\AI & COLOR AND ITS APPLICATIONS BY M. LUCKIESH DIRECTOR OF APPLIED SCIENCE, NELA RESEARCH LABORATORIES NATIONAL LAMP WORKS OF GENERAL ELECTRIC CO. AUTHOR OF " LIGHT AND SHADE AND THEIR APPLICATIONS," " THE LIGHTING ART," " THE LANGUAGE OF COLOR," "ARTIFICIAL LIGHT, ITS INFLUENCE UPON CIVILIZATION," " LIGHTING THE HOME," ETC. 150 Illustrations 4 Color Plates 34 Tables SECOND EDITION ENLARGED NEW YORK D. VAN NOSTRAND COMPANY EIGHT WARREN STREET IQ2I CQPYRIGHT 1915, IQ2I, BY D. VAN NOSTRAND COMPANY THE PLIMPTON PRESS NORWOOD 'MASS 'U'S 'A PREFACE The aim of this book is to present a condensed treatment of the science of color. An attempt has been made to cover as many phases of the subject as possible within the confines of a small volume. During several years of experimental work in the science of color I have been brought into contact with many persons interested in its applications, and the desire has been frequently expressed for a book that treated the science of color as far as possible from the viewpoint of those interested in the many applica- tions of color. These applications are constantly in- creasing in scope and interest. With this viewpoint in mind I have attempted to treat the subject, exercis- ing my judgment in drawing freely from the work of other investigators in order to make the volume as comprehensive as possible. I do not feel that the work comprises a complete treatment, for there are many interesting phases of color science that have been barely touched upon, and some that have been purposely omitted, because of the danger of straying too far afield. It is believed, however, that this treatise will be helpful to those interested in any of the arts involving the science of color. I have referred to my own investigations quite freely, but trust that this will not be attributed to a lack of per- spective. Naturally much of the text involves my own conclusions, but I have aimed to include only those that are supported by experimental data, be- cause only in so far as they are thus supported does iii 520416 iv PREFACE the work become authoritative. Many unsolved prob- lems have arisen throughout the text, which em- phasizes the need for more workers in the field. No attempt has been made to present a complete bib- liography of even the recent work in this branch of science ; but references have been given freely, which, if followed, will provide a substantial beginning to the almost endless chain of material available. It is a pleasant duty to record my acknowledg- ments to the management of the National Lamp Works of the General Electric Company, whose broad- minded spirit in establishing the Nela Research Laboratory has made this work possible, and to the director of the laboratory and members of the staff, who always have given freely of their time and counsel. SECOND EDITION Some changes have been made in the original text and an extensive chapter has been added. This con- sists of useful data and methods for their use. M. LUCKIESH September, 1920 CONTENTS CHAPTER I Page LIGHT 1 Wave Theory. Electro-magnetic Theory. Radiation and Light Sensa- tion. Temperature and Radiation. Spectra of niuminants. CHAPTER II TUB PRODUCTION OF COLOR 23 Refraction. Diffraction. Interference. Polarization. Reflection, Ab- sorption, and Transmission. Color of Daylight. Color Sensations Produced by Colorless Stimuli. Fluorescence and Phosphorescence. Useful Filters. CHAPTER III COLOR-MIXTURE 64 Subtractive Method. Additive Method. Juxtapositional Method. Simple Apparatus for Mixing Colors. CHAPTER IV COLOR TERMINOLOGY 69 Hue, Saturation, and Brightness. Tri-color Method. Color Notation. CHAPTER V THE ANALYSIS OF COLOR 86 The Spectroscope. The Spectrophotometer. The Monochromatic Colorimeter. The Tri-chromatic Colorimeter. Other Methods. Templates. Reflectometer. Methods of Altering Brightness Non- selectively. CHAPTER VI COLOR AND VISION fl6 The Eye. Brightness Sensibility. Hue Sensibility. Saturation Sensi- bility. Visual Acuity in Lights of Different Colors. Growth and Decay of Color Sensations. Signaling. Other Uses for Colored Glasses. vi CONTENTS CHAPTER VII THE EFFECT OF ENVIRONMENT ON COLORS ' 163 Illumination. After-images. Simultaneous Contrast. Irradiation. CHAPTER VIII THEORIES OF COLOR VISION.. Young-Helmholtz. * Duplicity.' Hering. Ladd-Franklin. Edrigde- Green. CHAPTER IX COLOR PHOTOMETRY 191 Methods of Color Photometry. Other Means of Eliminating Color Dif- ferences. Direct Comparison and Flicker Methods. Luminosity Curve of the Eye. CHAPTER X COLOR PHOTOGRAPHY 213 Lippmann Process. Wood Diffraction Process. Color Filter Processes. CHAPTER XI COLOR IN LIGHTING , 224 Artificial Daylight. Units for Imitating Daylight. Effect of Colored Sur- roundings. Color in Interiors. Color Preference. A Demonstration Booth. CHAPTER XII COLOR EFFECTS FOR THE STAGE AND DISPLAYS 272 Stage. Displays. CHAPTER XIH COLOR PHENOMENA IN PAINTING 282 Visual Phenomena. Lighting. Pigments. CHAPTER XIV COLOR MATCHING 302 The Illuminant. The Examination of Colors. CONTENTS vii CHAPTER XV THE ART OF MOBILE COLOR 312 Color Music. Its Relation to Sound Music. CHAPTER XVI COLORED MEDIA 327 Available Coloring Materials. Dyeing. Gelatine Films. Solvents. Lacquers. Celluloid. Phosphorescent Materials. Miscellaneous Notes. CHAPTER XVH CERTAIN PHYSICAL ASPECTS AND DATA 344 Three Types of Colored Media. Pigments. Optical Properties of Pig- ments. Applications of Spectral Analyses of Pigments. Reflection- factors of Pigments. Spectral Analyses of Dye-solutions. Applications of Spectral Analyses of Dyes. Laws Pertaining to Colored Solutions. Dichromatism. Graphical Method for Using Spectral Data. Spectral Analyses of Glasses. Red, Yellow, Green, Blue, and Purple Glasses. Use of Spectral Analyses of Glasses. Influence of Temperature on Transmission of Colored Glasses. Ultraviolet Transmission of Media. Compounds Sensitive to Temperature. Transmission of Light by Fog and Water. Color Temperature of Illuminants. INDEX . 407 COLORED PLATES Prismatic Spectrum Frontispiece Diffraction Grating Spectrum " Subtractive method of mixing colors Facing page 64 Additive method of mixing colors " " 54 Showing the effect of environment on the appearance of colors " " 163 Illustrating the effect of the spectral quality of the illuminant Daylight, below ; ordinary artificial light, above . . " " 282 LIST OF ILLUSTRATIONS Figure Page 1. Radiation curve of an incandescent solid 8 2. Showing the relation between radiant energy and light sensation ... 10 3. Showing the effect of temperature on the radiation from an incandes- cent solid (black-body) 12 4. Representative spectra 17 6. Distribution of energy in the visible spectra of various illuminants . . 20 6. Newton's experiment 23 7. Effect of the character of the slit of a spectrograph on the grating spec- trum of the mercury arc 24 8. Dispersion curves of various optical media 25 9. Young's double-slit experiment illustrating the principle of the diffrac- tion grating 26 10. Diagrammatic illustration of polarized light 31 11. The Nicol prism for obtaining plane-polarized light 33 12. Analyses of ordinary colors 36 13. Showing the variation in the spectral character of sunlight due to at- mospheric absorption 38 14. Benham disk for producing subjective colors by means of black and white stimuli 39 15. Diagrammatic illustration of the action of the rhodamine fluorescent reflector 44 16. Spectrophotographic analysis of the action of the rhodamine fluores- cent reflector 45 17. Screens for producing lights of the same hue but differing in spectral character 48 18. Ultra-violet spectra 60 19. Ultra-violet spectra 51 20. The subtractive method of mixing colors (colored plate) 21. The additive method of mixing colors (colored plate) 22. The color-wheel for showing complementary hues ' . . 59 23. Maxwell disks 62 24. An erratic color-mixing disk 64 25. A simple color-mixer 64 26. A simple color-mixer for transparent or opaque media 65 27. Lambert's color-mixer 65 28. A shadow demonstration of the additive and subtractive methods of color-mixture 66 29. Illustrating a disk for approximating a prismatic spectrum 68 30. Disk ' a,' for varying only the saturation of a color. Disk * b,' for vary- ing only the brightness of a color 71 31. The Maxwell color-triangle 73 32. Spectral complementaries 75 33. A color pyramid 75 34. The double pyramid (after Titchener) 76 36. A demonstration color-triangle 76 LIST OF ILLUSTRATIONS 36. The A. H. Munsell color tree "... 81 37. Prang's color and brightness scales 82 38. Ruxton's color mixture chart for printing inks 82 39. A direct-vision prism spectroscope 86 40. A simple grating spectroscope 86 41. The spectrophotometer 88 42. The Nutting pocket spectrophotometer 88 43. A small portable spectrophotometer for quantitative analysis .... 89 44. The variable sectored disk (after Hyde) 90 45. Scheme for reducing the amount of spectrophotometric work in ex- amining transparent colored media 91 46. Abney's spectrophotometric attachment for a spectrometer 93 47. Ives' spectrophotometric attachment for a spectrometer 93 48. Nutting's spectrophotometric attachment for a spectrometer .... 94 49. The Nutting monochromatic colorimeter 96 60. Analysis of two component color-mixtures 99 61. A simple method of converting a spectrometer into a combined mono- chromatic colorimeter, direct comparison photometer, flicker pho- tometer, and spectrophotometer 100 52. Illustrating the principle of the Maxwell ' color box ' 101 63. The F. E. Ives colorimeter 103 54. Kb'enig's sensation curves 104 65. Tri-color colorimeter measurements 104 66. Arrangement for using color filters before a photometer eyepiece . . 106 57. Arons colorimeter 108 68. Abney's template for carmine 110 59. Adaptation of Abney's scheme for the spectroscopic synthesis of color 111 60. The Nutting reflectometer 113 61. A vertical section of the human eye 116 62. Showing the effect of chromatic aberration in the eye 118 63. A simple achromatic lens 119 64. Limits of the visual field for colored and colorless lights 120 65. Brightness sensibility data. (See Table X) 121 66. Hue sensibility. (Steindler's Eye) 125 67. Hue sensibility, limen, and color scale 126 68. Apparatus for determining visual acuity in monochromatic lights . . 133 69. Visual acuity in monochromatic lights of equal brightness . . . . . 136 70. Visual acuity in the mercury spectrum, the lines being reduced to equal brightness 136 71. The growth and decay curves for white light sensation. (Broca and Sulzer) 138 72. The growth and decay curves of color sensations . 139 73. Showing the maxima attained by flickering lights at various frequencies 140 74. Showing the maxima of sensations produced by flickering red light on a steady green field (R), and vice versa (G) 141 75. Showing the relation between brightness and critical frequency for colored stimuli 145 76. Effect of contour of flicker on critical or vanishing-flicker frequency . 147 77. Effect of yellow-green glasses on vision under a bright sky 155 78. Ultra-violet transmission curves of various glasses 168 79. Effect of the intensity of illumination on the appearance of a pigment 166 80. Illustrating why a purple appears differently under two different illuminants 167 81. Effect of brightness on the duration of the after-image 171 LIST OF ILLUSTRATIONS xi 82. Showing the effect of simultaneous contrast. The V's are of equal brightness 174 83. Showing induction. Each band, though uniform in brightness, appears brighter at the right-hand edge 175 84. An arrangement for showing tjie reduction in the contrast effect by separating the two colored objects 176 85. An arrangement for showing the effect of simultaneous contrast and after-images 176 86. Illustrating irradiation 179 87. The evolution of the Ladd-Franklin gray molecule 187 88. The results of four methods of photometry. (Ives) 195 89. Spectral sensibilities of selenium and photo-electric cells compared with the spectral sensibility of the eye 200 90. Spectral sensibility of a panchromatic photographic plate 202 91. An accurate color filter for the panchromatic plate considered in Fig. 90. 203 92. Results by flicker and direct comparison photometers, illustrating dif- ferences including the Purkinje effect and a reversed effect . . 206 93. Visibility data. (See Table XVI) 209 94. Illustrating the standing waves produced in the Lippmann process . . 215 95. Illustrating the Wood diffraction process 216 96-98. Illustrating three processes of color photography 219 99. Illustrating the limitations of certain processes of color photography . 220 100. Ideal transmission screens for producing artificial daylight 230 101. Showing the loss of light when using the ideal artificial-daylight screens with the tungsten lamp operating at 7.9 lumens per watt .... 231 102. Showing the loss of light when using the ideal artificial-daylight screens with the tungsten lamp operating at 22 lumens per watt .... 232 103. Showing the spectral analyses of two subjective white lights com- pared with the spectral analysis of noon sunlight 235 104. Showing the additive method of producing artificial daylight .... 236 105. Showing the relative amounts of light of the character of A and B (Fig. 104) necessary to produce artificial daylight by addition . . 237 106. Illustrating the effect of multiple selective reflection of light from a green fabric 248 107. Showing the relative proportions of red, green and blue components in the reflected light from a green fabric after various successive reflections 249 108. Screen for altering tungsten light to the same spectral character as carbon incandescent electric light; c, d, e show the transmission curves of amber glasses of different densities 254 109. Comparison of ideal screen a, Fig. 108, with amber glass 265 110. Showing the preference or rank of a number of fairly saturated colors . 261 111. Wiring diagram of an experimental and demonstration booth .... 267 112. Showing dimensions and locations of lamps in the demonstration booth 268 113. Illustrating the effect of colored light upon the appearance of six colored papers 273 114. Illustrating the changing of scenery by the use of colored lights . . . 275 115. Illustrating the disappearing effects produced on a specially painted scene by varying the color of the illuminant 276 116. Illustrating a flashing sign produced by properly relating the hue and brightness of the pigments with the color of the illuminant . . . 279 117. Showing the reflection coefficents of fairly saturated colors for day- light and tungsten incandescent electric light. (See Table XV) . . 286 xii LIST OF ILLUSTRATIONS 118. Showing the effect of the illuminant upon the appearance of a colored frieze 288 119. Showing the effect of the spectral character of the illuminant upon the values of a painting 290 120. Effect of distribution of light on the expression of a painting 293 121. Illustrating the optics of picture lighting 294 122. Spectral analyses of pigments 298 123. Spectral analyses of pigments 298 124. Illustrating the effect of the amount of the green components in blue and yellow pigments on the amount of ' black ' in the mixtures . 299 125. Diagrammatic illustration of the results of mixing blue and green pig- ments containing various amounts of green 300 126. The ' Luce ' part for the ' Clavier a lumieres ' in Scriabine's ' Pro- metheus ' 315 127. Illustrating an instrument for studying the emotive or affective value of colors and color phrases ; Rimington's color code also shown 323 128. A color-mixture instrument for studying the emotive and affective value of colors and color phrases 324 129. Showing the relative positions of the colored lamps in the apparatus diagrammatically shown in Fig. 128 325 130. Michrophotographs of white cotton and silk fabrics against a black background 348 131. Spectral reflection-factors of pigments . 352 132. Spectral reflection-factors of pigments 353 133. Spectral luminosities of pigments 360 134. Spectral luminosities of pigments 361 135. A study of a pigment (light chrome yellow) 362 136. Reflection-factors of pigments 368 137. Relative reflection-factors of pigments 369 138. Influence of the illuminant on the appearance of a pigment 370 139. Relation between spectral transmission-factor and depth or concen- tration of a solution of methylengriin 381 140. Relation between spectral luminosity and depth or concentration of a solution of rosazeine 382 141. Complete relation between thickness, wave-length, and transmission- factor for a gold ruby glass 383 142. Spectral transmission-factors of selenium glasses 387 143. Spectral transmission-factors of copper, sulphur, chromium, and uranium glasses 388 144. Spectral transmission-factors of gold glasses and combinations with cobalt 389 145. Spectral transmission-factors of carbon glasses and combinations with cobalt glasses 390 146. Spectral transmission-factors of cobalt glasses 391 147. Spectral transmission-factors of iron and of manganese glasses . . . 392 148. Relations between spectral transmission-factor and thickness of a gold glass (23Au) 393 149. Relations between spectral luminosity and thickness of a gold glass (23Au) 394 150. Test of the relation between spectral transmission-factor and thick- ness of a blue-green glass \ . 395 COLOR AND ITS APPLICATIONS CHAPTER I LIGHT 1. The word Light has acquired two meanings; one pertains to sensation and is therefore physiological and psychological in character, while the other refers to the external cause of the sensation and is there- fore physical in nature. As both meanings are used in the study of color, they will be distinguished wher- ever necessary; for example, light rays when imping- ing upon the retina of the eye produce the sensation of light. In order to understand the phenomena of color, a fair knowledge of the physical nature of light must first be acquired. Unfortunately the field to be covered in this book is too extensive to permit of a detailed treatment of this interesting subject; only those phenomena will be discussed that are essential to an understanding of the subsequent chapters. Those wishing to pursue this line of study further can readily do so by consulting the many excellent treatises on the subject. 2. Wave Theory. --The passage of a beam of light from a source (flame, sun, etc.) to a receiver (the earth, the eye, etc.) involves a transfer of energy, and the question arises as to how this transfer takes place. All around us in Nature, energy is contin- ually being transferred from one place to another and whenever such a transfer does occur something is moving. On the ocean, for example, energy is COLOR AND ITS APPLICATIONS transferred by water due to its wave motion. Moun- tain streams are carrying energy, which fortunately can be made to do useful work by means of a water motor, but here the energy is transferred by the onward flow of the water. In air the same two meth- ods of transferring energy are found; currents in the case of winds and waves in the case of sounds. Solids also can be made to transmit energy in these two ways; by currents as in the sand blast and by waves as in the case of sound and other elastic dis- turbances. Since currents and waves are such com- mon methods of transmitting energy, it is quite natural that they should be called upon to explain the transfer in the case of light. Light travels in straight lines, casts (comparatively) sharp shadows, is reflected from a smooth surface as a regular succession of rubber balls would be if thrown against the same surface, and in many other ways acts much like a current of particles would act if projected from the source of light at a high velocity. There is one phenomenon, however, that cannot be explained by the assumption of a current of par- ticles; under certain conditions two rays of light of equal intensity can be sent to the same spot in such a manner that the spot will be dark and not twice as bright as it would be if either ray were present alone. This fact is explained by assuming that light energy is transmitted in the form of wave motion for it is seen that if two equal waves are made to pass in the same direction through any medium, but in such a manner that the crests of one wave coincide with the troughs of the other, the two waves will annul each other everywhere, there will be no resultant wave, no transfer of energy, and hence no light at the spot in question. To this phenomenon was given LIGHT 3 the name 'interference,' but the term has been extended to include all the phenomena that may take place when two or more waves travel in the same me- dium at the same time. The foregoing case and all others in which there is destruction of motion are now grouped under the term, destructive interference ; in contradistinction to this, there is constructive interference wherever the motion due to all the waves is greater than that due to one. The simplest case of the latter type is that of two equal wave trains traveling in the same . direction at the same time but in such a manner that the crests of one coincide with the crests of the other. The two -waves reinforce each other and the resultant wave has twice the am- plitude of the original waves. Another very important special case of interference is that to which the term 'standing wave 1 or 'stationary wave' has been applied. This occurs whenever two equal wave trains are passing in opposite directions through the same me- dium at the same time. The most common way of obtaining equal waves traveling in opposite directions is by means of reflection at a surface perpendicular to the direction in which one train is traveling. Stand- ing waves can be readily demonstrated by fastening one end of a long rope (preferably so that it hangs vertically) and by shaking the other end, timing the motion of the hand so that it is in unison with the reflected waves. It will be seen that some points of the rope remain at rest and others swing through a large amplitude. The points at rest are called nodes and the part of the string between the nodes is a segment. By varying the speed of the hand or the period of vibration the string can be made to vibrate in one, two, three, or more segments. A brief consideration of wave motion in general COLOR AND ITS APPLICATIONS and a few definitions may not be out of place here. In the first place, it is evident that in any wave motion, the parts of the medium do not travel as far as the wave. They remain each in its own region, each causing adjacent parts to move and in so doing gives up to the adjacent part some of its energy. The motion of the particles differ in various kinds of waves. 7 In waves in deep water each drop moves in a vertical plane in a circular orbit. In waves in shal- low water the orbit is an elongated ellipse. In media transmitting sound wav^s the motion of each particle is to and fro in a straight line in the direction in which the wave is traveling. For all waves the wave-length is the distance between any two suc- cessive particles that are moving through the same points in their orbits at the same instant. The ampli- tude is half a particle's path length (the diameter of the orbit). The period is the time taken for the wave to travel one wave-length. The frequency is the reciprocal of the period or the number of waves that pass a given point in a unit of time. If two waves are 'in step' so that a crest of one occurs at the same time and place as a crest of the other, the two waves are said to be in phase. If a wave is confined to a surface such as that due to a pebble dropped in a quiet pond of water, the waves will be circular; any circle is a wave front, and the direction in which the wave is traveling at any point is that of the radius drawn to the point and is therefore per- pendicular to the wave front. In the case of light under ideal conditions, the wave will spread out in all directions from a point, so that the wave front will be spherical. The direction of propagation will again be perpendicular to the wave front along the radii of the sphere. LIGHT Light energy or radiant energy passes through a vacuum. The phenomenon of interference has been explained by wave motion. Hence it is assumed that there is something in the vacuum that can move. This something is called the ether and is further assumed to penetrate all matter so that light waves always are ether waves; the properties of the waves may change as the matter imbedded in the ether is changed, but it is the ether and not the matter imbedded in the ether that is responsible for the propagation of the light waves. Some scientists, are not reconciled to this view but fortunately in this treatise we need not enter into the discussion. The adoption of the wave theory necessitates new and somewhat elaborate explanations for such simple phenomena as the rectilinear propagation of light; these have been made by the aid of Huy- ghen's principle, which states that each point on a wave front may be regarded as a new source of dis- turbance, sending out spherical waves, and that at any instant the new wave front will be the envel- ope of all of these secondary wavelets. By the aid of this principle it is at once evident that a light wave in going through a wide slit will pass on in such a manner that the sides of the slit cast a rather sharp shadow, whereas in going through a very narrow slit, comparable in width with a wave-length of light, it will pass on and spread out in all directions thus * turning a corner.' This phenomenon has been termed diffraction.' It is helpful to visualize light waves by means of the water wave analogy as has been done in the foregoing, but it is well to guard against being misled by following the analogy too closely. For example water waves dimmish in amplitude on account of 6 COLOR AND ITS APPLICATIONS dissipation of energy through molecular friction. In free space the amplitude of light waves (which is a measure of their intensity) has not been observed to decrease; in other words, there is no friction in the transmitting medium. 3. Electro-magnetic Theory. At first it does not appear that there is any relation between light and electricity, but such a relation was predicted by Max- well in about 1870. This theory assumes light rays to be identical with the electro-magnetic disturbances which are radiated from a body in which electrical oscillations are taking place. Some years later Hertz actually produced these waves and by this discovery the electro-magnetic theory expounded by Maxwell was supplied with the necessary physical foundation. In enunciating this theory it is customary to state that the oscillating electrons in the atoms which constitute a body send forth through space pulses of electro-magnetic energy. The electron at present is supposed to be an atom of electricity. These electric waves emanating from a radiating body whether it be the sun or a red-hot poker have many properties depending upon their wave-length. Al- though all travel at the same velocity, about 3 x 10 10 centimeters (186000 miles) per second, in free space, they differ somewhat in velocity in the ordinary trans- parent media. In glass, for instance, violet rays travel less rapidly than the red rays. All these rays represent energy and therefore regardless of wave-length have the property of producing heat when absorbed. Some- times the energy is not wholly converted into heat but enters into chemical reactions or is converted into electricity or radiant energy of other wave-lengths than those of the absorbed rays. Some of the rays, especially those of shorter wave-length than the visible LIGHT rays, are very active chemically, affecting a photo- graphic emulsion, destroying bacteria and animal, tissues such as the outer membrane of the eye and causing sunburn; they are also largely respon- sible for exciting phosphorescence and fluorescence. Other rays have varying effects on organisms and play a more or less important part in the growth of plants. Rays within a certain range of wave-lengths, separately and in groups, produce the sensation of light and color. In other words the retina of the eye can be likened to a receiving station in wireless telegraphy which is * tuned' to respond to electro- magnetic rays of a certain (limited) range of wave- lengths. 4. Radiation and Light Sensation. There are many ways of decomposing radiation into its various component rays. The rainbow is the result of one of_Nature's means of dispersing the radiation from the sun into rays of various wave-lengths. The eye sees in the rainbow many colors, the most conspicu- ous being violet, blue, green, yellow, orange and red. These are seen to be of different brightnesses. If the retina were sensitive to rays of an infinite range of wave-lengths the rainbow would appear much wider than it does. That is, colors whose appearance can not be imagined, would be seen in the now in- visible regions beyond the violet and red, because energy corresponding to those wave-lengths is present in sunlight. The distribution of the energy among the different wave-lengths radiated by a hot solid is shown in Fig. 1. This curve is technically known as a radia- tion curve and shows that energy of a great range of wave-lengths is present. Such a continuous spec- trum is characteristic of the radiation from solid 8 COLOR AND ITS APPLICATIONS bodies. On the basis of the electro-magnetic theory of light, it may seem strange that rays of all wave- lengths are produced by the vibrating electrons. This may be pictured sufficiently well for the present purpose by assuming that in a solid body there is considerable damping of the vibrations and that other influences are present which result in the emission V R WAVE LENGTH Fig. 1. Radiation curve of an incandescent solid. of no characteristic single ray or series of rays but, instead, of rays of all wave-lengths. The height of the curve at any point above the axis of abscissae or base line represents the relative amount of energy of that particular wave-length present in the total radia- tion. It will be noted that the amounts of energy of various wave-lengths are by no means of the same value. This characteristic of illuminants is of great importance in a study of colors as will become evident later. The region to which the eye is * tuned,' the visible spectrum, lies between V (violet) and R (red) which is exaggerated in relative extent for the sake of clearness. The relation between radiation and light sensa- tion is not simple. The ability of the various rays to produce light sensation is shown roughly by the dotted curve in Fig. 1. The maximum light sensa- tion is produced by rays in the middle of the visible spectrum, namely by those giving rise to a yellow- LIGHT. 9 green sensation. Beyond the limits of the visible spectrum, V and #, it is obvious that an infinite amount of electro-magnetic energy causes no sensa- tion of light. The total range of wave-lengths might be called the energy spectrum of this particular radi- ator. It is obvious that the greater the percentage of the total radiant energy confined to the visible spectrum, the greater is the 'luminous efficiency' of the radiating body. In the production of light the total *] range of wave-lengths is of interest, but in the con- sideration of Color, interest is very largely confined to the visible spectrum. As the temperature of an incandescent body is increased, the energy of the shorter wave-lengths increases more rapidly than the energy of the longer wave-lengths. Considering the visible spectrum, the violet and the adjacent rays increase in intensity more rapidly with increase of temperature than the red and its adjacent rays thus causing the light emitted by an incandescent filament, for instance, to become bluer as its temperature is increased. Here it is well to note that when the various visible rays are permitted to impinge separately upon the retina each produces its own color sensation but when all the visible rays simultaneously stimulate the retina, as in the ordinary viewing of colorless objects, an integral sensation is produced. In the case of most common illuminants the integral sensation is an unsaturated yellow, that is, a yellowish white, while the combined sensations aroused by average day- light produce the integral sensation of white light. In the discussion of Fig. 1 it has been seen that energies in the various wave-lengths .differ in light- producing effects. To this must be added the fact that the light-producing effect varies with the inten- 10 COLOR AND ITS APPLICATIONS sity. In Fig. 2, A represents the light sensation pro- duced in the author's eye by equal amounts of energy of various wave-lengths as measured with a direct comparison or equality-of-brightness photometer. The photometric field was a circle whose diameter sub- 100 so 80 I 70 o I 60 3 50 H 40 \ \ \ 0.40 044 045 052 Q56 Q60 064 0.68 M, WAVE LENGTH Fig. 2. Showing the relation between radiant energy and light sensation. tended an angle of about four degrees at the eye, and whose brightness was equivalent to that of a white surface illuminated to an intensity of 20 meter candles ; (a meter candle is the illumination received on a surface everywhere one meter distant from a source of one candle, the surface being perpendicular to the straight line from it to the light source). On decreasing the intensity and therefore the brightness of the photometer field to about one two-hundredth of its original value or to an equivalent of 0.1 meter candle on the foregoing basis, the relation between light and radiation become as shown in curve B. It will be noted that the maximum of the luminosity curve (as it is called) has shifted toward the shorter LIGHT 11 wave-lengths. In other words at the low illumina- tion the light-producing effect of visible rays of the shorter wave-lengths has not decreased as much as that of the longer wave-lengths. To illustrate by a simple experiment, suppose a red and a blue sur- face appear of equal brightness at a high illumina- tion. If the intensity of illumination is reduced to a very low value, the blue surface will appear much brighter than the red one. To further complicate matters it is found that even normal eyes differ some- what in spectral sensibility for experiment shows that the luminosity curves for various normal eyes do not exactly coincide (see # 56). Curve C is plotted from Koenig's 1 data, obtained at a very low illumination, practically at the threshold of vision. This phenome- non of shifting spectral sensibility which was discov- ered by Purkinje, and which bears his name, will be discussed in later chapters, and the quantitative rela- tion between radiation and light sensation will be fur- ther treated in the chapter on color photometry. 5. Temperature and Radiation. As already stated the effect of raising the temperature of a heated solid body is to increase the luminous effi- ciency and also the relative amount of energy in the rays of shorter wave-lengths. These effects are shown diagrammatically for a solid body in Fig. 3. The numbers on the curves indicate the absolute black-body temperatures. The wave-length is given in terms of ten-thousandths of a centimeter, this unit being usually expressed by the Greek letter, /x. The rays to which the eye is sensitive lie between V and /?, respectively about 0.4/z and 0.7^. The eye in reality is sensitive somewhat beyond these wave-lengths but for practical purposes the amount of light sensa- tion produced by rays beyond these limits is usually 12 COLOR AND ITS APPLICATIONS negligible. It is seen that as the temperature rises the maximum of the radiation curve shifts toward the shorter wave-lengths. The maximum of the radi- ation curve of sunlight lies in the visible region. This has brought forth the suggestion that the eye 80 234 WAVE LENGTH Fig. 3. Showing the effect of temperature on the radiation from an incandescent solid (black-body). in its process of evolution has become most sensitive to the rays of such wave-length as are a maximum in the radiation from the sun. As the maximum of the radiation curve shifts toward the shorter wave- lengths, it is seen that a relatively greater proportion of the total energy is found in the visible region between V and R which accounts for the increase, in luminous efficiency. One of the tendencies in light production is toward the development of materials and methods which will enable the light source to be operated at higher temperatures in order to appease the ever-present demand for higher luminous effi- ciencies. It is evident that the ideal light source emitting a continuous spectrum would be one that radiated no energy beyond V and R. The area under each curve is proportional to the total amount of energy emitted by the radiator at a certain tern- LIGHT 13 perature and the ratio of the area under that part of any curve included between V and R is propor- tional to the energy that can effect the eye. The ratio of the latter area to the former (for the same curve) is called the * radiant efficiency' of the radiator as a light source. To make the idea of radiant effi- ciency of practical value it must be combined with the relations between luminous sensation and radiation. 6. Spectra of Illuminants. The spectral distribu- tion of energy in the radiation from different illumi- nants is of great importance in the consideration of color owing to the fact that the appearance of the colored objects depends upon the spectral character of the illuminant under which they are viewed. The variation in the spectral character of illuminants is due to the temperature and composition of the radi- ating body and also to the state in which it exists when radiating luminous energy. A simple means of producing light is that of heating a solid conductor by passing an electric current through it. At first it will emit invisible radiant energy known as infra-red rays. As the temperature is raised it will finally become luminous, at first appearing a dull red. This is evident from an inspection of Fig. 3. If these light rays be studied by means of a spectroscope which disperses the radiation into its component rays, it will be found that deep red rays are the only visible rays present in appreciable amounts. As the temperature is in- creased the appearance of the body passes from red to orange, then to yellow and so on. If the body were sufficiently refractory to withstand higher tem- peratures and remain in solid form, at a certain tem- perature it would appear white and with increasing temperature would assume a bluish white appearance. 14 COLOR AND ITS APPLICATIONS The latter temperatures have never been reached in the production of artificial light. Notwithstanding the fact that all solids produce .a continuous spectrum and obey the general laws mentioned, it does not follow that they all emit the same amounts of light per unit area at equal temperatures. Kirchhoff has shown by the theory of exchanges that the emissive and absorptive powers of all bodies at the same tem- perature for rays of a particular wave-length are pro- portional to each other when the radiation is a pure temperature effect. For a particular kind of radiator called a black body or a full radiator, the relation between emission , the wave-length, X, and the absolute temperature J, has been deduced theoretic- ally. The black body is defined as a body that will absorb all radiation incident upon it and reflect none. When it radiates it emits in each wave-length more energy than any other body at the same tem- perature. The nearest approach to such an ideal radiator is a hollow space enclosed by emitting walls of uniform temperature provided with a small open- ing through which the radiation can escape. The laws deduced theoretically by various investigators do not agree entirely. The one that best fits exper- imental data is Planck's law given in E x = C^-'(e*T ..I)-' (1) Another law known as the Wien-Paschen law found to hold for the short-wave region of the visible spec- trum is shown in equation (2). x= CU-'e"^ (2) In the foregoing equations Ci and c 2 are constants. The values for these differ somewhat as determined by various investigators. LIGHT 15 A simple relation between the wave-length of the maximum of the radiation curve and the absolute temperature is derived from (2) and is expressed in equation (3). X m T = constant (3) Another interesting relation known as the Stefan- Bbltzmann law connects the total radiation, E, from a unit area of the radiator with the absolute temper- ature, T, and is expressed in equation (4). E= C7 14 (4) These laws are of chief importance in the theory of radiation but are given here as a matter of reference. A gaseous body is found to emit only certain definite rays and the spectrum is said to be a line spectrum. Sometimes the various lines (which are in reality the images of the slit of the spectroscope, (8) are found to be crowded together in such a manner as to give to the spectrum a fluted appear- ance. Such a spectrum is called a banded or fluted spectrum. A further striking fact is the constancy of the appearance of the spectra emitted by elements in gaseous form. For instance the spectrum of sodium is always recognized by the position of the emitted rays in the spectrum that is, by their wave-length. The visible spectrum of sodium con- sists of a double line (0.5890^ and 0.5896/0 and whenever this double line is found in a spectrum it is certain that sodium is present in the radiating substance. This constancy of the spectra of the elements forms the basis of spectrum analysis by means of which traces of elements far too small to be weighed by the most sensitive balance are readily detected. By means of the spectroscope helium was discovered on the sun before it was distinctly isolated 16 COLOR AND ITS APPLICATIONS by scientists on earth. The vacuum tube, the arc, the electric spark, and the flame are used in studying the spectra of elements and compounds. Sometimes both a line and a continuous spectrum are emitted by an illuminant. Such a case is found in the ordinary carbon electric arc. The crater of the arc being an incandescent solid, emits all visible rays while the incandescent gas of the arc between the electrodes emits a line spectrum the appearance of which depends upon the surrounding medium and the composition of the carbons. In Fig. 4 are shown several representative spectra photographed by means of a grating spectrograph on a Cramer spectrum plate which is sensitive in varying degrees to all the vis- ible rays. This particular brand of photographic plate is relatively less sensitive to blue-green rays so that on viewing the spectrograms the energy in this region appears to be less prominent than it really is. It is seen that the two gases, mercury and helium, emit line spectra. The arcs emit both continuous and line spectra, the latter as indicated above being emitted by the vapor in the arc itself. The relative prominence of the line spectra depends upon the relative intensities of the radiation from the arc as compared to that from the solid electrodes. For instance the line spectrum is much more prom- inent in the yellow flame arc than in the ordinary carbon arc and as is well known the arc vapor con- tributes a much greater proportion of the light in the former than in the latter illuminant. The line spec- trum of a carbon arc is subject to momentary changes both in character and intensity due to impurities and also to irregularities in the amounts of the chem- icals with which the carbons are impregnated. The injection of various chemicals into the arc as sug- Visible Spectrum a. Mercury Arc b. Helium c. Iron Arc d. Ye/ low Flame Arc e. Carbon Arc f. Carbon Arc g. Carbon Arc h. Tungsten Incandescent Lamp i. Skylight J. Skylight V B Y O R Visible Spectrum Fig. 4. Representative spectra. 18 COLOR AND ITS APPLICATIONS gested by the heating of metallic salts in a Bunsen flame, affords a means of varying the color or spectral character of the light from the carbon arc lamps. Spectra / and g were obtained from the same carbon arc within a period of a few seconds. The tungsten filament is seen to emit a continuous spectrum, ft, the dark band being due to the low sensibility of the photographic plate to blue-green rays. Two spectro- grams of light from the sky are shown in i and j in an effort to bring out the dark lines which cross the spectrum. The solar spectrum is of special interest. As already indicated a photograph of the spectrum of sunlight made with a narrow slit, shows practically a continuous band crossed by many fine dark lines (see Plate I). These lines were discovered by Wollaston in 1802 but were later studied with better instruments by Fraunhofer in 1814 who found several hundreds of them. These dark lines in the position of various colors show the absence of the corre- sponding images of the slit of the instrument and therefore the absence of these rays in sunlight. Their absence is attributed to absorption by vapor chiefly in the solar atmosphere. Luminous gases or vapors, as has already been indicated, emit only a limited number of rays, their spectra being dis- continuous in appearance. These vapors when lumi- nous are usually opaque to the particular rays which they emit and therefore the light from the sun is robbed of some of the rays in passing through its atmosphere. The Fraunhofer lines are often used as reference points in examining spectra, although electric discharges through gases in vacuum tubes and the heating of salts in a gas flame furnish convenient means of identification or reference spectra in the LIGHT 19 experimental laboratory. The chief Fraunhofer lines are given in Table I. TABLE I Principal Fraunhofer Lines Line Wave-length Color Source A 0.7594M Red Oxygen in atmosphere a .7185 K Water vapor in atmosphere B .6867 u Oxygen in atmosphere C .6563 K Hydrogen, sun Di .5896 Yellow Sodium, " D 2 .5890 14 < E .5270 Green Calcium, " b! .5184 Magnesium, " b 2 .5173 c w MOJ^ P. & E 1> "o w 8^ d g*S ^g6g S3 1 3 i Hefner Carbon incande lamp 3.: 1 Q f; Mixing Colors COLOR-MIXTURE 55 results in the residual colored light. The integral color of the light absorbed is said to be comple- mentary to the color of the light remaining if the total light in the beginning were white light, say noon sunlight. Of course in the foregoing case the ab- sorbed color has disappeared, so there is no oppor- tunity to view the complementaries. Any pair of complementary colors can be readily viewed by a comparatively simple apparatus. By means of a prism the spectrum of sunlight is produced at some point in space. A portion of this spectrum can be deflected from the original path by means of a prism of slight angle. The rays in each beam can be com- bined upon adjacent spots of a white surface by means of lenses, with the result that instead of a spot of white light two adjacent spots of colored light are seen. These two colored lights are obviously complementary, for if they are made to overlap they will be found to produce, by addition, a white light. By separating various portions of the spectrum all the pairs of complementary colors are readily pre- sented to view. As will be shown later, white light can be matched by mixing certain pairs of (and also by mixing three or more) spectral colors. This can readily be demonstrated by means of variable slits cut in a cardboard screen and held in front of the spectrum. If the slits have been placed in their proper position in the spectrum and properly adjusted in width, white light will result when the rays from these slits are combined on a white screen by means of a lens. The subtractive- primary colors have been termed red, yellow, and blue. In reality they would be more exactly described as purple, yellow, and blue-green. They are the complementaries of the additive pri- 56 COLOR AND ITS APPLICATIONS maries, as will be seen later. Some may prefer to use the term 'pink' or 'magenta' instead of 'purple', but the hue is a purple consisting of red and blue. The tri-color processes of printing and color photography are based upon the subtractive principle of mixing colors. The principle of the subtractive method is well demonstrated by Fig. 20 (Plate II). If the three subtractive primaries, purple, yellow, and blue-green, are carefully made by the use of transparent media, water colors or printing inks, and are superposed, the results shown in Fig. 20 are obtained. First let us take a simple case of a yellow pigment on a white surface. The light passes through the colored film and is reflected back through it by the white surface. As the light passes through the yellow pig- ment it is robbed of the violet and blue rays, there- fore the light which reaches the eye is white minus violet and blue rays, and produces a sensation of yellow. In the processes of painting and color print- ing the three disks may be assumed to be micro- scopic in size, each being a minute flake of pigment. If two flakes be superposed, a yellow above a blue- green, a green color is obtained. The yellow flake does not transmit blue rays, therefore the green rays are the only remaining rays that will be transmitted by the blue-green pigment. Ths.se will be reflected by the white surface, and will pass again through the blue-green and yellow pigments, undergoing further changes tending to purify them, so that only green rays reach the eye. If the blue-green flake is above the yellow flake, the explanation must be reversed, but with the same result. The blue-green flake trans- mits blue and green rays; however, the yellow flake does not transmit blue rays. Therefore, only the COLOR-MIXTURE 57 green rays will eventually be reflected to the eye. In the same manner the blue of the purple is sub- tracted by the yellow flake, and as purple consists of red and blue rays only, the red rays remain to be reflected to the eye. Therefore, yellow and purple flakes superposed produce red. Likewise the blue- green flake does not transmit red light, so that super- position of blue-green and purple flakes results in blue light being reflected to the eye. It is further seen that the superposition of the three subtrac- tive primaries results in a total extinction of light and black is the result. For instance, where the yellow and purple disks overlap, red results. The blue-green disk does not transmit red rays, so where it overlaps the red disk a total extinction results. Much interesting information may be obtained by carefully studying Fig. 20. Strips of colored gelatine laid over each other in checkerboard fashion present many striking examples of the subtractive method of mixing colors. In ordinary artificial light, screens made of ethyl violet (purple), uranin or aniline yellow, and filter blue-green, are excellent dyes for making the subtractive primaries for demon- strating the foregoing by superposition. Ethyl violet and naphthol green are practically complementary, so that when superposed no light rays are transmitted. 