 38
  • 44.
    Visible characteristics ofobjects 25 Solid-state matter The third state of matter is the solid state, in which atoms and molecules are not free to move, but vibrate about fixed posi- tions. The model for inorganic materials has these particles geo- metrically arranged in a crystalline lattice. Such solid materials are opaque, but as has been explained, some materials that are described as amorphous possess the transparency property of liquids while having other physical characteristics of solids. Except in rather unusual situations, the great majority of the objects and surfaces that form architectural interior spaces comprise 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Incident angle (degrees) Reflectance Unpolarized Perpendicular polarized Parallel polarized 90 80 70 60 50 40 30 20 10 0 Figure 1.13: Fresnel reflection at an air/glass boundary Normal Normal Normal i (a) (b) (c) r2 r1 r r Figure 1.14: Refraction and total internal reflection at a dense/rare boundary (Source: Bean, A.R. and Simons, R.H. Lighting Engineering: Applied Calculations, Architectural Press, 2001)
  • 45.
    Observation 26 opaque solid materials.Whereas these materials appear to us to have distinct boundaries, if we could reduce our scale of dimen- sions to that of an arriving photon, the molecular structures would present a view of an open lattice comprising an ordered array of molecules with an abundance of clear space between them. Arriving photons would be likely to penetrate some dis- tance into the lattice before interacting with particles. These par- ticles are large in relation to the wavelength of light, and while some photons will be absorbed, others will undergo reflection scattering whereby the particles act as tiny mirrors. In this way, some of the photons that have entered the surface layer of the crystalline lattice are re-emitted by back-scattering. Because the alignments of the mirrors are effectively random, the re-emitted light is totally diffused and independent of the direction of the incident photons and for this reason the process is known as iso- tropic re-emission. Such a surface is a uniform diffuse reflector, and is said to obey Lambert’s law. Figure 1.15 shows a small element of uniform diffuse reflector. The reflected light from this element has a cosine distribution, that is to say, the luminous intensity varies as the cosine of the angle measured from the normal. Also, the projected area of the surface from any viewing direction varies as the cosine of the same angle, so that element has the same luminance from all directions. A uniform diffuse reflector that reflects all of the inci- dent light would be a perfect diffuse reflector. Although no real materials achieve this, some come close to it. Reference white surfaces used in photometry laboratories may reflect 99% of inci- dent light at all visible wavelengths, and fresh white paint may be Light may be incident from any direction Reflected light always has a cosine distribution Perfect diffuse reflector Figure 1.15: The perfect diffuse reflector
  • 46.
    Visible characteristics ofobjects 27 as high as 95%. It is often assumed that a surface must be shiny to have high reflectance, but that is not so. Matt white paint reflects a higher proportion of incident light than does a silvered glass mirror or a polished metal surface, but of course the distri- bution of reflected light is quite different. More importantly, the diffuse reflector does not form a reflected image. If the reflected images formed by shiny surfaces include images of bright light sources, this can cause those surfaces to appear much brighter than adjacent diffuse reflectors and give rise to the misconcep- tion that they are reflecting more of the incident light. Photons incident on diffuse reflectors are either back-scat- tered (diffusely reflected) or absorbed. The reflectance of the surface is the ratio reflected to incident light, and this may be strongly wavelength-selective. Low-lightness surfaces absorb high proportions of the incident photons, and coloured surfaces may be described as wavelength-selective absorbers. This con- cept will be discussed further in the following section, but we should not lose sight of the fact that although we may describe some saturated surface colours as ‘bright’, particularly when referring to primary hues such as red or green, this ‘brightness’ is not achieved by red or green light being added to the sum of reflected light. It is due to the surface layer of the material strongly absorbing the complementary hues from the incident light spectrum. Our sensations of brightness are not simply determined by the amount of light arriving at the eye. The crystalline lattice structure applies generally to inorganic materials, but organic solid materials may take up different forms of structure with more scope for randomness. For example, white paper viewed through a microscope is seen to consist of a mass of fine fibres, which individually may be almost trans- parent. Although the molecular structure is quite different from the crystalline lattice, reflection occurs by isotropic re-emission and the matt surface of high-grade unglazed white paper is another close approximation of the perfect diffuse reflector. The effect of applying glaze to paper, or polish to flooring or furniture, is to add regular reflection to isotropic re-emission. The glaze and the polish are amorphous substances overlaying the structure of the solid material, and incident photons undergo a rare-to-dense medium transition, with some reflection loss, before undergoing the back-scattering and absorption that char- acterize the attributes of the perceived object. There are many examples of this combination of reflection characteristics. Paint comprises particles of pigments, which are wavelength-selective light absorbers, suspended in a clear vehicle, which traditionally
  • 47.
    Observation 28 was an oil-basedvarnish and now is more often a clear plastic coating material. The difference between gloss and matt paint is the vehicle. For a gloss paint, the vehicle dries out to a smooth, hard surface where regular reflection occurs. Figure 1.16 illustrates the processes at work when a beam of white light (W, comprising a combination of red, green and blue (R, G and B) components) is incident on a glossy red painted surface. The paint layer comprises particles of pigment suspended in a transparent amorphous vehicle that cures to form a smooth, non-electro-conductive surface. Both regular reflection and isotropic re-emission occur, giving differences of appearance to the observers A, B and C. For the regular reflection: ● direction is in accordance with laws of regular reflection ● quantity is in accordance with Fresnel’s reflection laws ● colour is the colour of the source ● luminance source luminance directional specular reflect- ance determined by Fresnel’s laws (Figure 1.13). Electric non-conductive material Glossy surface e.g. red paint A B C Reflection scattering 2) Isotropic re-emission Absorption W W B G R R 1) Regular reflection Figure 1.16: Reflection from a shiny dielectric surface, in this case gloss red paint (after Hebbelynck, 1987)
  • 48.
    Visible characteristics ofobjects 29 Note that the luminance of the reflected source image is inde- pendent of the distance of the source, and that this image will be seen by observer A but not observers B and C. For the isotropic re-emission: ● direction is in accordance with Lambert’s law ● quantity is determined by reflectance ρ, where ρ 1 α ● colour is determined by wavelength selective absorption ● luminance illuminance reflectance/π. Note that luminance is proportional to illuminance, and so is dependent on distance from the source. Observers B and C see the full saturation of the red pigment, but for observer A the redness is diluted, or even obliterated, by the regular reflection, depending on the relative luminances. A large diffuse light source would dilute the colour saturation equally for all observers. As shown in Figure 1.13, the proportion of incident light reflected from the surface of an amorphous material varies strongly with the angle of incidence. If the surface is smooth, regular reflection occurs and an image is formed, but generally this effect is apparent only for oblique viewing angles. Greatly increased levels of regular reflection can be achieved when an electro-conductive material is polished to an optically smooth surface, to the point where the high reflectance due to Fresnel reflection at high angles of incidence is achieved for all incident directions, so that variation with incidence angle is effectively eliminated. Examples of this have been referred to: polished metals such as silver, aluminium and chromium provide regular reflection that is independent of wavelength, while gold, brass and copper have wavelength dependent characteristics. Figure 1.17 illustrates the processes for an electro-conductive surface, in this case polished brass. There is no isotropic re-emis- sion, and all reflection is regular. Nonetheless, some absorption occurs, selectively at the shorter wavelengths, and this accounts for the characteristic metallic yellow colour of brass. In this case: ● direction is in accordance with laws of regular reflection ● colour is the colour of the source less the absorption losses ● luminance source luminance reflectance, where ρ 1 α.
  • 49.