19. Additive Method. -As already indicated, there are two distinct methods of mixing color, - the additive and subtractive, but close investiga- tion often reveals both processes entering into some part of the production of color. The additive method always tends toward the production of white, whereas the subtractive method tends toward the production of black. The additive primaries are red, green, and blue. Some prefer to use the term * violet* instead 58 COLOR AND ITS APPLICATIONS of 'blue.' Blue, however, appears satisfactory and is a safer term than violet, because there are a great many who apply the term violet to purples. Long ago it was demonstrated that, by proper mixtures of the three well-chosen primary colors, any color can be matched. This is largely due to the fact that the eye is a synthetic rather than an analytic instrument. In Fig. 21 (Plate II) are illus- trated the principles of color-mixture by the additive method. It is seen that red added to green produces yellow; and further, when blue is added to this com- bination white is produced. In other words, yellow and blue mixed by addition produce white. It is well known, however, that yellow and blue (in reality blue-green) pigments when mixed by the subtractive method, as is done in painting and color printing, produce green. This is a much confused point, but is very simply explained when the character of the procedure of mixture is analyzed. Red and blue when added produce purple; and blue and green produce blue-green. It is to be noted that combina- tions of two of the additive primaries produce the subtractive primaries and vice versa. The additive method can be readily demonstrated by the use of colored lights projected upon a white surface. Prop- erly selected color-screens are necessary, but can be readily made from aniline dyes by carefully mixing them. It is difficult to describe the procedure quan- titatively, but there is no difficulty in producing the proper colors. Owing to the very unsatisfactory state of color terminology, it is impossible to present an accurate and definite list of complementary hues. However, a few complementaries are given in Table IV. COLOR-MIXTURE 59 TABLE IV Complementary Hues 1 Red .Blue-green (Cyan blue) Orange-red Green-blue (bluish Orange Blue I Yellow .*. Blue-violet Yellow-green Violet-purple < Green * . Purple (magenta) Wave-length of Complementary Spectral Hues 0.6562 p 0.4921 ft .5671 fi .4645 .6077 .4897 .5644 .4618 .5853 .4854 .5636 .4330 .5739 .4821 An excellent scheme for showing the comple- mentaries is to arrange the spectrum around the circumference of a circle filling a gap between the ends of the spectrum, violet and red, with a series of purples from bluish purple to reddish purple. This has been called a color wheel, and is diagrammatically shown in Fig. 22. Here yellow and violet are shown as complementary. This may appear inconsistent with the foregoing discussion, but it will be noted that the terms 4 blue' and * violet' (as well as other color names) are indefinite. The term 'blue' will always mean a spectral blue, but when used as a primary color its hue is defi- nite, whether the term stands for blue, violet, or blue-violet, have been correctly applied Fig. 22. The color-wheel for show- ing complementary hues. If the complementaries to the color wheel, a neutral gray should be obtained when it is rapidly rotated. 60 COLOR AND ITS APPLICATIONS 20. Juxtapositional Method. If a color be broken up into its component colors and the latter be applied in small dots with the point of a brush, the sensation of the original color will be obtained if it be viewed from a distance at which the eye is un- able to resolve the individual dots and providing the relative areas covered by the various colored dots are correctly balanced. Colors, excepting those en- countered in the spectrum, are usually far from mono- chromatic (#12), (Figs. 122, 123). For instance, a colored fabric which may appear a pure red will be found to reflect rays throughout considerable range of wave-lengths. If these component colors be repre- sented as pure as possible in minute dots of proper relative amounts, the foregoing result is readily obtained. For instance, if one end of a pack of cards be painted red and the other end green, on revers- ing every other card and viewing an end of the pack at a distance of several feet, it will appear yellow in color. The brightness apart from hue will be an average brightness. Many interesting experiments can be performed by ruling alternate fine lines of different colors on paper or on glass. For instance, purple and green lines alternated on paper will, if well chosen, produce an appearance of gray at some distance. Such a method of breaking a composite color into more nearly monochromatic components and applying the latter in the form of minute dots is the foundation of the principle of impressionistic painting. The processes of color photography devised by Joly, Lumiere and others are also based on this principle. 21. Simple Apparatus for Mixing Colors. - There are very elaborate color-mixing instruments on the market for the purpose of demonstrating the COLOR-MIXTURE 61 theory and practise of color-mixture. Apparatus that deals with spectral colors is as a rule the most sat- isfactory for accurate study and demonstration. How- ever, inasmuch as the colors ordinarily available in practise are far from monochromatic, that is, far from spectral purity, there is much virtue in the simpler forms of apparatus that can be made at small expense. In fact, for the foregoing reason the results obtained with some of the simpler instruments for demon- stration are more readily interpreted and applicable in practise than those obtained with apparatus dealing with pure spectral colors. Maxwell's disks offer a ready means for mixing colors. A shaft is arranged so as to be revolved at high speed. Colors painted on a disk can thus be mixed by rotating it at a high speed owing to per- sistence of vision. Light sensations do not reach their full value immediately upon application of the stimulus, nor do they decay to zero immediately upon the cessation of the stimulus. An infinite number of mixtures of pigments, including black and white, can be made with such a simple disk. Colored papers cut in circles and slit along one of the radii can thus be overlapped to any degree, and by the use of circles of various sizes a number of mixtures can be produced upon the same disk. This method is not truly an additive one, excepting in the addition of hues. The brightness is the mean of the sepa- rate brightnesses, each weighted by its angular extent. In Fig. 23 are typical color disks for mixing colors to produce grays. In I and III are repre- sented pairs of complementary colors respectively, yellow and blue, and green and purple. The inner circle consists of black and white, which can be varied in angular amounts to produce a neutral gray to 62 COLOR AND ITS APPLICATIONS match the gray produced by the addition of the two hues. In II are represented the three primary colors which when mixed by rotation produce a neutral gray which is readily matched by means of the inner black and white disks. These matches made under one illuminant will not ordinarily remain matches under another illuminant. Much of the early work in the science of color was done by means of rotating disks and even today they are extremely valuable in some investigations. The disks represented in Fig. 23 can be readily made from Zimmerman's IL Fig. 23. Maxwell disks. colored papers. These papers are indicated in the catalogue by the letters of the alphabet and are given herewith as used in the disks already described. Yellow is designated as g, blue as o, green as i, red as by and purple as a. For the black and white sectors any neutral tint papers with dull finish are satis- factory for producing the grays. The additive and subtractive methods as illus- trated in Figs. 20 and 21 (Plate II) can readily be demonstrated in permanent charts. The imported colored papers have been found satisfactory, owing to their comparative purity and unglazed surfaces. For demonstrating the subtractive method by the three overlapping disks the six colors and black are surrounded with a white background. The Zimmer- COLOR-MIXTURE 63 man colors, designated by a, #, Z, &, z, and o, may be used respectively for purple, yellow, blue-green, red, green, and blue. These six colors, with black and white, are sufficient for the construction of charts for the additive and subtractive methods. For demon- strating the additive method the three disks should be surrounded with black background, but in the case of the subtractive method the background should be white. For demonstrating these two methods of color-mixture with artificial light by means of transparent media, purple and green are readily produced by using gelatines dyed with ethyl violet and naphthol green respectively. When these two colored gelatines are superposed in proper densities of coloring, no light is transmitted. When light is passed through these media in juxtaposition in proper relative amounts and combined on a neutral tint diffusing surface a white light is produced. These two colors afford an excellent example of comple- mentary colors when used with artificial light. In daylight the ethyl violet screen appears deep blue in color, instead of appearing purple as it does in the light from a tungsten incandescent lamp. Other transparent media for further demonstrating these methods are readily selected from the many organic dyes available. Uranin, fluorescein, carmine, patent blue, and filter blue-green are satisfactory. In Fig. 24 the construction of an erratic color- mixing disk is illustrated. To a disk of stiff card- board a sectored disk of cardboard is rigidly fastened by means of a circular rivet. The latter disk has two 60 openings, as shown in a. Another disk, arranged concentric with the other disks and between them, is permitted to slip at will about the rivet as an axis. If the latter disk is prepared as shown in 64 COLOR AND ITS APPLICATIONS b many striking colors are obtained on rotating the combination. Fig. 25 illustrates a simple arrangement for color- mixing. The wheel is similar to that employed in the Simmance-Abady flicker photometer. The periphery Fig. 24. An erratic color-mixing disk. of this wheel consists of truncated cones pointing in opposite directions. The axes of the cones are eccentrically placed at equal distances on either side of the axis of the wheel and parallel to it. In the angular position shown in the illustration, # L' * Fig. 25. A simple color-mixer. the eye, looking at the wheel in a direction at right angles to the axis, sees one conical surface illumi- nated by one light, L y and the other by the other light, L f . Colored screens, SS, are interposed between the wheel and the light sources. By having the lamps movable on a track any combination of brightnesses of two colors from transmitting media can be mixed COLOR-MIXTURE 65 by rotating the wheel rapidly. Pigments may also be applied directly to the wheel. In Fig. 26 is illustrated another simple instrument for mixing the colors from either opaque parent media. or trans- A wooden box is constructed as shown and painted black inside. G is a transparent plate glass and OO are ground opal glasses free from color. In mixing the colors of two transparent media, CC, the lamp, L, is moved to and fro on its track. Thus any propor- tions of the two colors can be mixed. If the colors of two opaque substances Fig. 26. A simple color-mixer for transparent or opaque media. are to be mixed, CC and OO are removed, the / /\ Eye \ \ \ \ C/ \ Fig. 27. Lambert's color-mixer. colored objects are placed at PP, and the lamp is moved to and fro as before. The range of mixtures in the last case is not infinite as in the case of transparent media; however, modifica- tions can readily be iade so that ie range is extended. A simple experiment devised by Lambert, though not having the flexibility of the foregoing instruments, / 66 COLOR AND ITS APPLICATIONS is of interest owing to its extreme simplicity. It is illustrated in Fig. 27. G is a plate glass and CC are colored objects. The colors are mixed one by re- flection, the other by transmission. By turning the glass and shifting the eye the proportions can be altered considerably. An apparatus of considerable use is a booth con- taining red, green, and blue incandescent lamps con- trolled by rheostats. If the colors are carefully made many interesting experiments can be performed, in- cluding the effect of quality or spectral character of light upon colored objects (#67). Many instructive experiments can be produced by the use of shadows cast by colored lights. One of especial interest is shown in Fig. 28, because it pre- sents the additive prima- ries, their complementaries (the subtractive primaries) and white light produced by the sum of the three primary colors, red, green, and blue. In the middle of a circle of white diffusing blotting paper stiffened by a board are erected three planes of white diffusing material about eight inches in height. The latter meet at the center of the circle at angles of 120 degrees with each other. At points several feet away, along the three arrows, red, green, and blue lights are placed somewhat above the plane of the circle. These should be small sources and quite powerful, concentrated tungsten filament lamps being quite satisfactory. The. experiment is best Fig. 28. A shadow demonstration of the additive and subtractive methods of color-mixture. COLOR-MIXTURE 67 seen if the plane of the circle is vertical. It will be seen that the nearly rhombic areas on the circle indicated by #, G, and B each receive light from only one source. These areas will then appear respectively red, green, and blue. The areas on the opposite sides of the circle, Z?G, P, and 7, each receive light from only two sources. They appear in colors com- plementary to the above primaries. They also rep- resent the subtractive primaries and the colors which remain after red, green and blue are subtracted respectively from three white lights. The remaining areas of the circle marked W represent the regions which receive light from each of the three sources, with the result that if the colors and intensities of the light sources are correct, and if the sources are sufficiently distant in comparison with the size of the circle, these areas appear a uniform white. This experiment is simple and is very satisfactory for dem- onstration before large audiences. The lights should be controlled by separate switches and rheostats. A rotating disk can be readily colored, so that it will appear, when viewed through a radial slit placed close to it, a fair approximation to the spectrum. The mixing of the colors by rotation obviates the neces- sity of the great care in blending colors in painting a spectrum that is to be viewed when stationary. The colors will not be of spectral purity, owing to the limitations of the pigments, but the disk will be instructive and affords a ready means of producing a spectrum for reproduction by color photography. An approximation to the prismatic spectrum can be readily produced as shown in Fig. 29. The approxi- mation can be made as close as desired by touching up various points with pigments where necessary or by varying the geometric figures. If a circle be cir- 68 COLOR AND ITS APPLICATIONS cumscribed about the inner square and a square in turn be circumscribed about this circle, and so on until four circles and four squares are pro- duced, the skeleton of the figure is ready for coloring. If the inner square be painted black and the succeed- ing hollow squares be painted red, yellow, green, and violet re- spectively and the re- mainder of the outer circle be Dainted black, Fig. 29. Illustrating a disk for approxima- . . ting a prismatic spectrum. & tair approximation to a prismatic spec- trum is obtained. The same results can be pro- duced with more difficulty with only three colors -red, green, and blue. If the spectrum pro- duced by rotation is not satisfactory at all points, it can be readily made so by the judicious use of pigments, or, as already stated, by altering the geo- metric figures. The more simple methods of mixing colors have been described in this chapter; however, it will be borne in mind that many of the instruments and methods considered in succeeding chapters are di- rectly or indirectly applicable to color-mixture. REFERENCES Captain W. de W. Abney, Colour Measurements and Mixture, London, 1891. O. N. Rood, Colour, 1904. Chevreul, Harmony and Contrast of Colours, 1839. v CHAPTER IV COLOR TERMINOLOGY 22. Hue, Saturation, and Brightness. One of the greatest needs in the art and science of color is a standardization of the terms used in describing the quality of colors and an accurate system of color notation. The term * color' in its general sense, is really synonymous to the term 'light.' It is used here by preference because it implies the consideration of the appearance of a surface or material object. The spectrophotometer is the most analytic instru- ment for examining colors (#26). By means of it the amounts of light of all wave-lengths reflected (or transmitted) by a colored medium may be obtained. These data are plotted in the form of curves shown by the dashed lines in Fig. 12. The full line curves represent the reflection (or transmission) coefficients of the pigments for energy for various wave-lengths. If the region under one of the curves indicated by a dashed line be integrated and this area compared with that obtained with the same illuminant for a white diffusing surface of known total reflecting power, the relative brightness of the colored medium under this illumination is obtainable. The domi- nant hue which is discussed below may be usually approximately determined by inspection of the curve, although in many cases it is impossible to estimate the dominant hue in this manner. It is thus seen that although the spectrophotometer is a valuable instru- ment for analyzing colors, there are further require- 69 70 COLOR AND ITS APPLICATIONS ments in color work better met by other instruments (Chapter V). The quality of any color can be accurately de- scribed by determining its hue, saturation or purity, and its brightness. (The latter term is analogous to the term * value' as used by the artist.) In the broadest sense, white, gray, and black are here con- sidered as colors, and a mere change in brightness alone is considered as a change in color. It appears necessary to assume this broad definition of color, inasmuch as brightness is distinctly one of the products of color analysis. Hue is suggested in the name applied to the color. The dominant hues of most colors are accurately represented by spectral colors ; however, there are composite colors, the purples, which consist of red and violet, for which no spectral colors are found to represent their hues. In these cases it is satisfactory to determine the domi- nant hue of the complementary colors. The satu- ration or purity is a measure of the relative amount of white light in the color. In other words, all colors excepting purples can be matched by diluting spectral light of a definite wave-length with white light. The greater the percentage of white light required in the mixtures, the less saturated the colors are said to be. The brightness of a color can be found by com- paring it by means of a photometer with a surface of known brightness. It is well to note that in the analysis of a color its absolute brightness is measured by comparing it with a brightness of known value. Inasmuch as its brightness depends upon the inten- sity of illumination of a given spectral character, its reflection coefficient for a standard white light should be determined in order to compare it with other colors in this respect. This latter measurement in- COLOR TERMINOLOGY 71 volves all the difficulties of color-photometry treated in Chapter IX. There is much confusion in the application of the terms 'tint,' 'tone,' 'shade,' 'intensity,' etc. Many use these terms wholly unjustifiably. It is true that the final usage is somewhat a matter of choice at the present time, but the terminology adopted here appears to the author to be consistent with other nomen- clature adopted by the physicist, photometrist, and Fig. 30. Disk 'a,' for varying only the saturation of a color. Disk b,' for varying only the brightness of a color. lighting expert, and best justified by usage and the dictates of common sense. On diluting a color with white light, tints are obtained; that is, tints are un- saturated colors. By the admixture of black to a color (in effect the same as reducing the intensity of illumination) the brightness is diminished without altering either the hue or the saturation, and various shades are produced. Only the relative brightnesses of shades are usually of interest, although for obtain- ing a basis of notation it may be desirable to deter- mine their absolute values. In a, Fig. 30, is shown a simple means of varying the saturation of a color without altering either the hue or brightness. On a 72 COLOR AND ITS APPLICATIONS circle of colored paper is glued a gray paper of the same brightness for the given illumination and of the form shown by the shaded area. On rotating the disk this gray will be mixed in various angular proportions from 360 deg. to deg. The gray paper, having been selected of the same brightness as the colored paper under the illuminant used in the experi- ment, does not alter the brightness upon mixing the two components by rotation; being non-selective in its reflection it does not alter the hue. Thus various degrees of saturation of the original color are obtained. The brightness can be varied, as shown in 6, Fig. 30, without altering either the hue or saturation by fastening to the original circle of colored paper a black paper cut in the same . form as the gray paper shown in a. If the paper were perfectly black it is seen that it cannot alter either the hue or satura- tion. As a matter of fact no available black papers are totally non-reflecting, so that some light is added to the color. This can be reduced to a minimum, however, by the use of a hole in a deep velvet-lined box. In this case the black sectors shown in b would be replaced by openings of the same contour in the disk. For convenience of construction the areas occupied by the black and colored papers may be reversed. In this connection it is well to emphasize that ordinary black surfaces are far from totally absorbing. This can readily be demonstrated by a box open at one end lined with black velvet. Over the open end place a black cardboard with an opening in it and it will be seen that the opening will appear very much darker than the black surface surrounding it. The foregoing demonstration may be easily performed by varying the brightness of a colored paper relative to that COLOR TERMINOLOGY 73 of a paper of the same color which surrounds it, by vary- ing the intensity of the illumination of the patch at the same time maintaining the absolute brightness of the surroundings constant. Instruments have been designed for the analysis of color quality into the three component factors, hue, saturation, and brightness. These are treated hi #27. 23. Tri-color Method. It is well known that any color can be matched by combining the three primary colors, red, green, and blue, in proper pro- portions. Many instruments have been devised for this purpose, the most elementary being the Maxwell disks, and the more elaborate and accurate are those employing spectral colors. The results of such a method are expressed mathematically in the equation xR + yG + zB= C, where the values of x, y, and z are the fractional parts of the red, green, and blue lights, respectively, that must be combined to match the color, C. This method has limitations because it does not give the results directly in terms of hue, saturation, and bright- ness. Some of these in- struments, however, can readily be adapted to the measurement of the last two factors. In order to plot the values of jc, z/, and z, it is necessary to employ tri-linear coordi- nates, there being three variables to be repre- sented. The results are Fig . 3 i.-The Maxwell coior-triangie. readily represented in the Maxwell color triangle, illustrated in Fig. 31. The green component increases from zero at the base 74 . . COLOR AND ITS APPLICATIONS line, RB, to 100 per cent at G. Likewise the red, Rj and green, G, components increase from zero at the base of the perpendiculars erected from the sides respectively opposite to their apexes. The data are plotted by erecting three perpendiculars proportional to the respective values of /?, G, and J5, starting at points in the opposite sides of the triangle respectively, such that the three perpendiculars intersect at the same point. Purples are found along the base line, RBj varying in the proportions of R and B from R = 0, B = 100, to R = 100, B = 0. Yellows are found along RG and blue-greens along GB. White, which is usually represented by J/?+^G+JZ? = W, is found at the center of the triangle. The curved line rep- resents the positions of the spectral colors in the color triangle; that is, each point on the curve rep- resents the primary sensation values of a particular spectral color. Some important lines of the spectra of cadmium and mercury are also shown. The less saturated colors are found near the center of the triangle and the more saturated ones near the sides. It is thus seen that spectral colors throughout a large range of wave-lengths arouse the three primary sensations, according to the Young-Helmholtz theory. (See #28, 47.) The primaries of course are found at the angles of the triangle. Complementaries are represented as being on opposite sides of the center of the triangle on a straight line passing through it. The dominant hue of a color is found by drawing a straight line from the center of the triangle through the point representing the color and continuing it until it intersects the curve representing the spectrum. The latter point of intersection represents the domi- nant hue of the color. The tri-color method in- volves the use of an invariable white light, that is COLOR TERMINOLOGY 75 0.50 0.60 WAVE LENGTH Fig. 32. Spectral complementaries. noon sunlight or its equivalent. A curve represent- ing spectral complementaries is shown in Fig. 32. "Results obtained by this general method with different instruments are likely to vary considerably. This is due in part to vari- ations in the spectral char- acter of the white light standard, and also to the transmission characteris- tics of the color-screens used in these instruments not employing spectral primary colors. The primary sensation values of the screens should be determined and the measurements be given in sensation values (#28). The use of the plane triangle is limited to the plotting of the analyses of colors of equal brightness. In order to include the brightness factor the figure takes the form of a solid inverted pyramid, shown in Fig. 33. The various triangular planes parallel to the base represent planes for plotting colors of different bright- nesses. The apex represents black. A line joining the point, W (white), with the apex passes through a complete range of shades of white, that is, of grays. Along the dotted line from x to the apex are a series of colors of constant hue and saturation, but varying in brightness. The color pyramid has been R Black Fig. 33. A color pyramid. modified in various rays to fit experimental results in- 76 COLOR AND ITS APPLICATIONS volving physiological and psychological influences. One of these modifications from Titchener l is shown in Fig. 34. At the two poles of this double pyramid are the extremes of white and black; upon the axis connecting the two poles are located the complete range of grays. Around the pe- riphery of the middle plane are located those colors of middle brightness and maximal saturation. Other points in the solid represent other colors of varying hue, satura- tion, and brightness. From the base toward white, tints are found; in the other direction shades are found. This pyramid, it will be noted, has four sides, the four angles representing red, yellow, Fi ^ 3 >- 7 h e double pyra- green, and blue. Obviously this mid. (After Titchener.) J solid does not directly represent color analyses as obtained by the tri-color method. Its significance will be better understood on referring to the Hering theory of color vision in # 49. The tri-color method is discussed further in Chapter V. A simple means 2 of dem- onstrating the Maxwell color triangle in actual colors is illustrated in Fig. 35. A box 6 inches in depth, and whose section forms an equilateral triangle about 18 inches on . - -I* j Fig 36. A demonstration color a side, is made of wood, triangle. with its back containing vent holes. A ground flashed-opal glass in the form COLOR TERMINOLOGY 77 of an equilateral triangle somewhat smaller than the section of the box forms the front side. In the three corners of the box are placed respectively, red, green, and blue spherical-bulb, concentrated- filament tungsten lamps. After proper adjustments of the position and color of the lamps, the diffusing glass, which has its roughed side inward, assumes the colors of a color triangle. A close approximation can be approached, depending upon the care exer- cised in adjusting the position of the lamps and the distribution, color, and intensity of the light. Inter- esting demonstrations of retinal fatigue and after- images are readily made with this simple apparatus. For coloring the lamps ordinary colored lacquers are satisfactory if properly mixed to obtain the exact hues. The aniline dyes can be' used with satis- faction. These colors are not permanent, but are sufficiently durable for such an apparatus. If the coloring is placed on separate plates of glass, it will remain unfaded for a long time with proper ventila- tion. 24. Color Notation. The need for a universal color notation is admirably illustrated by Munsell 3 in quoting from a letter by Robert Louis Stevenson, writing from Samoa to a friend in London, as follows : " Perhaps in the same way it might amuse you to send us any pattern of wall paper that might strike you as cheap, pretty and suit- able for a room in a hot and extremely bright climate. It should be borne in mind that our climate can be extremely dark too. Our sitting room is to be in varnished wood. The room I have particu- larly in mind is a sort of bed and sitting room, pretty large, lit on three sides, and the colour in favour of its proprietor at present is a topazy yellow. But then with what colour to relieve it? For a little work-room of my own at the back I should rather like to see some patterns of unglossy well I'll be hanged if I can describe this red - it's not Turkish and it's not Roman and it's not Indian, but it seems 78 COLOR AND ITS APPLICATIONS to partake of the two last and yet it can't be either of them because it ought to be able to go with vermillion. Ah, what a tangled web we weave anyway, with what brains you have left, choose me and send me some many patterns of this exact shade." t. Here is a man accustomed to present his thoughts in writing in a clear manner, yet he acknowledges failure in his effort to describe colors and closes his letter with the request, perhaps a bit sarcastic, that he be sent "patterns of this exact shade." Other sciences have exact and practically universally ac- cepted terminology. Music has its well-developed notation, which is definite and descriptive, and quite universal in adoption, but there is no universal scheme of color notation. Colors are named in very inexact, unwieldy, and often totally non-descriptive terms. We have rose, Indian red, Alice blue, pea green, olive green, cerise, taupe, baby blue, Copen- hagen blue, king's blue, royal purple, invisible green, etc. Thus flowers, vegetables, cities, the savage and the royal family, are used to describe colors. Is there a more ridiculous instance of neglect? Those who work in color often find themselves helpless in describing colors to others. Surely a color notation based upon color science should be acceptable, even though somewhat empirical. Musical notation is somewhat arbitrary, yet it has met with almost uni- versal adoption. An acceptable color notation must involve the factors which influence the quality of a colof, namely hue, saturation, and brightness. N An attempt was made by Runge as early as 1810 to build up a color notation by the use of a sphere with red, yellow, and blue, placed around the equator and separated from each other by 120 degrees, with white and black at opposite poles. Perhaps the greatest virtue in this attempt is the fact that it was COLOR TERMINOLOGY 79 one of the early constructive efforts. Chevreul, whose work on the effect of simultaneous contrast of colors in the practical textile industry is well known, constructed a hollow cylinder built up of ten sections perpendicular to the axis. Around the upper section red, yellow, and blue were equally spaced. The lowest cylindrical section was black, and the eight intervening sections were graded from top to bottom by adding increasing amounts of black. Munsell criticises these attempts, on account of the yellow being very light and the blue being very dark, which makes impossible any coherency in the brightness scales of the three colors. Inasmuch as the bright- ness scale of the yellow in the Chevreul color cylinder increases much more rapidly from the bottom toward the top than the brightness scales for blue and red, Munsell suggests that the yellow side of the cylinder be increased in length. This would result in the tilting of the sections more and more as the scale of brightness progressed from the bottom toward the top. Perhaps a general criticism for most of these schemes of color notation is that geometrical figures are chosen and the colors are made to fit. The latter method is perhaps partially justifiable from the standpoint of physical" measurements. There is another viewpoint in considering a color notation, and that is from the standpoint of harmony of color. In this treatise we are not so much concerned with the latter viewpoint, but it is of interest to consider a system of color notation devised by Munsell from the standpoint of the use of color in painting. In describing a color by this system the initial of the name of the color indicates the hue, and numerals represent the saturation and brightness. For example R? represents a color whose hue is red and whose 80 COLOR AND ITS APPLICATIONS saturation and brightness are respectively 7 and 5. The brightness scale is divided into ten parts, and the degrees of saturation shown vary with the bright- ness. For simplicity ten hues are balanced around the equator of the sphere somewhat after the manner shown in Fig. 22. The lower pole of the sphere is black and corresponds to zero on the brightness scale. The upper pole is white and corresponds to brightness 10 on the same scale. On slicing off a portion of the sphere through a plane corresponding to a certain brightness, various degrees of saturation are encoun- tered. The saturation decreases toward the center, the axis of the sphere consisting of a scale of gray S. However, the sphere does not completely satisfy Munsell. He therefore constructs a * color tree' so that varying numbers of steps in saturation can be represented, depending upon the hue and position in the brightness scale. The scheme is built up on the principle of the harmonious use of colors and in this respect departs somewhat from the scope of this book, which treats more with physical mixtures, regardless of the use of colors in harmony. The system is an interesting one and is the result of a noteworthy attempt to be freed from a state of color anarchy. MunselPs color tree is illustrated in simple form in Fig. 36. The base of the tree is black, the top white. In the small model illustrated three bright- ness levels are shown, namely 3, 5, and 7 in the arbitrary brightness (value) scale. The degrees of saturation shown vary with the 'brightness level.' At * level 3 J in the brightness scale blue is shown to the eighth degree of saturation. By the irregularity in the contour of the planes representing different brightness levels it is seen that the relative number COLOR TERMINOLOGY 81 of degrees of saturation shown for various colors depends upon the brightness level under considera- tion. At brightness level 3, PB (purple-blue) was shown with the highest degree of saturation, namely, 8. At brightness level 5, R ranked first in degree of saturation, its highest being 10. At the brightness level 7, yellow was shown with the highest degree White Yellow YeL Green Ye flow red Red Purple-blue Red-purple Fig. 36. The A. H. Munsell color tree. of saturation, namely 8. For a complete discussion of this system, the reader is referred to the original description by Munsell. 3 Numerous scales have been devised involving, either separately or combined, the factors hue, saturation, and brightness. All of these assist in bringing order out of chaos, but they constitute only the first steps toward a comprehensive system of color notation. The hues are usually expressed by names of spectral colors and purple, but the bright- ness is seldom more definite than is found in such expressions as W, HL, L, LL, M, HD, ' D, LD, and 82 COLOR AND ITS APPLICATIONS By which represent white, high light, light, low light, medium, high dark, dark, low dark, and black re- spectively. Examples of such charts (devoid of color) are shown in Fig. 37. These were taken from Book VI of Prang's text-book of art education. Such NEUTRAL VACUC CHART -B CHART - C Fig. 37. Prang's color and brightness scales. charts should pave the way toward a final scientific color notation. Another system of color notation is shown in Fig. 38. This is used by Ruxton in the mixture of XT OWS ~~>A A. R.G O COLOR^ C H A R.T I Fig. 38. Ruxton's color mixture chart for printing inks. printing inks. The chart is printed in colors, there being 144 colors, varying in hue, saturation, and bright- ness. The terminology is somewhat different than used in this text. The 144 colors are obtained from six fundamental colors, namely red, orange, yellow, green, blue, and purple. These six colors are de- scribed as spectral colors, so it is likely that purple COLOR TERMINOLOGY 83 is the name applied to a color meant to be violet. The starting point in obtaining the total array of colors is in the bottom row of Section 3. The larger rectangles represent the six fundamental colors, which are the purest or most saturated on the chart. The fundamental red is marked 820. The small square areas represent the intermediate hues and are ob- tained by mixing the fundamentals on either side. Red-orange for instance is obtained by mixing red and orange (820 and 840). The three horizontal rows above this row of twelve colors are made by adding white to the colors of the bottom row. Thus in the top row are found the least saturated colors in Section 3. Two degrees of saturation lie between the top and bottom rows. Thus in Section 3 there are 48 colors, six fundamental colors increased to twelve by mixing adjacent fundamentals (red and violet are mixed, producing 810) and these twelve colors decreased in saturation in three steps by the addition of white. Sections 1 and 2 are produced by adding black to the corresponding colors in Section 3, thus reducing their brightness. In Section 1 are the colors of lowest brightness. These are named 'hues' but in a different sense than that in which the term 'hue' is employed here. They could be termed 'Values' with better consistency. The 'bi- hues' (bi-values) in Section 2 are obtained by mixing one part by weight of the colors in Section 3 to one part of the corresponding color in Section 1. Thus it is seen that in a more correct sense there are twelve hues represented on the chart (bottom row in Section 3). With the hue and brightness constant saturation is found to be present in four degrees (moving vertically in Section 3). With the hue and saturation constant, the brightness is found 84 COLOR AND ITS APPLICATIONS to be present in three values (moving from left to right, Sections 3, 2, 1, so-called * colors,' 'bi-hues' and 'hues'). Besides these there are 72 other colors in which brightness and saturation appear in six combinations for each of the 12 hues. In other words, in a broad sense there are present 144 colors made up of twelve hues by varying the brightness and saturation. Six of the twelve hues are made by mixture of the adjacent hues in the bottom row of large rectangles in Section 3. Each rectangle being numbered, the chart systematizes the mixture of printing inks. Such progress is commendable and highly desirable, even though empirical. There are many other methods, but these few have been cited to show the lack of standardization of color notation and to illustrate that a system however empirical is just as desirable for the de- scription of color as a system is for music notation. There is much yet to be done before a system of color notation is devised which will be universally adopted. First there should be some definite terms adopted descriptive of the factors influencing the quantity of a color, namely ' hue,' ' saturation,' ' brightness.' The term 'hue' is used in a more definite sense than the terms applied to the two other factors. For satura- tion the terms 'chroma,' 'purity,' 'intensity,' and others are being used. For brightness the terms 'luminos- ity,' 'value,' 'hues' or 'bi-hues,' and others are being used. Purples are often called violets or reds. These are examples of usage from which general confusion arises. The problem of color terminology does not defy solution. As a matter of fact all the quantities involved in a scientific system of notation are readily measurable. Hue, saturation, and bright- ness are easily determined. The available hues, COLOR TERMINOLOGY 85 with the exception of purple, are invariable, consisting of the spectral hues. Scales of brightness (value) can be divided into any given number of parts and named in some consistent manner. The use of the terms 'high light,' 'low light,' 'medium,' 'low dark,' etc., is perhaps satisfactory, but the brightnesses that they represent should be standardized in absolute measurements in order to produce a universal scale of relative brightnesses. In fact all the terms re- quired in a satisfactory and scientific system of color notation can be measured for their absolute values. This would reduce the systems to one basis. Such a universal system must certainly be adopted even- tually, and those interested in color should put forth effort to hasten the day. REFERENCES 1. Primer of Psychology, 1899, p. 41. 2. Psych. Rev. 20, May, 1913. 3. A Color Notation. OTHER REFERENCES Sir William Abney, Color Mixture and Measurement. O. N. Rood, Textbook on Color. J. G. Hagen, Various Scales for Color-Estimates, Astrophys. Jour. 1911, 34, p. 261. K. Zindler, Color Pyramid, Zeit. f. Psych. 1899, 20, p. 225. R. Ridgeway, Color Scales. CHAPTER V ANALYSIS OF COLOR 25. The Spectroscope. - - As already indicated, colors can be analyzed in various ways. The method adopted in a given case will naturally depend upon data desired. The spectroscope affords a simple means of examining colored light, but the results of the visual inspection are only qualitative. There are various designs of spectroscopes available, all based upon either the principle of the prism or of the diffraction grating (#8 and 9). An ordinary prism spectroscope can be converted into a direct-vision instrument by combining two prisms made from different kinds of glass, so that dispersion is obtained for a certain ray without deviation. Crown and flint glasses differ in refractive index (Fig. 8), hence if prisms of each of these two glasses be made of proper refractive angles and combined so that their separate deviations prac- AF/\ |L] tically annul each other, ' v ^ LL I say, for the sodium line, Fig. 39. A direct-vision prism spec- dispersion is produced troscope - without deviation for this -. ray. Such a simple spec- troscope is shown in Fig. 1 39. A simple diffraction Fig. 40. A simple grating spectro- grating SpCCtrOSCOpe Can be readily made, as shown in Fig. 40. A replica of a diffraction grating (#9) is placed between two pieces of plate glass at G. By ANALYSIS OF COLOR 87 placing a lens at L the instrument is considerably shortened, so that it can readily be made of a pocket size. These, the simplest forms of spectroscopes, are only useful for rough qualitative analysis of the spectral character of light emitted by light sources or transmitted or reflected by colored media. A convenient form of spectroscope for qualitative analy- sis is the comparison spectroscope. This contains two or three distinct optical systems, so that two or three spectra may be viewed in juxtaposition. Such an instrument might be considered as roughly quan- titative in its analyses, owing to the opportunity of estimating relative intensities of a given light ray in the two or three spectra. More elaborate spectrometers will not be con- sidered here, for the function of the spectrometer is for qualitative analysis. However, in this respect such instruments are of considerable value in color work. Photographic accessories are readily attached in place of the eyepiece. If an absorption wedge be placed before the slit, so that its transmission varies along the length of the slit, the spectrograms will roughly indicate the relative spectral distribution of energy, providing proper filters are used to allow for the variation in plate sensibility. Sometimes it is advantageous to compensate for the unequal spectral distribution of energy in the illuminant, especially in the examination of colored media. 26. The Spectrophotometer. This instrument consists in principle of two spectroscopes, arranged so that the intensity of rays of the same wave-length in the two spectra can be photometrically compared. The results obtained are quantitative. A diagram- matic sketch of the optical system of a Spectro- photometer is shown in Fig. 41. Light enters the 88 COLOR AND ITS APPLICATIONS instrument from two sources at the slits S and S', respectively. At L is a Lummer-Brodhun photometer cube so constructed that through a part of the field light rays are trans- mitted directly from S' to the prism P and from the remainder of the field light rays from S are reflected s toward the prism. After being dis- persed by the prism the colored rays pass on to the eye placed at T. The wave-length of these rays depends upon the angular position of the prism which can be rotated. The photom- eter field is similar to that viewed in an ordinary Lummer-Brodhun photometer. A small direct-vision comparison spectroscope is of considerable use in color work. Such an instru- ment designed by Nutting l contains a pair of Nicol prisms, NN, for altering the brightness of one of the spectra, as shown in Fig. 42. A right-angled Fig. 41. The spectro- photometer. Fig. 42. The Nutting pocket spectrophotometer. prism, /?, reflects light into the slit from one of the sources. The instrument, which is called a pocket spectrophotometer, in itself is merely for qualitative analysis. It can be set up permanently and used for quantitative measurements. However, in order to make such an instrument portable and compact, yet available for obtaining qualitative data, the author devised the attachments shown in Fig. 43. An attachment, A, containing a miniature tungsten lamp ANALYSIS OF COLOR 89 which illuminates a ground flashed-opal glass, can be removed from its present position if desired and attached at B. When the comparison source in A Fig. 43. A small portable spectrophotometer for quantitative analysis. is in the position shown the unknown source is placed at B. The sleeve, O, was placed on the instrument to support A. S controls the slit widths. A mi- crometer screw with a graduated scale and drum is attached at M to the slide containing the observing slit, C, which is moved across the image of the spectrum. The drum is calibrated in terms of wave- lengths and the scale of the revolving Nicol prism in terms of transmission of light. The current through the lamp is obtained from a battery controlled by a rheostat and is measured by means of a small ammeter. The range of the instrument can be extended by varying the current through the lamp. This photometric field is not as satisfactory as might be desired, for it consists of two narrow bands juxta- posed at their ends. The instrument is very small, 90 COLOR AND ITS APPLICATIONS less than 8 inches long, is mounted on a tripod, and is really portable. There are many designs of spectrophotometers, but all have the same object. It is necessary to be able to vary the luminous intensity of one-half of the photometer field. This is done by varying the position of one of the light sources, by the use of Nicol prisms, by a neutral tint absorbing wedge, or by sectored disks. A convenient means is the variable sectored disk developed by Hyde, 2 a diagram of which is shown in Fig. 44. The disk is mounted upon a motor-driven shaft and arranged to be moved horizontally in its plane along the line CD in front of the slit S, by means of a micrometer screw. The transmission is nearly proportional to the lateral displacement. By means of the spectrophotometer, results can be obtained directly in terms of relative energy such as are plotted in Fig. 5 (Table II) and Figs. 122, 123. In this case the various rays in the unknown spectrum are compared directly with the corresponding rays in a spectrum of known distribution of energy. As has been previously stated the spectrophotometer is an analytical instrument, and by its use the spectral character of the light reflected or transmitted by colored media is readily obtained. An example of its use and a practical means of greatly reducing the number of readings is given below. During the Fig. 44. The variable sectored disk. (After Hyde.) ANALYSIS OF COLOR 91 development of a glass which could be used with the tungsten lamp to produce artificial daylight 3 the procedure involved the examination of many glasses containing various proportions of coloring ingredients. A glass which proved unsatisfactory at the thickness at hand might be found satisfactory at another thick- ness. Therefore it was necessary to grind and polish the samples as they came from the glass factory into many different thicknesses or in the form of a wedge. 0.01 01234567 THICKNESS OF GLASS IN MILLIMETERS Fig. 45. Scheme for reducing the amount of spectrophotometric work in exam- ining transparent colored media. This necessitated making a set of spectrophotometric readings for a considerable range of thicknesses. By utilizing the law relating transmission and thickness (density of coloring matter) of the glass, namely 7 = 7 e~ e ' z , a simple method was devised. 7 represents the original intensity of light of a certain wave-length and 7, its intensity after traversing a thickness d of the colored glass and e, the extinction coefficient. By considering the reflection from the two surfaces of the glass a relation was deduced in the form of 92 COLOR AND ITS APPLICATIONS Log T = log 0.92 +kd (where T is the transmission, d the thickness and k a constant) which is sufficiently accurate for ordinary purposes in the spectrophoto- metric analysis of the transmission characteristics of colored glasses and other media. The term 'log 0.92' can be eliminated by obtaining the transmis- sion of the colored glass in terms of a clear glass if so desired. This method necessitates an analysis of only one thickness, for, on plotting these data on logarithmic paper, as shown in Fig. 45, the data for various other thicknesses (even thicker than the sample) are readily obtained. Proof of the accuracy of this method is shown by the fact that the circles which represent data obtained on the same sample of glass at five different thicknesses lie close to the straight lines indicated by the mathematical relation expressed above. See Chapter XVII. The spectrophotometric examination of colored media is valuable inasmuch as the eye not being analytic, other methods fail to reveal the true spectral character of the light emitted by the colored medium. This was demonstrated by the three yellows in Fig. 17 which appeared of the same hue (and practically of the same saturation), but differed greatly in spectral character. A spectrophotometer is an elaborate and expensive instrument, therefore where the need for such an instrument is not great enough to warrant its pur- chase, an ordinary spectrometer with modifications can be made to serve the purpose. There are various ways of converting ordinary spectrometers into in- struments satisfactory for spectrophotometric work. A double bilateral slit and a combination prism for transmitting and reflecting respectively two juxta- posed beams of light from the different sources into ANALYSIS OF COLOR 93 1 S the collimator is a ready means of converting a spectrometer into a spectrophotometer. However, the comparison field which consists of narrow lines juxta- posed endwise is not very satisfactory. Abney used the scheme illustrated in Fig. 46 in his early studies in color. Two slits, SS, were placed in a plane at right angles to the col- limator. One slit was be- Fig. 46. Abney's spectrophotometric attachment for a spectrometer. low the other, so that their respective images could be reflected toward the collimating lens by the two right- angled prisms which were placed one below the other. This arrangement no doubt yielded a photometric field which was not divided by an invisible line, as is desirable for high sensibility. #6 cr -b -3- Fig. 47. Ives' spectrophotometric attachment for a spectrometer. A more satisfactory method is illustrated in Fig. 47. This arrangement, used by Ives, 4 was designed chiefly to avoid the errors due to instruments having two collimators becoming asymmetrical and also to avoid errors due to scattered light. At 1 in Fig. 94 COLOR AND ITS APPLICATIONS 47 is placed a combination of two right-angle prisms cemented together. The face b is entirely silvered and face a silvered halfway up. A lens at 2 forms an image in the field of the telescope tube at 3 which is observed by means of an ocular lens at 4. The two light sources are placed at 6 and 7 respectively. A large monochromatic field is obtained which is equally affected by scattered light if any is present. Further- more colored glasses can be judiciously used to elimi^ nate scattered light if necessary. A further improvement of the foregoing attach- ment, which was added by Nutting, 5 is illustrated in Fig. 48. The attachment consists of two reflecting prisms, Pi and P 2 , two Nicol prisms, Ni and N 2y and a lens arranged as \ _ \/ | shown in the figure. The -^/ p * ' whole can be attached to the slit of any spectrometer. The essential factor is that a real image of the photo- metric field (the common surface of the two reflect- the slit by an achromatic lens and is thus brought into the plane of the slit. The two beams of light to be compared, one passing through a portion of the photometric surface and the other being reflected by the other portion which is silvered, are brought to a brightness balance for any wave-length by rotating the Nicol prism M. High sensibility is claimed by Nutting for an instru- ment of this type. 27. The Monochromatic Colorimeter. Colorim- eters vary in design, depending upon the data to be ANALYSIS OF COLOR 95 obtained. In some industrial processes tintometers are employed which determine the color of substances in terms of arbitrary standards. Such instruments are colorimeters, but give no quantitative analyses of the colors. Their purpose is largely to keep the product within certain limits as to color, but they perhaps serve the purpose in many of these cases as satisfactorily as a more complex instrument. Of the instruments that analyze colors into the three terms 'hue,' 'satura- tion' and 'brightness,' the Nutting 6 colorimeter, being of the latest type, has been chosen for description. The optical system of this instrument, which has been called a monochromatic colorimeter, is shown in Fig. 49. Light entering the slit of collimator, A, which is movable, traverses the prism and is dispersed by prism P into its spectral components, thus furnish- ing the measurement of hue of the unknown light which enters through the slit of collimator C and is reflected by a portion of the diagonal surface in L, which is a Lummer-Brodhun photometer cube. White light enters the slit of collimator B and is reflected by the prism face and joins a portion of the beam from A. The eye placed at the ocular slit in D sees an ordinary photometric field, the two parts of which can be matched in hue, saturation, and brightness. The hue is matched by varying the angular position" of A and the saturation by varying the amount of white light added. The amounts of light entering the slits can be varied by changing the slit widths, by rotating sectors, or by rotating one of a pair of 96 COLOR AND ITS APPLICATIONS Nicol prisms placed just inside the slits. In analyz- ing a purple, for which no spectral match in hue exists, a spectral color is mixed with the unknown, the remaining procedure being obvious. The later instruments have been altered somewhat in con- struction, but the principle remains the same. The accuracy with which the dominant hue is obtainable is claimed to be about .001 to .002/* except in the extreme regions of the spectrum, for very unsaturated colors and dark shades. Data obtained by Nutting 6 are given in Table V. TABLE V Materials Hue Per cent white Reflection coefficient Sulphur 0.571/z 48 0.80 Cork .586 56 .26 Dandelion .580 9 Tobacco leaf (medium) .597 65 .14 Chocolate .595 70 05 Butter, light .580 45 Butter, dark .580 28 .64 Navy blue (U. S.) .472 90 .019 Paris green .511 56 386 Manila paper .682 65 .57 CoDDer .597 70 .23 Brass light .575 60 .32 Brass dark .583 61 25 Gold, medium .591 64 .21 Data obtained by Abney 7 in the analysis of the color of glasses and pigments are presented in Table VI. In Table VII are given some data on the color of illuminants obtained by L. A. Jones 8 with the mono- chromatic colorimeter. ANALYSIS OF COLOR 97 TABLE VI Hue Saturation Brightness Glasses Dominant hue Per cent white Transmis- sion coef- ficient Ruby . - . 0.622,u 2 131 Canary .585 26 .820 Bottle-green .551 31 106 Signal-green .4925 32 069 n .510 61 194 Cobalt .4675 42 .038 Pigments Dominant hue Per cent white Reflection coefficient Vermillion 0.610 M 2.5 0.148 Emerald-green .522 59 .227 French ultramarine blue .472 61 .044 Brown paper '. .594 50 .25 Orange .5915 4 .625 Chrome-yellow .5835 26 .777 Blue-green .5005 42.5 .148 Eosine dye .640 72 .447 Cobalt-blue .482 55.5 .145 TABLE VII Source Per cent white Hue Sunlight 100 Average clear sky 60 0472u Standard candle 13 593 Hefner lamp 14 .593 Pentane lamp . ... 15 .592 Tungsten glow lamp, 1.25 w. p. c. 35 .588 Carbon glow lamp, 3.8 w. p. c. 25 .5915 Nernst glower, 1.5 w. p. c 31 .5867 Nitrogen-filled tungsten lamp, 1.00 w. p. m. h. c Nitrogen-filled tungsten lamp, 0.5 w. p. m. h. c Nitrogen-filled tungsten lamp, 0.35 w. p. m. h. c Mercury vapor arc 34 45 53 70 .586 .5845 .584 .490 Helium tube .... . . 32 .598 Neon tube 6 .605 Crater of carbon arc at 1.8 amperes .... Crater of carbon arc at 3.2 amperes 59 62 .5846 .5846 Crater of carbon arc at 5.0 amperes 67 5834 Acetylene flame (flat) 36 5855 98 COLOR AND ITS APPLICATIONS In colorimetric work a standard white light is necessary. Jones used noon sunlight, which he found to be constant in color from 9 A.M. to 3 P.M., the obser- vations extending over several weeks. This light was reflected into the instrument from a magnesium car- bonate block. Many interesting studies in color-mixture can be made with such a colorimeter. An example is found in the work of L. A. Jones 9 in the analysis of mixtures of two component colors. Filters were chosen in several cases practically complementary in color. These filters were in the form of sectors of a circle and of equal angular extent. An opaque sector equal in size to one of the filters was varied in position over the sectors, so that they could be left open in any desired proportions. Lights passing through these filters were mixed in a complete range of ratios and the resultant mixtures were examined by means of a monochromatic colorimeter for hue and saturation or per cent white. For example, we will choose one of the pairs of filters, a red and blue- green of dominant hues 0.624^ and 0.497/x respec- tively. The saturation or purity of the colors are indicated by the per cent of white light (noon sun- light) that each transmitted, these being for the red and blue-green filters respectively 3.3% and 28%. The transmission coefficients of the two filters were respectively 24% and 16%. The data obtained in analyzing various mixtures of the two colored lights are shown in Fig. 50. It is seen that practically only two hues are obtained in a complete range of mix- tures and these are the dominant hues of the re- spective colored lights. The dominant hue of the mixtures changes abruptly from the hue of one of the colored lights to that of the other at the point ANALYSIS OF COLOR 99 near where the mixture contains the maximum amount of white light, or in other words where the two lights are nearest to being complementary. The per cent white reaches a maximum of 95% (indi- cating that the colored lights are here practically com- plementary) when the blue-green filter was open about 62% and the red filter about 38%. A con- clusion, among others drawn by Jones from this in- luu 90 80 70 UJ *0 -50 UJ ~ in / ^ 0.640 0.620 0.600 0.580 0560 0.540 0.520 fSCArt / S \ / X \ / | > \ / / \ _,. 10 *JO 20 10 n / \ 1 S 10 20 30 40 50 60 70 80 90 100 COM POSITION OF MIXTURE IN PER CENT. OF BLUE-GREEN FILTER Fig. 50. Analysis of two-component color-mixtures. vestigation, is that it is not possible with two filters that are complementary or nearly so to produce mix- tures that show appreciable color of more than two dominant hues, these hues being the dominant hues of the two components of the mixture. He points out other possibilities for this colorimeter in investiga- tions in color-mixture. The author has used an arrangement diagram- matically shown in Fig. 51 for the study of various problems, chiefly that of the influence of saturation of color in heterochromatic photometry. This ar- rangement has all the essentials of a colorimeter for 100 COLOR AND ITS APPLICATIONS analyzing colors into hue, saturation, and brightness. Light from a source L emitting light of a continuous spectrum enters the collimator of a Hilger spectro- scope and is dispersed by the prism. A standard white light illuminates the non-selective ground opal glass, O, an image of which is reflected into the objective telescope from the prism face as shown. A white sectored disk, Z>, which is smoked with magnesium oxide by holding near a burning mag- nesium ribbon, is placed so that it bisects the field, w u Fig. 51. A simple method of converting a spectrometer into a combined mono- chromatic colorimeter, direct comparison photometer, flicker photometer, and spectrophotometer. F, vertically. If the edges of the sectors are beveled and well sharpened, the dividing line can be made to disappear almost completely. The light from the unknown, 7, is reflected from the disk. By varying the intensity of the various lights the desired meas- urements can be made. The hue is determined by the position of the wave-length drum; the amount of white light can be measured by comparing with a standard at U or by a previous calibration. The brightness can be measured either by the direct comparison or the flicker method of photometry (# 55). The sectored disk provided with a motor drive is in reality a Whitman-disk flicker photometer. As al- ready stated the arrangement was originally devised for another investigation; however, it readily serves ANALYSIS OF COLOR 101 the requirements of a monochromatic colorimeter. Transparent colored media can be illuminated by a standard white light placed at U. Likewise opaque colored media can be placed on the disk D and illuminated by a white light. 28. The Tri-color Method. --It is well known that any color can be matched in hue by mixing the three primary colors, red, green, and blue, in proper proportions. The Young-Helmholtz theory of color vision is largely based on this experimental fact (#47). Kdenig found, by a rather complex method, the rela- tive amounts of the three primary color sensations aroused by the various spectral colors and determined iA Fig. 52. Illustrating the principle of the Maxwell * color box.* the so-called sensation curves of the eye (Fig. 54). Maxwell was one of the first to obtain quantitative data in matching colors by a mixture of three primary spectral colors. His apparatus, known as the * color box,' though somewhat more complex, involved the fundamental principle shown in Fig. 52, and is based upon the fact that an optical path is 'reversible.' For example if a spectrum is formed at A by means of a collimator and a prism by light entering the slit, S, of the collimator and traversing a prism, we can obtain a patch of light of any color by placing slits, /?, G, and 5, in the spectrum and combining the light from these on a distant screen. Conversely, if the latter slits be illuminated with white light on looking into the collimator slit, S, the prism face will appear of a color which is the result of the mixture of the colors of the slits which, in the first case, were 102 'COLOR AND ITS APPLICATIONS combined by means of a lens into one colored patch on the distant screen. Hence, instead of forming a spectrum at A, and producing colored light by mix- tures of /?, G, and B, by slits placed at these points and combining the three colored lights, Maxwell adopted the reverse process. He illuminated the three slits by sunlight reflected from a white diffus- ing surface placed in front of them, and on looking into the slit, S, he saw the prismfface appear in colors corresponding to the positions and proportions of R, G, and B. This composite color he compared with the original white light or any colored light. The Maxwell color box was actually constructed in a different manner, but the principles involved are the same as indicated above. By means of this instru- ment he obtained many color equations of the form xR + yG + zB = C. By a similar method Koenig obtained data which resulted in the production of the so-called sensation curves of the eye. Abney has employed the method in a great deal of work in color analysis, including the study of color vision, the analy- sis of pigment colors, and of the color of illuminants. ^A colorimeter based upon the tri-color method of analysis was developed by F. E. Ives. 10 Instead of spectral colors, red, green, and blue colored filters are employed in this instrument, which is illustrated in Fig. 53. By means of this instrument colors are analyzed in terms of the colors of the filters /?, G, and B. These can be reduced to sensation values as shown later. D is a variable slit which is illumi- nated by light of the color to be analyzed and A is an optical mixing wheel consisting of twelve convex lenses arranged to rotate. By means of this wheel the various amounts of the red, green, and blue com- ponents are mixed to match the light from D. F ANALYSIS OF COLOR 103 is the field lens and C a prism or small angle which divides the photometric field by a sharp line in the middle. H is the eyepiece, J is a hinged front carrying the objective lens, K, and prismatic lens L. These are unnecessary for some work and can be replaced by a non-selective ground opal glass. The procedure in making observations with this Fig. 53. The F. E. Ives colorimeter. instrument is obvious. If the colorimeter readings which are obtained from the position of the levers which control the slit widths of /?, G, and 5, be reduced to sensation values they become much more valuable. H. E. Ives u has done this in analyzing the color of illuminants, by using the sensation curves obtained by Koenig and modified by Exner. These are shown in Fig. 54. They are based upon experi- mental data which has afforded strong confirmation of the Young-Helmholtz theory of color vision which assumes three fundamental color sensations are responsible by different degrees of excitation for 104 COLOR AND ITS APPLICATIONS the perception of all colors (#47). It is noted that each of the three supposed primary color sensations 1TT Blue Red. Green 0.39 042 045 045 0.51 0,54 0.57 0.60 0.63 0.66 0.69 0.72 ^ Fig. 54. Ko'enig's sensation curves. is not excited by a limited portion of the spectrum. In fact, spectral rays in general are supposed to Fig. 65. Tri-color colorimeter measurements. excite the sensations in relatively different degrees, depending upon the wave-length. After a prelimi- nary investigation Ives concludes that these curves are a much nearer approach to the truth than those obtained by Abney. In reducing the colorimeter ANALYSIS OF COLOR 105 readings to sensation values it was necessary to obtain the red, green, and blue sensation values of the colorimeter screens. Spectrophotometric analy- sis of the screens combined with the data in Fig. 54 yield the primary sensation values of the screens which are obtained in relative values by integrating the areas under the sensation curves for the three screens and reducing the colorimeter readings accord- ingly. Each of the three colorimeter readings repre- sents a mixture of the three primary sensations, depending upon the primary sensation values of the colorimeter screens. The procedure is simple but more details, if desired, can be obtained from the original paper. The primary sensation values of various illuminants compared with average daylight as determined by Ives are found in Table VIII, some of which are plotted in Fig. 55. The data represent TABLE VIII Color of Illuminants by Tri-chromatic Colorimeter (See Fig. 65) Source Sensation values Red Green Blue 1. Black body 6000 abs 2. Blue sky (S) .... 33.3 26.8 32.0 34.6 37.7 64.3 61.1 48.6 48.3 49.2 42.5 45.4 47.2 41.0 29.0 62.0 31.3 33.3 27.2 32.0 33.9 37.3 39.5 40.5 40.8 40.8 40.7 40.8 42.0 41.8 36.3 30.3 37.6 31.0 33.3 46.0 35.8 31.5 25.0 6.2 5.4 10.6 10.9 11.1 16.7 12.6 11.0 22.7 40.7 10.5 37.7 Blue sky (C) 3. Overcast sky 4. Afternoon sun 5. Hefner lamp ... ... 6. Carbon incandescent lamp, 3.1 w. p. m. h. c 7. Acetylene 8. Tungsten incandescent lamp, 1.26 w. p. m. h. c. 9. Nernst 10. Welsbach, \ % cerium 11. Welsbach, f % cerium 12. Welsbach, 1 j % cerium 13. D. C. Arc 14. Mercury arc 15. Yellow Flame arc 16. Moore carbon-dioxide tube 106 COLOR AND ITS APPLICATIONS the means of the values determined by two methods, namely colorimeter readings and likewise spectro- photometric data reduced to sensation values. (The primary color sensation values of the spectral colors and principal lines of the cadmium and mercury spectra are plotted in Fig. 31.) The dotted line represents the color of a black body (or an incandes- cent solid emitting radiation non-selectively) for tem- peratures between 3000 and 7000 degrees absolute (C). Most of the artificial illuminants lie along this curve. Those radiating selectively in the visible spectrum, such as the yellow flame arc and Wels- bach mantle, do not lie upon it. Ives concludes that the spectral distribution of energy in noon sunlight which reaches the earth's surface is quite similar to that of the black body at 5000 degrees absolute (C) as computed from radiation laws (#6). s/ Another instrument for tri-color analysis which is extremely simple is illustrated in Fig. 56. This method, which has been applied by many in various color investi- gations, has been used by Bloch. 12 A disk con- taining four circular Fig. 56. Arrangement for using color ,-, * . filters before a photometer eyepiece. apertures, three being respectively covered by red, green, and blue screens, is pivoted Red Green Blue is pivoted so that the various screens can be brought before the ocular aperture in a photometer head. Pho- tometric balances are made while viewing the field through the various filters separately and the re- sults are plotted on rectangular coordinates, the ratio of red to green intensities being plotted against the ratio of blue to green intensities. Bloch presents ANALYSIS OF COLOR 107 plats containing his color analysis of many illuminants and the spectrophotometric analyses of the filters are also shown. Such results can hardly be consid- ered more than approximately comparative and of limited usefulness. In general, data concerning the color of illuminants or of colored media obtained by the tri-color method of analysis are limited in usefulness, owing to the fact that the method is not sufficiently analytical. The usefulness of such a method is broader than the tintometer with its arbitrary standards of color, but the spectropho- tometer and monochromatic colorimeter as a rule yield more useful data, the former being 'quite analytical for spectral examination and the latter rendering data in terms of the specific qualities of a color, namely, hue, saturation, and brightness. 29. Other Methods of Color Analysis. Many instruments have been devised for color analysis based on principles differing from the foregoing. A number of colorimeters employing colored solu- tions have been used, the measurements usually being made in terms of the depth of liquids of cer- tain concentrations. Purple and green solutions have been used by Fabry for eliminating color difference in photometry. In a sense such a procedure is a colorimetric method if it is desired to use it as such. The Kirchoff-Bunsen and Stammer colorimeters em- ploy colored solutions for the measurement of color. Leo Arons 13 has devised a colorimeter based upon the rotation of the plane of polarization by quartz plates (# 11) which have been cut perpendicu- larly to their crystallographic axes. This instrument is illustrated in /, Fig. 57. White light from a dif- fusely reflecting porcelain disk is reflected into the instrument through a circular hole, 5, and is rendered 108 COLOR AND ITS APPLICATIONS plane-polarized by the Nicol prism, P. A quartz plate at Q rotates the plane of polarization of various rays through various angles depending upon the wave-length. The beam then passes through another Nicol prism, Aj thence through the central portion of the Lummer-Brodhun photometer cube, W, and to the eye beyond R. The eye sees a circular patch of light of a certain color depending upon the thick- ness of the quartz plate and the relative angular posi- tions of the Nicol prisms. This colored patch is m I BLQ Fig. 57. Arons colorimeter. matched in color with the light entering the side tube, N. The latter beam is controlled in intensity by the two Nicol prisms, Pi and P 2 , and is reflected by the totally reflecting prism, Z>, to the photometer cube, which in turn reflects the light to the eye in a beam concentric with the first beam. The colored media are placed in front of tube AT, and are pref- erably illuminated by the same source that illumi- nates the porcelain disk in front of B. Mixed colors are obtained, the Nicol, A, subtracting certain rays depending upon its angular position leaving the remaining light colored instead of white. Six quartz plates are provided, of thicknesses 0.25, 0.5, 1.0, 2, 4, and 8 millimeters respectively, which are mounted in brass plates. These plates have two identical ANALYSIS OF COLOR 109 holes, one covered with the quartz plate, the other unobstructed. These are arranged to slide in or out of the instrument at or near Q. By sliding any of the brass plates to the side any number of quartz plates can be arranged one after another and thus the total thickness of quartz in the path of the beam from B can be adjusted in steps of 0.25 mm. to a total thickness of 15.75 mm. A still greater variety of colors can be obtained by using two sets of Nicol prisms and quartz plates in series. Therefore the tube in / can be removed at the plane xx, and tube II connected at the end yy. B' takes the place of B and lens U the place of L. Any thickness of quartz plates at Q f can be inserted; however, only a single plate is employed by Arons, this one being 3.75 mm. in thickness. In case transparent colored media are to be examined, a white diffusely reflect- ing porcelain disk similar to the other one is used in front of the tube N. The two disks should receive the same intensity of illumination from the same source. If opaque colors are to be examined for reflection, these are placed on the porcelain disk, and the observer sees an outer ring through R of the color of the unknown. This is matched in color and brightness by adjusting the thickness of quartz and the angular position of the Nicol prisms until the inner circle appears of the same color and brightness as the outer ring. The measurements are recorded in terms of the thickness of quartz, the angle between A and P and the angle between Pi and P 2 and also between P f and P if the tube II is in use. 30. Templates. - - Much of the early investigation in color was done with the rotating disks (Fig. 23) and it is quite natural that modifications of these would be made. Abney devised an ingenious method 110 COLOR AND ITS APPLICATIONS for showing the effect upon the color of the integral light of various spectral energy distributions and also of showing that a certain determined spectrophoto- metric curve was in reality the analysis of an integral color. On determining the relative amounts of light of various wave-lengths reflected by a pigment these, instead of being plotted on rectangular coordinates as shown by the dotted lines in Fig. 12, were plotted in a special manner on a disk. Along a portion, VR, of a radius of the circle in Fig. 58, a wave-length scale is laid off. The relative amounts of light of different wave-lengths reflected from the pigment as determined by means Fig. 58. Abney's of a spectrophotometer are laid off on circumferences of circles concentric with the center of the disk starting at a certain point of VR corresponding to the wave-length. The cardboard is now cut out along the boundary line, the template in Fig. 58 being Abney's template for car- mine. If this disk be carefully adjusted in the plane of a spectrum formed in space so that various wave- lengths along VR coincide with corresponding wave- lengths in the spectrum and the disk be rotated, on combining the colored rays passing through the rotating aperture upon a white screen by means of a lens the color of the integral light reflected from car- mine is seen. This patch will be exactly like the original color in appearance providing the optical parts of the instruments are non-selective and the same light is used in producing the spectrum as was used in illuminating the pigment when the spectrophotometric observations were made. Of course the irrational dispersion of the prism must be properly allowed for and the spectrum must be narrow. ANALYSIS OF COLOR 111 Instead of rotating the template before an actual spectrum Abney used the principle adopted by Max- well in his * color box* (Fig. 52), thus rotating the disk before a long narrow slit illuminated by the total light from the illuminant. The integral color was viewed through the eyepiece of the spectrometer. Abney made a number of these templates represent- ing pigments, illuminants, and the luminosity curve of the eye. ELCVATIOH Fig. 59. Adaptation of Abney's scheme for the spectroscopic synthesis of color. Recently Ives and Brady 14 applied Abney 's prin- ciple to the alteration of the light from a 4 w.p.m.h.c. carbon lamp to that of * average daylight' and also to that from the blue sky. A Hilger constant-devia- tion spectrometer was used, as shown in Fig. 59. The regular camera attachment B was placed in the position ordinarily occupied by the collimator, the latter being placed in the position of the objective telescope. The slit at S is long and narrow and is illuminated by light from the carbon incandescent lamp reflected from a white surface at F, the prin- ciple being the same as just presented in the description of Abney's work on templates. These templates were computed on the assumption that the 112 COLOR AND ITS APPLICATIONS relative spectral energy distribution in the spectrum of the carbon incandescent lamp operating at 4 watts per mean horizontal candle is that derived from the Wien equation (equation 2, #6) for a black body at a temperature of 2080 deg. absolute (C), and that of white light corresponding to a temperature of 5000 deg. absolute. The templates for converting the carbon light into blue sky light were made from relative spectrophotometric measurements. The disk in position is shown in the three views of the appa- ratus taken from the work cited above. The advan- tage of using the templates before a slit illuminated by white light is that a much greater amount of light is available than in the case of using it before a spectrum and recombining the transmitted light by means of a lens. A comparison field can be arranged by reflecting the light L into the instrument as shown. Abney cut a template corresponding to the luminosity curve of the eye which is of interest, but owing to the work of various modern investi- gators this has been more accurately established. The template scheme can be applied by using disks in which openings are cut corresponding to the lumi- nosity curve of the eye and replacing the surface at F before the objective slit by a straight incandes- cent filament; thus the transmissions of absorbing media can be determined by pure energy measure- ments. 31. The Nutting Reftectometer. In the study of color it is sometimes desirable to ascertain the reflection coefficients of colored media. This can be done if the object is diffusely reflecting by means of an ordinary brightness photometer, although the uncertainties of color photometry will be present in any case. However, Nutting 15 has devised a simple instru- ANALYSIS OF COLOR 113 ment shown in Fig. 60 that is very useful for deter- mining the reflection coefficients of any colored media for light incident from all possible directions simul- taneously. Two crown glass prisms of 21 deg. angle are fastened over the two apertures in the end of a Koenig-Martens p o 1 a r i z a- tion photometer and the lat- ter is inserted into a metal ring which is nickel-plated and polished inside. The light enters the apertures of the instrument along the dotted lines shown and is divided into two plane-polar- ized beams by a Wollaston prism. These beams can be balanced in intensity by rotating the Nicol prism. The surface whose re- flection coefficient is desired is placed on one side of the ring completely covering it and this is illuminated by a non-selective ground opal glass on the other side of the ring. The instrument is placed upon a wooden frame for convenience. The light is reflected back and forth between two planes of * infinite' extent made practically so by the polished ring. Simple theory shows that the ratio of the brightness of the unknown to that of the ground opal glass is a direct measure of the reflection coeffi- cient of the former for the character of the illumina- tion it receives. Certain precautions must be taken into consideration as explained by Nutting. 32. Methods of Altering Brightness of Colors Non-selectively. It is often desirable to alter the Fig. 60. The Nutting reflectom- eter. 114 COLOR AND ITS APPLICATIONS brightness of colored lights without altering them spectrally. A simple means is found in varying the distance of the light source and computing the rela- tive intensities from the * inverse square law.' How- ever, sometimes this is inconvenient. Sectored disks are often resorted to with satisfactory results. These are now being used in photometry to a great extent, the variable sectored disk devised by Hyde (Fig. 44) being especially convenient and reliable for spectro- photometry. The Brodhun variable sector is another device very often applicable. In this instrument a beam of light is rotated and is controlled in intensity by a variable stationary sector. Plate glass varied in its angular position with respect to the axis of the beam affords a means of obtaining a slight range of brightnesses, although non-selective glass is rarely found. Wire mesh and grids thoroughly blackened are satisfactory in some problems. Neutral tint wedges have been used, but it is difficult to obtain strictly non-selective smoke glass. Ives and Luck- iesh 1G studied the transmission characteristics of half-tone gratings (black lines on clear glass) and found them to be satisfactory if properly used. Pho- tographic screens are found to serve some purposes, but they must always be calibrated in position owing to their tendency to diffusely reflect light. These are a few methods which have proved helpful in the proper place. REFERENCES 1. Bul. Bur. Stds. 1906, 2, p. 317. 2. Astrophys. Jour. 1912, 25, p. 239. 3. Trans. I. E. S. 1914, p. 853. 4. Phys. Rev. 1910, 30, p. 446. 5. Bul. Bur. Stds. 7, p. 234. 6. Bul. Bur. Stds. 1913, 9, No. 187. ANALYSIS OF COLOR 115 7. Color Mixture and Measurement, p. 165. 8. Trans. I. E. S. 1914, 9, p. 687. 9. Phys. Rev. N. S. 1914, 4, p. 454. 10. Jour. Franklin Inst. July, Dec. 1907. 11. Trans. I. E. S. 1910, p. 189. 12. Electrotech. Zeit. 1913, 46, p. 1306. 13. L'Industrie Elec. July 25, 1911. 14. Jour. Franklin Inst., 178, p. 89. 15. Trans. I. E. S. 1912, 7, p. 412. 16. Phys. Rev. 1911, 32, p. 522. CHAPTER VI COLOR AND VISION 33. The Eye. Color vision is not essential, because achromatic vision serves the totally color- blind person well. However, 'the ability to perceive colors extends the usefulness of the sense of sight very much. It not only adds greatly to our pleasure but is utilized in many ways. The eye can be considered optically as a rather simple instru- ment, as indicated by the photograph of the middle vertical section of a human eye shown in Fig. 61. It is seen that the refracting Sj^^^^^ . V%v media consist of the cornea, >J^^ ik aqueous humor, lens, and vitre- & pb Bt ous humor. The retina, which I consists of the optic nerve ifr Jilf spread out over the interior of the eyeball, is a very thin ^B ^jjr membrane, and can be seen in the illustration partially de- F 1kfh^n V ^e alSeCti0n0f Cached from the wall. The radii of curvature, thickness, and refractive index of the various eye media as determined by Helmholtz l are given in Table IX. The normal eye, while being a wonderfully adapt- able instrument, is not free frpm errors, owing to the fact that it is optically quite simple. The chief error of interest here is its lack of achromatism. If an image of an object illuminated by light having a continuous spectrum be produced by a simple lens 116 COLOR AND VISION 117 TABLE IX Optical Constants of the Eye Distant vision Near vision (15 cm.) Index of refraction of the humors and cornea Index of refraction of the crystalline lens , 1.3365 1.4371 Effective index of refraction of lens surrounded by humors Radius of outer surface of cornea Radius of first lens surface Radius of second lens surface 1.0753 7.8 mm. 10.0 6.0 7.8 mm. 6.0 5.5 Thickness of cornea Thickness of crystalline lens Distance of first lens surface from cornea 0.4 3.6 3.6 0.4 4.0 3.2 Distance of second lens surface from cornea 7.2 7.2 it will be found to have a red, blue, or purple fringe. This is readily understood from Fig. 62, which repre- sents a schematic eye in which only the simple lens is considered. Owing to the difference in the refrac- tive index of a medium for rays of different wave- length, such a result as is exaggerated in Fig. 62 will obtain. The refractive index being greater for rays of shorter wave-length, the blue rays will be deviated or refracted more than the yellow rays, and the latter more than the red rays. Naturally the eye focuses for the brightest rays, which in ordinary light are the yellow-green or yellow rays. There- fore, the blue and red rays will be out of focus, with the result that the image of the point, P, will be surrounded by a purple fringe. This is of im- portance in vision, as will be shown later. The lack of achromatism of the eye can be demonstrated very simply. On viewing, by reflected light, the concentric circles shown at the right of Fig. 62 held close to the eye they appear colored. A very striking experi- ment is found in focusing a line spectrum that of mercury will suffice upon a ground glass. On 118 COLOR AND ITS APPLICATIONS viewing it at a normal distance (14 inches), the yellow and green lines will appear sharply focused, but the blue and violet lines will appear hazy and quite out of focus. On bringing the eye closer the latter lines will begin to appear clearer, and finally, when the eye is within about six inches of them, they will still appear clear-cut, while it will be quite impossible to accommodate the eye sufficiently to focus the yellow and green lines. In other words the eye is near- sighted (myopic) for blue rays and far-sighted (hyper- opic) for red rays. On viewing a narrow continuous spectrum at some distance the blue end appears to Fig. 62. Showing the effect of chromatic aberration in the eye. flare out. Another simple demonstration is found in viewing an illuminated slit through a dense cobalt glass which transmits extreme red and violet rays. On accommodating the eye for a point behind the slit a red image with a violet halo is seen. On accommo- dating for *a point in front of the slit a violet image with a red halo is seen. This defect plays a promi- nent, though usually unnoticed, part in vision. A lens can be made practically achromatic by combining a convergent lens of crown glass with a divergent lens of flint glass. The former is more strongly conver- gent for blue than for red rays, while the latter is more strongly divergent for blue than for red lights. It is thus possible to bring the red and blue rays in coincidence at a focus. Inasmuch as it is only pos- sible to bring two rays exactly into coincidence by COLOR AND VISION 119 a two-piece lens, such a lens is not truly achromatic, though practically so for most purposes. By com- bining more lenses the approach to true achromatism is brought /c\ Fi as near as desired. A simple / \ achromatic lens is illustrated in Fig. 63. The retina has been found to vary in its sensibility to colors. The central region is sometimes known as the yellow spot, be- cause it apparently absorbs the ^63. -A simple achromatic violet and blue rays to a greater degree than other rays. The effect of the yellow spot is often seen in viewing colors one after another, and it is quite noticeable at twilight illumination. It appears of somewhat irregular outline in after- images. Studies of the various zones of the retina as to their sensibility to various colors yield results in general similar to those shown in Fig. 64. The center of the fovea corresponds to the center of the circle. The solid line shows the boundary for the perception of light. The visual field for one eye extends outward about 90 deg. from the normal optical axis of the eye, inward about 60 deg., down- ward 70 deg., and upward 50 deg. The dashed line represents the extreme limits where blue can be perceived as such and the remaining two lines repre- sent respectively the limits for red and green per- ception. These facts must be reconciled with any satisfactory theory of vision. It might be noted here that each eye has a blind spot the point of entrance of the optic nerve which is totally insensitive to light. The retina, which consists of the optic nerve spread out, is covered with a mass of microscopic 'rods' and 120 COLOR AND ITS APPLICATIONS 'cones' (#48) projecting outward toward the lining of the eyeball which play an important part in theories of vision. 34. Brightness Sensibility. The sensibility of the retina to brightness differences is greatest over a wide range of intensities, falling off at extremely 10 120 135 22 315 300 Colorless .. Red - Green Blue Fig. 64. Limits of the visual field for colored and colorless lights. low and extremely high brightnesses. With decreas- ing intensities the sensibility diminishes more rap- idly for rays of longer wave-length than for those of shorter wave-length. Koenig and Brodhun 2 have done excellent work in this field as well as in many other fields pertaining to vision. They determined the least perceptible brightness increment for lights COLOR AND VISION 121 of various colors including white, for brightnesses of a neutral tint surface (' white') illuminated to various intensities from 1,000,000 meter-candles to nearly the -5 -2 -I 12 3 LOGARITHM OF ILLUMINATION, I Fig. 65. Brightness sensibility data. (See Table X.) threshold of vision, using an artificial pupil of 1 sq. mm. area. They started at 600 meter-candles and extended the illumination above and below by the various steps indicated in the accompanying table. The data for Koenig's eye after modification by Nutting 3 are shown in Fig. 65 and Table X. Koenig and Brodhun did not include the increment (5B) in the total brightness (B) in calculating the values dB/B. Nutting recomputed the data with the thresh- old value included. It is seen that the increment of brightness difference just perceptible, increases as the brightness decreases and more rapidly for the rays of longer wave-length. At high illuminations the minimal perceptible increment is about the same (1.6%) for all colors, including white. For the ordi- nary range of brightnesses 5B/B, is constant, which fact is known as Fechner's law, and the constant is called Fechner's coefficient. 122 COLOR AND ITS APPLICATIONS TABLE X Data of Koenig and Brodhun on Brightness Sensibility Recalculated by Nutting Wave- length = B = = 0.670/x = 0.060 0.605/i 0.0056 0.575 M 0.0029 0.505 M 0.00017 0.470 M 0.00012 0.430 M 0.00012 Meter Candles 8B B 200,000 0.0425 100,000 0.0241 0.0325 50,000 0.0210 0.0255 0.0260 20,000 0.0160 0.0183 0.0205 0.0195 10,000 0.0156 0.0163 0.0179 0.0181 5,000 0.0176 0.0158 0.0166 0.0160 2,000 0.0165 0.0180 0.0180 0.0175 0.0180 1,000 0.0169 0.0198 0.0185 0.0184 0.0167 0.0178 500 0.0202 0.0235 0.0180 0.0194 0.0184 0.0214 200 0.0220 0.0225 0.0225 0.0220 0.0215 0.0245 100 0.0292 0.0278 0.0269 0.0244 0.0225 0.0246 50 0.0376 0.0378 0.0320 0.0252 0.0250 0.0272 20 0.0445 0.0460 0.0385 0.0295 0.0320 0.0345 10 0.0655 0.0610 0.0582 0.0362 0.0372 0.0396 5 0.0918 0.103 0.0888 0.0488 0.0464 0.0494 2 0.1710 0.167 0.136 0.0655 0.0715 0.0600 1 0.258 0.212 0.170 0.0804 0.0881 0.0740 0.5 0.376 0.276 0.208 0.0910 0.096 0.0966 0.2 0.332 0.268 0.110 0.127 0.116 0.10 0.396 0.133 0.138 0.137 0.05 0.183 0.185 0.154 0.02 0.251 0.209 0.223 0.01 0.271 0.189 0.249 0.005 0.325 0.300 0.312 0.002 0.369 The value of the minimal perceptible increment depends largely upon the method of making the measurements. Usually the brightness of one of the two parts of the photometric field is varied until it appears just perceptibly brighter or darker than the comparison field. This procedure yields values of COLOR AND VISION 123 the least perceptible increment comparable with the foregoing value. In precision photometry the accu- racy is often as high as 0.1 per cent; however, another factor enters into such procedure. The brightness of one part of the field is varied between certain limits at which it is respectively distinctly brighter and darker than the comparison field, and these limits are gradually brought nearer together until finally an attempt is made to estimate the middle point. This cannot be considered a measure of brightness sensibility. However, P. W. Cobb has employed a method which is of considerable interest here inas- much as he obtains values for the minimal percep- tible increment for white light smaller than 0.5 per cent. In these experiments the test field was exposed to the view of the observer for a brief, but constant, period, after which his judgment was recorded. One side of the field appeared either brighter or darker, or no difference in brightness was distinguishable. This procedure was repeated for a range of aspects of the test field varying from that in which one side appeared distinctly darker for a number of succes- sive exposures to that in which it appeared definitely brighter. Obviously, by progressing in small steps between these two limits (presenting these various aspects in haphazard order) there were several near equality where the judgment was uncertain. After reducing the data by a special method Cobb con- cludes that the minimal perceptible increment is much smaller than that obtained by Koenig and Brodhun. The data of Koenig and Brodhun has been ex- tended by Nutting by computation to the point where 65/5 = 1; that is, to the threshold value. This computation is very interesting, though perhaps not 124 COLOR AND ITS APPLICATIONS entirely free from criticism. B Q in Table X repre- sents the threshold value of brightness measured as a fraction of the standard high brightness. Brightness By is proportional to illumination, 7, and inasmuch as it is a brightness that is perceived the symbol B is used. 35. Hue Sensibility. Notwithstanding the fact that the visible spectrum is generally considered to exhibit only six or seven colors, four of which, red, yellow, green, and blue are strikingly distinctive, there are theoretically present an infinite number of hues. The number of distinct hues that a person is able to distinguish depends upon the manner in which the experiment is conducted. Edridge-Green 4 states that he has * never met with a man who could see more than 29 monochromatic patches in the spectrum.' Rayleigh, 5 who is able to detect the dif- ference in hue of the sodium D lines (0.5890/x and 0.5896/0, could distinguish only 17 hues on Green's apparatus, and claims this is due to the method of comparing the patches. In Green's apparatus the principle is that of two opaque screens held over a spectrum and slightly separated from each other. One is then moved until the hue at its edge appears different from that at the edge of the other. With an apparatus employing the principle of the Maxwell color box Rayleigh was able to distinguish many more hues. By the use of spectral apparatus as high as 128 distinctly different spectral hues have been seen. It is not difficult to obtain by the use of dyed media a series of 25 distinct spectral hues. Ridgeway, 6 by beginning with papers dyed to represent six spectral hues and adding various intermediate hues, obtained 36 distinct hues. The data on hue sensi- bility vary considerably, which perhaps is due to COLOR AND VISION 125 variations in the refinement and nature of the experi- mental methods employed. Some excellent data have been obtained by Steind- ler 7 on hue sensibility for twelve subjects. The posi- tions of the maxima differed somewhat for the various \ 0.40 044 046 0.5Z 056 0.60 0.64 ^U. WAVE LENGTH Fig. 66. Hue sensibility. (Steindler's Eye.) observers. The hue sensibility curve for Steindler's eye is shown in Fig. 66 and the mean positions of the maxima and minima of the hue sensibility curves for the twelve observers and the wave-length limen of 'just perceptible difference' are given in Table XI. Nutting 8 has used the mean results obtained by Steindler in deriving a natural scale of color. These mean results, including Nutting's color scale, are plotted in Fig. 67. The hue sensibility curve, S, was plotted by connecting the mean positions of the minima and 126 COLOR AND ITS APPLICATIONS TABLE XI Steindler's Data on Hue Sensibility (The mean for twelve eyes) Position Perceptible limen First maximum . . . 0.456M 0.0293^ Second maximum 0.534 0.0334 Third maximum 0.621 0.0375 First minimum 0.440 0.0247 Second minimum - - - 0.492 0.0136 Third minimum 0.681 0.0139 Fourth minimum 0.635 0.0300 maxima for the twelve observers with smooth curves. The limen (least perceptible difference in terms of M) curve, L, is plotted in the same manner. For the 24 20 16 12 0.044 0.040 ^ 0.036 -A 0.032 t 0.02.8 0.024 5 0.020 ^ 0.016 < 0.01 2 0.006 0.00.4 n X x. \ s s^ \ / V 1 \ l r : \ i \ c A i \ \ i \ 1 V / \^ 1 \ / \ i / V > 1 / \ A A y ^ i y ^~ --- ^s / i ^ s \ s~ \^ / \ \. *s~ \ ^ ^x N 0.6Z 0.66 Fig. 67. Hue sensibility, limen, and color scale. > WAVE UMGTH details of the procedure adopted in obtaining the color curve, C, the reader is referred to the original paper. A difference of one unit in the color scale represents a difference in color that is just easily perceptible. It will be noted that the color curve COLOR AND VISION 127 indicates there are 22 of these colors 'just easily perceptible' within the spectral limits shown. 36. Saturation Sensibility. The data on the sensibility of the eye to changes in saturation are not very extensive or definite. Nutting 9 states that with his monochromatic colorimeter the probable error in the 'per cent white' observations on a nearly spectral matte orange pigment was about ten per cent. L. A. Jones 10 claims an accuracy of the order of three per cent for the 'per cent white' readings (Table VII) for this monochromatic colorimeter of improved type. The accuracy of course will vary with the hue, brightness, and degree of saturation of the colors. H. Aubert n determined the smallest sector of color that would be just apparent on a rotat- ing white disk to be 2 or 3 degrees less than one per cent. With black and gray disks he found that even smaller sectors were recognized. His experi- ments on the differential limen of color sensitivity indicated that on a black background the stimulus- increments for orange, blue, and red were respectively 0.95, 1.54, and 1.67 per cent in order to produce a just noticeable increase in saturation. Geissler 12 studied the problem whether the number and sizes of the colored stimulus-increments corre- sponding to just noticeable saturation differences would lend themselves to a measure of saturation. The problem was attacked from two extremes; one by gradually reducing a maximally saturated pigment color, and the other by introducing more and more color into a colorless stimulus. He employed the rotating double color disk with the Zimmerman col- ored and gray papers illuminated with an artificial daylight devised by Ives and Luckiesh. In the first method he used red beginning with maximal sat- 128 COLOR AND ITS APPLICATIONS uration 360 degrees of red for both the inner and outer concentric components of the double disk and gradually added small amounts of gray (of the same brightness as the red as measured with a flicker photometer) to the inner or smaller disk until it appeared just perceptibly less saturated than the outer or larger disk. This procedure was then re- versed, the outer disk being decreased in saturation until the change was just perceptible as compared with the inner disk whose saturation was kept con- stant. This was done for seven different degrees of saturation, ranging from 360 of red to 110 of red plus 250 of gray of the same brightness as measured by the flicker photometer. His results indicate that the stimulus-increments corresponding to just notice- able saturation-differences are approximately con- stant (about 4 of gray) at such different stages of saturation as 325 red plus 35 gray, 230 red plus 130 gray, and 110 red plus 250 gray. Geissler states that 'it seems fair to assume that the incre- ment-values would have remained constant at the intervening stages and perhaps also at a stage not far removed from the absolute color-limen,' which latter averaged for the four observers with the red paper about 1.2. That is, a sector of 1.2 of red when mixed with 358.8 gray causes a just percep- tible appearance of color. It appears from the fore- going that the estimated number of least perceptible differences in saturation of the red pigment under the conditions of the experiment is about 100. Another group of experiments was made with nine observers using red, yellow, green, and blue colored papers and their corresponding grays. These measurements were made for each eye separately and for binocular vision. Geissler places no great COLOR AND VISION 129 emphasis upon the absolute values of the results because of the lack of sufficient observers and the incompleteness of the investigation at present. How- ever, it is of interest to give the mean results for the nine observers. The averages for binocular vision were, as a rule, lower than for monocular vision. The results for all observers for monocular and binoc- ular vision gave as the mean limenal values of color saturation for red, yellow, green, and blue respec- tively, 2.23, 5.81, 7.19, and 2.99. That is, these values represent the smallest increments required to distinguish between * color and no color/ The comparison was made between a gray disk and a concentric disk of the same gray in which the color was introduced. The brightnesses were previously equated by means of a flicker photometer. The colored papers differed from each other in brightness and saturation, which appeared to have an influ- ence on the values of just perceptible saturation-dif- ference. Since the green requires a limen three times as great as that of red it appears to Geissler that it is reasonable to assume that its saturation is only one-third as great as the red and about one- half that of the blue. These figures agree approxi- mately with a number of estimates of saturations made by some of the observers, but in the absence of sufficient data little emphasis is given to this point. Experiments with a practically color-blind subject indicated that his limenal values were extremely high, being 37, 18, 140, and 8.25 respectively for the red, yellow, green, and blue papers. No analysis of his defect was made. There appears to be a need for a further explora- tion in this interesting field. 