    Observation 30 Note that inthis case the source image luminance is largely independent of direction, as well as being totally independent of source distance. Roughened or textured surfaces may par- tially or totally eliminate the reflected image, giving rise to vari- ous impressions of surface quality ranging from shiny, through sheen, to matt. Even so, the reflection process is quite different from isotropic re-emission, and so is the impression of colour. The metallic colours cannot be achieved by mixing pigments. Their appearance depends on modifying the source image, rather than the incident illumination. Interaction processes of light and matter To summarize the foregoing, the basic purpose of visual per- ception is to enable recognition of object attributes. The source of the information flowing through from the visual process is the interactions that occur when light encounters matter. The foregoing paragraphs describe the interaction processes that concern us, and in fact, they are quite limited in number. Wecanreducethenumberfurther.Althoughdiffractionscattering was mentioned, it involves interactions with very small particles Electric conductive material Glossy surface e.g. polished brass A B C 3) Regular reflection Absorption W B G R G R Y W 1) Regular reflection Figure 1.17: Reflection for an electro-conductive surface, in this case polished brass (after Hebbelynck, 1987)
  • 50.
    Visible characteristics ofobjects 31 and is not of concern for indoor lighting. This leaves the follow- ing processes: ● Absorption is almost inevitable; in fact, the only proc- ess mentioned that does not involve some degree of light absorption is total internal reflection. It is easy to dismiss absorption as an unfortunate source of inefficiency, but it should be recognized that this is the basis of surface light- ness, and selective absorption is the origin of surface colour. ● Regular reflection: – from the surface of liquid-state (including amorphous) materials, the Fresnel reflection enables us to distinguish glossy from matt surfaces – from electro-conductive surfaces, which give us reflected sparkle and the metallic colours. ● Refraction and dispersion within liquid-state materials, which give shape clues and may reveal spectral colours. ● Reflection scattering: – from particles in gaseous-state or liquid-state materials, giving cloudiness or translucency – isotropic re-emission in surface layer of solid-state materi- als, which reveal lightness, hue and saturation attributes. This is not a complete list of ways in which light may interact with matter, but it covers the interaction processes that con- cern us. Lighting is the source of energy that stimulates these optical phenomena, and provides much of the information that enables the perception process to discriminate differences of opaque, transparent and translucent materials. Ways in which lighting may be controlled to selectively promote or suppress these processes will be discussed in the following chapter, and that discussion will include recognition of object attributes such as form and texture. In the meantime, it should be noted that phenomena such as diffraction and polarization can be demonstrated in an optical laboratory, but it would be unusual for these to be of concern to an architectural lighting designer. 1.3 Object characteristics and perceived attributes The processes of interaction of light and matter described in the previous section cause the events in the luminous environment
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    Observation 32 that are thestimulus for vision (Figure 1.1). The visual process is the source of information for the perceptual process, which gives rise to the sensation of a perceived environment in the viewer’s brain, and comprises recognized objects with distinct characteristics. Whereas a single pixel in the luminous envi- ronment can be specified completely in terms of luminance and chromaticity, the object that contains this pixel may be perceived to have characteristics of substance, utility, beauty, value, affection, and so forth. These interpretations which occur during every moment of our waking hours derive from recogni- tion of perceived attributes. The perceived attributes that may be associated with any ‘thing’ that is seen depend on the mode of appearance in which it is perceived (Tables 1.1, 1.2). We are so dependent on vision for understanding our sur- roundings that it is difficult, or even impossible, to imagine the world as perceived by less vision-dependent species. A bat is, quite simply, ‘as blind as a bat’, and yet it can navigate at speed through forests and within caves. We know that it employs a sonar system similar in principle to that used by ships to locate submarines or shoals of fish, but what does the world ‘look’ like to a bat? Obviously they cannot experience colour, but we are able to recognize black and white images. However, can we even imagine a three-dimensional world with no light or shade? What is the image in a bat’s brain as it swoops between obstacles to intercept an insect in mid-air? Although this is not a lighting issue, it may cause us to think about how we perceive our environments, and the role of lighting in that process. It is important to appreciate that something that has the physi- cal properties of an object is not necessarily perceived in one of the ‘object modes’. Figure 1.18 shows two views of a lumi- naire. In case (a), the surface form, texture and lightness of the glass shade are all clearly visible. We could assess its lightness on a scale of zero to ten. We could make a reasonable guess of its reflectance. We perceive this shade in object surface mode. However, our perception of the shade is different in case (b). We recognize that it is still the same shade, but now that it glows we really have no idea of its texture, and it is quite meaningless to discuss its lightness. We could certainly discuss its brightness, and this lies at the heart of our changed percep- tion of the shade. We are now perceiving it in illuminant mode, and as indicated in Table 1.2, the range of associated attributes is different, and furthermore, the number of attributes is
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    Visible characteristics ofobjects 33 reduced. Of course we know that the glass shade is not self- luminous, but nonetheless, the ‘thing’ that our intellect informs us is a trans-illuminated object is perceived as if it is the source of light. Furthermore, an object may be perceived simultaneously in more than one mode. The glass object shown in Figure 1.19 is perceived to have both surface and volume attributes. The two cases show how a change of background can give a dif- ferent balance of the perceived attributes. Without disturbing the light sources, the object can be presented to give emphasis (a) (b) Figure 1.18: In view (a) the glass shade is perceived in object mode and has attributes of lightness and texture. In view (b) the shade is perceived in illuminant mode and has the attribute of brightness (a) (b) Figure 1.19: (a) and (b) How do we perceive transparent media? This glass vase is perceived in both surface and volume modes, and it can be seen in (a) that the directional lighting reveals the surface attribute of gloss while hue is perceived in volume mode. For view (b) only the background has been changed. The surface highlights are still evident, but it can be seen that the chromatic attributes visible in (a) are revealed not only by transmitted light but also by internally reflected light
  • 53.
    Observation 34 to its internalcolour or to the smooth glossiness of its sur- face. Differences of this sort are explored in the following chapters, but we should note that throughout these changes our understanding of the object’s fundamental nature remains intact. The differences of appearance may influence our sense of appreciation of the object, but basically it remains a coloured glass vase. Such is the power of the perceptual system to rec- ognize object attributes that, providing there is sufficient light to enable the visual process to operate effectively, viewing con- ditions have to be severely constrained for viewers to be con- fused over object recognition. Consider for a moment; if we could present the glass vase in Figure 1.19 so that the surface attributes were completely invisible, what would a viewer per- ceive? Is it possible to imagine the volume attributes without a bounding surface? Fortunately the perceptual process very rarely presents us with such confusion. The basic purpose of visual perception is to enable recognition of object attributes. Each attribute is associated with certain optical properties of the object, and is recognized by a charac- teristic interaction with light. Generally, the prime purpose of indoor lighting is to enable recognition of stable environments comprising recognized objects within which people can orien- tate themselves and navigate with confidence. However, the perceptual process is very adept at doing this, and copes well over a vast range of visual conditions. On one hand, this per- mits many lighting solutions that provide for no more than suf- ficiency of illumination to be found acceptable. On the other hand, it offers opportunities for designers to apply imagination to selecting object attributes for emphasis without compro- mising the basic requirements that lighting for occupied space must fulfil. It needs to be noted that the perceptual process involves plac- ing interpretations upon the visible effects of optical inter- actions. Every ‘thing’ that we perceive in our surroundings is recognized to have certain attributes, and the range of attributes that may be associated with a ‘thing’ depends upon the mode in which it is perceived. Objects perceived in the sur- face and volume modes have the greatest range of associated attributes, and this is where lighting designers often look to for opportunities to influence the appearance of surroundings. In Figure 1.19, the light that is reflected towards the viewer from the surface has undergone a different reflection process from the light that has been reflected or refracted within the volume of the object. While some lighting designers work on the basis
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    Visible characteristics ofobjects 35 of an intuitive understanding of this difference, a designer who understands the optical nature of this difference is in a stronger position to control the processes, and to select attributes for emphasis. To explore how this is done, we move on from characteristics of objects to characteristics of lighting, which opens up more opportunities for influencing the appearance of surroundings.
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    rare, probably unique.On taking it up to answer H. J.'s Query, I found some matter relating to the German academies of the seventeenth century, which I think may be interesting. Mr. Hallam (Literature of Europe, IV. v. 9.) says: The Arcadians determined to assume every one a pastoral name and a Greek birthplace; to hold their meetings in some verdant meadow, and to mingle with all their own compositions, as far as possible, images from pastoral life; images always agreeable, because they recall the times of primitive innocence. The poetical tribe adopted as their device the pipe of seven reeds bound with laurel, and their president, or director, was denominated General Shepherd or Keeper—Custode Generale. He slightly mentions the German academics of the sixteenth century (III. ix. 30.), and says: It is probable that religious animosities stood in the way of such institutions, or they may have flourished without obtaining much celebrity. The academy of Pegnitz-shepherds (Pegnitzshäfer-orden) took its name from the little river Pegnitz which runs through Nuremberg. Herr Sigmond von Birken was elected a member in 1645. He chose Floridan as his pastoral name, and the amaranth as his flower. In 1658 he was admitted to the Palm Academy (Palmen-orden), choosing the name Der Erwacsene (the adult?), and the snowdrop. In 1659, a vacancy having occurred in the Pegnitz-Herdsmen (Pegnitz-Hirten) he was thought worthy to fill it, and in 1679 he received the diploma of the Venetian order of the Recuperati. He died in 1681. This, and what can be hung upon it, is Die Betrübte Pegnitz, a dialogue of 406 pages. It opens with a meeting of shepherds and shepherdesses, who go in and out of their cottages on the banks of the Pegnitz, and tell one another, what all seem equally well acquainted with, the entire life of their deceased friend.
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    It would notbe easy to find a work more clumsy in conception and tasteless in execution. Herr von Birken seems to have been a prosperous man, and to have enjoyed a high pastoral reputation. His works are enumerated, but the catalogue looks ephemeral. There is, however, one with a promising title: Die Trockene Trunkenheit, oder die Gebrauch und Missbrauch des Tabacks. His portrait, as Der Erwachsene, is prefixed. It has not a shepherd-like look. He seems about fifty, with a fat face, laced cravat, and large flowing wig. There are twenty-four emblematical plates, rather below the average of their time. As so secondary a town as Nuremberg had at least three academies, we may infer that such institutions were abundant in Germany, in the seventeenth century: that of the Pegnitz shepherds lasted at least till the beginning of the eighteenth. In Der Thörichte Pritschmeister, a comedy printed at Coblenz, 1704, one of the characters is Phantasirende, ein Pegnitz Schäffer, who talks fustian and is made ridiculous throughout. The comedy is von Menantes. I have another work by the same author: Galante, Verliebte, und Satyrische Gedichte, Hamburg, 1704. I shall be very glad to be told who he was, as his versification is often very good, and his jokes, though not graceful, and not very laughable, are real. H. B. C. U. U. Club. MARRIAGES EN CHEMISE.—MANTELKINDER.— LEGITIMATION. (Vol. vi., pp. 485. 561.) The popular error on the legal effect of marriage en chemise is, I think, noticed among other vulgar errors in law in a little book published some twenty years ago under the name of Westminster Hall, to which a deceased lawyer of eminence, then young at the
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    bar, was acontributor. I believe the opinion to be still extensively prevalent, and to be probably founded, not exactly in total ignorance, but in a misconception, of the law. The text writers inform us that the husband is liable for the wife's debts, because he acquires an absolute interest in the personal estate of the wife, c. (Bacon's Abridgment, tit. Baron and Feme.) Now an unlearned person, who hears this doctrine, might reasonably conclude, that if his bride has no estate at all, he will incur no liability; and the future husband, more prudent than refined, might think it as well to notify to his neighbours, by an unequivocal symbol, that he took no pecuniary benefit with his wife, and therefore expected to be free from her pecuniary burdens. In this, as in most other popular errors, there is found a substratum of reason. With regard to the other vulgar error, noticed at the foot of Mr. Brooks' communication (p. 561.), that all children under the girdle at the time of marriage are legitimate, the origin of it is more obvious. Every one knows of the legitimatio per subsequens matrimonium of the canonists, and how the barons assembled in parliament at Merton refused to engraft this law of the Church on the jurisprudence of England. But it is not perhaps so well known that, upon such a marriage the premature offspring of the bride and bridegroom sometimes used to perform a part in the ceremony, and received the nuptial benediction under the veil or mantle of the bride or the pallium of the altar. Hence the children so legitimated are said to have been called by the Germans Mantelkinder. The learning on this head is to be found in Hommel's Jurisprudentia Numismatibus Illustrata (Lipsiæ, 1763), pp. 214-218., where the reader will also find a pictorial illustration of the ceremony from a codex of the Novellæ in the library of Christian Schwarz. The practice seems to have been borrowed from the form of adopting children, noticed in the same work and in Ducange, verb. Pallium, Pallio cooperire; and in Grimm's Deut. Rechts Alterth., p. 465. Let me add a word on the famous negative given to the demand of the clergy at Merton. No reason was assigned, or, at least, has been
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    recorded, but ageneral unwillingness to change the laws of England. As the same barons did in fact consent to change them in other particulars, this can hardly have been the reason. Sir W. Blackstone speaks of the consequent uncertainty of heirship and discouragement of matrimony as among the causes of rejection,— arguments of very questionable weight. Others (as Bishop Hurd, in his Dialogues) have attributed the rejection to the constitutional repugnance of the barons to the general principles of the canon and imperial law, which the proposed change might have tended to introduce,—a degree of forethought and a range of political vision for which I can hardly give them credit, especially as the great legal authority of that day, Bracton, has borrowed the best part of his celebrated Treatise from the Corpus Juris. The most plausible motive which I have yet heard assigned for this famous parliamentary negative on the bishops' bill at Merton, is suggested (quod minimè reris!) in an Assistant Poor-Law Commissioner's Report (vol. vi. of the 8vo. printed series), viz. that bastardy multiplied the escheats which accrued to medieval lords of manors. E. Smirke. A venerable person whose mind is richly stored with shreds and patches of folk-lore and local antiquities, on seeing the curious marriage entry (p. 485.), has furnished me with the following explanation. It is the popular belief at Kirton in Lindsey that if a woman, who has contracted debts previous to her marriage, leave her residence in a state of nudity, and go to that of her future husband, he the husband will not be liable for any such debts. A case of this kind actually occurred in that highly civilised town within my informant's memory; the woman leaving her house from a bedroom window, and putting on some clothes as she stood on the top of the ladder by which she accomplished her descent. K. P. D. E.