37. Visual Acuity in Lights of Different Colors. 130 COLOR AND ITS APPLICATIONS As has already been shown the eye is not achromatic; that is, rays differing in wave-length do not come to a focus at the same point, with the result that the image of an object illuminated by light of extended spectral character is not sharply defined upon the retina. Louis Bell 13 compared the acuity of the eye or its ability to distinguish fine detail in tungsten and mercury arc lights and obtained results indi- cating an advantage for the latter illuminant. This he attributed to the more nearly monochromatic light emitted by the mercury arc. It will be remem- bered (Fig. 4) that the preponderance of visible rays is confined to a rather narrow wave-length range in the yellow and green regions of the mercury spectrum. The author 14 verified these results and extended the investigation to lights of the same color but dif- fering in spectral character. By using the lights whose spectra are shown in Fig. 17, no difficulties of color photometry were encountered. Screens &, c, d, used with a vacuum tungsten lamp operating at 7.9 lumens per watt yielded lights of the same yellow color but of different spectral character. Like- wise screens e and / yielded two green lights, one purely monochromatic (mercury green line), and the other a green of extended spectral character. The data, except in case 4, Table XII, were not obtained as usual by using fine detail at the limit of discrimi- nation but instead, hi terms of equal ' readability ' of a page of type, which proved after some practise to be a rather definite criterion. Some such method should be applicable to many practical investiga- tions in lighting, for it renders results in terms of a criterion which, although apparently indefinite, is found to be quite definite and one which renders results full of significance. The results for the COLOR AND VISION 131 TABLE XII Relative Illumination for Equal Readability Case Source Screen Color Approx. foot candles Relative illumination 1 Mercury arc Tungsten lamp f e green line green 2.0 1.00 1.75 2 Tungsten lamp Tungsten lamp d c yellow yellow 4.0 1.00 1.33 3 Sodium lines Tungsten lamp none c yellow lines yellow 0.5 1.00 1.66 4 Mercury arc Tungsten lamp f e green line green 0.6 1.00 5.10 author's eye are shown in Table XII and are given in terms of the relative illumination required for equal readability of a page of type. In case 4 an acuity object proposed by H. E. Ives l5 and devel- oped by P. W. Cobb was used. Here the criterion was the ability to perceive fine lines at the limit of discrimination. Other observers obtained results of a similar nature with the same apparatus. No stress is laid upon the accuracy of the absolute values, but it is conclusively evident that monochromatic light is superior for discriminating fine detail. Later it was shown, 16 as was expected from the foregoing, that monochromatic light was superior to daylight for discriminating fine detail. In this case the Ives acuity object was viewed against a white magnesium oxide surface which was illuminated to an intensity of 10 meter candles (approximately one foot candle). The visual acuity on the Snellen scale was found to be 1.28 and 1.11 respectively for daylight, and mono- 132 COLOR AND ITS APPLICATIONS chromatic green light of equal intensities and results for tungsten light and daylight were practically iden- tical. Another experiment showed that for visual acuity of 1.28 on the Snellen scale the intensity of illumination with daylight or tungsten light was nearly three times that required for the same visual acuity with monochromatic green light. As the brightness of the background was increased it ap- peared that the difference in visual acuity under a given illumination of tungsten light and monochro- matic light decreased. The superior defining power of monochromatic light having been demonstrated, it is of interest to learn if there is any difference in the defining power of monochromatic lights of different colors. Dow 17 measured visual acuity in light of different colors using electric lamps screened with colored media and arrived at the conclusion that the blue-green region of the spectrum showed greater defining power. Ashe ls used red, green, blue and clear glasses with incandescent lamps and found visual acuity least for the red and increasing in the order green, blue and clear glass for the same illumina- tion; however, the data were too incomplete to war- rant any definite conclusions. Loeser 19 used red, green, and white papers on which black characters were printed. The papers were brought to equal brightness and visual acuity was determined by noting the greatest distance at which the observer could distinguish the details on the papers. He found acuity greater for green light than for red light, and also that the characters on the white card could be distinguished at nearly as great a distance as those on the green card. A serious defect in this method is the fact that, the distances not being constant, the COLOR AND VISION 133 change required in the accommodation of the eye complicates the results. Uhthoff 20 determined visual acuity in monochromatic lights of different wave- lengths, but gives no data on the relative brightnesses of the colored lights. A serious defect in most of the above work is the fact that the lights were neither monochromatic nor did their spectra extend over equal ranges of wave-lengths. The same criticism is applicable to the work of Rice, 21 who performed an extensive investigation of the problem. In order to determine visual acuity in mono- chromatic lights of different colors at ordinary bright- Fig. 68. Apparatus for determining visual acuity in monochromatic lights. nesses, the author 22 devised the apparatus shown diagrammatically in Fig. 68. The lines of the acuity object, 15 c, having a highly illuminated ground glass background, J, were focused crosswise on the slit of a Hilger wave-length spectrometer by the lens, /. On looking into the eyepiece these lines were viewed against a background whose color depended upon the position of the prism, the wave-length being indicated on the drum, n. On the pointer in the eyepiece was mounted a minute piece of magnesium sulphate, mm, at an angle leaning away from the eye at the top. This was illuminated by means of the frosted tung- 134 COLOR AND ITS APPLICATIONS sten lamp, j y the light being reflected downward by the mirror, o. Slides hh controlled the width of the photometric field, and an artificial pupil, &, was placed in front of the eyepiece. The drum, &, controlled by means of a belt the size of the lines of the test object which was read from drum e. The photo- metric balance was made, in the case of each mono- chromatic light used, by balancing it against the white surface mm, the lines of the acuity object at the time being too small to be visible. A feature of this acuity object which is essential for such a use is that the average brightness of the object is constant regardless of the width of the lines. Of course in making the photometric balance the un- certainties of color photometry are present, but these are not of much importance in this investigation, because visual acuity changes very slowly with change in brightness of the object at the illumination used; therefore, a large error in the photometric meas- urements would cause but a slight error in the visual acuity measurements. The brightness of the photo- metric field as seen by the eye through the artificial pupil was equivalent to the brightness of a white surface illuminated to an intensity of 4.2 foot candles. After the photometric balance was made by varying the current through the large lamp illuminating the test object, the lamp, j, was extinguished and a series of acuity settings were made by varying the size of the lines. The results obtained are shown in Fig. 69. Curves a, c, represent extreme series made by the author showing the fluctuation in the ability of the eye to distinguish fine details, and b is the mean curve of a great many observations. Curves d and e represent single series of observations (ten read- ings at each point) made by two other observers. COLOR AND VISION 135 In every case the observer was permitted to focus the instrument. These data indicate an advantage in the defining power of monochromatic yellow light over other monochromatic lights of equal brightness. In order to extend the observations into the violet end of the spectrum, the test object was illuminated by means of a mercury arc. The mean results for each of two observers are shown in Fig. 70, for three mercury lines. Curve F was combined with curve b in Fig. 69 (obtained by the same observer) which 040 0.44 045 0.52 0.56 0.60 0.64 0.66 Fig. 69. Visual acuity in monochromatic lights of equal brightness. extended the latter as indicated. This investiga- tion indicates that monochromatic lights differ in their defining power and that yellow monochromatic light is superior to others in this respect. It was also found that for a given change in brightness of the test object the change in visual acuity was least for yellow monochromatic light than for light of any other spectral hue. A striking experiment illustrating the effect of spectral character of light on visual acuity is given below. The test-object was viewed through an ethyl violet screen (purple under the illumination from a tungsten lamp) and visual acuity settings were made* After obtaining the mean of a series of observations 136 COLOR AND ITS APPLICATIONS a yellow screen was also placed before the eye. This screen absorbed the blue and violet rays transmitted by the purple screen, thus reducing the illumination at least 50 per cent. Notwithstanding this reduction in illumination visual acuity noticeably increased. In place of the yellow screen was now substituted a blue screen which absorbed the red rays transmitted J.H 1 T> < x-^ *** .^ 1 O X x ^ ^ X H r> , . ^X ^ .X ^ o .ll ^x ^ .X 3,0 x X . / cO W ' ^ X ^ > Q x x l~ - ^ v X U-l. K 5 s n 5 1 7 C; 1 A) 6 1 | || to a 30 0, )4 0.! )& Fig. 70.' Visual acuity in the mercury spectrum, the lines being reduced to equal brightness. by the purple screen, the resulting light being blue. Again visual acuity increased, notwithstanding the reduction in brightness. This experiment strikingly demonstrates the influences of chromatic aberration and spectral character of light on the ability to dis- tinguish fine detail. It is interesting to note some results on the legi- bility of colored advertisements. Le Courrier du Livre 23 reported the legibility of various combinations COLOR AND VISION 137 for reading at a considerable distance, the most leg- ible print being black on a yellow background. The order of merit was found to be as follows: 1. Black on yellow 8. White on red 2. Green on white 9. White on green 3. Red on white 10. White on black 4. Blue on white 11. Red on yellow 5. White on blue 12. Green on red 6. Black on white 13. Red on green 7. Yellow on black It is noteworthy that in this list the customary black-on-white combination is sixth on the list. These results are interesting, although perhaps not final, owing to the many variables that enter such a problem. 38. Growth and Decay of Color Sensations. Many investigators have studied the problem of the effect of time of exposure and intensity of the stim- ulus on the growth and decay of luminous sensations. It has been noted (# 14) that colors are seen on rotat- ing, at a proper speed, a disk composed of black and white sectors. It appears that this is due, in part at least, to the difference in the rate of growth and decay of the various color sensations excited by white light. Of the work in this field, that of Broca and Sulzer 24 is especially comprehensive. They com- pare the brightness of a white screen illuminated by a light of short duration with that due to a standard steady light. Some of their results which are plotted in Fig. 71 show that, excepting for lights of low inten- sity, the luminous sensation * overshoots' its final value; that is, the maximum luminous sensation is passed a comparatively short time after the begin- ning of the exposure and that the luminous sensa- tion reaches a steady value less than the maximum 138 COLOR AND ITS APPLICATIONS only after the elapse of an appreciable fraction of a second (depending more or less upon the intensity). The numbers on the curves indicate the final steady value of the various stimuli. Their data obtained with colored light, plotted in Fig. 72, indicates that under the stimulation of blue rays the luminous OJ5 0.20 SECONDS Fig. 71. The growth and decay curves for white light sensation. (Broca and Sulzer.) sensation overshoots very much more than in the case of red or green light, the latter showing the least overshooting. In studying the growth and decay of color sensa- tions in connection with the flicker photometer 25 some data of interest here were obtained. Red and blue- green lights, practically complementary, were matched by the ordinary direct comparison method of photom- COLOR AND VISION 139 etry. These were then separately flickered against darkness by means of a rotating disk with equal open and closed sectors. The maximum brightness of the flickering light was compared with a steady brightness of the same color for a large range of flicker frequencies. The data is shown in Fig. 73, JL\JV g 150 o 100 K 50 UJ / W" --, *- Re d / 9S ~^ / / 51 V /: ^_^ > 0.05 0.1 0.15 02 0.2 v^200 y Q t 50 / \ 72^ ^ 100 / Bi ue ^ ^8 -^ ""^^ - . --^ -^. Of / *s ^^ ~^- t! 50 s^ Jut ). IL LI / ^^ * 21 IOJ U ( ; 0.( )5 0. 0.1 5 0.2 QZi 0.1 0.15 SECONDS Fig. 72. The growth and decay curves of color sensations. the initials R and G representing the red and blue- green lights and the subscripts, high and lower in- tensities. The intensities used were those ordinarily considered satisfactory in photometry as is indicated by the frequency in cycles per second required to cause flicker to disappear. It will be noted that the colored lights were alternated against darkness, the steady values of the colored lights (sectors open) as 140 COLOR AND ITS APPLICATIONS determined by the direct comparison method being represented by unity on the relative brightness scale. The flicker of G L , /? L , G H , and # H completely dis- appeared at frequencies corresponding respectively to A, B, C, and D. Next red and blue-green brightnesses equivalent to the foregoing were placed so that on one side of RELATIVE MAXIMUM BRIGHTNESS O r- > ir> bo O ro 4*. I W V ft \ V \ \ in \ Vl \ 5 ^ \ \l*l \\s C \ N \ s. * ^ v> ^ *"^"*"" t 1 C 1 1 8 10 12 14 16 18 20 22 24 26 - FLICKER FREQUENCY (CYCLES PER SECOND) Fig. 73. Showing the maxima attained by flickering lights at various frequencies. the photometer field a red light flickered on a steady blue-green field and vice versa on the other side. This was done by means of identical sectored disks (180 opening) placed one on each side of the photometer. On one side a disk intercepted the blue-green light and on the other the red light was intercepted. On increasing the speed of rotation of the disks (which were fastened to the same shaft) the side on which blue-green light flickered upon a steady red field became quiescent long before the flicker COLOR AND VISION 141 disappeared on the other side. At all times when flicker was visible the side upon which red flickered on a steady blue-green field appeared to attain higher maximum values of brightness and to be more agi- tated. The brightnesses on either side were later s 20 UJ I 1.5 o CD | 1.6 | uJ 14 il.2 1.0 \ \ 04 8 12 16 20 Z4 2& FLICKER FREQUENCY Fig. 74. Showing the maxima of sensations produced by flickering red light on a steady green field (R), and vice versa (G)^ measured separately against a steady white light (there being little color difference excepting at low speeds) throughout a wide range of frequencies. These results are shown in Fig. 74, R and G indi- cating that red and blue-green were respectively the flickering components. The steady value reached at 142 COLOR AND ITS APPLICATIONS a high frequency is 0.75, unity being taken as the steady value at zero speed with the sectors open. The latter intercept only one of the two components which make up the brightness on either side; there- fore, the sectors being of 50% transmission, the final value at a high frequency of alternation is 0.75 of the original steady value with sectors open. Of course these experiments involve the measurement of the brightness of surfaces differing in color, but it is this problem that was involved in the study. All steady brightnesses were chosen equal as measured by an ordinary direct-comparison photometer. While these effects of different rates of growth and decay of color sensations are operative when there is an apparent flicker, evidence points to the disappearance of such influence upon the brightness of a mixture of colored light by alternately presenting the colored stimuli when the rate of alternation is so high that flicker has disappeared. For instance the foregoing red and blue-green lights were mixed by alternating them by means of a sectored disk (50% opening) and also by directly superposing the steady lights. The former mixture was found to be just one-half as bright as the latter, within the slight possible errors of the experiment. There was no color difference present in this experiment so the photometric data is correct to within one per cent. Other evidence of the same kind was obtained by comparing two yellow lights of the same hue, but differing in spectral character, by means of both the flicker and direct comparison methods of photometry. Identical results were obtained by the two methods. These results were also confirmed by comparing tungsten light by the two methods with a light of the same hue consisting of red and blue-green lights. (See # 55.) COLOR AND VISION 143 Talbot 26 long ago expounded the law that a sectored disk rotating at high speed transmitted light in direct proportion to the angular openings of the sectors. This law has been stated by Helmholtz 27 as follows: 'If any part of the retina is excited with intermittent light, recurring periodically and regularly in the same way, and if the period is sufficiently short, a continuous impression will result which is the same as that which would result if the total light re- ceived during each period were uniformly distributed throughout the whole period." Plateau, 28 Kleiner, 29 Weideman and Messerschmidt, 30 Ferry, 31 Lummer and Brodhun, 32 Aubert, 33 Hyde, 34 and others have inves- tigated the problem, and have generally agreed that the law holds for white light. Fick concluded that it holds only at moderate intensities and Ferry veri- fied the law for white light but found discrepancies when one side of the photometer field was bluish as compared to the other side. Hyde, after a thorough investigation of the problem, concluded that the law holds within the accuracy of the work (about 0.3%) for the range of sectors used by him, namely from 288 to 10 in opening. He further concluded that the law held for red, green, and blue lights within the accuracy of precision photometric apparatus, and found that when a color difference existed on the two sides of the photometer field no appreciable deviation from the law was observed. The author has had many opportunities to test the law for colored lights and found no deviations within the accuracy of the experimental work, which was usually well within one per cent. The sectored disk, there- fore, affords a means of altering the intensity of colored light in definitely measurable amounts. Lights of very short duration are perceptible if 144 COLOR AND ITS APPLICATIONS intense enough. For instance, a lightning flash as short as one-millionth of a second is visible and by rotating mirrors flashes of light as short as one eight- millionth of a second have been perceived. Blondel and Rey 35 studied the perception of lights of short duration at their range limits. Bloch 36 had pre- viously contended that the excitation necessary for the production of the minimum sensation was per- ceptibly constant and proportional to the product of the brightness and the duration. Charpentier 37 veri- fied the law within certain limits. Blondel and Rey conclude that Bloch's law can be applied only to intense lights of very short duration. After a very extended investigation they deduce a simple law, (B Bo)t=aB , where B is the minimum per- ceptible brightness of the field, t the duration of the stimulus in seconds, and a is a constant of time equal to 0.21 second. They show by simple integration one can deduce from the law of the flashes which are not uniform, their range and the intensity of the equivalent constant light from the point of view of range, where 7 h represents the photometric intensities of the luminous points measured in a horizontal section of the beam and referred to unit distance. They conclude by taking into consideration the curves of sensation of Broca and Sulzer 24 'that the maximum utilization of a source of light must demand short flashes without its being necessary to take any notice of an inferior limit of the period of the signals, except in the case of telegraphic signals. It more- COLOR AND VISION 145 over suffices that the period of the flash, 2 -i, should become a negligible quantity in the presence of the constant a, in order that a maximum efficiency may be assured.' On alternating a given brightness with darkness by means of a sectored disk with 50% openings, a violent flicker is evident at low speeds; however, there is a certain minimum frequency, called the critical frequency, at which the flicker just disappears. The critical frequency depends upon the intensity of illumination or brightness of the observed field and increases with the brightness. Porter 38 has found LOGARITHM OF BRIGHTNESS Fig. 75. Showing the relation between brightness and critical frequency for colored stimuli. that the relationship, / = a log 7+6, holds for white light where / is the critical frequency, /, the illumi- nation, and a and b are constants; that is, there is a straight line relation between the critical frequency and the logarithm of illumination. The constant, a, has two values, one for brightnesses above those resulting from illuminations on a white surface greater than about 0.25 meter candles and one below. It is thought by adherents to the von Kries 'duplicity 146 COLOR AND ITS APPLICATIONS theory' (#48) that this point of abrupt change in slope corresponds to the change from cone to rod vision. Haycraft 39 has studied the critical frequency for spectral lights, but the results are complicated, be- cause the intensity of the various rays was not con- stant throughout the spectrum. Ives 40 studied the relation between critical frequency and brightness for various spectral rays and obtained results which he expresses diagrammatically as shown in Fig. 75, the logarithm of brightness being plotted against critical frequency. It is noted that the 'red' curve shows no change in direction at low intensities. The blue curve changes from a diagonal to a horizontal straight line; that is, at low illuminations the critical fre- quency becomes constant for blue light of various feeble intensities. Intermediate curves represent spec- tral colors between red and blue. It is significant to note that the slopes of the curves are different for the higher illuminations, the 'blue' slope being steeper than the 'red' slope, which indicates that the Purkinje phenomenon is operative. The author 25 has shown that the critical frequency depends upon the wave form of the stimulus or the contour of flicker. Some of the data for white light is shown in Fig. 76. In cases a, 6, c, the maximum, minimum, and mean cyclic illumination were re- spectively the same. A difference in critical fre- quency was obtained throughout a wide range of illumination, the critical frequency being higher the greater the period of darkness in a given cycle. This also appeared to hold for colored lights, but no extensive study has yet been made. 39. Signaling. The chief requisite of a colored light for signaling purposes is high intensity, because its range depends largely upon this factor.. This COLOR AND VISION 147 precludes the use of very pure colors owing to low intensities obtainable in practise, and for this reason signal glasses are a compromise between saturation of color and transparency. As is seen by the redness of the setting sun, red rays are less absorbed by smoke and dust in the atmosphere than the blue rays, therefore, a red signal should have a greater range than a blue signal through a smoke and dust laden CRITICAL FREQUE.NCYCCYCLES PER SECOND) 3^O>Oro^,* ^ ^ ^ _^ / It 1 5 / ^ x^| ^ ' ^* ^ ^ / 1 / / \ ^ ^ .X ^ / / T\ / x ^ y / I/ / / /J / / ^ x ____. -' ^ -T- =. -sT "7 "^ *~ "6 / y /9 "> -^ -^.. s ^ ^ / -zr ^ x >.5U UM 0.34 0.36 0.58 0.40 042 044 046 048 JJ;. WAVE LENGTH 0.5 Fig. 78. Ultra-violet transmission curves of various glasses. of glasses in the ultra-violet region 45 has been de- termined by using a wide slit, the spectrum of the quartz mercury arc, and a combined photographic and photometric method. For qualitative analysis an iron arc is a satisfactory source rich in ultra-violet rays. Some specimen transmission curves for various optical glasses, employed for protecting the eyes from ultra-violet energy, are shown in Fig. 78, the ultra-violet region being represented to the left of the heavy vertical line at 0.40/x. It is interesting COLOR AND VISION 159 to note the difference in ultra-violet absorption of the two samples of ' smoke ' glass. Of course the absorp- tion depends upon the thickness of the specimen and its density of color. All the glasses excepting three specimens, 3, 5, and 6, transmitted 50% or more of the total light from a tungsten lamp. Spectrophoto- graphic analyses of various glasses are shown in Fig. 18. In general it is no doubt advisable to use glasses as free from color as possible and yet providing protection if they are to be worn for long periods. Yellow-green glasses when otherwise filling the re- quirements appear to distort colors (more commonly encountered) less than medium amber. A striking instance was found in the lap-welding department of a steel mill, where the operators judge tempera- ture visually. They became confused when wear- ing amber glasses, but found no difficulty in using yellow-green glasses. This brings to mind the fact that through a yellow-green glass transmitting only a limited region of the spectrum the relation of bright- ness and temperature appears practically the same as to the unobstructed eye when the luminous sub- stance radiates light approximately the same as a black or 'gray' body. Years ago Crova suggested a method of photometry involving this principle (# 54). Schanz and Stockhausen, Voege, Crookes, Parsons, and others have studied the subject of protecting the eye from harmful rays. Crookes 46 concludes with his associates that the relatively great amounts of infrared energy emitted by molten glass is responsible for glass-blowers' cataract, although this conclusion is questioned by some. He has made an exhaustive study of the manufacture of glasses for eye-protection and has published the valuable results. 160 COLOR AND ITS APPLICATIONS Colored glasses are often used for bringing out certain colored portions of an object in more striking contrast with the surroundings. For instance, if a black-line drawing be made on blue-lined coordinate paper and viewed through a dense blue glass, the blue lines practically disappear. If the drawing be photographed through this glass the coordinate lines will not appear on the negative. In the same manner if blue and red appear upon the same background, one or the other can be made practically to disappear by using a colored screen of exactly the same color. Of course the degree of change in contrast will depend upon the purity of the colors and the care exercised in choosing the colored screen. In using field glasses distant vision can be im- proved sometimes by the use of a light yellow screen which eliminates the blue haze from the visual image. In this connection it is well to note also that blue rays are normally out of focus at the retina. The author has experimented with colored screens for use with field glasses for detecting colored objects at a distance by altering their contrast with the sur- roundings by the use of colored screens. For in- stance, a khaki uniform (yellow-orange in color) can be made to appear either lighter or darker than the green foliage surrounding it by respectively using a yellow-orange screen or one of a complementary hue. For instance if the ratio of the brightness of a piece of khaki cloth to that of a certain green leaf be taken as unity under daylight illumination, through an ordinary orange filter this ratio became 1.5 and through a blue-green filter, 0.7. With care the con- trast can be made practically a maximum. In the case of objects more striking in color the problem is not as difficult. Whether or not the reduction of COLOR AND VISION 161 brightness more than offsets the advantage of in- creased contrast in distinguishing distant objects can be solved by actual trial. The point is mentioned here to illustrate the possibilities in the use of colored glasses as an aid to vision. REFERENCES 1. Physiol. Optik. 1896, p. 140. 2. Sitz. d. Berliner Akad. 1888, p. 917. 3. Bui. Bur. Stds. 1907, p. 59. 4. Lancet, Oct. 2, 1909. 5. Proc. Roy. Soc. A, 84, p. 464. 6. Color Scales. 7. Wein. Sitz. 1906, II a, 115, p. 1. 8. Bui. Bur. Stds. 6, p. 89. 9. Bui. Bur. Stds. 9, p. 59. 10. Trans. I. E. S. 1914, 9, p. 700. 11. Physiol. d. Netzhaut, Breslau, 1865, p. 138. 12. Amer. Jour. Psych. 1913, 24, p. 171. 13. Elec. World, 1911, 57, p. 1163. 14. Elec. World, 1911, 58, p. 450. 15. Elec. World, 1910, 55, p. 939. 16. Elec. World, Dec. 6, 1913. 17. Lon. Ilium. Engr. 2, p. 233. 18. Elec. World, Feb. 25, 1909. 19. Graefe Arch. f. Ophth. 69, p. 479. 20. Graefe Arch, f . Ophth. 26, p. 40. 21. Columbia. Cont. to Phil, and Psych, 20, No. 2. 22. Elec. World, 1911, 58, p. 1252; Trans. I. E. S. 1912, p. 135. 23. Sci. Amer. Sup. Feb. 2, 1913. 24. Jour, de Physiol. et de Path. Gen. No. 4, July, 1902; Comp. Rend. 2, 1903, p. 977, p. 1046. 25. Phys. Rev. 1914, p. 1; Elec. World, May 16, 1914. 26. Phil. Mag. 1834, 5, p. 327. 27. Physiol.. Optik. II, p. 483. 28. Pogg. Ann. d. Phys. 1835, 35, p. 457, 29. Pfliiger's Archiv. 1878, 18, p. 542. 30. Wied. Ann. 1888, 34, p. 465. 31. Phys. Rev. 1895, 1, p. 338. 162 COLOR AND ITS APPLICATIONS 32. Zeit. Inst. 1896, 16, p. 299. 33. Physiol. der Netzhaut, p. 351. 34. Bui. Bur. Stds. 1905, 2, p. 1. 35. Acad. Sc. Paris, July 3, 1911; Trans. I. E. S. 1912, 7, p. 625. 36. Comp. Rend. Soc. Biol. 1885, 2, p. 485. 37. Comp. Rend. Soc. Biol. 1887, 2, p. 5. 38. Proc. Roy. Soc. 1902, 79, p. 313. 39. Jour, of Physiol. 21, p. 126. 40. Phil. Mag. 1912, p. 352. 41. Meeting Ry. Signal Assn. 1905. 42. Ann. d . Hydrographie, 1886. 43. Proc. Phys. Soc. London, 1913, 24, p. 379. 44. Elec. World, Dec. 6, 1913. 45. Elec. World, Jan. 15, 1912; Trans. I. E. S. 1914, p. 472. 46. Proc. Roy. Soc. London, A, 214, p. 1. CHAPTER VII EFFECT OF ENVIRONMENT ON THE APPEARANCE OF COLORS 41. Colors have been largely treated in other chapters as if they were invariable in appearance. However, the study and application of the science of color is rendered very complex owing to the fact that the appearance of a color is so modified by its environment. The intensity, spectral character, and distribution of the light illuminating it, the adaptation of the retina for light and color, the duration of the stimulus and the character of the stimulus preceding the one under consideration, the surroundings, the size and position of the retinal image, the surface character of the colored medium, all affect the appear- ance of a given color. Thus an analysis that holds for a color in a certain environment does not in general hold for the identical colored object viewed under other conditions. The size of a colored image and its position and duration on the retina affects its appearance, owing to the variation of sensitivity of the various retinal zones. MacDougal 1 found that with small colored areas (squares from 1 to 16 sq. cm. in area viewed from a distance of one meter) the larger areas ap- peared more saturated than the smaller. He found the saturating effect of increasing the area greatest for violet and decreasing in the order, blue, green, yellow, orange, red. He even concludes that a color field is not fully saturated until it extends over the 163 164 COLOR AND ITS APPLICATIONS whole field of vision. This can hardly be true, for an observer in a room with neutral tint surround- ings illuminated with pure red light is not conscious of a saturated red color. Similarly if a white paper on a black velvet ground be illuminated by a moder- ately intense red light it will appear quite unsaturated owing to the lack of anything with which to contrast it in color. The loss in saturation appears to progress with time, no doubt largely due to 'adaptation.' Whether or not this adaptation is psychological or physiological there is a lack of agreement. However, the effect of area is of importance, although there is much work to be done in this field before definite conclusions can be drawn. Another experiment of importance in viewing colors which is connected with the rate of growth of color sensations and, perhaps, to a slight degree, with chromatic aberration, is found in viewing a red piece of paper on a blue-green ground held at an arm's length under a moderate illumination. If the paper be moved back and forth without relaxing fixation at a point in the plane in which the card is moved, the red patch will appear to shake like jelly and will appear not to be in the same plane as the blue-green paper. Thus there are numerous visual phenomena associated with the appearance of colors. 42. Illumination. It has already been shown that the maximum spectral sensibility of the eye shifts toward the shorter wave-lengths at low inten- sities (Purkinje effect #4, Fig. 2). Therefore colors ordinarily encountered appear to shift in hue under low illumination. For example, a green pigment appears to assume a more bluish hue as the illumi- nation is greatly decreased. On referring to Fig. 2 it is seen that the relative values of luminous sensa- EFFECT OF ENVIRONMENT ON COLORS 165 tion produced by equal amounts of radiant energy depend upon the wave-length. A colored pigment has the ability to reflect certain proportions of the rays of various wave-lengths. The latter is a purely physi- cal operation which remains invariable regardless of the intensity of illumination. However, the relative physiologic effect of the different rays change so that the maximum luminosity is produced by energy of a shorter wave-length at low intensities than at high illumination. By multiplying the reflection coefficients r\ 0.50 Q54 058 A, WAVE LENGTH 0.4Z 0.46 Fig. 79. Effect of the intensity of illumination on the appearance of a pigment. of a pigment for various rays by the luminosities of the corresponding rays at high and low intensities an idea of the shift of the dominant hue is obtained. This was done for a green pigment by using the luminosity curves in Fig. 2 for high (H) and low (L) illumination. The results plotted with equal maxima are shown in Fig. 79. Colors appear more saturated at low than at high intensities of illumination. In fact, intense illumina- tion causes colors to appear very much less saturated. For instance, a deep red object illuminated by direct sunlight is painted orange-red by the artist. The employment of this illusion is successful in conveying 166 COLOR AND ITS APPLICATIONS to the observer the idea of intense illumination. Simi- larly colors appear more saturated when exposed only for a very short interval of. time. Quality or spectral character of light affects the appearance of colored objects very much. Except in very special cases a red fabric for example appears red because it has the ability to reflect chiefly the red rays (# 12, Fig. 12). Such a fabric must appear black when viewed under an illuminant which con- tains no red rays. This is the case under the light from the mercury arc, which contains practically no visible rays longer than 0.579^ (yellow). It is a fundamental principle that, excepting in special cases, a colored fabric cannot appear the same under two different illuminants. Therefore two colors that ap- pear alike under one illuminant will not match when viewed under another illuminant, unless the colors in each case show the same spectral character by spec- trophotometric analysis. In other words, because the eye is not capable of analyzing a color spectrally, it is possible to produce colors which appear the same but whose spectral compositions differ. Such a match will not in general remain a match under another illuminant differing in spectral character. In Fig. 80 the effect of the illuminant upon the appearance of a colored pigment (purple) is shown diagrammatically. The relative luminosities (dotted lines) produced by the relative amounts of energy (full lines) of cor- responding wave-lengths are shown for daylight in the illustration on the left. The result as shown by the dotted curve is to give to the pigment the appear- ance of a blue-purple. However, when this same fabric is illuminated by ordinary artificial light of continuous spectral character, the excessive amounts of energy of the longer wave-lengths and the defi- EFFECT OF ENVIRONMENT ON COLORS 167 ciency in short-wave energy as compared with day- light alter the spectral character of the light reflected (or transmitted) by the fabric as shown in the dotted curve on the right. The appearance under the artificial light is red-purple. It is difficult to distin- guish a blue fabric as blue under ordinary artificial light owing to the scarcity of blue rays in most of the artificial illuminants. Of course a truly mono- chromatic pigment (if such existed) would not be NOON SUHLIGHT \ V B G Y OR BLUE -PURPLE ORDINARY ARTIFICIAL LIGHT V B G Y R RED-PURPLE Fig. 80. Illustrating why a purple appears differently under two different illuminants. changed in hue under various illuminants but would be altered in brightness. In the special case where no energy existed in the illuminant of the wave- length corresponding to that .-^3 50 0.5Z 0.54 0.56 05& 0.60 0.62 0.64 0.66 Fig. 88. The results of four methods of photometry (Ives). extreme insensitiveness and the influence of eye fa- tigue and adaptation. The influence of the spectral character of light further complicates the visual acuity method. The direct comparison method, though claimed by many to yield measurements of 'true' brightness is unpopular, owing to the uncertainties in the measurements. The flicker method, however, owing to its elimination of color difference and high 196 COLOR AND ITS APPLICATIONS sensibility, had won many ardent supporters even before extensive investigations of the method had been made. In Fig. 88 are shown data obtained by Ives x with the four methods. In each case the standard was the total light from a tungsten lamp. Spectral colors were compared with this white standard. Curve V was obtained by the visual acuity method; D, by the direct comparison; F, by the flicker; and C, by the critical frequency method. If the four methods gave identical results, the curves would coin- cide. The general shapes and positions of the max- ima are similar, but the areas under the curves are very different. The enormous area under curve V is in accord with the previous work of Bell 2 and of Luckiesh, 3 which showed that acuity was much better in monochromatic light than in light of ex- tended spectral character. 54. Secondary Methods of Color Photometry. - Various schemes have been proposed and developed for eliminating color difference in heterochromatic photometry, such as the use of colored filters, and physical and chemical photometers used with filters that properly weigh the energy of various wave- lengths according to their light-producing effects. Among the latter possibilities are the radiometer, thermopile, selenium cell, photo-electric cell, and photographic plate. Obviously, in order to reduce measurements to absolute values, the transmission coefficients of the colored filters must be determined by some acceptable method. Likewise, determina- tions of the relation between radiation of various wave-lengths and the corresponding luminous sensa- tions and of the sensibility of the instruments to energy of various wave-lengths must be made before COLOR PHOTOMETRY 197 the results obtained with the selenium cell, the radiom- eter, filters, etc., are useful in measuring brightness. Crova 4 suggested as a method of comparing lights possessing continuous spectra, but differing in color, the determination of their intensities at one wave- length, 0.582/z. The lights to be compared in this manner must not differ much in spectral energy dis- tribution from the black body. Rayleigh, 5 Nernst, 6 Fery and Cheneveau, 7 Lucas, 8 Rasch, 9 and others have made various applications and modification of Crova's original proposal. The filter used by Crova consisted of an aqueous solution of anhydrous ferric chloride (22.321 grams) and crystallized nickelous chloride (27.191 grams), the total volume being 100 c.c. at 15 C. A thickness of 7 mm. of this solu- tion was used which transmits energy from 0.63^- to 0.534,u with a maximum of 'transmission at 0.582/*, the wave-length which Crova found to be satisfactory for carrying out his proposed scheme. Ives 10 tested Crova's method by comparing the luminous intensities of a tantalum and a carbon incandescent lamp at various wave-lengths. He found that the wave-length for such a comparison lies between 0.56^ and 0.58/4, depending on the range of temperature. The latter wave-length was found to hold best of all within the limits of temperature represented by ordinary incan- descent lamps of that time. Twelve years ago Fabry n recommended the use of two or more col- ored solutions for eliminating color difference, having first calibrated these solutions for thickness and trans- mission by an acceptable method. Aniline dyes were not used, because of the need for definite and re- producible solutions. By using two solutions, A and Bj he was able to match the Carcel lamp with almost any illuminant. The solutions were made as follows: 198 COLOR AND ITS APPLICATIONS A. Crystallized copper sulphate 1 gram Commercial ammonia (density 0.93) 100 c. c. Water sufficient to make one liter. B. Potassium iodide 3 grams Iodine 1 gram Water sufficient to make one liter. Ives and Kingsbury 12 have recently investigated the problem of obtaining suitable solutions that would eliminate color difference after the manner pro- posed by Fabry. They developed a yellow solution containing 100 grams of cobalt ammonium sulphate, 0.733 grams of potassium dichromate, 10 c.c. of 1.05 sp. gr. nitric acid, and distilled water to make one liter at 20 deg. centigrade. The method of preparation is considered very important and is pre- sented in detail in the original paper. Of course a given depth or concentration of the solution has a different transmission for illuminants of different spectral character. The transmission values were determined by means of a flicker photometer by averaging the results obtained by specially selected observers. The transmission of the solution was found to vary considerably for different temperatures and the character and cleanliness of the glass sides of the containing cell were found to be of consid- erable importance. It was found possible to eliminate color difference in comparing many illuminants with the carbon lamp standard by placing the solution on either one side or the other of the photometer. Many have used aniline dyes and colored glasses. In practical photometry the use of colored glasses appears to be satisfactory for a large amount of work. The carbon lamp operating at about 4 w.p.m.h.c. is the present standard of luminous intensity. Properly COLOR PHOTOMETRY 199 tinted bluish glasses used with this standard will eliminate the color difference when comparing tung- sten lamps with it. The transmissions of the tinted glasses can be obtained by averaging the determina- tions of a large number of observers, using the direct comparison method. Such a procedure is being used successfully in several laboratories for the above work where the color difference is not excessive. However, it is not a solution of the general problem of color photometry. Houston 13 in 1911 proposed the use of a filter composed of two solutions copper sulphate and potassium dichromate for closely approximating in transmission the luminosity curve of the eye, this filter to be used with an energy-measuring instru- ment. Koenig's visibility data were used as a basis for developing the solution. A proper solution would transmit rays of various wave-lengths in the propor- tions corresponding to the relative light-producing values of the various rays. It is necessary to cut off both the infra-red and ultra-violet rays and to re- duce the visible rays in just the correct relative pro- portions so that an energy-measuring instrument (bolometer, thermopile, or radiometer) will record data proportional to the luminous intensity. A dis- advantage of such instruments is found in their extreme sensitiveness to outside disturbances. For instance, the galvanometer used in the procedure must be of a high order of sensibility and therefore must be set up where it will be free from mechanical and magnetic disturbances. Karrer, 14 recently follow- ing Houston's lead, similarly employed the visibility data obtained by Ives. By using three solutions he was able to produce a screen whose transmission curve closely approached this luminosity curve of the 200 COLOR AND ITS APPLICATIONS eye. The solutions were made by dissolving (1) 57.519 grams of cupric chloride, (2) 1.219 grams of potassium bichromate, and (3) 9.220 grams of ferric chloride, each in one liter of water. A triple cell was used, each compartment being 1 cm. thick. The selenium cell has been used for stellar pho- tometry and for other special work, owing to its change in resistence on being illuminated. However, it has not yet found a place in color photometry, be- cause at present it is too erratic and undependable. Photo- Electric 040 0.45- Selenium 0.50 0.55 0.60 0.65 0.70 0.75 V Fig. 89. Spectral sensibilities of selenium and photo-electric cells compared with the spectral sensibility of the eye. Its sensibility to energy of various wave-lengths ap- pears to depend upon the method of making the cell, and is in general far different from that of the eye. A sensibility curve is shown in Fig. 89, compared with the luminosity curve of the eye. The maximum change in resistance is usually due to energy of the longer visible wave-lengths. Obviously a filter that properly weighs the energy of various wave-lengths according to its light value and to the spectral sen- sibility of the cell, must be used for the photometry of illuminants of extended spectral character. The photo-electric cell has been used in special cases of scientific investigation for detecting the COLOR PHOTOMETRY 201 presence of radiant energy. Surfaces of potassium, zinc, and other elements and compounds in vacuo exhibit the property of emitting electrons when illumi- nated. The maximum effect is usually found in the short-wave visible region, as illustrated by a sensi- bility curve of a photo-electric cell, shown in Fig. 89. As in the case of the selenium cell, the photo-electric cell is too erratic at the present time to be adopted as a means of photometering lights of different colors. The strengths of the electronic currents measured by means of a sensitive electrometer or galvanometer afford a measure of the relative intensities of the illumination of a given spectral character when the characteristics of the cell are shown; that is, when the relation between the intensity of illumination and the photo-electric effect is known. Lights differing in spectral character cannot be compared by means of the photo-electric cell unless a correcting filter is used after the manner necessary with the selenium cell. The photographic plate affords another possible method for the photometry of lights of different color, but its general adoption is discouraged, owing to lack of uniformity of the emulsion both as to thickness and sensibility. Some of the difficulty could be obviated by using plates made of plate glass. The panchromatic plates must be used, because the ordi- nary plate is not appreciably sensitive to rays of longer wave-length than 0.48ju, the maximum of sensibility being in the extreme violet region of the spectrum. The relative sensibility of a certain com- mercial panchromatic plate, for equal amounts of energy of various wave-lengths, is shown in Fig. 90 compared with the spectral sensibility of the eye. In order to make the plate record the values of col- 202 COLOR AND ITS APPLICATIONS ored brightnesses as determined with a flicker pho- tometer, an accurate filter was made which consisted of aesculine, tartrazine, rhodamine, naphthal green, and glass three-eighths of an inch thick. How nearly this filter performs its intended purpose is shown in Fig. 91 by the circles in comparison with the lumi- nosity curve of the eye which is represented by the full line curve. This filter was used with the pan- 044 0.4& 0.5Z 0.56 0.60 0.64 0.66 Ms, WAVE LENGTH Fig. 90. Spectral sensibility of a panchromatic photographic plate. chromatic plate considered above, for which it was made by Ives and Luckiesh 15 for various photometric problems. In using the photographic plate for pho- tometric purposes it must be remembered that, in general, the product of intensity of illumination and time of exposure is not a constant for equal photo- graphic effect. The relation between exposure and intensity of illumination for a constant photographic effect as discovered by Schwartzchild is It* = i T p where I and i are the larger and smaller intensities and T and t are the larger and smaller periods of COLOR PHOTOMETRY 203 exposure. The value of p varies with different plates, generally lying between 0.75 and unity. The manner of development, the temperature, and other obvious factors influence the results so that the photographic method becomes unattractive except for special prob- lems. As already stated, the use of these so-called physical or chemical photometers, while obviating color difference in practise, does not preclude the necessity of establishing the relation between lumi- nous sensation and radiation of various wave-lengths by an acceptable method of color photometry. RELATIVE BRIGHTNESS W tf> / . f - 7 \ / \ / v s ? / X x, *^ ^^ >< s. 0.40 0.44 048 0.52 0.56 0.60 0.64 0.68 0.72 Fig. 91. An accurate color filter for the panchromatic plate considered in Fig. 90. 