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    In that amusingwork, Burn's History of the Fleet Marriages, p. 77., occurs the following entry:—The woman ran across Ludgate Hill in her shift; to which the editor has added this note:—The Daily Journal of 8th November, 1725, mentions a similar exhibition at Ulcomb in Kent. It was a vulgar error that a man was not liable to the bride's debts, if he took her in no other apparel than her shift. J. Y. Saffron Walden. EDITIONS OF THE PRAYER-BOOK PRIOR TO 1662. (Vol. vi., pp. 435. 564.) As Mr. Sparrow Simpson invites additions to his list from all quarters, I send him my contribution: and as I see that he has included translations of our Liturgy into other languages, I do the same: 1552. Worcester. Jo. Oswen. Folio. 1560. London. Jugge and Cawood. 4to. 1565. London. Jugge and Cawood. 8vo. 1607. London. Folio. 1629. London. Folio. 1629. Cambridge. Folio. 1632. London. 4to. 1633. London. 4to. 1634. London. Folio. 1635. London. 4to. 1638. Cambridge. 4to. 1639. London. Folio. 1641. London. 4to. 1660. Cambridge. Folio. 1644. The Scotch, by Laud and the Scotch bishops. Printed by John Jones. 8vo.
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    1551. Latine versa,per Alex. Absium. Lipsiæ. 4to. 1594. London. 8vo. S. A. by Reginald Wolfe. London. 4to. 1638. In Greek. London. 8vo. 1616. In French. London. 4to. 1608. In Irish. Dublin. Folio. 1612. In Spanish. London. 4to. 1621. In Welsh. London. 4to. All the foregoing editions are in the Bodleian Library. I may add to them the following three: 1.—1551. Dublin, by Humfrey Powell. Folio 2.—1617(?). Dublin. Company of Stationers. 4to 3.—1637. Dublin. The first of these, which is the first book printed in Ireland, is extremely rare. I believe only two copies are certainly known to exist; one of which is in the library of Trinity College, Dublin; and the other in that of Emmanuel College, Cambridge. Both are in very fine condition. The second is in my possession. The book is quite perfect; but some wiseacre has carefully erased the date. The Almanac for xxvi Yeares tells nothing, being for the years 1603 to 1628. But the book contains a prayer for Frederick, the Prince Elector Palatine, and the Lady Elizabeth, his wife, with their hopeful issue. He married the princess in 1613; and in 1619 he was elected King of Bohemia, and thenceforward would be prayed for under his higher title. If the Sunday letter in the calendar is to be trusted, the book was printed (according to De Morgan's Book of Almanacs) in 1617. The Dublin Society of Stationers was established in that year; and it is not unlikely that they commenced their issues with a Prayer-Book. I have never seen nor heard of another copy, with which I might compare mine, and thus ascertain its date.
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    The third, of1637, is reported; but I have never met with it. H. Cotton. Thurles. ETYMOLOGY OF PEARL. (Vol. vi., p. 578.) The inquiry of your correspondent Ifigfowl respecting the etymology of the word pearl does not admit of a simple answer. The word occurs in all the modern languages, both Romance and Teutonic: perla, Ital. and Span.; perle, French and German, whence the English pearl. Adelung in v. believes the word to be of Teutonic origin, and considers it as the diminutive of beere, a berry. Others derive it from perna, the Latin name of a shell-fish (see Ducange in perlæ; Diez, Grammatik der Romanischen Sprachen, vol. i. p. 235.). Neither of these derivations is probable: it is not shown that beere had a diminutive form, and perna was a local and obscure name: see Pliny, N. H. xxxii. ad fin. Salmasius (Exercit. Plin., p. 40. ed. 1689) thinks that perla is formed from perula, for sperula, the diminutive of sphæra. A more probable origin is that the word is formed from the Latin pirum, as suggested by Diez, in allusion to the pear-shaped form of the pearl. Ducange in v. says that the extremity of the nose was called pirula nasi, from its resemblance to the form of a pear. But pirus was used to denote a boundary-stone, made in a pyramidal shape (Ducange in v.); and this seems to have been the origin of the singular expression pirula nasi, as being something at the extremity. Another supposition is, that the word perla is derived from the Latin perula, the diminutive of pera, a wallet. A wallet was a small bag hung round the neck; and the word perula, in the sense of a small bag, occurs in Seneca and Apuleius. The analogy of shape and mode of wearing is sufficiently close to suggest the transfer of the name. Perula and perulus are used in Low Latin in the sense of pearl. Ducange cites a passage from a hagiographer, where perula
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    means the whiteof the eye, evidently alluding to the colour of the pearl. The choice seems to lie between perula as the diminutive of pera or of pirum. Neither derivation is improbable. It is to be observed that the modern Italian form of pirum, the fruit of the pear, is pera; the modern feminine noun being, as in numerous other cases, formed from the plural of the Latin neuter noun (see Diez, ib. vol. ii. p. 19.). The analogy of unio (to which I shall advert presently) supports the derivation from the fruit; the derivation from pera, a wallet, is, on merely linguistical grounds, preferable. The Greek name of pearl is μαργαρίτης, originally applied to a precious stone, and apparently moulded out of some oriental name, into a form suited to the Greek pronunciation. Scott and Liddell in v. derive it from the Persian murwari. Pliny, H. N. ix. 56., speaking of the pearl, says: Apud Græcos non est, ne apud barbaros quidem inventores ejus, aliud quam margaritæ. The Greek name Margarita was used by the Romans, but the proper Latin name for the pearl was unio. Pliny (ibid.) explains this word by saying that each pearl is unique, and unlike every other pearl. Ammianus Marcellinus (lib. xxiii. ad fin.) thinks that pearls were called uniones, because the best were found single in the shell; Solinus (c. 53.) because they were always found single. The more homely explanation of Salmasius seems, however, to be the true one; namely, that the common word for an onion, growing in a single bulb, was transferred to the pearl (Exercit. Plin., pp. 822-4.; Columella de R. R. xii. 10.). The ancient meaning of unio is still preserved in the French ognon. L. Your correspondent asks the etymon of our English word pearl. It would not be uninteresting to learn, at the same time, at what period pearl came into general use as an English word? Burton, who wrote his Anatomy in the reign of James I., uses the word union (from the Latin unio) instead of pearl (Anat. Melanc., vol. ii. part 2. sec. 3. mem. 3., and ib., p. 2. sec. 4. mem. 1. subs. 4.). In the latter
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    passage he saysThose smaller unions which are found in shells, amongst the Persians and Indians, are very cordial, and most part avail to the exhilaration of the heart. The Latin term unio differs from margarita, in so far as it seems to have been applied by Pliny to distinguish the small and ill-shaped pearls, from the large round and perfect, which he calls margaritæ. And in his ninth book, c. 59., he defines the difference philologically, as well as philosophically. Philemon Holland, who published his translation of Pliny in 1634, about thirteen years after Burton published the first edition of his Anatomy, uses the word pearl indifferently as the equivalent both of margarita and unio. Query: Was the word union generally received in England instead of pearl in Burton's time, and when did it give place to it? J. Emerson Tennant. MARTIN DRUNK. (Vol. v., p. 587.) Has not the following song something to do with the expression Martin drunk? It is certainly cotemporary with Thomas Nash the Elizabethan satirist, and was long a favourite three man's song. It is copied from Deuteromelia, or the Second Part of Musick's Melodie, 4to., 1609: MARTIN SAID TO HIS MAN. Martin said to his man, Fie! man, fie! O Martin said to his man, Who's the foole now? Martin said to his man, Fill thou the cup, and I the can;