55. Direct Comparison and Flicker Methods. - Only two primary methods for the photometry of lights differing in color are worthy of consideration, namely the direct comparison and flicker methods, the other two being ruled out of consideration for reasons already given. These two methods have been compared by many observers, but much of the work is so incomplete that it yields little data for a thorough comparison. It is desirable that the method finally acceptable for photometering lights of different colors should measure light-value with the same order of definiteness as other physical measurements are obtained. Dow 16 compared these two methods by using 204 COLOR AND ITS APPLICATIONS colored lights produced by means of red and green glasses at different intensities and with different field sizes. He found with the direct comparison method that the ratio of the red to the green bright- ness decreased with decreasing illumination, the decrease being rapid below 0.3 meter candle, the well-known Purkinje phenomenon. With the flicker method this decrease was slight. With a small pho- tometer field the change in the ratio of red to green was considerably less. In general Dow's results indicate that the flicker method is less influenced by the size of the field or by a change in the illumina- tion than the direct comparison method. P. S. Millar 17 compared mercury vapor arcs with incandescent lamps over a wide range of illumina- tions. The Purkinje phenomenon was in evidence in the direct comparison measurements but absent in the results obtained with a flicker photometer. In other words, with the former method the apparent brightness of the side of the photometer field illumi- nated by light from the mercury arc did not decrease as rapidly as the brightness of the other side illumi- nated by light from an incandescent lamp, as the illumination decreased. Stuhr 18 compared the four methods namely, visual acuity, critical frequency, direct comparison, and flicker. He found the critical frequency and flicker methods to yield identical results, but these differed from the results by the other two methods. Various physiological factors, such as field size and illumination, were not considered. Ives 19 carried out an extensive series of investi- gations which represent the most elaborate and thorough work yet done on the problem. He con- cluded that the flicker method is more sensitive than the direct comparison method and that the results COLOR PHOTOMETRY 205 are more reproducible. He discovered that the flicker method exhibited a 'reversed Purkinje effect' and found, as other investigators had, that the two methods yielded different results in general, but con- cludes that the flicker method yields, under certain specified conditions, a measure of true brightness. Much evidence obtained throughout these investiga- tions and some obtained by the author and others point favorably to the flicker method as the best method of photometry. However, notwithstanding the extensive investigations, some take the stand that the case has neither been decided against the direct comparison method nor in favor of the flicker method. This conclusion is perhaps justifiable. However, considering the unsatisfactoriness of the former method, there is considerable virtue in the adoption of the latter method with its many satisfactory fea- tures in default of a method which has been definitely proved to yield the desired measurements. Ives in his early papers did not emphasize the differences in the results obtained by the two methods. His results were plotted in the form of luminosity curves of the eye, so that without careful inspection, the results by the two methods, under certain conditions of high illumination and small field size, do not appear to differ greatly. In order to determine the magnitude of these outstanding differences the au- thor 20 carried out an investigation, a portion of the results (L) being plotted in Fig. 92. Red and blue- green lights were used. The ratio of the intensity of the red to that of the blue-green light is plotted for a wide range of illuminations. The illumination values are those obtained with the flicker photometer and a standard tungsten lamp, but are not corrected for the absorption of the photometer, which, owing to 206 COLOR AND ITS APPLICATIONS a complex optical path, was considerable, or for reduction due to the small artificial pupil. It is seen that the flicker method exhibits a reversed Purkinje effect and the direct comparison method the true Purkinje effect, and further that the ratio of the red to the blue-green brightness obtained by the direct comparison method is only about 62 per cent of that obtained by the flicker method for 1.0 1.4 1.3 1.2 I.I 1.0 0.9 0.7 0.6 0.5 0.4 0.3 0,2 0.1 ( J -rf< 1 ^<. < v ( Y F L CKi F/ 1 ?) Observer L. IQU ALI TYt >FB RIG/ frfi ESS) Ob sen 0! ser L f X c { ) 1 Z 3 4 5 6 1 METER CAttDLESCON 8 9 10 II 12 13 14 15 PHOTOMETER SCREEN) Fig. 92. Results by flicker and direct comparison photometers, illustrating differences including the Purkinje effect and a reversed effect. a large range of illuminations. The results as to the reversed Purkinje effect were verified in general by another observer (K). It is seen that he did not obtain the same results as the author, even with the flicker photometer, by about 13 per cent. A similar difference, though in general not as great, is found in Ives* data when the lights of the correspond- ing dominant hues (0.64/i and 0.52/z) are compared, even though in his measurements the spectral colors were always balanced against a white light. The same extreme difference in the results by the two COLOR PHOTOMETRY 207 methods was confirmed by the writer using the same glasses a year later. The field size in the foregoing experiments was rather large about ten degrees - but a large difference persists even with smaller fields, though not to such an extent. Morris-Airey 21 suggested that the differences between the two methods might be due to the dif- ferent rates of rise of the sensation with different colors. The author 22 studied this factor and showed that the maximum of a flickering red light was con- siderably greater than that of the blue-green light for a large range of flicker frequencies when the brightnesses of the two lights were those obtained by a direct comparison balance. This in itself did not prove which, if either, is the correct method. However, another experiment was performed which is perhaps as convincing as any yet performed in indicating that the flicker photometer when properly used is inappreciably influenced by the different rates of growth and decay of color sensations. Lights differing greatly in spectral character, but alike in hue, were compared by the two methods and identical results were obtained. Two yellow lights were ob- tained by means of filters of aqueous solutions of potassium dichromate used before a tungsten lamp. In one of the solutions was dissolved some neodym- ium ammonium nitrate, which absorbed all the spec- tral yellow. The two lights now nearly matched in hue and were readily brought to an exact color-match by altering the concentration and by adding a little ordinary yellow or orange dye to one solution. The spectral characters of these two lights are shown in c and dj Fig. 17. Two 'white' lights were also com- pared, one consisting of the total light from a tung- sten lamp, the other being made up of narrow regions 208 COLOR AND ITS APPLICATIONS 4 o the spectrum, respectively in the red and blue- green. In both cases no difference in the ratio of the intensities of the two lights of the same color was detected in the results by the two methods, the accuracy being well within one per cent. It was also shown that red and blue-green lights add, whether by direct superposition or by alternately flickering them as in the flicker photometer, when the flicker is not more than barely apparent. Further investigations may show that the flicker photometer is influenced by the different rates of growth and decay of color sensations, but the fore- going experiments indicate that such influence is slight. Ferree 23 has attacked this problem and has reported some interesting preliminary results. The flicker method possesses many desirable character- istics, yet at present it can hardly be accepted as yielding 'true' measurements of brightness unless the difference in the results, obtained by this method and by the direct comparison method, be ignored. Where color differences are large just where such a method as the flicker method is most desired the results by the two methods vary most widely. 56. Luminosity Curve of the Eye. Ives found that the spectral luminosity curve obtained with the di- rect comparison photometer by the * cascade' method (involving small steps of slight hue difference) agrees at high illumination for a small field with the curve obtained by the flicker photometer. He also found that the latter method fulfilled certain funda- mental axioms, namely, that the sum of several indi- vidual brightnesses of different hue must equal the brightness of the whole and that if each of two bright- nesses of different hue equal a third brightness, they must be equal to each other, while the direct com- COLOR PHOTOMETRY 209 parison method did not. While these experiments point with favor to the flicker method, it is true that a method can fulfill these requirements and yet not yield measurements of 'true' brightness. However, it appears at the present time that the balance of experimental data is strongly in favor of the flicker photometer. For this reason the relation between radiation of various wave-lengths and their physi- ologic effect in producing luminous sensation as i.o 0.9 0.8 t 0.7 to | 0.6 1.0.5 ^ 0.4 1 \ 0.3 0.2 0.1 040 0.44 045 0.5Z 056 0.60 0.64 060 J6. WAVELENGTH Fig. 93. Visibility data. (See Table XVI.) obtained with the flicker photometer is of consid- erable interest. Ives 24 determined the luminosity curves of 18 observers which he has published in comparison with the mean curve. Later he deter- mined the luminosity curves for 25 more observers, which mean, he states, agrees well with that of the previous eighteen. Nutting 25 has recently obtained such data with 21 observers. The apparatuses used by both Nutting and Ives were practically the same as shown in Fig. 51, the source W being eliminated. 210 COLOR AND ITS APPLICATIONS The data as presented by Nutting are shown in Fig. 93 and Table XVI compared with Koenig's 26 origi- TABLE XVI The Visibility of Radiation (See 92) Wave length 0) Nutting mean visibility Ives mean Koenig mean Computed from Nutting's formula 0.400 0.002 0.410 0.003 0.420 0.008 0.430 0.012 0.440 0.023 0.029 1 0.450 0.038 0.047 l 0.158 0.460 0.066 0.073 i 0.201 0.470 0.105 0.107 i 0.250 0.480 0.157 0.154 0.302 0.135 0.490 0.227 0.235 0.370 0.232 0.500 0.330 0.363 0.476 0.358 0.510 0.477 0.596 0.670 0.514 0.520 0.671 0.794 0.830 0.675 0.530 0.835 0.912 0.950 0.824 0.540 0.944 0.977 0.996 0.933 0.550 0.995 1.000 0.990 0.994 0.560 0.993 0.990 0.945 0.993 0.570 0.944 0.948 0.875 0.939 0.680 0.851 0.875 0.780 0.839 0.590 0.735 0.763 0.680 0.717 0.600 0.605 0.635 0.585 0.585 0.610 0.468 0.509 0.492 0.456 0.620 0.342 0.387 0.396 0.343 0.630 0.247 0.272 0.300 0.235 0.640 0.151 0.176 0.210 0.158 0.660 0.094 0.104 0.128 0.108 0.660 0.051 0.068 i 0.070 0.072 0.670 0.028 0.044 1 0.032 0.680 0.012 0.026 * 0.690 0.007 0.700 0.002 1 Extrapolated. nal data which was obtained by the direct comparison method. Nutting has extended the observations well into the red and violet regions of the spectrum by COLOR PHOTOMETRY 211 using sources emitting line spectra. Koenig's data are shown in curve K, Ives* data in curve /, and Nutting's data in curve N. Nutting developed a formula of the form V= V m R* e a(I ~ R) from which the values given in Table XVI and represented in Fig. 91 by the circles have been computed. V m in the formula represents the maximum light-producing ef- fect, R = *, a = 181, and V the visibility or rela- tive light-producing value of energy of any wave- length, A. The maximum sensibility is at X max = 0.555^. The computed values are found to coincide practically with Nutting's mean luminosity curve be- tween wave-lengths, 0.48/z and 0.65^. REFERENCES 1. Phil. Mag. 1912, 24, p. 847. 2. Elec. World, 1911, 58, p. 637. 3. Elec. World, 1911, 58, p. 450, p. 1252. 4. Comp. Rend. 93, p. 512. 5. Phil. Mag. June, 1885. 6. Phys. Zeit. 1906, 7, p. 380. 7. Bui. Soc. Inst. Elec. 1909, p. 655. 8. Phys. Zeit. 1906, 6, p. 19. 9. Ann. d. Phys. 1904, 14, p. 193. 10. Phys. Rev. 1911, 32, p. 316. 11. Comp. Rend. Nov. 1913; Trans. I. E. S. 1913, 8, p. 302. 12. Trans. I . E. S. 1914, 9, p. 795. 13. Proc. Roy. Soc. A, 1911, p. 275. 14. Lighting Jour. Feb. 1915; Phys. Rev. 1915, 5, p. 189. 15. Trans. I. E. S. 1912, p. 90; Elec. World, 1912, 60, p. 153. 16. Phil. Mag. 1910, 19, p. 58. 17. Trans. I. E. S. 1909, 4, p. 769. 18. Kiel, Phil. Diss. Vol. 19, 1908, p. 50. 19. Phil. Mag. 1912, 24, p. 149, p. 170. 20. Elec. World, Mar. 1913, p. 620. 21. Electrician (Lon.), Aug. 20, 1909, p. 758. 22. Phys. Rev. N. S. 1914, 4, p. 11. 212 COLOR AND ITS APPLICATIONS 23. Before. I. E. S. 1914. 24. Phil. Mag. 1912, 24, p. 853. 25. Trans. I. E. S. 1914, 9, p. 633. 26. Ges. Abhandlungen. OTHER REFERENCES On the Photo-electric Cell: H. Dember, Beiblatter, 1913, No. 16; p. 1044. Nichols and Merritt, Phys. Rev. 1912, 34, p. 475. F. K. Richtmeyer, Trans. I. E. S. 1913, p. 459; Phys. Rev. July, 1915. H. E. Ives, Phys. Rev. N. S. 1914, 3, p. 68, p. 396. On the Selenium Cell: Seig and Brown, Phys. Rev. N. S. 1914, 4, p. 48, p. 85; 5, p. 65, p. 167. F. Townsend, Sci. Abs. A, 7, 2869. A. H. Pfund, Phys. Rev. 1912, 34, p. 370; Light. Jour. 1913, p. 128. T. Torda, Electrician, 1906, 56, p. 1042; Sci. Abs. 9, 771. Joel Stebbins, Astrophys. Jour. 1908, 27, p. 183. On Color Photometry: E. P. Hyde and W. E. Forsythe, The Visibility of the Red End of the Spectrum, Phys. Rev. July, 1915 ; Astrophys. Jour. Sept. 1915. Irwin J. Priest, A Proposed Method etc., Phys. Rev. July, 1915. E. F. Kingsbury, A Flicker Photometer Attachment for a Lummer-Brodhun Photometer, Jour. Frank. Inst Aug. 1915, p. 215. H. E. Ives and E. F. Kingsbury, Flicker Photometer Measure- ments on a Monochromatic Green Solution, Phys. Rev. 1915, 5, p. 230. CHAPTER X COLOR PHOTOGRAPHY 57. At the present time no processes of color photography have been developed which employ the simple principle of fixing the colors of Nature directly upon the photographic plate by chemical means. O. Wiener 1 discusses the use of body colors which would assume the colors corresponding to the rays of light by chemical modification. Carey Lea 2 ob- tained a form of silver photochloride which assumed different colors on exposure to various rays, but no means was found for fixing them. Most of the com- mercial methods employ colored media which repro- duce colors by one of the common methods of color- mixture. In the first place the emulsion must be sensitive to all visible rays, and preferably the plate should be sensitive to light rays, in closely the same manner as the eye. There are no commercial plates endowed with the latter characteristic, so panchro- matic plates are usually used with an approximate color filter. Ray filters of the accuracy approaching that illustrated in Fig. 91 are rare, but for accurately photographing colored objects in their true values of light and shade, carefully made filters must be used with panchromatic plates, because the latter differ greatly in spectral sensibility from the eye (Fig. 90). In other words, a plate must be rendered of the same relative sensibility to the various visible rays as the eye by the use of sensitizing dyes and ray filters. 213 214 COLOR AND ITS APPLICATIONS Fortunately ordinary photography does not require such a high degree of accuracy. About a century ago Seebeck discovered that silver chloride becomes tinted by exposure to light with an accompanying chemical action. It is also possible by properly selecting luminescent salts to produce a series of tints after exposure which are very effective. Such colors cannot be fixed, and therefore are of little practical interest. The devel- opment of color photography has been confined largely to two methods. In one the phenomenon of interference of light waves is utilized to reproduce colors directly, while the other method is based upon the principles of color-mixture -- both additive and subtractive (#18, #19). In the latter method artificial color-screens are used. Sometimes these are of minute size, as will be shown later. 58. Lippmann Process. The method employ- ing interference of light waves is originally due to Becquerel, 3 but Lippmann's name is usually asso- ciated with the process, owing to the improvements which he devised after extensive investigation. 4 Zenker 5 in 1868 explained the colors sometimes ex- hibited by spectrograms made on silver chloride plates as due to the interference of light waves reflected from layers of metallic silver which are originally produced by stationary light waves. Among those who have investigated the process are Wiener, 6 Neuhaus, 7 Valenta, 8 Lehmann, 9 and Ives. 10 In the Lippmann process the sensitive film is backed by a film of clean mercury which acts as a reflector. As light which has passed through the thin film strikes the layer of mercury it is reflected back on its path, and owing to the disappearance of energy at certain points through interference, the Reef Green oooooooo< Blue COLOR PHOTOGRAPHY 215 silver compound is acted upon only in layers at the antinodes. The phenomenon is diagrammatically shown in Fig. 94. The silver compound, instead of being acted upon throughout the thickness of the film, is largely reduced in thin laminae the dis- tance between which is one-half a wave-length of the light producing them. Especially fine-grain plates mUSt be USed in Order tO produce duced in the Lippmann the very minute structure. The emulsion must be sensitive to all colors which are so made by the use of certain sensitizers. This discovery is due to H. W. Vogel, in 1873, who found that silver bromide by treatment with certain sensitizing dyes, such as eosine and cyanine, was rendered sensitive to rays of longer wave-length than when untreated. If the plate after exposure in the Lippmann process be developed and illuminated by white light, from various parts of the film only colored light escapes to the eye and a photograph in colors is seen. It is easy to account for the reproduction of pure spectral colors, but the general theory has been the subject of much discussion too extensive to dwell upon here. 59. Wood Diffraction Process. - - This method, invented by R. W. Wood in 1899, depends upon the phenomenon of interference, though in a different manner. It depends upon the principle that all colors may be matched in hue by mixtures of three primary colors, red, green, and blue, each consisting of a narrow band of the visible spectrum. These spectral primaries lie near the regions of the spectrum cor- responding respectively to 0.65ju, 0.52/z, and 0.45/z. This process utilizes diffraction gratings for the pro- 216 COLOR AND ITS APPLICATIONS duction of the primary colors. If a point source of light or an illuminated slit be viewed through a dif- fraction grating (#9), not only will an image of the source be seen, but displaced on either side a series of spectra will be seen. The displacement of the spectra from the line joining the eye and light source will depend upon the number of lines per inch in the grating; the fewer lines per inch the less is the dis- placement. The primary spectral colors are produced as shown in Fig. 95. If a source of light S, a lens L, and a grating G, be arranged as shown, an image of the source will be seen on a screen at 7. With a Fig. 96. Illustrating the Wood diffraction process. fine grating a spectrum of the source will be seen on the screen extending between aa. With a coarse grating a spectrum of the source will be formed between cc and a medium grating will produce a spectrum at bb. If the three gratings have different rulings, the eye at E will see the lens face illuminated by a monochromatic color depending upon the grating interposed at G. If all three spectra be produced simultaneously in proper intensities, the eye at E would see the lens face illuminated by white light providing gratings of proper rulings are chosen. Such a scheme was used by Wood for viewing the photo- graphs. The latter, which appear colorless, really consist of images of the object made on plates of bichromated gelatine through three properly chosen COLOR PHOTOGRAPHY 217 gratings. In making the photographs one of the gratings was placed in contact with the bichromated gelatine film and the image of the object was pro- jected upon the sensitive film through the grating. This grating was then replaced by another and the procedure repeated. It was then repeated a third time with the remaining grating, but usually with another sensitive plate. On superposing the two exposed plates and viewing by a proper combination of lens and light source the picture was seen in colors. Copies can be made by contact printing. It was found, however, that while satisfactory pictures could be made there was no certainty about obtain- ing them. This was later found to be due to super- posing the three grating exposures. Ives 11 improved the process by printing the grating pictures through a very coarse grating placed at right angles to the lines of the three gratings. The coarse grating had opaque lines twice the width of the transparent strips. After making an exposure through one of the gratings the coarse auxiliary grating was moved in a direction perpendicular to its lines a distance equal to the width of one of its open slits and an exposure was made through the second grating. This procedure was repeated for the third grating. The lines of the coarse auxiliary grating were as in the so-called Joly process to be discussed later, so narrow as to be just unresolved by the eye about 2*0 to the inch. This process yielded satisfactory results. Ives further simplified the process by making one grating answer the purpose of the original three by using the finest grating 3600 lines per inch and rotating it in its plane respectively 21.5 and 42 degrees for the other two exposures. Thorp, unknown to Ives, had previously suggested the use of one grating for a 218 COLOR AND ITS APPLICATIONS similar purpose. When using three gratings, one with 2400 lines per inch furnished the red component, one with 3000 the green, and one with 3600 the blue. F. E. Ives worked out a viewing apparatus involving important improvements over Wood's original scheme. 60. Color Filter Processes. If an object be separately photographed on three panchromatic plates respectively through properly chosen red, green, and blue filters (which collectively transmit all visible rays) and these three photographs be separately projected upon a white screen by means of three projection lanterns equipped with the foregoing colored filters, three separate ' monochromatic ' photographs will be seen. If, however, the three colored images -red, green, and blue be superposed in exact coincidence, a picture in natural colors will be seen. The principle is that of adding colors as shown in Fig. 21. F. E. Ives, who was a pioneer in this field, developed an apparatus for viewing the three-colored photographs simultaneously and also the so-called chromoscope for tri-color projection of photographs made in this manner. Charles Cros independently developed a similar method. In 1868 Louis Ducos du Hauron described a process for three-color photog- raphy (since known as the Joly process) which in- volved the ruling of red, green, and blue lines of transparent dyes on a transparent screen. The lines were too fine to be distinguished by the eye. The procedure involves the juxtapositional method of color-mixture, a principle long used in the textile industry and in painting. If a photograph be made through such a screen and a positive made there- from, the latter will appear in colors when viewed through the original screen when properly superposed. The screen is diagrammatically shown largely mag- COLOR PHOTOGRAPHY 219 nified in Fig. 96. (In Figs. 96, 97, and 98 red, green, and blue are represented respectively by the hori- zontal lines and the diagonal lines running in direc- tions perpendicular to each other.) There have been numerous variations of this scheme commercialized. The Paget screen is illustrated in Fig. 97. An inter- esting development is the Lumiere process. Minute grains of dyed starch are used in a thin layer over a sensitive emulsion. Three batches of transparent starch grains are dyed respectively orange-red, green, du Hauron Paget Lumiere FIG. 96 FIG. 97 FIG. 95. Green Red Blue Illustrating three processes of color photography. and blue. These are mixed in such proportions as to give a mixture of neutral color and are spread on the plate in a single layer. A portion of the plate greatly magnified is shown diagrammatically in Fig. 98. The light passes through the minute color filters of dyed starch before striking the plate. The plate is developed in the ordinary manner, and by chemical means the negative is converted into a positive. The reversal may take place in a bath of potassium per- manganate acidified with sulphuric acid and is later developed again in the same developer as used in the first development. After drying, the plate is varnished and the color photograph is ready for 220 COLOR AND ITS APPLICATIONS viewing. The process is a very ingenious one and reproduces natural colors quite satisfactorily. A de- ficiency of the process of no great importance in most work is shown in Fig. 99. It is also of interest in showing the inability of 0.40 050 0.60 0.70 the eye to analyze colors. ^. WAVE LENGTH The approximate trans- Fig. 99. -nitrating the limitations of m i ss i on of the three dyes certain processes of color photography. are diagrammatically shown. It is evident on photographing the solar spectrum that the dyes used are somewhat too monochromatic, because the colored spectrogram which consists of red, green, and blue bands shows gaps between the blue and green, and also between the green and red where little color is visible. For instance in a spectrogram of the mercury spectrum the yellow lines at 0.578/x appear an orange-red. This defect is evident in a greater or less degree in the foregoing processes, depending upon the spectral character of the colored dyes. However, owing to the fact that colors ordinarily en- countered are far from monochromatic, this deficiency is unimportant and practically negligible in ordinary color photography. This defect is encountered with regret when one desires to reproduce spectra for demonstration purposes. For the latter purpose the methods employing the three colored transparencies about to be described are satisfactory. The fore- going methods are based upon the additive and juxta- positional processes of color-mixture. The processes using the minute color filters shown in Figs. 96, 97, and 98 have a disadvantage in loss of light. For instance, if the process be analyzed it will be seen that a red object will be recorded upon the photo- COLOR PHOTOGRAPHY 221 graph in general in the proportion of one red patch to two black patches. That is, no red light will be transmitted by the minute blue and green filters, so in the final photograph these will appear as black spots. This is a decided disadvantage in the making of colored lantern slides for projection unless an exceedingly powerful arc lamp is available. A num- ber of processes employing subtractive method (#1.8) have been developed. Sanger Shepherd developed a method wherein three differently colored films such as are indicated in Fig. 20 are superposed in a single transparency. F. E. Ives was also a pioneer in this field. The process is identical in principle with the tri-color printing process in use at the present time, with the exception that in the latter case a black- white record is sometimes used with the three color records. Three negatives are made respectively through red, green, and blue filters from which posi- tives are made on special thin transparent films of celluloid coated with gelatine sensitized by immersion in a solution of bichromate of potash. The trans- parencies are each dyed a color complementary to that of the taking filter, the red record being colored blue-green (cyan-blue); the green record, purple (magenta); and the blue record, yellow. These transparencies are free from opaque silver deposit, the gradation being from a maximal transparency to the deepest color of the dye on each film. On super- posing them the natural colors are produced by the subtractive method, as will be readily understood from an inspection of Fig. 20. F. E. Ives devised a method after this principle whereby the three plates were exposed simultaneously with one lens. Shep- herd first employed a repeating plate holder so that the three plates were successively exposed through 222 COLOR AND ITS APPLICATIONS the proper filters. A few years ago a process of producing moving pictures in colors known as Kinema- color was launched. In order to simplify the matter only two colors are used, namely a blue-green and an orange-red. The different colored images are alter- nately thrown on the screen at the usual rate. It is obvious that the use of three colors would render the problem exceedingly complex. Such a two- color method cannot reproduce all colors with fidelity, but the results are quite satisfactory considering the simplification that is obtained. Recently another scheme, employing only two colors, has been devel- oped, known as the Kodachrome process. By means of a repeating back two plates are successively ex- posed through red and green filters respectively. These are developed in the ordinary manner and after being washed they are bleached and fixed, at this stage appearing transparent. They are next given a final washing in a weak aqueous solution of ammonia and dried. Finally the plates are dyed, the one made through the red filter being dyed a bluish-green and the one made through the green filter an orange-red. In general the processes employing dyed trans- parencies superposed yield more brilliant color records, but are obviously more dependent upon the skill of the photographer. In much work the processes employing the juxtapositional method of color-mixture are more satisfactory owing to the simplicity, notwith- standing the less brilliant results. Of the latter methods those employing the ruled screens are some- what more flexible; however, the adjustment of the viewing screens requires some patience. It is thus seen that at the present time the prob- lem of color photography has been solved by rather COLOR PHOTOGRAPHY 223 indirect methods involving color-mixture. Most of the methods will be completely understood on refer- ring to Chapter III. REFERENCES 1. Wiedmann's Ann. 1895, p. 335. 2. Amer. Jour. Sci. 1887, p. 349. 3. Ann. d. Chimie et Phys. 1848, p. 451. 4. Comp. Rend. 114, p. 961; 111, p. 575. 5. Lehrbuch der Photochrome, 1868. 6. Ann. d. Phys. 1899, 69, p. 488. 7. Des Farbenphotographie nach Lippmann's Verfahren, 1898. 8. Die Photographic in naturlichen Farben, 1894. 9. Beitrage zur Theorie und Praxis der director Farben-photo- graphie, 1906. 10. Astrophys. Jour. 1905, 27, p. 325. 11. Jour. Franklin Inst. June, 1906. OTHER REFERENCES Louis Ducos du Hauron, Les Colours en Photographie, 1868. R. Child Bailey, Photography in Colours, 1900. E. Konig, Natural Color Photography, 1906. E. Konig, Beiblatter Ann. d. Phys. 1909, p. 1027. J. A. Starcke, Sci. Amer. Sup. Mar. 9, 1913, p. 158. A. Byk, Phys. Zeit. Nov. 22, 1909, p. 921. G. E. Brown, Photo Miniature, No. 128, 1913. G. L. Johnson, Photography in Colours, 1914. CHAPTER XI COLOR IN LIGHTING 61. Lighting is of great importance, because it is essential to our most important and educative sense vision and color is intimately associated with lighting and vision. Color in lighting is rapidly grow- ing in interest in the science and art of illumination. The recent increase in the luminous efficiency of light sources and the rapid strides in the development of the art of lighting are largely responsible for the growing interest in color and quality of light. Much is yet to be learned regarding the physiological and psychological effects of color, and the laws for its proper use are hazy and not well understood. How- ever, equipped with a full knowledge of the physics of color, an aesthetic taste and a comprehensive view of what is known and unknown regarding the physio- logical and psychological influence of color, a person is capable of utilizing many of the possibilities of color in lighting. The illuminant plays a very im- portant part in the appearance of colors, as has been seen in Chapter VII. The spectral character of the illuminant influences the hue and relative brightness of colors, and the intensity influences the hue and apparent saturation. At low intensities the hue shifts toward the shorter wave-lengths and at high intensities there is an apparent decrease of satura- tion. The distribution of the light affects the appear- ance of colors, owing to the character of these surfaces. All of these factors are of importance in 224 COLOR IN LIGHTING 225 considering the proper illuminant for accurate color work in the dye-rooms of textile and paper mills, in the mixing of pigments for color printing and for painting, for the matching of colors, and in many other places. The spectral character of illuminants is of importance (#37) in the discrimination of fine detail, for it has been seen that monochromatic light is superior in defining power to light of any other spectral character. There are many important problems as yet un- solved which involve color in its application to light- ing. There are practically no data on the influence of color on eye fatigue, although it is known that colors are of influence psychologically. There is a prevalent idea that the kerosene lamp is 'easy on the eyes,' owing to its yellowish color. However, the low intrinsic brightness of the kerosene flame as compared with more modern illuminants is a fact worthy of consideration. When it is further noted that there is no general objection to daylight on account of its color and it is far whiter or more bluish than ordinary illuminants it must be ad- mitted that the virtue of the kerosene lamp based upon its color is on a rather shaky foundation. It is likely that the eye having evolved under daylight is better adapted to it than to any other illuminant and that the nearer an artificial illuminant approaches daylight in spectral character the more likely is it to be satisfactory physiologically. Misuse of common illuminants is perhaps responsible for eye-fatigue to a greater extent than any spectral characteristics. One cannot look directly at the sun and state con- scientiously that daylight is ideal. It has been found that visual acuity is better in monochromatic light than in daylight (#37), and it may appear from this 226 COLOR AND ITS APPLICATIONS that daylight is not ideal. However, these experi- ments were carried out at ordinary intensities con- sidered satisfactory in artificial lighting, and daylight intensities are ordinarily very much greater, which means that, for the discrimination of ordinary details, the intensity is many times the minimal amount re- quired, so that the limit of defining power is seldom reached. For years many have held that the eyes are less fatigued when reading from yellow paper than from white paper. In a biography of Joaquin Miller we read that 'he wrote on yellow paper with a pencil because white paper hurt his eyes.' Bab- bage many years ago strongly advocated the use of yellowish paper in reference books, such as logarithm tables, where the eyes are severely taxed. Javel later advocated the same procedure, claiming that eye-strain was decreased, owing to a decrease in contrast. Many are of the same opinion although, as already stated, quantitative data relating directly to the problem are lacking. After reading from white paper the eyes seem to welcome a change to yellow paper, but this may be due to a decrease in contrast, owing to a lower reflection coefficient of the yellow paper than that of the white paper. However, meas- urements show only a slight difference in the bright- ness of pale yellow copy paper as compared with white, especially under ordinary artificial light. There is no doubt that a yellow or yellow-green light of less extended spectral character than daylight or ordinary artificial light is of superior defining power, due to the reduction of the effects of chromatic aberration in the eye. This fact may partly account for the contention that yellow paper is * easier on the eyes.' It is difficult to focus blue light at a normal reading distance, and impossible to do this at the same time COLOR IN LIGHTING 227 keeping the most luminous rays in focus, therefore, the elimination of the blue rays by means of yellow paper may actually increase the definition. However, reading does not ordinarily involve the discrimination of fine detail, but instead the recognition of groups of characters. Furthermore, the eye is found to pro- gress across a page in a series of jumps, being sta- tionary only a few times per line. It has been found that there is practically no difference in visual acuity when the detail is viewed against a white ground and a ground consisting of yellow copy paper when both receive the same intensity of illumination, that is the same density of light flux. Colored surroundings, such as foliage, brick walls, the wall coverings of the room, etc., alter the spectral character of light before it arrives at the useful plane. Such effects must be considered in any lighting prob- lem requiring a light of a certain spectral quality and are also of importance from the aesthetic viewpoint. Many uses of illuminants of different color and colored media are found in the problems of lighting. 62. The Production of Artificial Daylight. The arts having developed largely under daylight illumi- nation, the daylight appearance of colors is naturally considered as standard. With the production of artificial light man became less dependent upon day- light; nevertheless, owing to the impracticability and perhaps impossibility of a dual criterion of color, there has always been a demand for artificial daylight. The efforts in the production of artificial light have been directed toward the production of light of day- light spectral quality. The principal reason, no doubt, is that such a procedure in our most important method of producing light (by high temperature radiation) at the present time tends toward an ever-increasing 228 COLOR AND ITS APPLICATIONS luminous efficiency. Nevertheless each increment in the steady approach toward daylight has been loudly acclaimed by reason of the better ' color-value ' of the illuminant. However, there is a method which has been applied whereby light of a daylight character can be obtained by excluding from an illuminant con- taining all the rays found in daylight, those portions which are present in excessive amounts. Such a subtractive method is wasteful of light, but is made practicable by the recent increase in the luminous efficiency of illuminants. However, it is well to remember that efficiency in lighting as in any other case is 'the ratio of satisfactoriness to cost and not the reciprocal of the cost.' In order to produce artificial daylight it is neces- sary to determine the spectral character of natural daylight. First it is well to distinguish between sun- light and skylight. The latter is scattered sunlight, but owing to the relatively greater scattering of the rays of short wave-length (#13) skylight is more bluish in color than sunlight. Daylight varies tre- mendously with time and place, although north blue skylight and clear noon sunlight, when unaltered by reflection from immediate surroundings, are fairly constant in color. However, the modification due to selective absorption of the particles in the atmos- phere and selective reflection from foliage, buildings, etc., make daylight rather indefinite in spectral char- acter. E. L. Nichols 1 has published interesting accounts of his investigations on the spectral char- acter of daylight under different conditions of weather, cloudiness, location, and time of day. He found among other things unmistakable evidence of the coloring added to daylight by reflection from green foliage by noting the characteristic absorption spec- COLOR IN LIGHTING 229 trum of chlorophyl (a substance in green foliage) present in observations made on land in the summer time. This effect was absent on the sea. Koettgen, 2 Nichols and Franklin, 3 Crova, 4 Vogel, 5 Ives, 6 and others have studied the spectral character of day- light. The data on noon sunlight and skylight plotted in Fig. 5 is a weighed mean of the results of the foregoing investigators as presented by Ives. The distribution of energy in the visible spectrum of clear noon sunlight as it reaches the earth corre- sponds closely to that of a black body at 5000 deg. absolute (C). A number of investigators, including Dufton and Gardner, 7 Mees, 8 Pirani, Ives, 9 Hussey, 10 and Luckiesh 11 have devised colored screens for producing artificial daylight by altering the light from an artificial source emitting a continuous spectrum. In order to demon- strate the procedure and illustrate the advantage of first choosing a light as close to daylight as possible, the production of daylight screens for two tungsten lamps of different luminous efficiencies as considered by Luckiesh and Cady n will be presented. The visible spectrum of the light from a tungsten lamp being continuous, it has all the rays present that are found in daylight. The difference in their spec- tral characters is due to the difference in the relative amounts of the various rays present. First let us consider the production of light of noon sunlight quality from a vacuum tungsten incandescent lamp operating at 7.9 lumens per watt (1.25 w.p.m.h.c.). It is found sufficiently accurate to consider no rays of shorter wave-length than 0.42/z. An ideal screen for altering the tungsten light to a noon sunlight quality will therefore transmit all the rays of wave-length 0.42^. It will partially absorb rays of longer wave- 230 COLOR AND ITS APPLICATIONS length in increasing proportions from 0.42^ toward the long-wave end of the spectrum. The reduction of the intensity of the rays of various wave-lengths is readily computed from the ratios of the amounts of these rays present in noon sunlight to the amounts of the corresponding rays present in the tungsten light under consideration. The resultant transmis- sion curve of a colored screen for thus altering the Fig. 100. Ideal transmission screens for producing artificial daylight. tungsten light (7.9 lumens per watt) to noon sunlight quality is shown in b, Fig. 100. The ideal transmis- sion curve of a colored screen for producing artificial noon sunlight by means of a nitrogen-filled tungsten lamp operating at 22 lumens per watt (0.5 w.p.m.h.c.) is shown in c. In order to produce artificial north skylight it is seen in Fig. 5 that the visible rays of long wave-length must be reduced by relatively greater amounts than in producing artificial noon sunlight. The ideal transmission curve for producing artificial north skylight by means of the tungsten COLOR IN LIGHTING 231 lamp operating at 7.9 lumens per watt is shown in a. The ideal transmission curve for producing artificial north skylight with the gas-filled tungsten lamp oper- ating at 22 lumens per watt coincides closely with b. That is, a screen which produces artificial noon sun- light with the older type of tungsten lamp operating at 7.9 lumens per watt will produce artificial skylight when used with the gas-filled tungsten lamp operat- 100 0.40 044 0.65 0.72 Fig. 101. Showing the loss of light when using the ideal artificial-daylight screens with the tungsten lamp operating at 7.9 lumens per watt. ing at 22 lumens per watt. This fact has been taken advantage of by the author in developing daylight units. These curves show the increased daylight efficiency of the tungsten lamps operating at higher luminous efficiencies. This is further illustrated in Figs. 101 and 102. In the former E l represents the luminosity curve of the eye for light from a tungsten lamp operating at 7.9 lumens per watt, that is, the relative light values of the rays of various wave- lengths. This curve may be found directly or by mul- 232 COLOR AND ITS APPLICATIONS tiplying the mean luminosity curve of the eye (Fig. 93) for equal amounts of energy of all wave-lengths by the amounts of energy of various wave-lengths in the spectrum of the light under consideration. In this case it is the 7.9 lumens per watt tungsten lamp whose spectral energy distribution is found in Fig. 5. On multiplying curve 1 by the transmission values of curve a, Fig. 100, curve a' is obtained. The 0.65 072. Fig. 102. Showing the loss of light when using the ideal artificial-daylight screens with the tungsten lamp operating at 22 lumens per watt. areas under curve Ei and a' are proportional to total luminous sensations, and the ratio of the area of a' to that of Ei represents the skylight efficiency of the 7.9 lumens per watt tungsten lamp as based upon the foregoing computations. The reduction in lumi- nous intensity when screen b is used with the source under consideration is found on comparing b' with E^ in Fig. 101, and the ratio of the areas represents the sunlight efficiency of the 7.9 lumens per watt tungsten lamp. The corresponding data for screens COLOR IN LIGHTING 233 b and c used with the 22 lumens per watt gas-filled tungsten lamp are shown in Fig. 102, where E 2 repre- sents the luminosity curve of the eye for this tung- sten light. Screen b produces skylight and reduces the luminous intensity an amount represented by the difference between the area of b' and E 2 in Fig. 102. Screen c produces noon sunlight with an efficiency represented by the ratio of the area of c' to that of E. The daylight efficiencies for the two lamps con- sidered in the foregoing were found by determining the relative areas. For the 7.9 lumens per watt tungsten lamp (vacuum type) the noon sunlight efficiency is 14% and the skylight efficiency 4%. However, for the 22 lumens per watt tungsten lamp (nitrogen-filled type) the corresponding values are considerably higher, being 25% and 13% respectively. It has been found in actual practise that the consideration of 0.42/z as the starting point for the computations just described conduces to a higher accuracy than necessary in most cases, therefore beginning with a screen of 100% transmission at 0.45/x the daylight efficiencies are very considerably increased. Under these circumstances for the 7.9 lumens per watt lamp the sunlight and skylight efficiencies are respectively 18% and 9% and for the 22 lumens per watt lamp 33% and 19%. It is thus seen that very accurate artificial noon sunlight can be obtained with an ideal colored trans- mission screen with the 22 lumens per watt lamp at an efficiency of 25% or at 5.5 lumens per watt. This is a higher efficiency than that of the ordinary carbon incandescent lamp operating normally at the present time. Artificial daylight sufficiently accurate for nearly all purposes can be made at a much higher efficiency. The author has developed bulbs for the 234 COLOR AND ITS APPLICATIONS high efficiency tungsten lamp that produce artificial daylight satisfactory for general illuminating pur- poses. Thus the advent of the high efficiency lamps has made artificial daylight available, and now that it is practicable it is surprising how many places are found for it. Besides in the general field of store lighting, artificial daylight is useful for mixing pig- ments, matching artificial teeth and buttons, cigar sorting, medical examination of manifestations of skin diseases, green houses where botany classes study at night, observations of chemical reactions, and for many other operations. The production of colored media for the above purpose requires spectrophotometric apparatus. Mis- takes have been made by using colorimeters or by using merely the eye to judge the color. As has already been seen, the eye is undependable for such purposes, because it is not an analytical instrument for the examination of color. Two lights may appear white to the eye, yet differ considerably in spectral character. For instance, ultramarine blue of a proper density will so alter tungsten light by transmission that a white paper will appear quite the same as under daylight, yet colored objects will appear greatly different. Such a screen is very useful for demon- stration purposes. The distribution of energy in the visible spectrum of a white light produced with an ultramarine filter screening a tungsten lamp operating at 10 lumens per watt as compared with that of noon sunlight, S, is shown in U, Fig. 103. This unit was once seriously proposed as a 'daylight lamp,' but was short-lived for the reason shown. Another white light is shown in curve C, which is produced by the addition of red and blue-green light. It is similar to the ultramarine white light, yet more ex- COLOR IN LIGHTING 235 treme. These three illuminants are called 'white,' because a white object appears the same under all of them; however, a colored object does not. A quartz mercury arc will cause a white paper to appear nearly white, yet its spectral composition is known to consist chiefly of four lines in the visible region. 