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    Thou hast welldrunken, man, Who's the foole now? I see a sheepe shering corne, Fie! man, fie! I see a sheepe shering corne, Who's the foole now? I see a sheepe shering corne, And a cuckold blow his horne; Thou hast well drunken, man, Who's the foole now? I see a man in the moone, Fie! man, fie! I see a man in the moone; Who's the foole now? I see a man in the moone, Clowting of St. Peter's shoone; Thou hast well drunken, man, Who's the foole now? I see a hare chase a hound, Fie! man, fie! I see a hare chase a hound, Who's the foole now? I see a hare chase a hound, Twenty mile above the ground; Thou hast well drunken, man, Who's the foole now? I see a goose ring a hog, Fie! man, fie! I see a goose ring a hog, Who's the foole now? I see a goose ring a hog, And a snayle that did bite a dog;
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    Thou hast welldrunken, man, Who's the foole now? I see a mouse catch the cat, Fie! man, fie! I see a mouse catch the cat, Who's the foole now? I see a mouse catch the cat, And the cheese to eate the rat; Thou hast well drunken, man, Who's the foole now? Edward F. Rimbault. GÖTHE'S REPLY TO NICOLAI. (Vol. vi., p. 434.). Had M. M. E. gone to the fountain-head, and consulted Göthe's own statement in his autobiography, he would have seen in the Werke, vol. xxvi. p. 229., that Mr. Hayward's note was not written with that writer's usual care. Göthe does not say that his reply to Nicolai's Joys of Werter, though circulated only in MS., destroyed N.'s literary reputation: on the contrary, he says that his squib (for it was no more) consisted of an epigram, not fit for communication, and a dialogue between Charlotte and Werter, which was never copied, and long lost; but that this dialogue, exposing N.'s impertinence, was written with a foreboding of his sad habit, afterwards developed, of treating of subjects out of his depth, which habit, notwithstanding his indisputable merits of another kind, utterly destroyed his reputation. This was most true: and yet all such assertions must be taken in a qualified sense. Nearly thirty years after this was written I partook of the hospitality of N. at Berlin. It was in 1803, when he was at the head, not of the Berlin literati, but of the book-manufactory of Prussia. He was then what, afterwards and elsewhere, the Longmans, Murrays, Constables, Cottas, and
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    Brockhauses were,—the greatpublisher of his age and country. The entrepreneur of the Neue Deutsche Bibliothek may be compared with the publishers of our and the French great Cyclopædias, and our Quarterly Reviews. It was unfortunate for the posthumous reputation of the great bibliopolist that he, patronising a school that was dying out, made war on the athletes of the rising school. He assailed nearly every great man, philosopher or poet, from Kant and Göthe downwards, especially of the schools of Saxony, Swabia, and the free imperial cities. No wonder that he became afterwards what Macfleckno and Colly Cibber had been to Dryden and Pope. In some dozen of the Xenien of Göthe and Schiller, in 1797, he was treated as the Arch- Philistine. M. M. E. characterises him as the friend and fellow-labourer of Lessing. Now Lessing was incomparably the most eminent littérateur of the earlier part of that age,—the man who was the forerunner of the philosophers, and whose criticisms supplied the place of poetry. The satirists of the Xenien affect to compassionate Lessing, in having to endure a companion so forced on him as Nicolai was, whom they speak of as a thorn in the crown of the martyr. The few who care for the literary controversies of the age of Göthe in Germany will be greatly assisted by an edition of the Xenien, with notes, published at Dantzig, 1833. H. C. R. PHOTOGRAPHIC CORRESPONDENCE. Processes upon Paper.—The favourable manner in which the account I have given of the Collodion process has been received, not only by your readers in general, as has been evinced by many private letters, but also by the numerous correspondents it has drawn forth, induces me, after some little delay, to request space for a description
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    of the followingprocesses upon paper. In giving these I wish it to be understood that I may offer but little that is original, my object being to describe, as plainly as I possibly can, these easy methods, and to make no observation but what I have found to be successful in my own hands. I have had the good fortune to obtain the friendship of some of the most successful photographers of the day; and taking three very eminent ones, I find they have each some peculiarities in his mode of manipulation, varying with each other in the strength of the solutions employed, and producing results the most agreeable to their respective tastes. Reviewing these different processes in my own mind, and trying with patience the various results, I conclude that the following quantities are calculated to produce an adequate degree of sensibility in the paper, and yet to allow it to be prepared for the action of light for many hours previous to its use, and yet with more certainty than any other I am acquainted with. I think I may always depend upon it for twenty-four to thirty-six hours after excitement, and I have seen good pictures produced upon the third day. I believe it is a rule which admits of no contradiction, that the more you dilute your solution, the longer the excited paper will keep; but in proportion to its diminished sensibility, the time of exposure must be prolonged, and therefore I am, from this waste of time and other reasons, disposed to place much less value upon the wax-paper process than many do. The process I am about to describe is so simple, and I hope to make it so intelligible to your non-photographic readers, that a perfect novice, using ordinary care, must meet with success; but should I fail doing so upon all points, any information sought through the medium of N. Q. shall meet with explanation from myself, if not from other of your experienced correspondents, whose indulgence I must beg should the communication be deemed too elementary, it being my earnest desire to point out to archæologists who are desirous of acquiring this knowledge, how easily they themselves may practise this beautiful art, and possess those objects they would desire to preserve, in a far more truthful state than could be otherwise accomplished.
  • 69.
    I have notmyself met that uniform success with any other paper that I have with Turner's photographic of Chafford Mills: a sheet of this divided into two portions forms at the same time a useful and also a very easily-managed size, one adapted for most cameras, forming a picture of nine inches by seven, which is adequate for nearly every purpose. Each sheet being marked in its opposite corners with a plain pencil-mark on its smooth side (vide antè, p. 372.), the surface for all future operations is in all lights easily discerned. In my instructions for printing from collodion negatives, a form of iodized paper was given, which, although very good, is not, I think, equal to the following, which is more easily and quickly prepared, exhibits a saving of the iodide of potassium, and is upon the whole a neater mode. Take sixty grains of nitrate of silver and sixty grains of iodide of potassium; dissolve each separately in an ounce of distilled water; mix together and stir with a glass rod. The precipitate settling, the fluid is to be poured away; then add distilled water to the precipitate up to four ounces, and add to it 650 grains of iodide of potassium, which should re-dissolve the precipitated iodide of silver, and form a perfectly clear solution; but if not, a little more must be carefully added, for this salt varies much, and I have found it to require 720 grains to accomplish the desired object. The fluid being put into a porcelain or glass dish, the paper should be laid down upon its surface and immediately removed, and being laid upon a piece of blotting-paper with the wet surface uppermost, a glass rod then passed over it to and fro ensures the total expulsion of all particles of air, which will frequently remain when the mere dipping is resorted to. When dry, this paper should be soaked in common water for three hours, changing the water twice or thrice, so as to remove all the soluble salts. It should then be pinned up to dry, and, when so, kept in a folio for use. I have in this manner prepared from sixty to eighty sheets in an evening with the greatest ease. It keeps good for an indefinite time, and, as all experienced photographers are aware, unless you possess good iodized paper,
  • 70.