0.72 Fig. 103. Showing the spectral analyses of two subjective white lights compared with the spectral analysis of noon sunlight. These examples illustrate the importance of spectro- photometric measurements in such problems. Another method of producing daylight is to add to a continuous-spectrum illuminant the correct amounts of certain rays which are not present in sufficient amounts. To most artificial illuminants of this char- acter violet, blue, and blue-green rays must be added. To illustrate the procedure the two tungsten lamps considered previously will be used. In Fig. 104 curve S represents the spectral distribution of energy in 236 COLOR AND ITS APPLICATIONS noon sunlight. Curves A and B represent respec- tively the spectral distributions of energy for the two tungsten lamps operating at 7.9 and 22 lumens per watt. These three curves are plotted with their energy values equal at 0.70/x, a point near the prac- tical limit of visibility for long-wave energy. By subtracting the ordinates of A and B respectively 260 Fig. 104. Showing the additive method of producing artificial daylight. from the ordinates of S and plotting the remainders, curves A' and B' are obtained. These curves are complementary to A and B respectively; that is, the light produced by A when added to the light pro- duced by A' gives the same amount of light and of exactly the same spectral character as the light pro- duced by S, which is assumed to be white light. By multiplying the ordinates of S, A, and B by the light values of energy of corresponding wave-lengths the curves in Pig. 105 are obtained. For example, S is COLOR IN LIGHTING 237 the luminosity curve of the eye for noon sunlight. On integrating these curves the relative areas under S, B y and A are respectively 100, 50, 33. Thus it is seen that equal amounts of light from a nitrogen-filled tungsten lamp operating at 22 lumens per watt and light of such a spectral character as 5', Fig. 104, will produce artificial noon sunlight. However one part 240 220 200 180 UJ 3160 ^140 12120 100 5 80 LJ tt 60 40 20 0.40 \ 044 046 \\ 0.52 0.56 0.60 0.64 07? Fig. 105. Showing the relative amounts of light of the character of A and B (Fig. 104) necessary to produce artificial daylight by addition. of light from a vacuum tungsten lamp operating at 7.9 lumens per watt must be added to two parts of light of the character of A' y Fig. 104, to produce artificial noon sunlight. These data have proved of value in the use of colored lamps with clear lamps for the lighting of paintings and other decorative colored objects. In Table VII the 'per cent white' values obtained by L. A. Jones 12 for various artificial illuminants with a monochromatic colorimeter are presented. His 238 COLOR AND ITS APPLICATIONS values show higher daylight efficiencies for the tung- sten incandescent lamps than obtained by Luckiesh and Cady. 11 The difference may be partly due to a difference in the standards of white light used and in part to the possible fact that the author's computa- tions were made for artificial daylight of too great accuracy. That is, it is possible that the extremely low luminosity of rays at 0.42/z makes it unnecessary to produce a screen that begins to absorb light at that extremely short visible wave-length. The com- putations for screens beginning to absorb rays of longer wave-length than 0.45/z more nearly agree with the data obtained by Jones. It is unfortunate that Jones did not rate his tungsten lamps in lumens per watt, which is more definite because the mean hori- zontal candle-power of a tungsten lamp depends so much upon the manner of mounting the filament. Ives 13 obtained data on the daylight efficiency of illuminants several years ago, but his standard of daylight used at that time does not agree with a standard later arrived at by him by weighing the observations of various investigators, so that m's values are not presented here. 63. Practical Units for Imitating Daylight. Lu- minous efficiency in artificial daylight production is a minor matter in a unit developed for very accurate color-matching. However, there are many cases where light approximating daylight quality is desired for general lighting. Here the wattage is an important consideration, although illuminating engineers and consumers alike must learn that the efficiency of a lighting unit or installation is a measure of how well it fulfills its purpose. This means a broader concept than watts per square foot or effective lumens per watt. If a light source is used for illuminating dress COLOR IN LIGHTING 239 goods, and blues cannot be distinguished from blacks, and greens as seen in daylight are confused with yellow and brown fabrics under the artificial light, then the efficiency of the lighting installation falls close to zero in these particular cases. As illuminat- ing procedure becomes more refined, and as the effi- ciency of light production increases, more attention is being given to the importance of quality of light, which is an important factor in many lighting prob- lems. For these reasons glassware for use with tungsten lamps of high efficiency was developed by the author 11 in 1914 which greatly improves the qual- ity of the light, and does so without such an excessive loss of light as would be impractical for purposes of general lighting. Three phases of daylight have been considered, with the result that three classes of units have been developed. The latest color-matching unit, in which the gas-filled tungsten lamp operating at 22 lumens per watt is used, produces light of a deep blue skylight quality at about 3 lumens per watt. With the multiple lamps of the same type operating at 15 lumens per watt the light corresponds to that of sky- light not quite as blue, and the luminous efficiency is about 2 lumens per watt. This unit is used for the purpose of accurate discrimination of color in textile mills, laboratories, color-printing shops, etc. The colored screen is entirely of glass, and as there is no excessive temperature rise in a well-ventilated unit, the glass is permanent and the unit is entirely safe. The next class of units are intended to imitate clear noon sunlight. This might be considered an average outdoor daylight. There are many cases indoors where the daylight quality is a mixture of sunlight and skylight, and this unit is designed to 240 COLOR AND ITS APPLICATIONS produce a satisfactory artificial sunlight at an effi- ciency of about 7 lumens per watt when multiple tungsten lamps operating at an efficiency of about 16.5 lumens per watt are used. It will be noted that the luminous efficiency at which this artificial sunlight is produced is practically the same as that of the older type of tungsten lamps. Thus sunlight quality is available for general lighting purposes. The applications for such units are to be found in color factories, lithographing plants, wall paper and paint stores, paint shops, cigar factories, art galleries, etc. Other units have been made by combining this colored element with ornamental glassware, by casing with light-density opal, or by mixing the two inti- mately. These units are intended for use in general store lighting, where a better quality of light is often desirable than can be obtained from any practical light source available for general store lighting. Any desired step toward sunlight quality can be produced, the magnitude of the step, of course, depending upon the permissible overall luminous efficiency and the color desired. Notwithstanding the blue or white appearance of daylight, when such a quality of light is produced artificially, there is some objection to its use in stores because of the 'cold' appearance, notwithstanding its necessity for the proper appear- ance of colors. By this means a quality of light better than can be obtained from any unaltered light source for general use is produced at a luminous efficiency sufficiently high to meet with favor. Ob- viously a quality of light approximately midway be- tween that from the new high efficiency tungsten lamps and sunlight can be obtained at a higher efficiency than that of the older types of tungsten COLOR IN LIGHTING 241 lamps. Lamps operating at a higher efficiency emit a whiter light, to begin with, thus giving the gas- filled tungsten lamps a dual advantage over those of the older type for the purpose of artificial daylight production. Recently this glass, with a slight modi- fication, has been incorporated into bulbs for the gas-filled tungsten lamps for the purpose of general lighting. As already stated, any light source having a continuous spectrum, or one nearly so, can be used for the purpose of making artificial daylight. Other desirable characteristics are high luminous efficiency and steadiness of light both as to quality and in- tensity. The arc lamp early entered the field and has been used considerably, although fluctuations in both the color and intensity have been serious draw- backs. A unit developed by Dufton and Gardner 7 in 1900 appears to be the first practical use made of the colored screen for subtractively imitating day- light. Doubtless there have been many more or less approximate reproductions made by others. Many are familiar with the beautiful white light of the Moore carbon-dioxide vacuum-tube lamp. 14 No better approximation of average daylight could be desired; however, at present the luminous effi- ciency of the small units for color-matching purposes is quite low. Certain difficulties have prevented the general adoption of the longer tube, although wher- ever this unit has been used the quality of the light appears to be very satisfactory. In 1909 the mercury arc lamp was combined with the tungsten lamp in proper proportions, with the result that a white light was produced. However, this is only an approximate imitation of daylight, the blue lines of the mercury spectrum supplying the 242 COLOR AND ITS APPLICATIONS blue rays in which the old tungsten lamp was quite deficient. This combination cannot result in a true daylight as considered spectrally, because the spectrum of the mercury arc consists of only a few lines. The addition of the fluorescent reflector to the mercury vapor lamp greatly improved this illuminant by adding red rays, but this is done partially at the expense of green light. (See Figs. 4, 15, and 16.) Early in 1911 Ives and Luckiesh, 9 by means of two commercial glasses and an aniline dye, produced a screen for use with the old tungsten lamp operating at 1.25 w.p.m.h.c. for the purpose of producing 'aver- age daylight,' that is, noon sunlight. Later the two glasses were replaced by a single glass, but a cor- recting aniline dye was still necessary. In 1912 R. B. Hussey 10 described a screen for use with an intensified arc which produced sunlight quality. This was done by means of pieces from two colored glasses arranged in a checkerboard fashion, with suitable diffusing glasses to mix the light. Owing to the unsteadiness of the arc, spectrophotometric measurements were difficult to make, therefore a colorimeter developed by F. E. Ives was used (#28, Fig. 53). It will be noted that colorimeter measure- ments are not sufficiently analytical for the purpose of determining the character of the spectrum of a light source. For instance, this instrument will indi- cate that the quartz mercury arc gives approximately white light, yet this light source emits a line spectrum consisting chiefly, in the visible region, of four spec- tral lines, as shown in Fig. 4. However, the colorim- eter measurements are of interest where the light is known to have an approximately continuous spec- trum. This instrument gives readings in terms of red, green, and blue components, which when mixed COLOR IN LIGHTING 243 produce the same color on a white surface as the illuminant under examination. In Table XVII are TABLE XVII Colorimeter Measurements on Units for improving the Spectral Quality of Artificial Light toward Daylight Source Colorimeter reading Red Green Blue Average daylight (noonday sunlight) North blue skylight Hussey daylight arc lamp Intensified arc lamp (bare) Ives and Luckiesh (artificial daylight) Tungsten 1.25 w. p. m. h. c. (7.9 lumens per watt) Tungsten 0.65 w. p. m. h. c. (16.4 lumens per watt) . . . Tungsten 0.50 w. p. m. h. c. (22 lumens per watt) Tungsten 1.25 w. p. m. h. c. in tinted reflector Tungsten 0.65 w. p. m. h. c. in tinted reflector New color matching unit (with 0.65 w. p. m. h. c. tungsten lamp) Artificial sunlight units (with 0.7 w. p. m. h. c. tungsten lamp) 100 78 93 147 100 183 164 157 145 120 80 110 100 82 111 102 93 96 102 103 103 102 84 103 100 138 96 51 107 21 34 40 52 78 136 87 shown the results obtained with this instrument on Hussey's daylight arc and other data of interest comparable only in a rough manner. The daylight arc examined was a near approach to daylight as far as colorimeter measurements can be trusted, although it shows an excessive greenish component. This could be easily remedied. Sharp and Millar, 15 in 1912, by means of colored screens and tungsten lamps, also produced a daylight effect. About this time several units, designed to produce artificial daylight, appeared, but no examina- tion of these has been made and no quantitative data are to be found regarding them. The author 16 has successfully used colored lamps combined with clear tungsten lamps by the additive method, as illustrated in Fig. 104. Blue, green, and 244 COLOR AND ITS APPLICATIONS blue-green lamps were used with success for pro- ducing daylight effects in combination with clear tungsten lamps. A notable installation was the light- ing of the paintings at a large temporary art exhibit in 1913, where more than 400 colored lamps were used. This is perhaps the first large exhibition of paintings where any attempt has been made to produce a day- light appearance by means of artificial light. In order to produce a practical method for obtaining a light of better color value for lighting paintings and other colored objects, many experiments have been made, 11 with the result that, besides the glassware already described, metal reflectors have been used having a tinted surface of such a character as to alter the reflected light to a color complementary to the direct light from the tungsten lamp. Obviously this method results in altering the distribution curve of the reflector, producing in general a less concentrated distribution. This indicates that focusing and inten- sive reflectors of this character should be used instead of those of extensive type. The results obtained with tinted reflectors show that a very good quality of light is obtained at a loss of about 50 per cent of the original useful light. With coatings of less depth of color the loss of light is less, but the improvement in quality is also less. By changing the shape of the reflector the amount of the altered light can be varied within wide limits. For lighting mural paintings, for instance, the reflectors proved satisfactory. No attempt has been made to reproduce skylight or even sunlight, but a very desirable in- crease in blue and blue-green rays has been obtained, as shown in Table XVII. The same scheme has been applied to the prismatic glass reflector, a glass coating being applied in this case. COLOR IN LIGHTING 245 In 1914 Ives and Brady 9 produced a glass for accurate color-matching for use with the Welsbach gas lamp or the tungsten lamp. Other units have been developed more or less approximating daylight, but some have not fulfilled the claims made for them. There appear to be two fields for artificial daylight units: one where accurate discrimination of colors requires a correct repro- duction of skylight, and another field where coarser color work is done, such as in the paint shops and lithographing plants. Light approximating sunlight quality has been found to fill the requirements in the latter field. The lighting of paintings is treated in Chapter XIII and other colored lighting effects in Chapter XII. 65. Effects of Colored Surroundings. The color value of illuminants has been a subject of consider- able discussion and investigation during recent years. Most of the work has been done with colorimeters, which, owing to their limited power of analysis, furnish data which are likewise limited. However, the light that reaches the object is ultimately of greater im- portance in lighting. This can be greatly altered by selective reflection from surrounding colored objects, but the effect has been a much neglected phase of lighting. G. S. Merrill 17 measured the color value of daylight on the working plane in a room after some of the light had been reflected from the colored sur- roundings. The interior measurements were made on clear and cloudy days. They showed consid- erable alteration in the color of outdoor daylight. The author 18 made a study of this factor in a minia- ture room lighted by means of a tungsten incan- descent lamp operating at 7.9 lumens per watt, green, yellow, and white wall papers in various combinations 246 COLOR AND ITS APPLICATIONS on the walls and ceiling and direct and indirect lighting systems having been used. In order to illustrate the possible color change in light due to reflection from a colored surface, it is possible to take an actual case and utilize spectro- photometric data, but for simplicity we will take a hypothetical case. Assume a light source radiating equal amounts of monochromatic red, green, and blue light, and that this source is placed at the center of a hollow sphere the walls of which are covered with a perfectly diffusing green paper. The colorimetric analysis of the illuminant may be expressed as RGB 100 100 100 The reflection coefficients of this paper for the particular illuminant are assumed to be in per cent, RGB 25.2 47.2 27.6 The light received by the green paper in the sphere is reflected an infinite number of times. If the walls of the sphere are temporarily assumed to be white and if N is the reflection coefficient of the paper, then the total light falling on the walls will be Q = Q' + NQ' + WQ' + N*Q' + . . . =^-x (1) where Q = total light falling on the walls and Q' = direct light from the light source falling on the walls. The color of a paper is generally determined by measuring the color of the light after it has been reflected once from the paper. It is seen that a total reflection coefficient of 33^% has been assumed for the green paper for this particular illuminant. The coefficient of reflection may vary within wide limits COLOR IN LIGHTING 247 without any change in the color values. Based on the foregoing assumptions the reflection coefficient of this paper for the monochromatic red light is 25.2% of the original 100 units; 47.2% of the total 100 units of green light; 27.6% of the total 100 units of blue light. For this case the total red, green, and blue components in the light incident on the wall paper after an infinite number of reflections will be respectively, CR = Q'v + N R Q' R + N&'* + N&'* + . . . = -- (2) Q G = Q' G + N G Q' G + WG + N G Q' G + . . . = j- (3) CB - Q'* + N B Q' B + N&' B + N*Q' B + '...- j-^; (4) and Q = PR + QG + QB = total light on walls (5) Q' = Q' R + Q' G + Q' B = total direct light on walls (6) W R , N GJ N B are respectively the reflection coefficients for the monochromatic red, green, and blue compo- nents of the original illuminant. N*Q f R> ^G^G'J NB(?'B are the color values of the wall paper as determined by a tri-color method of colorimetry under the light, Q'. Computations yield the results given in Table XVIII. On plotting these percentages (shown in the column on the right) in a color triangle, it is shown graphically as indicated in the table that the reflected light rapidly approaches pure green by successive re- flection, but of course the intensity rapidly diminishes* as is shown in Fig. 106. It is also instructive to plot the logarithm of the intensity against the number of the reflection which gives a straight line. All three 248 COLOR AND ITS APPLICATIONS TABLE XVIII Computations According to Equations (2), (3), and (4), showing the Changes produced in the Light from a Special Source by Successive Reflections from a Green Paper The terms in Equations (2), (3) and (4) Actual values Percentages R G B R G B Q' 100.00 100.00 100.00 33.3 33.3 33.3 NQ' 25.20 47.20 27.60 25.2 47.2 27.6 N 2 Q' 6.35 22.30 7.62 17.5 61.5 21.0 N 3 Q' 1.60 10.45 2.10 11.3 73.9 14.8 N 4 Q' 0.40 4.93 0.58 6.8 83.4 4.8 N 5 Q' 0.10 2.33 0.16 3.8 90.0 6.2 N 6 Q' 0.03 1.10 0.04 2.6 94.0 3.4 N 7 Q' 0.52 0.01 N 8 Q' 0.25 N 9 Q' 0.12 components decrease rapidly in intensity with the number of reflections, but the green component does I Z 3 4 5 6 7 NUMBER OF. REFLECTION Fig. 106. Illustrating the effect of multiple selective reflections of light from a green fabric. not decrease as rapidly as the others. In Fig. 107 are shown the relative values of the three components after various successive reflections. It will be noted COLOR IN LIGHTING 249 that the color of the light approaches saturated green as the number of reflections is increased. In the original paper various computations were made which relate to conditions of so-called indirect and direct lighting which will not be presented here. However, these indicate, as is shown by actual measurements described below, that the color of the walls and ceiling alter the color of the light in so-called indirect systems very much. RELATIVE PERCENTAGE OF COM FOMENTS r\)(>l-No-i-JCPc0C OOOOOOOOOOC ^ ^ ^^ <*- ^ x" X X / ^^ =^.^ ^^^^ -^3 ^ ^R ^=^ := ) 1 3456 MUMBER OF REFLECTIONS Fig. 107. Showing the relative proportions of red, green and blue components in the reflected light from a green fabric after various successive reflections. Actual measurements were made in a miniature room illuminated by tungsten light (7.9 lumens per watt, 1.25 w.p.m.h.c.) of the color of the total light reaching the working plane. The room was four feet square and four feet high and the floor was assumed to be the working plane. The results are presented in Table XIX reduced so that the colorimeter read- ings for the tungsten lamp used in the investigation equal 100 for each of the three components. This course is considered legitimate inasmuch as only 250 COLOR AND ITS APPLICATIONS TABLE XIX Colorimeter Measurements in a Miniature Room under Various Conditions of Surroundings Red Green Blue 1. Tungsten lamp, 1.25 w. p. m. h. c. (7.9 lumens per watt) 100 100 100 2. Tungsten lamp, 0.65 w. p. m. h. c. (17 lumens per watt) 78 96 126 3. Carbon lamp, 3.1 w. p. m. h. c 116 104 80 4. Carbon lamp, 4.0 w. p. m. h. c. 129 101 70 5. Color of dull yellow wall paper 6. Color of dull green wall paper 131 104 115 119 54 77 (Results with tungsten lamp, 7.9 lumens per watt) 7. Yellow walls and yellow ceiling, indirect 8. Yellow walls and yellow ceiling, direct 159 143 111 107 30 50 9. Yellow walls and white ceiling, indirect 130 107 63 10. Yellow walls and white ceiling, direct 111 106 83 11. Green walls and green ceiling, indirect 12. Green walls and green ceiling, direct 108 109 139 113 53 78 13. Green walls and yellow ceiling, indirect 14. Green walls and yellow ceiling, direct 15. Green walls and white ceiling, indirect . . 145 119 110 128 119 102 27 62 88 16. Green walls and white ceiling, direct 106 104 90 Reduced Colorimeter Readings the relative magnitudes of the alterations on color are desired. For the sake of comparison the color- imeter readings in the same scale for other incan- descent lamps are presented. - It is seen .'that .-ordinary wall paper of dull yellow -xolor /may .-alter /the : color of tungsten light so that the useful light is more yellow than the old carbon incandescent lamps. This is a factor too often neglected, and there are cases where lighting experts have striven to improve the color of artificial light by partially correcting glass- ware, yet this light was permitted to be largely re- flected from yellowish walls and ceiling. In stores and other interiors where attempts are made to cor- rect the artificial light the surroundings should be COLOR IN LIGHTING 251 of a neutral shade or of a slightly bluish tint if this is compatible with the color scheme of decoration. Many possibilities arise where the tinting of light by reflection can be utilized, for, as is seen by the fore- going, the effect can be of considerable magnitude. 65. Color in Interiors. This subject is largely of interest to the decorator, and inasmuch as this book is chiefly confined to the science of color, the aesthetic side of color will not be considered except- ing in so far as lighting aids the decorator. Some first principles of interior decoration, however, may not be out of place here. A room has been likened to a painting: the floor representing the foreground; the walls, the middle distance; and the ceiling, the sky. A ceiling may be lowered apparently by treating the walls horizontally, that is by finishing the lower por-* tions of the walls a dark shade and the next section a lighter shade to within two or three feet from the ceiling and permitting the ceiling finish to extend down the walls. Some decorators insist that color has much to do with the apparent size of a room, the lighter tints seemingly enlarging the room. The color of a room creates its atmosphere. No single color can produce the best effect any more than one note can produce a melody in music. It is the artistic variation in values and tints that satisfies the eye. The principles of masses, spaces, and con- trasts, as well as sequences in hue and brightness, play their part in harmonies of color. The law of appro- priateness is as important here as in other fields, yet color and brightness are largely matters of individual taste, thus limiting the artist in formulating rules which at best are not thoroughly understood. North rooms, or those shielded from direct sun- light, are in general more satisfactory when colored 252 COLOR AND ITS APPLICATIONS in rose, cream, yellow, buff the 'warm' colors. Yellowish tints in the window curtains aid in giving the effect of sunshine. On the sunny side, rooms will perhaps be more satisfactory when colored pale blue, gray-green, or shades and tints of other 'cool' colors. In introducing color into the illuminant by means of colored shades or lamps the color scheme of the room should be considered. Apparently many prefer bright red wall coverings, if one may draw conclusions from observations. This again is a matter of personal taste, but extremely pure and bright colors in lighting effects in interiors are to the author like living with a brass band. Many of the lighting effects in pure colors certainly arise from a lack of study of the use and influence of color. If a room is decorated for natural lighting, theoretically it should receive the same artificial lighting both as to direction and spec- tral character. Yet the change in the lighting - from natural to artificial may be just the thing to relieve monotony. There are many statements on this subject that cannot be reconciled with the facts. For instance, a person may be satisfied with daylight, living under it from day to day without any other comment than that it is ideal. The same person, however, may object to the increasing 'whiteness' of modern artificial illuminants. He insists that we must go back to the color of the carbon incandescent lamp, or even further to that of the candle flame. Is there a dual standard? Can daylight be satisfactory and the light of the tungsten lamp or Welsbach mantle be too 'white'? As a matter of fact all modern illu- minants used in ordinary interiors the gas and incandescent filament lamps are in the same class and far yellower than daylight that enters interiors. Color is certainly the keynote of lighting in many COLOR IN LIGHTING 253 interiors, but let us not base its use upon incorrect premises. If we prefer * warmer' colors in our arti- ficial illuminants, let us have them, but let us attribute this desire to the proper cause, which may be a love for change in color. Slight tints of rose and yellow may add something pleasing to the complexion, but deep yellow, orange, or red have an obliterating effect upon the flesh tints of the face. They also tend to make colors appear further from their daylight appearance than untinted artificial lights. Using color for color's sake is a legitimate procedure, and in the absence of sufficient physiological and psychological data the use of color must remain, for the present, largely a matter of taste. In lighting it is well to bear in mind the effect of surroundings in coloring the useful light. Let us take a particular case the use of amber glass with the tungsten lamp for aesthetic purposes. A combination fixture had an * indirect' bowl from which hung some direct units with yellow silk shades. The indirect light first passed through an amber glass, then after various reflections from ceilings and walls reached the useful plane. Inasmuch as the majority of living rooms have wall coverings tending toward the yellow, brown, buff, or so-called 'warm' colors, the indirect component is likely to* be considerably altered toward yellow in one of these rooms without the use of amber glass. If the wall coverings are of a 'colder' tint why are they satisfactory under daylight and not under the far yellower artificial light? The result obtained with the amber glass would have been obtained without it by the use of a more yellow- ish wall and ceiling coverings. The color of the surroundings depends upon the spectral character of the illuminant. A yellowish paper may appear the 254 COLOR AND ITS APPLICATIONS same under a deep yellow light as a yellower paper under a pale yellow light. The object of these re- marks is to illustrate that there is some scientific or physical basis for discussing any alteration of the color of artificial light tending away from daylight in color. Inasmuch as amber glass is often used as in the foregoing, it is of interest to analyze it. The author 100 90 80 TO 60 |40 530 C H 20 10 040 044 0.46 Q5Z 056 0.60 0.64 0.68 Fig. 108. Screen for altering tungsten light to the same spectral character as carbon incandescent electric light, c, d, e show the transmission curves of amber glasses of different densities. has considered it unsatisfactory for the above pur- pose because of its greenish tinge, and has therefore sought for a yellowish glass or dye without this greenish tint. Inasmuch as amber glass is usually used for the purpose of altering the present illumi- nants to a color approximating the yellow light of the carbon filament lamp, kerosene or candle flame, let us take the case of altering the light of a tungsten lamp operating at 7.9 lumens per watt to the color of the old carbon lamp operating at 4 w.p.m.h.c. This COLOR IN LIGHTING 255 can be done at a loss of not more than 20% of the total light; that is, the tungsten lamp operating at 1.25 w.p.m.h.c. will, with a yellow screen, produce light closely approaching that of the above carbon lamp at about 1.5 w.p.m.h.c. Thus light similar in color to carbon incandescent lamp light can be ob- tained at a high efficiency with the tungsten lamp. Curve a, Fig. 108, represents the transmission curve of an ideal screen for altering the tungsten 120 1 10 o 100 CO CO 90 CO 1 80 h- uj 70 > 60 5j <* 50 40 ArYl Yier Q y$ A -**. ^ ^^^ / 'deal (a. i / ^ *^ ^ / , / / L f \ / & Y y / 040 044 045 0.5Z 0.56 0.60 0.64 0.68 Fig. 109. Comparison of ideal screen a, Fig. 108, with amber glass. light to a spectral character close to that of the old inefficient carbon lamp. Curve c is the transmis- sion curve of a light-density sample of amber glass. Curves d and e are the transmission curves of thicker specimens of the same amber glass. Curve d is the result of reducing curve a at all points by 8%. It is seen that the amber glass is far from ideal, there being an excessive transmission of green light com- pared with the ideal curve a (or b). As the density or thickness of the amber glass is increased the trans- mission of green rays decreases relatively more than 256 COLOR AND ITS APPLICATIONS the yellow, orange, and red rays; that is, the dominant hue shifts toward red, which is apparent by casual observation. In Fig. 109 the transmission curve of the ideal yellow glass for the above purpose is plotted as a straight dashed line, and the transmissions of light and dark amber glasses relative to those of the ideal screen are plotted as shown. It is seen here, expressed analytically, what careful observation indi- cates to be true: that amber glass is far from ideal for altering modern illuminants to a color similar to that of the older illuminants which many claim to be the more aesthetic. By means of present day tungsten lamps used with a proper colored screen the light of the kerosene flame can be closely imitated at efficiencies from 5 to 10 lumens per watt depending upon the efficiency of the unaltered source employed. The light of the old carbon incandescent lamp can be imitated in the same manner at efficiencies from 7 to 13 lumens per watt. The author has treated this subject elsewhere. 24 Screens for this purpose are readily made by the use of dyes, although they will in general lack per- manency. Most yellow dyes are objectionable for the above purpose for the same reason as amber glass. Potassium bichromate is, under moderate conditions, a permanent yellow. To this may be added a pink dye, which will usually yield a com- bination which is satisfactorily yellow. For the pink some very dilute red dyes may be used. Rhodamine is satisfactory in color, but is very fugitive under the influence of light and heat. Many yellow dyes are quite permanent if used on a sheet of glass instead of directly on the lamp bulb, but usually these must be corrected as already indicated. Much pleasure can be derived from the use of COLOR IN LIGHTING 257 tinted illuminants, for they lend themselves to deco- rative effects and afford an easy means for eliminat- ing monotony in lighting. Two- or three-circuit pendant units (preferably indirect or semi-indirect) are convenient for this purpose, for by using clear and colored lamps various combinations of color and in- tensity can be obtained which are very pleasing. Silk shades of various tints are readily applied to lamps, and colored gelatines are easily concealed and afford a ready means of obtaining pleasing colors. (Colored media are discussed in the last chapter.) Very brief descriptions of a few uses of colored light in interiors may aid in showing the possibilities of such application of the art and science of color. There are many indications that we are at the beginning of an age of color appreciation. It has already as- serted itself in modern painting; and the gorgeous display of color that greets the visitor to such mag- nificent architectural structures as the Congressional Library in Washington and the Allegheny County Soldiers' Memorial at Pittsburgh indicates that this century is likely to witness a renaissance in the use of colors in decoration. Color was the keynote in the plans of the Panama-Pacific Exposition 19 much of which is obtained by lighting effects. Col- ored jewels reflecting millions of images of light sources, colored flames, moving color filters, and lights of various colors were woven into a gorgeous spectacle. W. A. D'a Ryan, who planned the color effects, has also used the ' scintillator ' with con- siderable success. Powerful searchlights arranged to point radially upward illuminate clouds of steam in various colors. The beams diverge from each other in a fan-like manner. The possibilities in spectacular lighting are manifold. 258 COLOR AND ITS APPLICATIONS A notable use of colored illuminants is found in the Allegheny County Soldiers' Memorial. In this splendid lighting installation, which was designed by Bassett Jones, 20 mercury arc lamps, tungsten incan- descent lamps, Moore tubes, and yellow flaming arcs were used. The ceiling of the auditorium, which is sixty feet above the floor, is composed largely of glass in decorative panels. The central panel is outlined by means of the pinkish light of the nitrogen tube. Over the corner panels yellow flame arcs are hung, and their flicker adds charm to the colored ceiling which would not be present with perfectly steady light sources. The outer panels are lighted by the bluish light of mercury arc lamps, and tungsten lamps stud the ceiling, adding a touch of brilliancy. The contrasting of colors is so harmoniously accomplished that the result is exceedingly artistic. Thus the beauty of this monument of decorative art is visible at night as well as by daylight, which is too often not the case. There are many other interesting applica- tions of color which make this beautiful work of art a worthy mecca for those interested in color and lighting. Art galleries offer excellent opportunities for in- troducing the science of color lighting. As already mentioned, more than four hundred colored tungsten lamps were used with clear tungsten lamps in cor- recting the lighting of a temporary art exhibit. The results were extremely encouraging, inasmuch as they met with the approval of artists and critics alike. This was perhaps the first notable attempt ever made to furnish illumination of a daylight quality for light- ing paintings. This field offers a splendid oppor- tunity for development, which can readily be done by means of the color-correcting lamps and acces- sories now available. COLOR IN LIGHTING Many artistic effects can be obtained by the use of colored light in the home. A slight rose or orange tint in the light is very pleasing and attractive, although the choice of tints is of course a matter of taste. A rather interesting case is found in a dining room of a pretentious residence. A large oval panel of dif- fusing glass is set into the ceiling, and behind this a great many red, green, and blue lamps of low voltage are placed in the approximate proportions of two red, three green, and five blue lamps. The lamps of dif- ferent colors are controlled by means of dimmers set in the wall, so that by varying the proportions of red, green, and blue light various qualities of light may be obtained and also a large range of intensities. A person who enjoys color can readily devise many simple schemes for obtaining tinted light. An experiment which the author found of interest was the production of an artificial moonlight effect. A high decorative window in the living room was removed and placed in the normal position of the storm sash, thus providing space for tubular lamps in reflectors. The window was covered on the inside with a cardboard of bluish-green tint and in the open- ing before the window, a stained wooden lattice was placed, over which an artificial rambler rose was twined. The lamps, which were tinted a light blue- green, illuminated the bluish-green cardboard, which as viewed through the foliage produced a charming effect of moonlight. As the space was narrow the cardboard was not uniformly bright, owing to the proximity of the lamp, but this defect was readily overcome by stippling the surface with a black water- color. Such effects are readily applied to bay win- dows and other convenient places. Many possibilities present themselves to those 260 COLOR AND ITS APPLICATIONS interested in color lighting. The many colored media available and the diversity of the color of commercial illuminants provide the means for carrying out many ideas. Electrically excited gases, such as carbon dioxide, neon, helium and mercury vapor, contained in glass tubes, are commercial possibilities which have not yet been applied to the fullest advantage for elaborate colored effects. In the average case, how- ever, requirements are readily fulfilled by means of ordinary light sources and colored media. 66. Color Preference. It may be of interest here to record the results of some simple experiments, in- asmuch as such data may indicate eventually the effect of the illuminant upon our preference for certain colors and may throw some light upon the relation of lighting to the pleasing effect of colors. The ex- periments represent the beginning of an investiga- tion begun with an object in view which is discussed in Chapter XV but are described here as a matter of interest. The Zimmerman colored papers were used, but as there was no saturated green paper one was dyed and placed in the series. This is designated as g, the other letters indicating the catalogue desig- nation of the various colored papers. Fifteen col- ored papers, each four inches square, were spread out haphazardly upon a white surface, the individual papers being from six to ten inches apart. The ob- server was asked to study the colors and pick them out in the order of his preference. He was asked to isolate the individual colors from everything as far as possible, choosing the color for color's sake alone. In other words, if possible he was not to associate the colors with wearing apparel or anything else. The experiments were carried out under ordinary tungsten light (7.9 lumens per watt) and also under daylight COLOR IN LIGHTING 261 entering the window, in the latter case no direct sun- light being present. The intensity of illumination in each case was of such value as would be considered sufficiently high for viewing saturated colors. The two observations were carried out at least a week apart and usually several weeks intervened. The general consistency of the preference orders of the V abcdefg h kiqnopm DESIGNATION OF COLORED PAPERS Fig. 110. Showing the preference or rank of a number of fairly saturated colors. fifteen observers was somewhat surprising. The mean results are plotted in Fig. 110, the ordinates representing color preference. There may be some question regarding the legitimacy of the definition of color preference, but the procedure adopted here pro- vides a simple means of plotting the data. There being fifteen colored papers, the least preferred would be placed last and ranked fifteen, the highest 262 COLOR AND ITS APPLICATIONS preference therefore being unity. It is seen that the least preferred colors were those of highest lumi- nosity and in general of lowest saturation. That is, purples and highly saturated colors having hues cor- responding to the regions near the ends of the visible spectrum, namely blue and red, were definitely fa- vored. This confirms a conclusion previously arrived at from other observations. According to E. B. Titchener 21 there are two types of observers: one type prefers the saturated colors and the other definitely prefers unsaturated or 'ar- tistic' colors, but the former type constitute a majority. The author's observations indicate that, when colors are chosen for 'color's sake' alone, the saturated colors are almost invariably chosen. E. J. G. Brad- ford, 22 in experimenting with twenty-six university students with a set of fifteen papers each about 30 inches square, found that saturated colors were most preferred. He also found that the admixture of a small proportion of another color lowered the posi- tion of the color in the preference order. Cohn 23 has also contended that increase of saturation tended to make a color more pleasing. Bradford found that the order of preference remained reasonably con- stant by performing the same experiments on three observers after an interval of two weeks and again after a lapse of twelve months. The subject of color preference will be treated further in Chapter XV, but it may be of interest here to compare the results obtained by Bradford with those obtained by the author. In the latter's experiments nearly all the colors were as saturated as possible, while only the first eight of Bradford's were 'pure.' Bradford does not state the character of the illuminant used, but presumably it was daylight, so the daylight preference COLOR IN LIGHTING 263 order taken from Fig. 110 is used for comparison in Table XX. TABLE XX Color Preference Rank Bradford Luckiesh 1. Dark blue Dark blue 2. Saturated green Blue 3. Chocolate-brown Red-purple 4. Pale blue Green 5. Slate blue gray Violet-purple 6. Saturated crimson Deep red 7. Pale green Orange-red 8. Coffee-brown Crimson 9. Bluish green Dull yellow-green 10. Ink-red Orange 11. Cinnamon-brown Orange-yellow 12. Pale pinkish brown Dull green 13. Bluish green Slate blue gray 14. Pink Yellow 15. Yellowish Green Lemon-yellow A word of caution is necessary regarding drawing conclusions from Table XX. The colors are de- scribed so indefinitely and the two series of colors differed very much. In one series practically all colors were as saturated as it is possible to obtain them by means of pigments, but in the other series about half of the colors were tints and shades. For instance, in the latter series chocolate-brown is a saturated red of a dark shade. Furthermore, as seen by Fig. 110, the reds ranked fairly high, but in placing them in order, as in Table XX, they are near the middle of the list because several colors ranked just above them. Notwithstanding the foregoing there is a similarity in the two preference orders. Fig. 110 serves as an indication of the similarity of the preference order of the various observers. For 264 COLOR AND ITS APPLICATIONS instance, there being 15 colors if every observer placed h (lemon-yellow) last, its rank Would be 15. The mean rank for h was nearly 14, indicating that nearly all the observers placed it last. Dark blue was placed first by most of the observers. As far as the limited results indicate, there was no general difference in the preference orders under the tungsten light and daylight, excepting under the former illuminant the reds were definitely placed higher in the preference order than in daylight. This has seemed apparent from previous observations as well as the indication that of a series of saturated colors the most saturated are usually the most preferred. There is some indication from other experiments that the relatively few who prefer tints instead of saturated colors, when asked to choose the colors for color's sake alone, are those that are unable to overcome the tendency to associate the colors with other things. It is just this associational preference order that is of more interest in this chapter. That is, in lighting there is no doubt that tints are more proper or more aesthetic. The data which is dis- cussed in Chapter XV from the viewpoint for which they were obtained are inserted here merely to illustrate some points in the matter of color preference. The data on this subject are rare and the danger of draw- ing definite conclusions at the present time is clearly recognized. Observation during the past few years has led the author to conclude that in the matter of color preference for color's sake alone, the colors near the ends of the spectrum and the purple series are in general favored. Artificial illuminants are usually poverty-stricken in blue and violet rays. Therefore these colors can probably be made to app.ear more COLOR IN LIGHTING 265 attractive by means of an illuminant having more blue and violet rays and less red and orange rays than ordinary artificial light. Strictly, the artificial day- light already described is in general the correct artificial illuminant, but experiments indicate that, in the illumination of colors for pure decoration, a c white ' light in which violet and red rays predominate pro- duces very pleasing results. A glass of this char- acter was made of a proper density so that white objects had the same white appearance as under natural skylight, yet such color as the pinks, purples, blues, violets, deep reds, appeared richer. Inasmuch as in the decorative use of color, exactness in hue is not usually essential, and as the color is employed for our pleasure, it is legitimate to use the illuminant that produces the most pleasing result. It is well to have white objects appear white, yet if those colors which please us most can be made more pleasing by the use of ' white ' light of such a spectral character as described above, it is within the province of the lighting expert to use such a light. Cobalt-blue glass, in the absence of a specially made glass, will produce these results fairly well if chosen of proper density. An ultramarine blue screen used with ordi- nary artificial light will produce an extreme * white' light of this character. Prussian blue added to it forms a satisfactory screen for this purpose. In prescribing such an illuminant one is not committed to the opinion that the pigments used in ordinary decoration are not 'rich' enough to begin with. Such handicaps are not uncommon in many of the arts employing color, and furthermore colored decorations are often dimmed by exposure. In any event the matter is one that will be governed largely by taste and the adoption of such a lighting procedure as 266 COLOR AND ITS APPLICATIONS indicated above is legitimate if it pleases those concerned. The foregoing experiments are not described here with the intention of suggesting that saturated colors should be used in lighting. They could not be used without endangering the appearance of many colors. These various comments have been made with the object of suggesting fields for thought and experi- menting. Of course it is realized that the matter of color preference is exceedingly complicated by all the phenomena of color vision and environment, yet the foregoing experiments are instructive if the limi- tations of the results are recognized. 67. A Demonstration Booth. The most effect- ive manner of studying and demonstrating lighting effects is found in the use of a booth specially de- signed for the purpose. Having employed such a booth for several years very successfully it appears of interest to describe one in detail. A number of different types have been constructed, but the one described here has been most successful. In Fig. Ill is shown the wiring diagram covering the prin- cipal features. The dotted line enclosing a rec- tangular space represents the front dimensions of the booth, the center being represented by the mal- tese cross. The lamps represented by the larger circles are placed in their relative positions. Fourteen clear 40-watt tungsten lamps indicated by numbers were spaced as shown around the inside of the box near the front side, thus providing light from various directions. These are controlled by a contact arm arranged to rotate. The control apparatus is dia- grammatically shown at the right, spread out for con- venience. These switches are actually placed in a small recess in the right end of the box, as shown in COLOR IN LIGHTING 267 Fig. 112. Twelve snap switches are shown above the rotating contactor, of which the upper six control clear lamps as indicated by the numbers. The middle switches Si and S 2 in the upper two rows control, respectively, the four lamps on the left and right. 268 COLOR AND ITS APPLICATIONS These must be special switches and the wiring con- nections have been omitted for the sake of simplicity. The clear lamps are very useful in demonstrating effects of light and shade and for showing the effect of diluting colors, or decreasing their saturation, for which purpose a variable resistance is placed in series with the rotating contactor. Two single-pole double- throw switches are shown at the left of the rotating o f > f\ r~* X \.s G o O- , G i p .1 3 M " , , r ' 1 1 1 T N. > \ 1 \ * it n ii ii H H n M "<* \ s 1 *i 1 > j; ll > \ , \ \ V O io q 1 f\ w G O G O O O O O V.X o % Fig. 112. Showing dimensions and locations of lamps in the demonstration booth. contactor switch, which provide for quickly changing the lighting from above to below or from the left to the right side. A motor Af, controlled by switch M r , is placed on an extension at the back of the booth, so that its elongated shaft can be projected through the back side in the center. (A motor operating on direct or alternating current and capable of rotation at a very high speed is desirable.) On this rotating shaft such experiments as those indicated in Figs. 14, 23, 29, 30, 31 are readily performed. General lighting of many colors can be obtained COLOR IN LIGHTING 269 on the objects placed at the center of the back from the rows of colored lamps eight above and eight below by controlling the relative intensities of the red, green, and blue lights by the corresponding rheostats indicated at the right. The switches con- trolling these lights are /?, G, and B in the bottom row of the twelve snap-switches at the right. The purity of the primary colors is of great im- portance. These can readily be made satisfactory by any one acquainted with coloring materials and the science of color-mixture. Perhaps the easiest procedure in most cases is to begin with a set of lamp colorings. Gelatine filters made as described in Chapter XVI may also be used; however, for demon- stration purposes colored lamps provide a more com- pact apparatus, although in the latter case constant care of the lamps is necessary, owing to the fading of the colorings due to heat and light. By using the rheostats the mixing of colored lights can be done on a white diffusing surface hung on the back of the booth at the center, which provides a very satis- factory means for the synthesis of colors. The effects of quality of light on colored objects can be readily demonstrated, and daylight effects can be easily shown by adding blue-green light to the clear tung- sten light. In fact practically any demonstration involving color-mixture is possible with such an apparatus. ' Single red (#')> green (GO, and blue (B') lamps controlled by corresponding switches on the right, are placed as shown at the angles of an equilateral tri- angle, the green lamp being placed at the upper apex. Interesting colored shadow demonstrations are easily shown, the shadow experiment illustrated in Fig. 28 showing the primaries, complementaries and 270 COLOR AND ITS APPLICATIONS white light having been developed for use with these lamps. Many of the effects described in this book, especially those in Chapter XII, have been developed in the booth just considered. Other electric circuits are also used, but these will readily occur to the experimenter. Two views of the booth are shown in Fig. 112 with dimensions. Those interested in such a field will find the use of such a booth exceedingly interesting and instructive. A number of booths, perhaps not as compact and universal, have been used by pioneers in the study of color effects. Sev- eral years ago Basset Jones, a pioneer in the art of lighting, employed such an apparatus in very inter- esting demonstrations. REFERENCES 1. Jour. Franklin Inst. 1912, 173, p. 315. Phys. Rev. 26, p. 498; 25, p. 123; Trans. I. E. S. 1908, p. 301. 2. Wied. Ann. d. Phys. 1894, 53, p. 807. 3. Silleman's Jour. 1889, 38, p. 100. 4. Ann. d. Chimie et d. Phys. Ser. 6, 1889, 20, p. 480. Comp. Rend. 1889, 109, p. 493; 112, p. 1176, p. 1246. 5. Berl. Berichte, 1877, p. 104; 1880, p. 801. 6. Trans. I. E. S. 1910, p. 189. 7. Brit. Assn. Report, 1900, p. 631. 8. Lon. Ilium. Engr. 1912, 5, p. 79. 9. Elec. World, 1911, 57, p. 1092; Lon. Ilium. Engr. 1911, 4, p. 394. Lighting Jour. (U. S.), 1913, 1, p. 131. 10. Trans. I. E. S. 1912, 7, p. 73. 11. Trans. I. E. S. 1914, p. 839; Elec. World, Sept. 19, 1914. 12. Trans. I. E. S. 1914, p. 687. 13. Bui. Bur. Stds. 1909, p. 265. 14. Trans. I. E. S. 1910, 5, p. 209. 15. Trans. I. E. S. 1912, 7, p. 57. 16. Lighting Jour. (U. S.), April, 1914; Lon. Ilium. Engr. March, 1914. 17. Proc. A. I. E. E. 1910, p. 1726. 18. Trans. I. E. S. 1913, p. 61. COLOR IN LIGHTING 271 19. N. E. L. A. Bui. Feb. 1915, p. 87; Elec. World, 1915, 65, p. 391. 20. Trans. I. E. S. 1913, 6, p. 9. 21. Experimental Psychology, New York, 1910, p. 149. 22. Jour, of Psych. 1913, 24, p. 545. 23. Phil. Stud. 1900, 15, p. 279. 24. Elec. Rev. and W. E. 1915, 67, p. 161. By M. Luckiesh: The Lighting Art, 1917. Artificial Light, 1920. Lighting the Home, 1920. CHAPTER XII COLOR EFFECTS FOR THE STAGE AND DISPLAYS 68. The Stage. It is not the intention to treat the use of colored light in stage and display effects as they have been practised heretofore, but to point out some interesting new possibilities that have been developed by the application of the science of color. By the use of red, green, and blue lights any desired color effect may be produced, but the purity of these primary colors is very important. Apparently there has been little exact science of color-mixture applied to the stage. It is true that wonderful effects have been produced^ but it is just as certain that the pos- sibilities in color effects have scarcely been touched upon. The color effects of today have not passed beyond the play of colored lights upon colored scenes in a more or less haphazard manner, the final effects, which are often very attractive, being arrived at by a 'cut and try' method. Examination of colored media used for such effects show that very often impure colors are used. In fact, satisfactory com- mercial colorings are rare, and it is usually necessary to alter them in order to obtain colored lights of satisfactory purity. As already stated, only pure primary colors red, green, and blue are essential for producing a large variety of colored effects. Colored effects are based upon the principle that the appearance of colored objects depends largely upon the spectral character of the light which illumi- nates them; that is, the color of an object is not 272 EFFECTS FOR THE STAGE AND DISPLAYS 273 inherent wholly in the object itself. Things are vis- ible only by virtue of the light which passes from them to the eye. For instance, a red fabric appears red because it has the property of reflecting only red light. Obviously, if red rays are not present in the light under which the fabric is viewed, it will appear black. Colors can be made to disappear on a light background if they are sufficiently pure and free from * black ' by viewing them through a glass of proper color. Pale blue lines on white paper will practically disappear under a deep blue light, and red pencil marks on white paper will be invisible under (Red light) (Green light) (Blue light) Fig. 113. Illustrating the effect of colored light upon the appearance of six colored papers. a pure red light of proper color. This principle has been applied in stereoscopic drawings, the picture for one eye being printed in blue-green ink and that for the other in red ink. On placing a blue-green glass before one eye and a red glass before the other, a stereoscopic effect is produced. In Fig. 113 are shown the relative brightnesses of * six colored papers under red, green, and blue lights, the colored papers being in the same relative posi- tions in each group. The photographs were made through a very accurate filter specially made for the panchromatic plate used (Fig. 90), and 'therefore the brightnesses are shown as nearly in true relation to each other as the limitations of photographic repro- 274 COLOR AND ITS APPLICATIONS duction permit. It is interesting to note some of the changes; for example, the two middle colors re- verse in brightness when respectively illuminated by red and green (or blue) lights. Carrying this principle further the author 1 has developed some colored effects which show promise of application as the applied science of color becomes more thoroughly understood and as the cost of pro- ducing suitable colored light decreases. In making these applications it must be remembered that a color is only completely defined when analyzed into the three factors, hue, saturation, and brightness. For the purpose of producing the disappearing effects to be described, a simpler analysis can be used. That is, it is convenient here to consider separately the hue of the light that the pigment reflects and the amount it reflects, the first involving the spectral hue and the latter its brightness (or value). A group of col- ored patches on a gray ground can be made to dis- appear that is to become indistinguishable from each other and the background when the colored patches are illuminated by light of such a spectral character that they reflect rays of exactly the same character and in equal amounts as the background. This condition will not hold for another illuminant; therefore, some of the colored patches will be dis- tinguishable under another illuminant. This dis- appearance can be produced in another manner. By using a light of such character that the colored patches will reflect practically none of it they will disappear if placed on a black or dark gray background. Both methods have been used in developing these colored effects. The success of the scheme depends largely upon the choice of pigments properly related to each other and to the colored lights employed. Pure EFFECTS FOR THE STAGE AND DISPLAYS 275 transparent pigments are quite essential. In mixing the colors it is necessary to understand the principles of color-mixture, for in mixing pigments there is always a tendency toward black (Fig. 20). A large supply of pure pigments is desirable, so that a pure pigment may be selected instead of obtaining the necessary hue by mixture. For example green can be made by mixing yellow and blue-green. This subtractive method often results in a green plus black; Fig. 114. Illustrating the changing of scenery by the use of colored lights. that is, a muddy green. If the green can be obtained directly as a pigment instead of by this mixture, the black component is not present. Attention to these finer points is what distinguishes the scientific colorist from those who arrive at results without heeding the fundamental principles. Owing to the confused state of color terminology and the indefinite notation of pigments, it is impossible to describe accurately how these disappearing effects are produced. They in- volve the science of color and can be produced readily if the principles are thoroughly understood. 276 COLOR AND ITS APPLICATIONS The modern tendencies toward the use of color and color effects point to great future possibilities in the application of the science of color. Already in some European theaters the stage scenery has been revolutionized, and lighting effects are playing a greater part in the drama than heretofore* The experiments described below suggest the possibility Fig. 116. Illustrating the disappearing effects produced on a specially painted scene by varying the color of the illuminant. that rays of light, swift and noiseless, might take the place of some of the present-day cumbersome methods of scene-shifting. Possibilities are also sug- gested for representing the supernatural, heretofore unrealized on the stage. In Fig. 114 are shown, as well as can be represented in black and white, two appearances of a mountain scene. The mountain and entire background can be made to disappear at will by changing the color of the illuminant. The EFFECTS FOR THE STAGE AND DISPLAYS 277 appearance on the left is that under the ordinary yellowish light from tungsten incandescent lamps. The other appearance is that under an orange-red light. The colors in the foreground are violets, grays, blues, greens, and touches of yellow. Those in the background are white, yellow, orange, red, and pink. Lightning effects can be obtained by flashing reddish light on the painting. No attention was paid to congruity in the use of colors, for the painting was designed merely to illustrate the pos- sibilities of the scheme. Further striking effects can be obtained by the use of illuminants of other colors, especially pale blue-green light. Thus a scene can be changed by rays of light. It is also possible to make the mountain disappear and in its place to have some other scene appear, for instance a seascape. In Fig. 115 the first picture appears to be a Jap- anesque arrangement of foliage. This is the appear- ance under a deep orange-red light. Gradually, by introducing blue light, the figure appears, and on adding green light or clear light it appears fully in view. On extinguishing the red component in the illumination the figure, and especially the flowing robe, stands but in strong contrast and beautiful effects are produced by changing gradually from blue-green to a deep blue. By gradually introducing orange-red light and extinguishing the other components the figure slowly disappears. Such effects show the possibility in scenic effects in fairyland plays. \ It is well to understand that the photographic repro- ductions just shown only illustrate the brightness contrast. In the originals the contrasts are more striking, because they are due to differences in hue as well as in brightness. In fact, it is difficult to illus- trate in black and white the effects produced with this 278 COLOR AND ITS APPLICATIONS particular subject, because in the center illustration most of the contrast is due to differences in hue alone. Another changing scene that was produced is that of a summer landscape gradually merging into a snowy wintry scene. By painting the body and branches of the trees a gray, and covering these and the ground with a bluish-green foliage, they appear in their abundant garb of summer under ordinary light. By changing the color of the illuminant to a 'cold' pale blue-green the summer foliage disappears from the trees and from the ground, and barren trees and a ground covered with snow appears. These are the chief features of this scene. Of course touches of color added judiciously here and there greatly en- hance the beauty of the scene. Many other effects have been produced, but no attempt has as yet been made in applying them on a large scale in stage scenery. However, the problem in the theater is comparatively simple owing to the perfect control of the illumination. Certainly the possibilities of such applications of the science of color are very exten- sive. Only the simpler ones have been described here, owing to the necessity for demonstrating the principle as simply as possible. The more elaborate effects require more perfect interrelation of colors and illuminants. A field not to be overlooked is that of legerdemain, in which such disappearing and changing effects should prove valuable. 69. Displays. The foregoing effects are also applicable for advertising displays. In fact, it is strange that colored light has not been applied more to illuminated signs. Large tungsten lamps equipped with color filters and operated on flashers should add considerable to the attractiveness of ordinary scenic signs. The filters could be such as to produce moon- EFFECTS FOR THE STAGE AND DISPLAYS 279 light, daylight, and sunset effects upon a scene with great effectiveness. Colored lights pursuing each other in waves around the border of a sign represents a very simple application of colored light in adding movement to an ordinary illuminated sign. It seems that the introduction of changing and disappearing effects on illuminated signs should become popular, owing to the lower cost as compared with the cost of an elaborately wired sign studded with incandescent lamps. There is no doubt that the latter signs are visible at a greater distance, but there are a large majority of signs that cannot be viewed from a THE SOCIETY FOR ELECTRICAL DEVELOPMENT INC. NEW YORK THE SOCIETY FOR ELECTRICAL PEVELOPMENT INC. NEW YORK "DO IT ELECTRICALLY" "DO IT ELECTRICALLY" Fig. 116. Illustrating a flashing sign produced by properly relating the hue and brightness of the pigments with the colors of the illuminant. great distance owing to obstructions. In Fig. 116 is represented a possibility outrivaling the ordinary illuminated sign, for actual disappearing effects are produced. The copy in the first view is in red, orange, and pale yellow. This disappears under orange-red light, the whole surface appearing of a uniform tint. The copy in the second view is in blue-green. On illuminating the sign with ordinary tungsten light the sign appears as in the third view. By alternat- ing with this clear tungsten light an orange-red light the copy shown in the first view appears and disappears. By alternating with the clear light a blue-green light the copy shown in the second view 280 COLOR AND ITS APPLICATIONS disappears and appears. Thus various effects can be produced. This is a very simple sign, requiring a most simple wiring scheme. The flashing lettered sign has also been effectively combined with a scenic painting. Practically an endless variety of effects can be produced as rapidly as desired. Many other effects have actually been produced, such as a smiling and frowning face, a gesticulating speaker, a waving flag, and a rotating wheel, by this method of chang- ing the spectral character of the illuminant. These have been widely exhibited. This scheme has already been applied in displays. The haphazard play of colored lights upon colored patterns not designedly chosen is productive of catchy attractiveness; however, the actual disappearing ef- fects are more striking. For window displays the copy is placed in a darkened recess resembling the demonstration booth described in #67 so that it is protected from extraneous light. The colored lights are operated by flashers designed to bring about the proper sequence of appearances. Such demonstra- tions have been built and successfully operated. Successful experiments have been carried out with the color effects appearing by transmission through trans- lucent glass, in this case the colored lights being behind the glass. The colored scene or pattern is painted in transparent colors related as before on the back of the glass. It is also possible to project the colored light from a distance by means of parabolic reflectors, which would be an advantage in some cases for out-door displays. From the foregoing simple illustrations several advantages in the scheme are obvious. Apparent motion is obtained without elaborate wiring or me- chanical devices excepting the usual flasher. Copy EFFECTS FOR THE STAGE AND DISPLAYS 281 can be changed continually if desired or a sign can be repainted often. The greatest difficulty lies in the initial preparation of the colors in proper rela- tion as to brightness and hue. In order to produce elaborate effects it will perhaps be necessary to use water colors or carefully prepared oil paints, protect- ing these with a covering of weatherproof varnish. The only expense involved in the change of copy is in the painting of it, for the color scheme can always be retained in proper relation to the colored illumi- nants. A flashing sign of this character is very simple, and the possibilities of scenic effects are greater than in any other method, simplicity being taken into consideration. The scheme adds to the possibilities of stage effects where it can be carried out with ease and sometimes be employed to sup- plant the jarring interruption due to shifting scenery. Of course the necessity of screening extraneous light if present will be a disadvantage in the application of this scheme to out-door displays, but there are .many places where this will be unnecessary, because numerous bill-board sites can be found where there is little or no scattered light. The possibilities of the use of colored light in applying the science of color to displays, advertising and stage effects, have barely been touched. With the increasing efficiency of light production the utiliza- tion of color in lighting effects will become more elaborate. REFERENCES . 1. Elec. World, April, 1914. Amer. Gas. Inst. 1913. Lon. Ilium. Engr. 1914, p. 158. International Studio, April, 1914. Gen. Elec. Rev., March, 1914, p. 325. CHAPTER XIII COLOR PHENOMENA IN PAINTING 70. Visual Phenomena. The artist has often shown an antipathy toward science, apparently under the impression that art goes further than the mere mixture and grouping of colors and shadows and produces effects beyond scientific explanation. By no means is it contended here that art can be produced by 'rule of thumb,' or by scientific formulae. Never- theless, facts are the basis of all art and, while scientific investigation has not yet revealed all its hidden secrets, scientific explanations can be pre- sented for many supposedly mysterious effects. It is proposed in this chapter to present the results of analyses and to indicate that science has been a great aid to art, and that it will perhaps render a much greater service in the future. The artist is in reality a link between two light- ings. He endeavors with chisel or brush to record an expression of light. The record is therefore an expression of light. Inasmuch as both the original scene and the painted record make their appeal through the visual sense, it is well to inquire into the process of vision. Seeing involves the discrimination of differences in light, shade, and color. In the ordi- nary sense no eye ever sees more, and no painting however 'soulful' has more for its foundation, than differences in light, shade, and color. (In the general sense white, gray, or black are colors of complete unsaturation and varying brightness. It will perhaps 282 ' Plate IV. Illustrating the effect of spectral quality of the illuminant. Daylight, below; ordinary artificial light, above 4 f: COLOR PHENOMENA IN PAINTING 283 be more convenient in this chapter to use the terms 4 colors' and * values' but with a clear understanding that the term * color' is here used in a restricted sense and that value is in reality included in the term 'color' as used heretofore. See Chapter IV.) The funda- mentals of a painting therefore are colors and values. It is by grouping these elements that the artist makes his appeal to emotional man. However, science can aid the artist by analyzing the influences which alter these fundamentals. It took the artist many years to learn that the eye is far less perfect in definition than a simple lens and screen. In other words everything in the whole visual field is not seen distinctly at the same time. Definition is best at the point of the retina where the optical axis of the eye meets it, but outside of a small area surrounding this point, objects are not seen dis- tinctly. Further, the eye sees only the beginning and end of an ax stroke and it does not see all the move- ments of a galloping horse or splashing water. Pho- tography was hailed by many as being a useful means for recording a scene. But photography has done much to teach the artist what he should not paint - and that is the realistic picture recorded by the pho- tographic plate. Thus it is seen that material facts are often represented by artistic lies; that is, in reproducing a scene the artist does not record what he knows to be there, but rather what he sees. In- stead of recording details over the whole scene, the artist's task is to paint what the eye sees and in addition, by a sort of legerdemain, to record in colors and values, as far as is within his power, the impres- sions gained through the other senses. Thus the problem grows more complex, departing from the physical and entering the physiological and psycho- 284 COLOR AND ITS APPLICATIONS logical realms. The physical laws are comparatively well understood, but the phenomena underlying the other fields are still hazy, owing to the lack of suffi- cient experimental data. In viewing a painting the problem becomes still more complex, for what the observer sees in a painting he must largely supply himself through the associational mental process. There are many vague terms used by artists, per- haps definite to those who use them, but the lack of systematic usage is confusing. It has been seen that the eye is far from being a perfect optical instru- ment. One of its faults is chromatic aberration; that is, an inability to focus different colors at the same time (#33). Naturally when viewing a group of different colors the eye is focused for the brighter colors. The eye is also constantly shifting under normal conditions. We are not conscious of these minute involuntary movements, but this shifting surely influences the appearance of paintings. The effects of after-images are also of importance (#43). If one views a red line on a green or blue ground, the effect is that of unrest. Both colors may not be exactly in focus at the same time, but perhaps of greater im- portance are the effects of overlapping after-images caused by involuntary eye-movements, which result in a 'lost edge.' The latter effect is sometimes very striking at the horizon of a landscape painting. The after-image caused by a green stimulus is an un- saturated purple or pink. At the edge where green foliage meets a gray or pale blue sky a hazy pink fringe is often seen. The eye, in shifting slightly up and down, causes an overlapping of these after- images (approximately complementary to the original stimulus), thus forming a 'lively' edge. The result of an after-image sometimes is to alter the saturation COLOR PHENOMENA IN PAINTING 285 as well as the hue of a colored area. The phenom- enon of simultaneous contrast (#44) is very influen- tial, and of course is carefully studied by the artist. This effect of one area upon another is of consider- able magnitude under some conditions. Two adja- cent colored areas can mutually so influence each other that they each appear differently in hue, sat- uration, and brightness than if viewed separately. On considering these influences and those due to intensity and spectral character of the illuminant, it becomes evident that no color has any definite and fixed appearance after it is out of the tube. These are facts which should be of great interest to the artist. Indeed, the great artists understood some of these influences very well. Most artists recognize many of them, but in general some of the influ- ences are unknown to the vast majority of painters. These various phenomena are treated elsewhere. (See Plate III.) 71. Lighting. Light has been termed the soul of art. The body of a landscape consists of the material things, but as Birge Harrison states, 'its soul is the spirit of light of sunlight, of moonlight, of star- light which plays ceaselessly across the face of the landscape veiling it at night in mystery and shadow, painting it at dawn with the colors of the pearl-shell, and bathing it at midday in a luminous glory.' But of scarcely less importance is the lighting of the painted record of an expression of light. In various chapters it has been shown what a great influence the illuminant exerts on colors and values the very essence of a painting; however, slight attention has been given to this important factor. 1 The lighting artist should be to art what the musician is to music. His duty is to render the color symphony as the 286 COLOR AND ITS APPLICATIONS composer intended it to be rendered. This only can be done exactly by lighting the work both as to dis- tribution and spectral character of light just as it was lighted when the artist gave to it the final touch. This of course is impossible, but it is easy to light it by an artificial daylight which will render its appear- ance more nearly that which is had when completed by the artist, and to overcome certain limitations of pigments by properly distributing the light. (The i* ! 1 1 Orange 1 \ 1 YellowGr: DullGreei\ I I.I O C ou UJ o 70 \ C o ' v 2: ^ 60 / ^_ ^' \ \ \ 0.9 H O UJ tO // \ \ / 0.8 3 ou u_ y 40 / j 1 ~~^ ^ s 0.7 TLA // \ \ 0.0 OU J ^ N \ / in *** '^ X \ \" --^ ^ n bcd m ghkiqnop COLORED PAPERS Fig. 117. Showing the reflection coefficients of fairly saturated colors for daylight and tungsten incandescent electric light. (See Table XV.) influence of the illuminant is seen in #42, 68, and Table XV.) Not only does the spectral character of the illuminant alter the hue, but likewise the bright- ness or value. In Table XV it is seen that pigments COLOR PHENOMENA IN PAINTING 287 differ tremendously in reflecting power or relative brightness when illuminated with daylight and ordi- nary artificial light. These data have been plotted in Fig. 117 to emphasize this important point. The reflection coefficients of the various colored papers for daylight are shown by the dashed line R D and for the light from a (vacuum) tungsten lamp operating at 1.2 w.p.m.h.c. by the full line R T . The ratios of the reflection coefficient under tungsten light to that under daylight are shown by the upper curve. It will be noted that those pigments which predomi- nantly reflect red, orange, or yellow rays are con- siderably brighter under the tungsten light than under daylight, but those pigments which predominantly reflect violet, blue, and green rays are brighter under daylight. The curve shows that these ratios vary from 0.69 to 1.57; that is, some of these pigments change in relative ' value' more than 50 per cent. However in painting, the relative 'values' of adjacent and other areas are of importance, and such changes in relative brightness are often as high as 100 per cent. For instance, assume that clouds are adjacent to an area of blue sky in a certain painting and that these pigments are represented respectively by e and n. The ratio of the brightness of the clouds to that of the sky is 1.6 when the painting is illuminated by daylight. Under the light from the tungsten lamp this ratio is 2.8 or nearly doubled. In fact cases have been found where such ratios have doubled, as will be seen later. The hue changes are in some cases enormous, but these cannot be readily shown here. The artist recognizes the difficulty of painting under artificial light, yet he apparently does not raise his voice in protest when his work is illuminated by artificial light. In Fig. 118 are shown the effects of 288 COLOR AND ITS APPLICATIONS COLOR PHENOMENA IN PAINTING 289 different illuminants upon the values of a frieze painted with ordinary water colors. The illuminants were daylight, ordinary tungsten light (vacuum in- candescent lamp), and an orange-red light. In this frieze the upper rectangles were alternately tinted a reddish purple and a bluish purple. The lotus flowers and buds were tinted a pale blue, the stems, dark green, and the alternate sectors of the lower circular patterns were colored respectively a yellow- ish orange and a reddish orange. The background was white. An extreme example is shown in Fig. 113. An example of the difference in the appearance of a painting under natural and ordinary artificial light is shown in Fig. 119 (see Plate IV). A photom- eter was used to measure the relative brightnesses of adjacent patches of pale blue sky and pale yellow clouds at about the center of the sky area in this picture, indicated by the circle. Under tungsten light the two patches were of equal brightness, but under daylight the light under which practically all paint- ings are done the patch of sky was twice as bright as the adjacent clouds. The filter used in taking these photographs was quite accurate, so that the values are faithfully represented. It is seen that the sky was much brighter than the foreground when the painting was illuminated by daylight. It is only fair to state that the difference in foregrounds is due somewhat to the lack of sufficient range of grada- tion in the photographic paper. Note also the relative brightnesses of the low-hanging clouds. If a paint- ing will stand such enormous changes in the relative 4 values' of its parts (and the accompanying changes in hue), it is indeed flexible. Under most artificial illuminants the hues in a painting shift toward the red as compared to their appearance under daylight 290 COLOR AND ITS APPLICATIONS a < COLOR PHENOMENA IN PAINTING 291 illumination. That is, a deep yellow appears orange, a bluish purple appears a reddish purple, blues and violets approach gray, and the reds are relatively brighter. Accompanying this shift in hue is a cor- responding shift in brightnesses or values. That is, yellow, orange, and red appear brighter, and violet, blue, and green appear relatively less bright, as shown in Fig. The c^Bribution of light on a painting has a great influenc^Tpon the expression of the painting. Meas- ureme^^show that the range of relative brightnesses in a laf scape is often as high as five hundred to one. the brightest spot (for instance, cumulus receiving direct sunlight) are often several d times brighter than the deepest shadow, igments employed by the artist will not record a physical contrast. In any landscape painting brightest spot is seldom more than forty times hter than the darkest spot when both receive actically the same amount of light as is usually pproximately the case. A white paper is no more an fifty times brighter than a so-called black paper. In order to overcome this handicap due to the limi- tations of pigments the artist may resort to illusions if possible. For instance, a highly illuminated red object is not painted red but an orange-red, because it is true that under intense illumination colors appear less saturated. Thus, by painting the highly illumi- nated red object an orange-red, the illusion of in- tense illumination is produced. A hot desert scene is depicted in the same manner, with the additional illusion of short or minimal-length shadows. Thus the feeling that the sun is at the zenith helps to L produce the illusion of a hot desert scene. L R. W. Wood performed an interesting experiment 292 COLOR AND ITS APPLICATIONS in accentuating contrast in a painting by projecting a positive lantern slide image of a painting upon the original in exact coincidence. In this manner the high lights received relatively very much more light, and the shadows less light than in the ordinary case, where the painting is uniformly flooded with light. This scheme, though interesting and instructive, can- not be used in practise. Extensive experiments on the effects of distribution of light over paintings indi- cate that a proper distribution is a legitimate and an effective aid to the artist in bringing forth the proper expression of a painting. In Fig. 120 are shown some effects of different distributions of light on a painting, although the limitations of the photo- graphic process prevent a very satisfactory illustra- tion of these effects. It is seen, however, that the mood can be changed enormously by altering the distribution of the light. The scheme is difficult To carry out in cases where the wall space is crowdei with paintings, and it is unfortunate for various sons that such crowded conditions must exist. How ever, the principle is easily applied to individual paintings, and at the same time a correction of the light to daylight quality can be made. This has also been carried out in the case of trough lighting, which is often a practical and convenient procedure, because most paintings have their chief high lights in the upper portion. The predominant light can be directed from a point in the trough near the middle of the upper edge or near one of the corners of the painting, de- pending upon the requirements. This has been found very effective. The lighting of paintings depends also upon the hanging, which is too often done with a view toward keeping the bottom edges on a hori- zontal line instead of with a view toward placing COLOR PHENOMENA IN PAINTING 293 I 0> I 294 COLOR AND ITS APPLICATIONS them in the proper position for lighting and observ- ing them. The wall covering is of importance and should be a dull, neutral, diffusing surface and pref- erably rather dark from a lighting viewpoint. This prevents undue annoyance from its image, as seen in the glass coverings of pictures on the opposite side of the room. The illusion produced by dark sur- roundings is striking, and there are many who advo- cate such wall coverings; however, others contend that the appearance of a gallery so hung is unaes- thetic. Much could be written on the daylighting of galleries. There have been some extensive studies of the problem made in various countries, but there is no general agreement so far as quality of light is concerned. Some advocate southern exposure, others a northern exposure. In general, artificial light is more readily controlled than daylight, and therefore lends itself more readily to obtaining proper effects. Inasmuch as paintings are very often poorly lighted, a simple illus- tration of the geometrical principle of lighting is shown in Fig. 121. Every problem is readily solved by such methods. In the design \ of the lighting, both natural and '** artificial, it is well to determine graphically the positions of the light sources and the expanse of sky- light (if the latter be diffusing), so that images of these bright sources cannot be reflected from Fig. 121. illustrating the th e glazed surfaces of paintings optics of picture lighting. . ., directly into the eyes. 72. Pigments. A number of satisfactory pig- ments, among which are vermilion, indian-red, and COLOR PHENOMENA IN PAINTING 295 the ochres, are derived from minerals; the animal kingdom supplies such pigments as carmine and sepia; and a large number of pigments, such as indigo, gamboge, and madder, are derived from the vegetable kingdom. Many of the aniline dyes are derived from coal tar. Many pigments are made artificially, such as ultramarine, cobalt-blue, zinc-white, Prussian-blue, chrome-green, and the lakes. The natural pigments derived from minerals are prepared by calcining and grinding and are purified by washing. For oil paint- ing these pigments are ground in such vehicles as linseed or poppy oil. For water colors the medium is usually gum water. The latter fixes the pigments on the surfaces to which they are applied and serves as a varnish. Such vehicles should preferably be colorless, because, for instance, the yellow color of linseed oil is likely to impart a greenish tinge to pale blue pigments. Turpentine is used as a thinner for oil paints. Varnish is employed to protect pigments from destructive agents usually present in the atmos- phere and from marring by abrasion. Oil varnishes are less liable to crack than spirit varnishes, and the quality of a varnish depends largely upon the resin of which it is composed. (See Chapter XVI.) There are three general classes of pigments used in paintings. The pastel pigments are quite destruct- ible. Water colors lend an airy delicacy to a paint- ing and are quite appropriate in some classes of work. They are difficult to use, owing to their transparency and to the change in color that they undergo on drying. Oil colors, which according to some authori- ties were first used in canvas painting in about the year 1400, are the most durable an important and necessary property of pigments for use in painting. Many pigments are permanent under moderate illumi- 296 COLOR AND ITS APPLICATIONS nation when used alone, but there is always the danger of interaction between pigments when mixed. The permanency of the older paintings is no doubt due in part to the fact that the palette was rather poverty- stricken many years ago. Today there are several hundred pigments available, and therefore there is considerable danger of mixing pigments that interact. Anyone who has searched for pigments that are per- manent under excessive illumination and heat will perhaps readily agree that the permanency of pigments is only a matter of degree and that under severe conditions many so-called permanent pigments readily deteriorate. Gases and coal dust in the atmosphere and especially the products of the combustion of illuminating gas are known seriously to affect pig- ments. Light has a bleaching action and paintings often turn yellow when kept in the dark. A simple method of restoring paintings is to clean them with a cloth and set them in the sun for a day or two. This treatment, however, is rather severe for water colors and modern lake colors and is only satisfactory in some cases. Doubtless tests are being made con- tinually on the permanency of pigments, but there are few available data on the subject. In general min- eral colors are more stable than vegetable colors. Gases, moisture, interaction, heat, and light are the common causes of the deterioration of pigments. It has been found that most pigments are more per- manent in vacuo, protected from harmful gases and moisture. Some of the results of experiments indi- cate that the destructive rays in sunlight are chiefly the violet and ultra-violet rays ; that is, pigments have been found to deteriorate practically as quickly under blue glass as under clear glass. However, the most commonly used blue glass, namely cobalt-blue, trans- COLOR PHENOMENA IN PAINTING 297 mits deep red and infra-red rays almost as freely as clear glass, so it is possible that heat was responsible for some of the deterioration. Of the great number of available pigments the following are found to be most durable : Indian-red, largely ferric oxide; Venetian red, iron oxide; Burnt sienna, calcined raw sienna; Raw sienna, a clay containing ferric hydroxide; Yellow ochre, hydrated iron oxide; Emerald-green, a mixture or compound of copper arsenate and acetate; Terra verte, a natural green pigment found in Italy; Chromium oxide, green; Cobalt-blue, usually a mixture of the arsenate, phos- phate, or oxide of cobalt with alumina; Ultramarine ash, now made from soda, sulphur, charcoal, and kaolin. (Pigments are far from spectral purity; that is, they reflect light of many wave-lengths.) Interesting data obtained by Abney are shown in Table VI and spectrophotometric analyses of a number of these pigments, including those just described as being quite permanent, are shown in Figs. 122 and 123. It is seen that the mixing of colors is complicated owing to the complexity of the spectral character of the light reflected by pigments. For the sake of clearness it will be noted that the reflection curve of a neutral tint (white or gray) surface would be a straight line parallel to the base line in the last two illustrations. The colorist should be somewhat fa- miliar with the spectral characteristics of his pigments, because such knowledge is very useful in mixing pig- ments. ;The production of different hues by mixing pigments is possible because pigment colors are not 298 COLOR AND ITS APPLICATIONS monochromatic, that is, not of spectral purity. For instance, suppose monochromatic pigments were avail- a -Ye I low Ochre e- Indiqo b- Co bait Blue f- Terre Verte c-Chromous Oxide g- French Ultramarine d- Antwerp Blue h- Emerald Green 044 045 0.52 0.66 0.60 0.64 sU, f WAVE LENGTH Fig. 122. Spectral analyses of pigments. i -Mercuric Iodide x- , ,/ ,, j - Vet-million m ' Cod m turn Yellow k- Gamboge n " Indian e - Indian Ye Ho w O - Carmine 066 vv 90 80 70 fc 60 1*0 240 ri 30 a: 20 10 c ,\~ /- ^ IN v e / r, \ f*^ -,- N Js- ^ ^*- -Vl ^ ***** *N, k\ _ ^ I 'Z x^ m o / // nV / f' M -** " -/ /' * ->- ^.. ^<^ n r m ^ --* / // e .-' ^ / J ^ x^" ~***- __ ^ 40 044 048 0.52 056 060 0.64 0.65 Fig. 123. Spectral analyses of pigments. able and yellow and blue were chosen for mixing. Instead of obtaining green from the mixture, black COLOR PHENOMENA IN PAINTING 299 would be obtained, because the light transmitted by pure yellow flakes would be a monochromatic yellow which would not be transmitted by pure blue flakes; thus by the combination no light would be trans- mitted. One virtue of the poverty-stricken palette is the scanty possibility of the interaction of pigments; however, such a palette cannot be the source of a large variety of highly luminous and pure colors. Where high brightness and full saturation are desired it is well to avoid the production of the de- sired hue by mixture, as far as possible. This can V B G Y R WAVE LENGTH Fig. 124. Illustrating the effect of the amount of the green components in blue and yellow pigments on the amount of ' black ' in the mixtures. be illustrated by means of the mixture of blue and yellow. When these pigments are pure their mix- ture must result in the production of black. For instance, suppose the two pigments transmit light rays respectively as shown diagrammatically by B l and Yi in Fig. 124. Neither pigment transmits green rays nor does one pigment transmit any rays that are transmitted by the other; therefore the resultant transmission will be zero and * black' results. If the so-called blue and yellow pigments are less pure, they may be found to transmit some green rays. These may be represented diagrammaticaUy as B 2 and Y 2 in Fig. 124. It is seen that the resulting mixture of these two pigments will be a green of rela- 300 COLOR AND ITS APPLICATIONS tively low brightness corresponding to a green to which ' black' pigment has been added. The greater the proportions of green rays transmitted or reflected by the two pigments, the less ' black' will be present in the green resulting from their mixture, or, more correctly, the brighter the resultant mixture will be. Obviously, if the two components are selected suc- cessively closer and closer to green, finally the limit- 1.0 D z:. 0.6 QC O U- S- 06 z:o I s * 02 / V 0.2 0.4 06 0.8 10 SUM OFTHEGREEN COMPONENTS I M TH E BLUE Ah D YELLOW PIGMENTS Fig. 125. Diagrammatic illustration of the results of mixing blue and green pigments containing various amounts of green. ing case would be that in which both components were green, G, and their mixture of course would produce green. The results of such a theoretical procedure are shown diagrammatically in Fig. 125, where the sum of the green components in the blue and yellow pigments is assumed to vary from zero to 100 per cent; meanwhile the amount of green in the mixture varies from zero to 100 per cent, and the amount of * black' from 100 per cent to zero. This simple diagram illustrates a very important point in COLOR PHENOMENA IN PAINTING 301 the mixture of pigments. For the reason that the subtractive method of color-mixture always tends toward the production of * black' it is well to have available a large number of fundamental pigments representing as many hues as possible. This is especially desirable in the production of effects de- scribed in Chapter XII. Before J;he advent of modern art such a stock of pigment was not essential be- cause the tendency in the past was not to employ colors as pure as those found in common use today. A study of the curves in Figs. 122 and 123 is recom- mended in connection with the discussion presented in connection with Figs. 124 and 125 in order to obtain an idea of the relative brightnesses of various mixtures. See Chapter XVII. ^ REFERENCES 1. M. Luckiesh, Light and Art, Lighting Jour. (U. S.), March, 1913. Light in Art, International Studio, April, 1914. The Lighting of Paintings, Lond. Ilium. Engr. March, 1914. Lighting Jour. (U. S.), April, 1914. The Importance of Direction, Quality, and Distribution of Light, Proc Amer Gas. Inst. 1913, 8, part 1, p. 783. Ostwald, Letters to a Painter. C. Martel, Materials Used in Painting. E. N. Vanderpoel, Color Problems, 1903 C. J. Jorgensen, The Mastery of Color, 1906. CHAPTER XIV COLOR MATCHING 73. Nearly all the phenomena influencing the appearance of colored objects have been treated else- where, but inasmuch as color-matching is a special art and is also of interest to everyone at times, a summary of those factors that influence the appear- ance of colors may not be out of place. The expert colorist is fully aware of the influence upon the ap- pearance of a color of retinal fatigue or after-images, surrounding colors, difference in sensibility of various parts of the retina, the spectral character and intensity of the illuminant, the surface character of the fabric, the peculiarities of dyes, and other factors. Every- one has encountered difficulties in distinguishing or matching colors. For instance it is difficult to dis- tinguish some blues under ordinary artificial light, owing to the relatively low intensity of these rays. Many colored objects that have appeared pleasing in daylight are so changed under artificial light as to be quite unsatisfactory. Usually under artificial light the dominant hue of most colors shifts toward the longer wave-lengths. For instance, some purples will appear quite red under artificial light and bluish under daylight (Fig. 80). Such an example is methyl- violet. The surface character of a fabric plays an im- portant part in the appearance of the color. A col- ored fabric is ordinarily seen by reflected light, the light falling upon it being robbed of some of its rays 302 COLOR MATCHING 303 by the selective absorption of the dye. If the surface is porous like wool, the light can penetrate deeply and will therefore suffer more internal reflections (# 64), finally reaching the eye quite pure in color. The degree of transparency of the fiber also exerts an influence. It is seen that such dichroic dyes as methyl-violet, cyanine, and dilute solutions of rhoda- mine or eosine pink will be very much influenced by the surface character and composition of the fiber. Wool and silk fibers are transparent, but those of cotton are not, hence light cannot penetrate as far into the latter as into silk or wool. Therefore, when these three fabrics are dyed in the same solution of a dichroic dye such as methyl-violet the cotton will appear bluer than the other fabrics. The finish of the surface is also of importance, because of the re- flection of unchanged light which dilutes the colored light of the fabric. Many aniline dyes exhibit the property of fluorescence, which alters the appearance of the fabric under different illuminants. A fabric colored with such a dye will appear differently at grazing incidence than when viewed normally to the surface. The actual distribution of light is of im- portance for the last two reasons. 