    which should beof a primrose colour, you cannot meet with success in your after-operations. Iodized paper becomes sometimes of a bright brimstone colour when first made; it is then very apt to brown in its use, but tones down and improves by a little keeping. To excite this paper, dissolve thirty grains of nitrate of silver in one ounce of distilled water, and add a drachm and a half of glacial acetic acid; of this solution take one drachm, and one drachm of saturated solution of gallic acid, and add to it two ounces and a half of distilled water. The iodized surface of the paper may then be either floated on the surface of the aceto-nitrate of silver or exciting fluid, and afterwards a rod passed over, as was formerly done in the iodizing, or the aceto-nitrate may be applied evenly with a brush; but in either instance the surface should be immediately blotted off; and the same blotting-paper never used a second time for this, although it may be kept to develop on and for other purposes. It will be scarcely needful to observe that this process of exciting must be performed by the light of a candle or feeble yellow light, as must the subsequent development. The excited paper may be now placed for use between sheets of blotting-paper; it seems to act equally well either when damp or when kept for many hours, and I have found it good for more than a week. The time for exposure must entirely depend upon the degree of light. In two minutes and a half a good picture may be produced; but if left exposed for twenty minutes or more, little harm will arise; the paper does not solarize, but upon the degree of image visible upon the paper depends the means of developing. When long exposed, a saturated solution of gallic acid only applied to the exposed surfaces will be sufficient; but if there is little appearance of an image, then a free undiluted solution of aceto-nitrate may be used, in conjunction with the gallic acid, the former never being in proportion more than one-third. If that quantity is exceeded, either a brownish or an unpleasant reddish tint is often obtained. These negatives should be fixed by immersing them in a solution of hyposulphite of soda, which may be of the strength of one ounce of
  • 71.
    salt to eightounces of water—the sufficiency of immersion being known by the disappearance of the yellow colour, and when they have been once immersed they may be taken to the daylight to ascertain this. The hyposulphite must now be perfectly removed by soaking in water, which may extend to several hours; but this may be always ascertained by the tongue, for, if tasteless, it has been accomplished. If it is deemed advisable—which I think is only required in very dark over-done pictures—to wax the negative, it is easily managed by holding a piece of white wax or candle in front of a clean iron rather hot, and passing it frequently over the surface. The superabundant wax being again removed by passing it between some clean pieces of blotting-paper. Although the minuter details can never be acquired by this mode which are obtained by the collodion process, it has the advantage of extreme simplicity, and by the operator providing himself with a bag or square of yellow calico, which he can loosely peg down to the ground when no other shade is near, to contain spare prepared papers, he can at any future time obtain a sufficient number of views, which afterwards he can develop at his leisure. It requires no liquids to be carried about with you, nor is that nice manipulation required which attends the collodion process. The wax-paper process has been extolled by many, and very successful results have been obtained: the paper has the undoubted advantage of keeping after being excited much longer than any other; but, from my own experience, just so much the weaker it is made, and so as to safely rely upon its long remaining useful, so it is proportionally slower in its action. And I have rarely seen from wax negatives positives so satisfactory in depth of tone, as from those which have been waxed after being taken on ordinary paper. It is all very well for gentlemen to advocate a sort of photographic tour, upon which you are to go on taking views day after day, and when you return home at leisure to develop your past proceedings: I never yet knew one so lukewarm in this pursuit as not to desire to know, at his earliest possible opportunity, the result of his labours; indeed,
  • 72.
    were not thisthe case, I fear disappointment would more often result than at present, for I scarcely think any one can exactly decide upon the power of the light of any given day, without having made some little trial to guide him. I have myself, especially with collodion, found the action very rapid upon some apparently dull day; whilst, from an unexplained cause, a comparatively brighter day has been less active in its photographic results. As in the previous process, I would strongly advise Turner's paper to be used, and not the thin French papers generally adopted, because I find all the high lights so much better preserved in the English paper. It may be purchased ready waxed nearly as cheap as it may be done by one's self; but as many operators like to possess that which is entirely their own production, the following mode will be found a ready way of waxing:—Procure a piece of thick smooth slate, a trifle larger than the paper to be used; waste pieces of this description are always occurring at the slate works, and are of a trifling value. This should be made very hot by laying it close before a fire; then, covered with one layer of thick blotting-paper, it will form a most admirable surface upon which to use the iron. Taking a piece of wax in the left hand, an iron well heated being pressed against it, it may rapidly be made to flow over the whole surface with much evenness, the surplus wax being afterwards removed by ironing between blotting- paper. When good, it should be colourless, free from gloss, and having the beautiful semi-transparent appearance of the Chinese rice-paper. To iodize the paper completely, immerse it in the following solution: Iodide of potash 200 grains. Mannite 6 drachms. Cyanide of potash 5 grains. Distilled water 20 ounces. Allow it to remain three hours, taking care that air-particles are perfectly excluded, and once during the time turning over each sheet of paper, as many being inserted as the fluid will conveniently cover, as it is not injured by after keeping. It should be then removed from
  • 73.
    the iodide bath,pinned up, and dried, ready for use. When required to be excited, the paper should, by the light of a candle, be immersed in the following solution, where it should remain for five minutes: Nitrate of silver 4 drachms. Glacial acetic acid 4 drachms. Distilled water 8 ounces. Being removed from the aceto-nitrate bath, immerse it into a pan of distilled water, where let it remain about a quarter of an hour. In order to make this paper keep a week or two, it must be immersed in a second water, which in point of fact is a mere reduction of the strength of the solutions already used; but for ordinary purposes, and when the paper is to be used within three or four days, one immersion is quite sufficient, especially as it does not reduce its sensitiveness in a needless way. It may now be preserved between blotting-paper, free from light, for future use. The time of exposure requisite for this paper will exceed that of the ordinary unwaxed, given in the previous directions. The picture may be developed by a complete immersion also in a saturated solution of gallic acid; but should it not have been exposed a sufficient time in the camera, a few drops of the aceto-nitrate solution added to the gallic acid greatly accelerates it. An excess of aceto-nitrate often produces an unpleasant red tint, which is to be avoided. Instead of complete immersion, the paper may be laid upon some waste blotting-paper, and the surface only wetted by means of the glass rod or brush. The picture may now be fixed by the use of the hyposulphite of soda, as in the preceding process. It is not actually necessary that this should be a wax-paper process, because ordinary paper treated in this way acts very beautifully, although it does not allow of so long keeping for use after excitement; yet it has then the advantage, that a negative may either be waxed or not, as shall be deemed advisable by its apparent depth of action.
  • 74.
    Hugh W. Diamond. Exhibitionof recent Specimens of Photography at the Society of Arts. —This exhibition, to which all interested in the art have been invited to contribute, was inaugurated by a conversazione at the Society's rooms, on the evening of Wednesday, the 22nd of December: the public have since been admitted at a charge of sixpence each, and it will continue open until the 8th of January. We strongly recommend all our friends to pay a visit to this most delightful collection. By our visit at the crowded conversazione, and another hasty view since, we do not feel justified to enter into a review and criticism of the specimens so fully as the subject requires; but in the mean time we can assure our archæological readers that they will find there such interesting records of architectural detail, together with views of antiquities from Egypt and Nubia, as will perfectly convince them of the value of this art with reference to their own immediate pursuits. Those who feel less delight in mere antiquity will be gratified to see, for the first time, that there are here shown photographs which aim at more than the bare copying of any particular spot; for many of the pictures here exhibited may rank as fine works of art. We feel much delicacy and hesitation in mentioning any particular artist, where so many are entitled to praise, especially in some particular departments. We could point out pictures having all the minute truthfulness of nature, combined with the beautiful effects of some of the greatest painters. We must, however, direct especial attention to the landscapes of Mr. Turner, the views in the Pyrenees by Mr. Stewart, and one splendid one of the same locality by Le Gray. Mr. Buckle's views in paper also exhibit a sharpness and detail almost equal to collodion; as do the various productions of Mr. Fenton in wax paper. The effects obtained also by Mr. Owen of Bristol appear to be very satisfactory: why they are, with so much excellence, called experimental, we cannot tell. In collodion Mr. Berger has exhibited some effective portraits; and we think the success of Mr. De la Motte has been so great, that in some of his productions little remains to be desired. We cannot conclude
  • 75.
    this brief noticewithout directing attention to the minuteness and pleasing effect of the views in Rome by M. Eugène Constant, which are also from collodion; as also the specimens from albumen negatives of M. Ferrier; and, lastly, to the pleasant fact that lady amateurs are now practising this art,—very nice specimens being here exhibited by the Ladies Nevill, whose example we shall hope to see followed.
  • 76.
    Replies to MinorQueries. Quotation in Locke (Vol. vi., p. 386.).—The words Si non vis intelligi non debes legi were, I believe, the exclamation of St. Jerome, as he threw his copy of Persius into the fire in a fit of testiness at being unable to construe some tough lines of that tough author. I set down this reply from memory, and am unable to give the authority for it. W. Fraser. Pic-nic (Vol. vi., pp. 152. 518.).—The Query of A. F. S. (p. 152.) as to the etymology of pic-nic still remains unanswered. The Note of W. W. (p. 518.) merely refers to the time (1802) when pic-nic suppers first became fashionable in England. Under a French form, the word appears in a speech of Robespierre's, quoted in the British and Foreign Review for July, 1844, p. 620.: C'est ici qu'il doit m'accuser, et non dans les piques-niques, dans les sociétés particulières. An earlier instance occurs in one of Lord Chesterfield's letters (No. 167.), dated October 1748. Jaydee. Discovery at Nuneham Regis (Vol. vi., pp. 386. 488. 558.).— Nuneham Regis was granted to John Dudley, Duke of Northumberland, in the seventh year of King Edward VI.; but as it was forfeited on his attainder, in the first year of Queen Mary, and immediately granted by her to Sir Rowland Hill, knight, and citizen of London, from whom Sir Thomas Leigh, knight, and alderman of London, almost immediately acquired it; and as he exercised the right of presentation to the vicarage in the first year of the reign of Queen Elizabeth, there is no probability of the body of John, Duke of Northumberland, being removed from the Tower of London to Newnham.
  • 77.
    The letters T.B. on the clothes on the body at Nuneham are distinctly worked in Roman capitals, like those on a common sampler. I have seen them. J. S.s. Door-head Inscriptions (Vol. vi., p. 543.).—
  • 78.
    Sit mihi necglis servus nec hospes hirudo. From servant lazy as dormouse, Or leeching guest, God keep my house. Mr. Woodward tells us that he quotes this inscription from memory: it is so very pertinent that it seems a pity even to hint a correction, but, as I read it, it seemed partly familiar to me, and I find something so like the latter part of it in two ancient authors, that I am tempted to inquire whether he may not have omitted one letter, which alters the sense as given above, and yet gives a sense as good. Among the Symbols of Pythagoras, I read the following: Ομωροφιους χελιδωνας μη εχειν. Domesticas hirundines ne habeto. To the same effect (but, strange to say, without any reference to Pythagoras' dictum), we find it in the proverbia of Polydore Virgil (A.D. 1498): Hirundo suscipienda non est. and the exposition is the same in both: Hirundo garrula semper, i.e. garruli et tumigeri homines recipiendi non sunt. I find no original for the former part of the inscription. Probably Mr. Woodward will agree with me, that it is difficult to decide whether a greedy or a gossipping guest would be the worst household infliction; but as a careful householder might well deprecate either, as matter of curiosity perhaps he would refer to the original
  • 79.
    inscription again, anddecide whether he has or has not omitted an n. A. B. R. Belmont. Stratford Parsonage, Wilts: Parva sed apta Domino. 1675. Montacute House, Somerset: Through this wide opening gate None come too soon, none go too late. And yours. Sudbury House, Derbyshire: Omne Bonum Dei Donum. At Verona: Patet Janua, Cor magis. The next I have seen somewhere: Detur digniori. H. T. Ellacombe. Clyst St. George. Cross and Pile (Vol. vi., pp. 386. 513.).—The pile is invariably on the obverse or head side of a coin; and pile or poll both mean the head, from whence the poll tax and poll groat—a tax paid by the head, or a personal tax, of which we have an historical example of its collector in the case of Wat Tyler.
  • 80.
    Ruding, in Annalsof the Coinage, vol. ii. p. 119., 8vo., edit. 1819, states that Ed. I. A.D. 1304, in the delivering out the stamps for the coinage, orders that three piles and six crosses shall be given. It is well known to all numismatists that all, or most early coins, both Saxon and English, had a head on the obverse and a cross on the reverse—the latter being placed on the coins as symbolical of Christianity. Pile also means the hair, or any filament: as the pile of velvet, the nap of woollen cloth, c. And Jamieson, in his Scotch Dictionary, says: Pile. The soft hair which first appears on the chins of young men. Coles, Ashe, Webster, and others give the same meaning. The superstitious effect of the cross as a charm or amulet is well known; from whence the saying: I have never a cross in my purse to keep the Devil away. Again: Priests were coin-proof against the Devil, they never being without money; of course, always had a cross in their pocket.— Gilpin's Beehive of the Romish Church, 1636, p. 251. And Nash, in the Supplication of Pierce Penniless to the Devil, makes Pierce to say: Whereas your impious excellence hath had the poore tenement of my purse anytime this half year for your dancing schole, and he, notwithstanding, hath received no penye nor crosse for farme, c. And the poet Skelton says:
  • 81.
    . . .. . . . . and in his pouche, The Devil might dance therein for any crouche. P. 71. Trusting the above will be satisfactory to D. W. S., I beg to conclude, thinking, you will say I have already made much ado about nothing. Goddard Johnson. Rhymes upon Places (Vol. vi. p. 281.).—Perhaps you will think the following rhymes upon places worth insertion: I stood upon Eyemouth Fort, And guess ye what I saw? Fairmiside and Furmintong, Neuhouses and Cocklaw, The fairy fouk o' Fosterland, The witches o' Edincran, The bly-rigs o' Reston; But Dunse dings a'. Near the seaside village of Eyemouth, in Berwickshire, is a promontory marked with a succession of grassy mounds, the remains of a fort built there in the regency of Mary of Lorraine. A number of places are represented as visible from the fort: but here fact is not strictly adhered to. Fosterland once existed in the parish of Bunkle as a small village; but even its vestiges are not now visible on the brown moor where it once stood. Edincran, properly Auchinchran, is an estate in the vicinity of Fosterland, as is Reston also. There is a variation as follows: The fairy fouk o' Fosterland, The witches o' Edincran, And the rye-kail o' Reston Gar'd a' the dogs die.
  • 82.
    The rye-kail alludedto must have been a broth chiefly made from rye, which grain, it is well known, is sometimes so much tainted as to be poisonous. C. Benson. Birmingham. Ἀρνί ο ν (Vol. vi., p. 509.).—Probably your correspondent is aware of the explanation given by Dr. Wordsworth in his book on the Apocalypse, but does not think it satisfactory. Still, as he does not allude to it, I venture to transcribe it: The Apocalypse abounds in contrasts. For example, the LAMB, who is always called Ἀμνὸς, never Ἀρνίον, in St. John's Gospel, is called Ἀρνίον, never Ἀμνὸς, in St. John's Apocalypse, in which Ἀρνίον occurs twenty-nine times. And why does ὁ Αμνος here become τὸ Ἀρνίον? To contrast Him more strongly with τὸ Θηρίον, that is, to mark the opposition between the LAMB and the Beast. To this a note is appended: This contrast is even more striking in the original, where it is aided by an exact correspondence of syllables and accents. On one side are— 'Ἡ πόρνη καὶ τὸ Θηρίον:' On the other— 'Ἡ Νύμφη καὶ τὸ Ἀρνίον.' See Rev. xxi. 2. 9., xxii. 17.—Is the Church of Rome Babylon? p. 58.: London, 1851. A. A. D.
  • 83.
    Ἀρνίον and ἀμνὸςboth denote a lamb. In John i. 29. 36., the latter is applied to Jesus by John the Baptist. In Acts viii. 32., and 1 Pet. i. 19., the term is manifestly derived from Isa. liii. 7., the Septuagint translation. But, in the Revelation, the word selected by the apostle is simply to be viewed as characteristic of his style. Taken in connexion with John i. 29. 36., the difference presents one of those points which so strikingly attest the authenticity of the Scripture. If the writer had drawn upon his imagination, in all likelihood he would have used the word ἀρνίον in the Gospel; but he employed another, because the Baptist actually made use of a different one, i. e. one different from that which he was in the habit of employing. B. H. Cowper. Who was the greatest General (Vol. vi., p. 509.).—In reply to the following Query, Who was the greatest general, and why and wherefore did the Duke of Wellington give the palm to Hannibal? I think the following note appended to the eloquent sermon of Dr. Croly, preached on the death of the Duke, Sept. 19th, not only shows the humility of the Duke in giving preference to Hannibal over himself, but it contains so just a comparison between the two generals, that it deserves recording in the valuable and useful pages of the N. Q. as well as being a perfect and true answer to C. T.: It has been usual, the note says, to compare Wellington with Hannibal. But those who make the comparison seem to forget the facts:— Hannibal, descending from the Alps with a disciplined force of 26,000 men, met the brave Roman Militia, commanded by brave blockheads, and beat them accordingly. But, as soon as he was met by a man of common sense, Fabius, he could do nothing with him; when he met a manœuvring officer, the Consul Nero, he was outmanœuvred, and lost his brother Asdrubal's army, which was equivalent to his losing Italy; and when he met an active officer, Scipio, he was beaten on his own ground. Finally,
  • 84.
    forced to takerefuge with a foreign power, he was there a prisoner, and there he died. His administrative qualities seem to have been of the humblest, or of the most indolent, order. For fourteen years he was in possession of, or in influence with, all the powers of southern Italy, then the richest portion of the peninsula. Yet this possession was wrested from him without an effort; and where he might have been a monarch, he was only a pensioner. His punic faith, his flight, his refuge, and his death in captivity, might find a more complete resemblance in the history of Napoleon. The following, concluding sentence of Dr. Croly's note conveys a truer and far more just comparison with another great general: The life of the first Cæsar forms a much fairer comparison with that of Wellington. Both nobly born; both forcing their way up through the gradations of service, outstripping all their age; forming their characters by warfare in foreign countries; always commanding small armies, yet always invincible (Cæsar won the World at Pharsalia with only 25,000 men): both alike courageous and clement, unfailing in resources, and indefatigable in their objects; receiving the highest rewards, and arising to the highest rank of their times; never beaten: both of first-rate ability in council. The difference being in their objects; one to serve himself, the other to serve his country; one impelled by ambition, the other by duty; one destroying the constitution of his country, the other sustaining it. Wellington, too, has given the soldier and statesman his 'Commentaries,' one of the noblest transcripts of a great administrative mind. J. M. G. Worcester. Beech-trees struck by Lightning (Vol. vi., p. 129.).—On Thinnigrove Common, near Nettlebed, Oxon, a beech-tree, one of three or four
  • 85.
    growing round apit, was shattered by lightning about thirteen or fourteen years ago. A gentleman who has lived sixty years in the neighbourhood of the beech woods near Henly, tells me that he remembers three or four similar cases. Single beech-trees, which are very ornamental, generally grow very low and wide-spreading, which may be the reason why they often escape. On the other hand, in the woods where they run up close and very high, they present so many points of attraction to the electric fluid, that probably for that cause it is not often the case that one tree in particular is struck. Corylus. Portsmouth. Passage in Tennyson (Vol. vi., p. 272.).—It appears to me that Tennyson has fallen into the error of a Latin construction. I call it an error, because in that language the varied terminations of the cases and numbers make that plain which we have no means of evidencing in English. I should translate it Numenii strepitus volantis—The call of the curlew dreary (drearily) gleams about the moorland, as he flies o'er Locksley Hall. The summer note of the curlew is a shrill clear whistle, but in winter they sometimes indulge in a wild melancholy scream. Corylus. Portsmouth. Inscriptions in Churches (Vol. vi., p. 510.).—I differ from your reply to Norwood's Query, in which you refer to the colloquy between Queen Elizabeth and Dean Nowell as the origin of these inscriptions. No doubt they were derived from the custom of our ante- Reformation ancestors, of painting figures and legends of saints upon the walls of churches; but the following instance will suffice to prove that they originated in the reign of Edward VI., and not in Queen Elizabeth's. In the interesting paper by the Rev. E. Venables in the Transactions of the Cambridge Camden Society, on The Church of St. Mary the
  • 86.
    Great, Cambridge, hegives, under the year 1550, the following extracts from the churchwardens' accounts: For makyng of the wall where Saynt George stood in the chyrche vjd It. payd for wythynge ye chyrch xxs iiijd It. payd for wryghtynge of ye chyrch walls with Scriptures iiijlib iijs iiijd . Shortly after the accession of Queen Mary in 1553, the following entry occurs: Payd to Barnes for mendyng over the rode and over the altar in the chapell, and for washing oute the Scriptures 4s 4d . They do not appear to have been restored after this, for in the year 1840 some of the plaister between two of the windows of the south aisle peeling off, discovered traces of wryghtynge beneath; and I and another member of the Cambridge Camden Society spent some time in laying it bare, and after much difficulty made out that it was the Lord's Prayer in English, headed, The Lord's Prayer, called the Paternoster, and written in the church text of the period, the whole enclosed in a sort of arabesque border; it was not merely whited over, but had evidently been partially effaced, or partly washed oute, before being concealed under its dreary shroud of whitewash. On examination there were traces of more of this writing between the other windows, but we had not time to make any further investigation, for the church was then being cleaned, and in a few days all that we had laid bare was again concealed under a veil of whitewash. Thus, I think, we may assign to the reign of Edward VI., not merely the obliteration of the numerous frescoes of St. Christopher, the great dome, c., which are now so constantly coming to light, but also the origin of wryghtynge of ye chyrch walls with scriptures in
  • 87.
    their stead, someten or twelve years earlier than the remarkable colloquy between Queen Elizabeth and the worthy Dean of St. Paul's. Norris Deck. Cambridge. Dutensiana (Vol. vi., p. 376.).—Lowndes gives a list of Dutens' works, which does not include Correspondence interceptée, of which he was the author; and I have seen a presentation copy of it proving this. W. C. Trevelyan. Early Phonography (Vol. vi., p. 424.).—Have the modern phonographists ever owned their debt of gratitude to their predecessors in the phonetic art? The subjoined advertisement may perhaps be considered an answer to this Query: Hart's Orthography, 1569; or, 'An Orthographie conteyning the due order and reason, howe to write or paint thimage of manne's voice, most like to the life or nature. Composed by J. H. [John Hart], Chester Heralt;' reprinted from a copy in the British Museum. Cloth, 2s. An unanswerable defence of Phonetic Spelling, and one of the earliest schemes of Phonetic Orthography. A considerable portion of the book being printed in the author's Phonetic Alphabet (given in the present edition in Phonetic Longhand), we have thus exhibited the pronunciation of the age of Shakspeare. W. C. Trevelyan. Kentish Local Names; Dray (Vol. vi., p. 410.).—In the low embanked land in the west of Somersetshire, between Bristol and Taunton, the
  • 88.
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