74. The Illuminant. Inasmuch as the appear- ance of a color is so influenced by its environment, the question might be asked, Under what conditions is its appearance considered standard? Daylight has always been the accepted standard, because the arts have developed under daylight. Furthermore day- light of a certain quality is considered white light; that is, it has no dominant hue. Such an illuminant is logically a better standard than an illuminant which of itself will impart a definite hue to the colored fabric. The color of an illuminant (the color of a white sur- 304 COLOR AND ITS APPLICATIONS face) is largely a matter of judgment which is influ- enced by many factors, and, inasmuch as daylight is quite variable, there has been a lack of agreement as to a standard daylight. Some have taken a white mist as representing such a standard, others have insisted upon the adoption of clear noon sunlight, and some have advocated the integral light from the sky and noon sunlight. Nevertheless a great many col- orists have adopted north skylight for color-matching. The difference between sunlight and skylight is demonstrated on viewing objects in the sunlight. Colors receiving direct sunlight appear * warmer,' and the shadows which receive only light from the blue sky appear of colder hues, although this comparison is not wholly justifiable, because of the difference in intensity. Of course the relative intensities of sun- light and skylight vary considerably, but on a clear day a shadow on a white blotting paper, which re- ceives light from the unobstructed blue sky, is only one-fifth or one-sixth as bright as the portion of the paper receiving both direct sunlight and skylight. The color of daylight varies -throughout the day (# 62). Many colorists favor the use of daylight in the fore- noon, although the morning light is often of a pinkish tint. On cloudy dark days a purplish tint is often quite noticeable. Anyone engaged in accurate color discrimination is aware of the continual changes in the spectral character of daylight. Smoke and dust also alter daylight toward a reddish hue, and it is likely that the conditions in the upper stratum of the atmos- phere vary from time to time, producing a variation in the character and amount of scattered sunlight. The influence of colored surroundings in altering the color of daylight is very important. Clouds, adjacent buildings, green foliage, and the color of interior COLOR MATCHING 305 walls exert a very noticeable influence on the color of daylight. Perhaps the most annoying feature of daylight is its unreliableness. In some climates the actual hours that a satisfactory daylight is available for color-matching are few. In congested and smoky cities this useful period is further reduced, so that there has always been a demand for an ' artificial daylight.' Satisfactory units of this kind are now available (#62), since the advent of highly efficient steady light sources such as the gas-filled tungsten lamp. It is of course impractical to furnish an arti- ficial daylight to match the different kinds of daylight, to which various colorists have become accustomed. It is too much to ask any manufacturer to supply artificial daylight which will exactly match daylight, altered by reflection from adjacent colored objects, which will be different in most cases. If, however, an artificial daylight is available to fill the demand of the colorist, he should be willing to compromise and give the unit a fair test. If it differs slightly from the daylight to which he is accustomed, yet shows no peculiar spectral characteristics, the colorist can readily adapt himself to the slightly altered condi- tions. If the artificial daylight is composed of inde- structible color-screens the colorist can be assured that he has an invariable standard that will serve him twenty-four hours a day a most desirable char- acteristic. There is some advantage in the produc- tion of a colored glass screen for use with tungsten lamps because the quality of light can be varied be- tween sunlight to blue skylight by varying the tem- perature of the lamp filament. As was shown in # 62 the glass developed by the author alters the light from the vacuum tungsten lamp operating at 7.9 lumens per watt to noon sunlight quality; however, 306 COLOR AND ITS APPLICATIONS when used with the gas-filled lamp operating at about 16 lumens per watt, artificial skylight is produced. This is a very convenient method of utilizing the same glass to produce artificial daylight of different kinds. Spectrophotometric tests afford the only thor- ough means of analyzing an artificial daylight. Di- chroic dyes and some mixtures of aniline dyes are greatly influenced by the spectral character of the illuminant and therefore afford a ready means for determining approximately the satisfactoriness of an illuminant for color-matching purposes. Such mix- tures can be readily made so that fabrics dyed with them will appear of the same hue under a certain illuminant, yet under another illuminant they will appear quite unlike. The two dyes used in screens c and dj Fig. 17, are examples of this character. Mixtures that appear green under daylight but quite different under another illuminant can be readily made by mixing naphthol-yellow with acid-violet and an orange with a deep bluish aniline dye. Two blue dyes can be readily made to appear practically alike under daylight, one consisting of a rather pure blue and the other having the common characteristic of transmitting deep red rays. Under an artificial illu- minant, rich in red rays, the latter will appear quite reddish as compared to the former. A weak solu- tion of erythrosine or rhodamine when added to a weak solution of potassium bichromate will produce a yellow in artificial light ; however, in daylight it will appear quite pink. Such combinations can be readily produced by examining the spectra of the dyes and by combining them judiciously. Excellent dichroic dyes are methyl-violet and cyanine. These striking instances of the effect of the illuminant are well known to dyers and other colorists. COLOR MATCHING 307 75. The Examination of Colors. In examining colors it is well to understand the peculiarities of vision. The fovea centralis of the retina, where vision is most acute, is directly opposite the middle of the pupillary aperture. A small area around this point has been named the macula lutea. The center of this region, which is called the * yellow spot,' owing to its color, often manifests itself in the examination of colors (#55). It apparently absorbs blue rays somewhat, and its effect is quite noticeable on view- ing bright colors. The effect is particularly notice- able roughly outlined in after-images produced by large bright colored areas. Bright colors are difficult to examine, owing to retinal fatigue and to the promi- nence of after-images and successive contrast. This annoyance can be reduced by decreasing the intensity of illumination or by the use of neutral tint glasses. These methods are perhaps questionable, but are certainly less objectionable than the use of blue- green glass, as is used by some for the examination of bright red and orange colors. Paterson 1 recom- mends the use of a gelatine film dyed with malachite green (a blue-green) for the examination of such highly luminous colors. A practically neutral tint screen can be made of a solution of nigrosine in gelatine. The effect of simultaneous contrast is often very great, for colors are apparently altered in hue and brightness by the influence of an adjacent color (Plate III). A black pattern on a red ground will appear of a blue-green tint. A white surrounded by green will appear brighter and of a pinkish tint. Chevreul 2 published an extensive work on this subject many years ago which goes into elaborate detail concerning contrast. In order to examine the 308 COLOR AND ITS APPLICATIONS color of a thread or portion of a variegated color pat- tern, it is well to isolate the portion to be examined by means of a gray mask. The effect of contrast is so great that a colored thread may appear rich and pure in one pattern, yet quite dull amid other sur- roundings. This effect cannot be overlooked without inviting trouble in color-matching. If it were not for the effects of fluorescence, color- matching glasses could be used which have been especially adapted to the artificial illuminant. How- ever, as many of the aniline dyes fluoresce they should receive light of the standard daylight quality while being examined. This would not be the case with the combined use of an artificial illuminant and correcting spectacles. Of course the intensity of the artificial light must be several times greater than ordinarily required for seeing in order to compensate for the unavoidable absorption of the color-matching spectacles. This scheme is not new, for it has been practised in special cases by many expert col- orists. Colored fabrics are examined both by transmitted and reflected light. Colors are usually viewed by reflected light, the change in the color of the incident light being due to selective absorption. In a loose fabric of porous surface the light penetrates more deeply and is colored by many multiple reflections. As already stated silk and wool fibers are more trans- parent than cotton, and therefore permit a deeper penetration of the light. This means a greater num- ber of multiple reflections, and, for example, as in the case of a dichroic dye, it results in a color corre- sponding to that which would be obtained with a cotton fabric dyed in a denser solution of this dye. The luster of silk is attributed to the smoothness of COLOR MATCHING 309 the fibers. In examining colors by reflected light the distribution of the incident light is of importance inasmuch as some of the regularly reflected light is but slightly changed, owing to the fact that it does not penetrate the fabric. This tends to dilute the colored light and to make it appear less saturated. In installing artificial daylight it is well to distribute the light in a manner found satisfactory in day- light. Many aniline dyes in solid form reflect light complementary in color to that which they transmit. Crystals of some purple dyes appear green by reflected light. If the crystal be ground into a fine powder, the latter appears purple in color, because the light penetrates it and by transmission and multiple re- flections appears different than by specular reflec- tion. A borax bead containing cobalt may appear almost black, but when ground into a powder it ap- pears blue. Pigments when in a dense homogeneous mass are quite opaque and reflect light selectively. The phenomena of surface color is intimately related to the coefficient of absorption and the refractive index of the substance. Inasmuch as the phe- nomenon is not of sufficient importance to go into details regarding it, the reader is referred to any standard text-book in physics for an analysis of the phenomenon. Some fabrics exhibit changeable colors owing to their nap, which ends in a certain direction. If it ends toward the light, the latter penetrates the fabric to a considerable depth and is deeply colored by multiple reflections. If the nap ends away from the direction of the light, there is more specular reflec- tion and therefore less penetration, which results in a smaller change in color. The fibers of a fabric may be considered to hold 310 COLOR AND ITS APPLICATIONS the dye in a state of suspension or solution, and there- fore fabrics are sometimes examined by transmitted light. In a special case of this kind of examination the fabric is held between the eye and the light and is viewed at a grazing angle. This is sometimes called the overhand method. By thus looking through the fibers, hues can be distinguished that are quite imperceptible in an examination by reflected light. This method is especially applicable to the examina- tion of colors of the darker shades. The rich appear- ance of these dark colors viewed in this manner is very striking. The change in color that a dyed fabric undergoes on drying is of great importance and often quite annoying. This need not be treated here, because the novice will learn very quickly that dyes in solu- tion and freshly dyed fabrics often undergo great changes in color. It is a point to be considered in color-matching. A great many dyes exhibit the prop- erty of fluorescence, among which are the eosines, phloxine, rhodamine, uranine, fluorescein, rose bengal, naphthalin-red, resorcin-blue, and chlorophyl. In matching strongly fluorescent colors it is seen that there is quite a difference in hue between the re- flected and transmitted light. For instance, by reflected light an eosine pink will appear redder than by transmitted light by the overhand method. This is due to the fact that the reddish fluorescence is most predominant in the light reaching the eye when the fabric is examined by the ordinary method of reflected light. By the overhand method this fabric will appear decidedly bluer, owing to the fact that the fluorescent light does not reach the eye in appreciable amounts. This effect may readily be demonstrated by dyeing two fabrics respectively by fluorescent and COLOR MATCHING 311 non-fluorescent dyes so that they match by reflected light. They will be found to appear different by the overhand method. REFERENCES 1. David Paterson, Colour Matching on Textiles, London, 1901. 2. M. E. Chevreul, Principles of the Harmony and Contrast of Colours. CHAPTER XV THE ART OF MOBILE COLOR 76. This subject will be treated from two view- points: first as to the relation of colors and sounds, and second, from the viewpoint of an art of mobile color independent of any other art. The treatment from the first viewpoint is not entirely one of choice. In fact one feels compelled to discuss the possibility and justification of such a relation because in the few instances that colors have been related to sound music the superficiality has been quite apparent. It took centuries of scientific study and analysis to mould musical chaos into a uniform art of measured music, and even today there are composers who are not reconciled to the generally accepted state of affairs. Even with this example of slow evolution in sound music before them, there have been a few who have had the temerity to relate colors and music before the public notwithstanding the meager data available. It is significant that the names of these ' inventors ' are not found among the experimental psychologists and other investigators who are un- earthing information that may some day form the foundation of an art of mobile color. Rimington, in a book entitled * Colour-Music,' re- peatedly compares colors and sounds, owing to the fact that both 'are due to vibrations which stimulate the optic and aural nerve respectively.' He further states that 'this in itself is remarkable as showing the similarity of the action of sound and color upon 312 THE ART OF MOBILE COLOR 313 us.' He presents other ' similarities ' but in fairness it should be noted that he states that too much weight should not be given to them. Nevertheless, owing to the repeated citations by Rimington of these 'simi- larities' one concludes that they influence him con- siderably in developing his so-called 'color organ.' If no stronger reason for interest in the art of mobile color existed, space would not be given to a discus- sion of this subject, but there are indications that such an art is waiting to be evolved. Furthermore, the relation between sound and color forms such an in- significant part in the author's thoughts regarding color music, that space would not be given to such a discussion if it did not appear necessary to clarify the matter by dispelling some of the superficial ideas regarding such a relation and by pointing out the limitations of certain attempts to present such a combination. There is no physical relation between sounds and colors. Sounds are transmitted by waves in a mate- rial medium, as proved by many experiments. Light rays are supposed by many to be transmitted by a hypothetical medium called the ether, but scientists are divided in their opinions regarding the existence of an ether. Furthermore, the two kinds of wave motion that are used to represent sound and light waves are necessarily different, because the former cannot be polarized while the latter can be. Light waves pass through what we term a vacuum, but sound waves cannot. These few fundamental differ- ences are sufficient to illustrate the futility of any claims that sounds and colors are produced in similar ways. Next let us consider the respective perceiving organs. The ear is analytic, for a musical chord 314 COLOR AND ITS APPLICATIONS can be analyzed into its components. This is not true of the eye. In other words, the eye is a syn- thetic instrument incapable of analyzing a color into its components. Many examples have been cited in previous chapters of colors that appeared identical to the eye, yet differed greatly in spectral character. This difference in the two organs must necessarily influence the choice of a fundamental mode of pro- ducing ' color music.' As already stated, it is noteworthy that those few persons who have actually written ' color-music ' are not found among the large group contributing to the development of the science of experimental psychol- ogy or to sciences closely akin to it. The relation between colors and sound music, if any exists, some day will be revealed, but only through systematic experimentation by investigators well versed in phys- ics, physiology, and psychology. There is value in experiments directly relating colors and music, but certainly it is too early to experiment before the public. Such procedure jeopardizes the chance for ultimate success, but, fortunately, past exhibitions of this character will have been forgotten long before color-music evolves into a form in which it will be recognized ultimately. For some time the author has been interested in the subject of mobile color as a mode of expres- sion similar to the fine arts, and has therefore watched with interest some attempts in relating colors and music. This interest has been almost entirely in an art of mobile color independent of any other art, but, besides preliminary experiments bearing on the sub- ject, some experiments with colors and music have also been performed. These will be touched upon later. Recently a musical composition by A. Scria- THE ART OF MOBILE COLOR 315 bine entitled ' Prometheus ' was rendered by a sym- phony orchestra with an accompaniment of colors according to the 'Luce* part as written by the com- poser for the 'Clavier a lumieres' (Fig. 126). No clue is found in the musical score regarding the colors represented by the notes in the 'Luce' part, or the A.SCR1ABINE _PROMTHEE_ LEPOEMEDUFEU POUR GRAND OR(mSTRK ET PIANO AVEC ORtillK. CHOHURS ETCL'WlLRAlJJMII-RliS r^dg* . Fig. 126. The 'Luce' part for the 'Clavier a lumieres' in Scriabine's ' Prometheus.' (Upper staff in each portion is the ' Luce ' part.) manner in which a 'colored chord' is to be played whether by juxtaposition or by superposition. The latter point is of fundamental importance, inasmuch as the eye is not analytic and a mixture of the colors of a 'color chord' results in only a single hue. Some of those responsible for the rendition of this music, with color accompaniment, had, at different times previous to the final presentation, accepted both the Rimington scale and Scriabine's code (the latter having been discovered later in a musical journal published at the time of a previous presentation of the same selec- 316 COLOR AND ITS APPLICATIONS tion in London) as being properly related to the music. The acceptance of the Rimington scale, in the absence of Scriabine's code, as being adapted to the music, and the final acceptance of the latter code, which was used in the public presentation, shows that at the present time there is no definite relation between colors and sound music, even in the minds of artistic interpreters of music. It must not be assumed that the colors in Table XXI bear any abso- lute relation to the corresponding musical notes. Rimington's scale apparently was chosen arbitrarily, as shown, merely for convenience in writing a * color score/ This is probably true of Scriabine's scale. Those familiar with the science of color would hardly consider it probable that a composer of sound music would hold the key to ' color music ' when they freely acknowledge their helplessness in definitely relating colors and musical sounds. Everything pointed to failure, and if one may judge from the criticisms of the rendition of * Prometheus' with the accompaniment of colors, after allowing for a considerable degree of conservatism and inertia, the 4 relation of the colors and musical sounds was indefinite, unsatisfactory, and distracting. Con- sidering that the experimental work has not yet been done which should form a basis for expres- sion and arousing emotion by means of colors, no other outcome of superficially relating colors to sound music could have been expected. Even though this be an extremely progressive age,- it is not likely that color music can evolve, in an acceptable form, from the imagination of a few persons. 77. While it appears that the art of mobile color must evolve from fundamental experimental data THE ART OF MOBILE COLOR 317 TABLE XXI Color Codes Rimington Scriabine C Deep red C# Crimson D Orange-crimson D# Orange E Yellow F Yellow-green F# Green G Bluish green G# Blue-green A Indigo A# Deep blue B Violet C Invisible Red Violet Yellow Glint of steel Pearly blue and shimmer of moonshine Dark red Bright blue Rosy orange Purple Green Glint of steel Pearly blue and shimmer of moonshine on the ' emotive value' of colors, of simultaneous and successive contrasts in brightness and hue, of sequences in hues, tints, and shades, of rhythm, etc., it is interesting also to experiment with colors in rela- tion to music. However, a safe elementary procedure in the latter experiments is to use colored light merely to provide the * atmosphere' and gradually to intro- duce the element of varied intensity and, possibly, rhythm. Certainly it is far less presumptuous to use color in this manner in the absence of experimental data than to attempt to play a 'tune' in colors as a part of a musical score. If it is only a matter of individual taste, any procedure is, perhaps, legitimate, but when the object is to develop an art of mobile color only cautious procedure is commendable. In providing 'atmosphere' for a particular motif such superficial associational relations as blue-green for rippling water and red for fire (because artists paint 318 COLOR AND ITS APPLICATIONS them thus) are insufficient. It is the deeper emo- tional relation that is desired which, perhaps, cannot be determined with certainty without many careful experiments on a large number of subjects. In developing an independent art of mobile color, what procedure shall be adopted? Certainly the fundamental experiments will be found to lie largely in the realm of psychology. The aim of the modern artist is not totally unrelated to the subject, and a group of such artists perhaps would form a most interested audience for such experiments. The new movement in the theater which is striving for har- mony in action, lighting, and setting is not wholly unrelated to the subject under consideration. In experimenting with colors for the purpose of devel- oping an art of mobile color it may be profitable and encouraging to study the evolution of sound music. In Baltzell's 'History of Music' we read ' When we think of music we have in mind an organization of musical sounds into something definite, something by design, not by chance, the product of the working of the human mind with musical sounds and their effects upon the human sensibilities. So long as man accepted the various phenomena of musical sounds as isolated facts, there could be no art. But when he began to use them to minister to his pleasure and to study them and their effects, he began to form an art of music. The story of music is the record of a series of attempts on the part of man to make artistic use of the material which the ear accepts as capable of affording pleasure and as useful in expressing the innermost feelings.' The leading principles in music are rhythm, melody, harmony, and tone quality, and in the execution of a musical composition dynamic contrast is an essential factor in expression. 'For ages after the birth of music, rhythm and melody were the only real elements, rhythm being first recognized. Music THE ART OF MOBILE COLOR 319 that lacks a clearly-defined rhythm does not move the masses. It was not until harmony appeared that music was able to claim a position equal to that accorded to poetry, painting, sculpture, and architecture.' * These principles, rhythm, melody, and harmony, became, when couched in the forms of expression adopted by the great masters, what we call modern music, and the story is one of a development from extreme simplicity to the complexity illustrated in modern orchestral scores.' The lesson we gain from the foregoing is to proceed patiently. Sound music had an elementary begin- ning evolving into its present form only after many centuries of experiment. A thought that naturally comes to us is this: Is there anything in Nature that suggests color music? Perhaps scenes full of color are suggestive of ' atmos- phere' colors for musical compositions. Perhaps if the cycle of appearances of such a scene throughout a day were compressed into a period of five minutes, it might suggest what a composition in color music would be. Being unaccustomed to thinking of color apart from form, perhaps such studies would be fruitful. Certainly at first, in thinking of color for color's sake alone, one has a feeling that all solid foundation has been removed from beneath him. When it comes to experimental work one feels that the foundation has been restored, but is appalled at the immensity of the work to be done. The avail- able psychological literature yields some interesting information. Some work on affection pertaining to colors has been done, and the studies of rhythm are very extensive; however, the work, which eventually will form a definite basis for developing an art of mobile color, has hardly been begun. The meager data in color preference partially described in #66 were obtained as a beginning of an inquiry into some of the elementary impressions produced by colors. 320 COLOR AND ITS APPLICATIONS It appears from this work, which supports conclusions arrived at by others, that in general saturated colors are more preferred than tints or shades, the latter perhaps being generally more preferred than tints. There is some evidence that subjects who are less capable of isolating the colors, that is, more inclined to associate them with other experiences, prefer the tints and shades or so-called * artistic' colors. Some study has been made of combinations of colors, but without definite results at the present time. Of course all the known principles of harmony and con- trast of colors are available for use by the pioneer in the art of mobile color. However, no application of these principles can be made until extensive ex- periments have been performed. The * emotive value ' of various hues, tints, and shades, of simultaneous and successive contrasts in hue and brightness, and of rhythmic sequences in hue and brightness must be determined. Bradford found that saturated colors were most preferred and that the admixture of small proportions of another color have a lowering effect upon the preference of a color. Regarded objectively, the pure colors were found first in the preference order while those which appear to be adulterated with another color, were placed last. Cohn had pre- viously claimed that increase in saturation tended to make a color more pleasing. Titchener obtains results of a similar nature with the majority of his subjects, who definitely reject tints and shades of colors in favor of the saturated colors. While a color may be most highly preferred among a large number of colors the * emotive value' of this color is perhaps rather low as compared with many other things. For instance a dark blue color may be dis- tinctly more preferred than any other color in a cer- THE ART OF MOBILE COLOR 321 tain group, yet it can hardly be compared in emotive value to a song by one of our operatic artists. As Titchener states, when compared in pleasantness with a good dinner or the scent of a flower the color patch will seem practically indifferent. Of course results of impressions are only relative and there is perhaps sufficient emotive value in colors alone to afford pleasure when combined to form color music. How- ever, the foregoing point is of interest in combining colors and sound music. Certainly a * color instru- ment' cannot compete with a symphony orchestra, which leads to the tentative conclusion that color in such a relation should be subordinated to the role of merely providing ' atmosphere.' A * color instrument' of definite form is conspicuous in its feebleness when in the midst of a symphony orchestra. It was sug- gested that the colors be used in the rendition of 1 Prometheus' by combining them on the whole background of the orchestra setting without any arbi- trary limits, thus providing the atmosphere. The use of diaphanous curtains, draped in loose folds and per- haps kept moving gently by electric fans placed at a considerable distance, was recommended. However, neither of these suggestions was adopted, the colors having been played on a relatively small white screen. 78. The mechanical construction of experimental apparatus for studying ' color phrases' is simple. There are two general methods of procedure which immediately occur to the experimenter. In one, the various colors composing a * color chord' are separated physically by playing them on different parts of a white screen, thus introducing the factor of harmony and overcoming the lack of analytic ability of the visual apparatus. In the other the component colors of a color chord are mixed by superposition. Obvi- 322 COLOR AND ITS APPLICATIONS ously, in the latter case harmony is limited to the presentation of colors successively and the predomi- nant factor in 'composing' color music to be rendered by such an instrument would be that of color-mixture. In the former case the predominant factor would be that of the harmony of colors. In both proce- dures the element of rhythm and variation in bright- ness can be introduced. A decision regarding the mode of presenting colors --by juxtaposition or superposition must be made before any serious attempts at composing color music can be made. Doubtless instruments employing both principles should be investigated, and with this in mind two simple instruments were constructed. One similar to that illustrated in Fig. 127 was used by Rimington, who employed arc lamps for sources of light. The various colors indicated in Table XXI were played in arbitrarily selected positions relative to each other. Obviously no purples appeared when the colors of the Rimington scale were played in this manner. This omission is inexcusable, for purple is of a definite hue and perhaps nearly as full of emotive value as any spectral color. The colors could also be mixed on a screen. A mechanical dimming apparatus was em- ployed for controlling the brightness of the colors. Rimington evidently has experimented considerably with such an apparatus, but gives little data that supplies fundamental information from which to develop an art of mobile color. Such an instrument was constructed by the author, using tungsten incan- descent lamps and fairly pure color filters, the wiring diagram being as shown in Fig. 127. Either mechani- cal or electrical dimmers may be used for control- ling the brightness of the colors. A similar instru- ment was used in the rendition of the 'Luce' score THE ART OF MOBILE COLOR 323 in * Prometheus,' cited early in the present chapter. In order to overcome the arbitrariness of the relative positions at which the colors appeared upon the white reflecting screen an oscillating motion was given to the colors. By this means the colors never appeared completely superposed and appeared on various occa- sions on different parts of the screen. * c? 1 ^ 1 Qj ^ ^ ^ 1 ^ Crimso \ I Yellow I 5 > ^ Oii % x vS j | f X ^ 1 -^ Q ^ . \ \ \ i \ \ J i i \ J uJ * J m $m V 1 ,T~ *9 i... . .; m t 1.._ ,._-; t 1 ^-=1" 4 Fig. 127. Illustrating an instrument for studying the emotive or affective value of colors and color phrases ; Rimington's color code is also shown. Another instrument constructed by the author, on the principle that any color can be matched by a mixture of three primary colors, namely red, green, and blue, is illustrated in Figs. 128 and 129. Red, green, blue, and clear tungsten lamps are respectively placed in series with specially constructed resistors. Each of the resistors, a, fe, c, and d, contain ten mov- able contacts which are respectively connected to the corresponding keys on the keyboard. On pressing a given key the circuit is completed through the 324 COLOR AND ITS APPLICATIONS corresponding lamps and a certain amount of resist- ance wire. The line voltage is applied at 7, and C is a common terminal for the four circuits. The clear tungsten lamps are in reality * daylight' lamps, thus producing white light. This light is used to dilute the colored light to any degree of saturation represented by the ten steps in intensity produced Fig. 128. A color-mixture instrument for studying the emotive and affective value of colors and color phrases. by pressing the corresponding keys in the upper row marked W. Thus ten steps in intensity can be obtained for light of each primary color and white. Such a combination is, of course, arbitrary, but is sufficiently elaborate for preliminary experimental purposes. Hundreds of different colors are obtain- able, varying in brightness from that just perceptible to the maximum brightness, which is at the limit of comfortableness. The lamps are placed inside a THE ART OF MOBILE COLOR 325 velvet-lined box (Fig. 129) around the rectangular aperture. The colors are mixed by superposition and viewed at present on a circular white diffusing surface placed on the back of the box opposite the viewing aperture. The movable contacts are adjusted so that any corresponding set of three keys in the /?, G, and B rows will produce white light. Thus white light of ten degrees of brightness can be made B G R G B O O O O O o B R G R B ._.X- PROMT VIEW END-VIEW Fig. 129. Showing the relative positions of the colored lamps in the apparatus diagrammatically shown in Fig. 128 in this manner. The upper row of keys for producing white light has been installed in order to produce greater flexibility. Considerable personal experimenting has been done with these forms of apparatus, but little definite information has yet been derived. The foregoing has been presented to illustrate the procedure considered desirable in this work. The amount of experiment- ing that can be done with such apparatus is very ex- tensive, but the first question to decide concerns the character of the data desired. Many have dreamed 326 COLOR AND ITS APPLICATIONS of color music, some have written about it, and a few have attempted to present it. The objects of this discussion have been to show that there is no art of mobile color at present; that meager constructive data exists concerning it; that there have been hardly more than superficial attempts made to present it; that psychological studies must be relied upon to point the way toward its development; that it is a field worthy of cultivation; and that there are defi- nite problems that must be studied in order to obtain foundation material for building up an art of mobile color. REFERENCES E. J. G. Bradford, On the Relation and Aesthetic Value of the Perceptive Types in Color Appreciation, Amer. Jour, of Psych. 1913, 24, p. 545. J. Cohn, Gefuhlston und Sattigung der Farben, Phil. Stud. 1900, 15, p. 279. D. R. Major, On the Affective Tone of Single Sense Impression, Amer. Jour. Psych. 1895, 6, p. 57. W. H. Winch, Color Preferences of School Children, Brit. Jour. Psych. 1909, 3, p. 42. L. R. Geissler, Experiments in Color Saturation, Amer. Jour, of Psych. 1913, 24, p. 171. E. B. Titchener, Experimental Psychology, New York, 1910, p. 149. A. W. Rimington, Colour-Music, London. C. A. Ruchmich, A Bibliography of Rhythm, Amer. Jour, of Psych. 1913, 24, p. 508. G. H. Clutsam, The Harmonies of Scriabine, London Musical Times, March, 1913, p. 157. For a discussion of the rendition of 'Prometheus' with an accompaniment of colors, see New York papers of March 22, 1915. J. D. MacDonald, Sounds and Colours, 1867. J. Aitken, On Harmony of Colour, Trans. Roy. Scot. Soc. of Arts IX, 1873. Mrs. E. J. Hughes, Harmonies of Tones and Colours, 1883. Arnold Ebet, Farbensymphonie, Alleg. Musik Zeit. 1912, 39, Nos. 34 and 35. M. Luckiesh, The Language of Color, 1918. CHAPTER XVI COLORED MEDIA 79. Available Coloring Materials. In any kind of work a knowledge of the tools and materials avail- able is quite important. If one may judge from the questions that are asked by many interested in va- rious phases of color science, a brief outline of colored media and means of manipulating them should be of interest. The available coloring materials are very numerous, yet it is often difficult to find satisfactory pigments for a given purpose. It is of considerable advantage to have at hand a large variety of these materials; therefore a list of useful colored media are presented below. Colored glasses. --Sets of samples can be ob- tained from various supply houses. Signal glasses afford a limited number of fairly pure colors, usually red, yellow, green, blue-green, blue, and purple. Colored gelatines. - - Very elaborate sets of colored gelatines can be obtained from theatrical supply houses. These are exceedingly useful, though lack- ing in permanency. If mounted between sheets of glass and kept in a ventilated position, many of them will be fairly durable. Complete sets of samples are very convenient. Colored lacquers. Those intended for coloring incandescent lamps are very useful, although it is often desirable to mix these carefully in order to obtain colors of greater spectral purity. Such col- ored lacquers vary considerably in permanency, and 327 328 COLOR AND ITS APPLICATIONS wherever possible it is well to apply the coloring to sheets of glass which can be mounted at some dis- tance from the lamp. This insures a much greater permanency. Aniline Dyes. For coarse work the cheap dyes used for coloring cloth will afford a fairly satisfactory range of hues. By judiciously mixing these dyes some fairly pure colors can often be obtained, al- though as mixing usually tends to produce muddy colors the better procedure is to have at hand a variety of fundamental pigments from which perhaps a satisfactory color can be selected. A variety of dyes of the better grade is almost indispensable for accurate color work. Such dyes are usually pure and fairly reproducible, and are the best coloring media for making photographic and other screens requiring pure colors. Sets of stains for tinting lantern slides are available. These coloring media can be purchased in various forms, liquid, powder, sheets, etc. It is probably surprising to the unini- tiated what a variety of coloring materials can be obtained in the stores of a city of moderate size. Artists' Pigments. Such pigments are classed as pastel, water colors, and oil paints. These all have their uses in color science. Water colors are now available in opaque moist pastes, which have advantages in some classes of work. Printers' Inks. Such a set of pigments will be found useful by those desiring to collect a variety of coloring media. They are especially adaptable to applications similar to those found in the print shop. Colored Papers. The ordinary colored tissue papers are useful in demonstrating color effects, but in the study of the science of color no series is equal in purity and uniformity to the imported colored COLORED MEDIA 329 papers, such as the Wundt colored papers supplied by Zimmerman which are mentioned in this work on various occasions. 80. Pigments. As stated in # 72 pigments are derived from mineral, animal, and vegetable matter, and in general the inorganic pigments are the most durable. The organic dyes are often more brilliant, and for a great many purposes are more satisfactory, than inorganic pigments, because the latter are usu- ally more opaque. The durability of pigments is a matter of degree, and depends upon the protection provided against moisture and other destructive agents in the atmosphere, such as gases and smoke* Few pigments will withstand excessive amounts of heat and light. An extended discussion of pigments is outside the scope of this chapter, but a few details regarding common pigments should prove helpful. The chemistry of pigments obviously is complex, so no simple rules can be formulated which will always guide the colorist in making mixtures of pigments that will not interact. Blue. In general blue pigments reflect or trans- mit an appreciable amount of deep red rays, which becomes quite noticeable under ordinary artificial light. Ultramarine is considered by the artist to be a close approach to spectral blue in hue, yet it trans- mits a considerable proportion of red rays (Fig. 103, 122). Natural ultramarine is obtained from a min- eral, but owing to its scarcity an imitation has been produced artificially in various grades. The artificial ultramarine is quite permanent and is insoluble in water, alcohol, turpentine, and oil. Ultramarine ash is a blue-gray pigment derived as a by-product in the preparation of natural ultramarine. Cobalt-blue is readily prepared quite pure and is 330 COLOR AND ITS APPLICATIONS very durable, although it is far from a pure blue. Its reddish appearance under artificial light indicates that it reflects a large proportion of deep red rays, which conclusion is supported on analyzing the re- flected light (see 6, Fig, 122). Smalte is a powdered cobalt glass of a brilliant, transparent color that is quite durable. Prussian blue, like most artificial pigments, varies in quality. It is not generally as durable as the pre- ceding blue pigments, but is fairly permanent if used alone. It interacts with many pigments. In oxalic acid it forms a satisfactory writing fluid. It can be made by mixing a ferric salt with potassium ferro- cyanide. It can be deposited intimately in contact with a fabric if the latter be dipped first into one solution and then into the other. An excess of the potassium ferrocyanide forms a compound known as soluble prussian blue. Indigo is derived from the vegetable kingdom and belongs to the lakes. It is insoluble in water, ether, oils, and cold alcohol. It dissolves in boiling concentrated alcohol and fuming sulphuric acid. In the latter solvent it forms saxon blue. There are numerous blue aniline dyes, but most of them transmit red rays as well as blue. Green. -- Chromium oxide is a durable, opaque, deep green pigment. Emerald-green usually is a carbonate of copper mixed with alumina. It is quite opaque and durable and of a brilliant green color. Many greens are made by mixtures of such pig- ments as chrome-yellow and prussian blue, but the luminosity of such a mixture depends upon the amounts of green in the two pigments (#72). Terra verte, a native mineral found in various COLORED MEDIA 331 parts of Europe, is opaque and durable and quite satisfactory in color. Malachite green is a natural carbonate of copper. It is pale green in color and moderately permanent. This pigment is being imitated artificially. There are several beautiful greens among the organic dyes, but of less durability. Yellow. -- Gamboge closely represents spectral yellow. It is a gum resin employed very extensively in water colors. It is a bright, transparent, per- manent yellow. Cadmium-yellow is a brilliant, opaque pigment which forms fairly satisfactory greens by mixing with a number of the greenish blue pigments. It contains sulphur, and therefore should not be mixed with pigments containing lead. It is satisfactory in combination with zinc-white. Indian yellow is a permanent, fairly transparent, orange-yellow. The pure pigment burns easily, which is a means of detecting fraudulent adulteration or substitution. Chrome-yellow is lead chromate and varies in color from a lemon-yellow to a deep orange, depend- ing upon the chemical constitution and the admixture of other substances. It is used in both oil and water colors. Zinc chromate is an opaque, permanent yellow pigment which mixes well with other pigments. Potassium bichromate is a permanent yellow having many uses. It dissolves in water and appears a greenish yellow in slight concentration, but ap- proaches a deep amber in a saturated solution. Satisfactory spectral yellows are rare, even among the large number of organic dyes available. Tart- razine, aurantia, martius-yellow, and naphthol-yellow 332 COLOR AND ITS APPLICATIONS are representative of these dyes. They have a green- ish tinge. The ochres, which are earthy combinations of iron oxides, yield several yellow pigments. The native ochres are yellow and red. Red. Carmine, which is obtained from the cochi- neal insect, closely imitates spectral red, and is con- sidered by many as the most beautiful red pigment known. It is opaque and mixes well with other pig- ments but is not very permanent. Vermilion is a natural compound of sulphur and mercury found in many places, and in mineralogy is called cinnabar. It is available in several hues, vary- ing from orange-red to deep red in both oil and water colors. Indian red and Venetian red are native ochres. Some of the yellow ochres are converted into light red pigments by calcining. The madder pigments, which are lakes, include various reds. The coloring matter is extracted from roots and united with alumina. These pigments are not very permanent. Lakes. The coloring elements used in lakes are generally of vegetable origin. These possess the property of being precipitated from .an aqueous solu- tion by metallic oxides, with which they combine. They have alumina, and sometimes other oxides associated with them, for a base to give them body. If it were not for the affinity of these oxides for many organic coloring matters many colors would not be available. For example, indian lake contains a col- oring matter extracted from lac; the coloring element in yellow lake is derived from berries; and the color- ing matters in cochineal and madder are extracted as described above. COLORED MEDIA 333 White. White pigments are used in diluting colored pigments for obtaining tints. White lead is carbonate of lead. It has many commercial names, but perhaps flake white is the most common. It is a very opaque pigment. Oil gives it a yellowish tint and should not be used very freely when a pure white surface is desired. White lead is attacked by sulphur and converted into black lead sulphide. It is more liable to react with other pigments than zinc-white, which is a formidable rival. Zinc-white is oxide of zinc. It possesses all of the good qualities of white lead and perhaps none of the objectionable features. It is claimed that the covering power of zinc-white is greater than that of white lead. Its sulphide is white, so that sulphur does not discolor it. Black. Black pigments are usually carbons. Ivory-black is obtained from ivory waste and pos- sesses a rich black appearance. It produces excel- lent grays when mixed with white. Bone-black is a cheaper substitute. Lamp-black is obtained by burning certain sub- stances in an atmosphere containing little air or oxygen. Kerosene and coal gas yield soot which makes a satis- factory black. Nigrosine is a black pigment, soluble in water, which is very useful. It can be readily incorporated into various mediums and makes a fairly satisfactory neutral tint screen in a gelatine film. 81. Solvents. In making lacquers various sol- vents are available, the properties of some of them being given below. (See Table III.) Methyl alcohol (wood alcohol) mixes with water in all proportions. It is similar to grain alcohol as a solvent. 334 COLOR AND ITS APPLICATIONS Ethyl alcohol (grain alcohol) dissolves many resins, oils, soaps, glycerol, camphor, celluloid, phenol, iodine, and many chlorides, iodides, bro- mides, and acetates. Acetone dissolves fats, oils, gums, resins, celluloid, and camphor, and mixes in ethyl alcohol and water. Ether, produced by distilling alcohol and sulphuric acid in proper proportions, dissolves fats, oils, resins, iodine, bromine, and many alkaloids. It mixes with alcohol, benzine, chloroform, and slightly with water. Amyl alcohol mixes with benzol, ether, alcohol, and slightly with water. It dissolves oils, camphor, resins, alkaloids, and iodine. Amyl acetate (artificial banana oil) mixes in all proportions with alcohol, amyl alcohol, and ether. It dissolves celluloid and is used in the preparation of collodion varnishes. Benzine should not be confused with benzene or benzol, the latter being derived from coal tar. It is a substitute for turpentine in paints, pils, and driers. Glacial acetic acid (pure acetic acid) mixes with water, alcohol, and ether. It dissolves oils, phenols, resins, and gelatine. Linseed oil is used as a vehicle in oil pigments. It dissolves hard resins, amber, and copal and is used for making varnishes. Poppy oil replaces linseed oil in oil pigments where the yellow color of the linseed oil is objectionable. Benzene is derived from coal tar. It mixes with alcohol, ether, petrolic ether, turpentine, and dissolves oils, fats, waxes, iodine, and rubber. It loosens paint. Benzol is an impure benzene. Toluol, toluene, and methyl-benzol are similar to it. Gelatine is soluble in hot water and concentrated acetic acid, forming, in the latter case, an adhesive COLORED MEDIA 335 paste. Potassium bichromate renders it insoluble on exposure to light. Formalin added to a warm aqueous solution and permitted to dry renders the gelatine insoluble in hot water. Turpentine dissolves fats, oils, and resins. It is used for thinning paints and varnishes. Venice turpentine is slowly soluble in absolute alcohol, but is readily soluble in ether, acetone, pe- trolic ether, benzol, and glacial acetic acid. It is used in fixing colors, in printing inks, and in spirit varnishes to give elasticity. Canada balsam is soluble in ether, chloroform, petrolic ether, benzol, turpentine, and gasoline. It is used to cement glasses, and owing to the fact that its refractive index is close to that of glass it practi- cally eliminates reflection and refraction of light at the surfaces in contact with it. For this reason it is excellent for cementing cover glasses on color filters. 82. Varnishes. A varnish is usually made by dissolving a resin in a medium such as alcohol, tur- pentine, or oil, the first forming a spirit varnish, the second a turpentine varnish, and the third an oil varnish. The so-called resins most commonly em- ployed are copal, sandarac, mastic, dammar, shellac, and amber. The properties of a varnish depend largely upon the resin and somewhat upon the solvent. If the solvent is volatile, like alcohol and turpentine, after the varnish dries the resin is left in the same state as before it was dissolved. These are quick drying varnishes. If the solvent be an oil, then both the oil and resin remain and the coating after drying is pliable and tough. Copal is soluble in hot linseed oil; sandarac in alcohol; mastic in ether, in hot alcohol, and in tur- 336 COLOR AND ITS APPLICATIONS pentine; dammar in alcohol and in turpentine; shel- lac in alcohol and in a solution of borax; amber in boiling linseed oil; gum arabic in water forming a varnish for water colors; gum kauri in hot ether, in turpentine, in amyl alcohol, and in benzol; common resin in ether, alcohol, turpentine, benzol, acetone, or hot linseed oil. Common resin in wood alcohol forms a cheap varnish. An excellent spirit varnish is obtained by dissolving dammar in alcohol and turpentine, the proportions of the latter being respectively about four to one. A weather-proof varnish can be made of dried copal 7%, alcohol 15%, ether 77%, and tur- pentine 1 %. 83. Lacquers. -- Shellac dissolved in alcohol and decanted after settling provides a cheap lacquer and solvent for some aniline dyes. Ordinary shellac is quite yellowish in color, so that the use of bleached shellac is sometimes advisable. The latter, however, does not dissolve as readily as the yellow shellac, but satisfactory proportions are one part of bleached shellac to eight parts of 90% alcohol. In making colored lacquers the aniline dyes are usually more satisfactory on account of their transparency, although they lack permanency when exposed to radiant en- ergy. The inorganic pigments are more opaque, but are usually more permanent. In general they do not dissolve, although they can be held in suspension. They are not as generally satisfactory as the aniline dyes for coloring media, excepting for their greater permanency. Photographers' ordinary collodion, which consists of pyroxylin (soluble guncotton) dissolved in ether and alcohol, can be used as a solvent for aniline dyes for coloring incandescent lamp bulbs. COLORED MEDIA 337 Ordinary photographic film (from which the emul- sion has been removed) dissolved in amyl acetate, alcohol, or acetone, provides a satisfactory lacquer for lamp colorings. Ordinary clear celluloid scraps containing a large percentage of camphor are readily dissolved in acetone, thus providing a cheap lacquer for dyeing purposes. The latter celluloid scraps dis- solve in wood alcohol and in amyl acetate, but when the lacquer drys it becomes white; however, this provides a means of making a cheap though not very satisfactory opal lacquer. Ordinary white celluloid scraps dissolved in wood alcohol provide a very cheap opal lacquer. A permanent opal solution can be made by mixing pure zinc-white to a fair consistency, using but little oil with a few drops of gold size. This can be applied by stippling with a flat-headed brush. Obviously this solution can be readily colored by mineral pigments, but such mixtures are not very transparent. If it is desirable to make a transparent glass dif- fusing or translucent, a saturated solution of epsom salts in warm water is satisfactory. After applying this solution and permitting it to dry, a surface is obtained similar to that produced by etching or sand blasting. This can be colored with some of the dyes soluble in water. Such a surface is not permanent. 84. Dyeing Gelatine Films. Perhaps the most convenient manner of making color filters for a large variety of uses is in applying the coloring matter to gelatine. A simple scheme is found in placing a photographic plate in an ordinary fixing solution for a few moments, and, after thoroughly washing it, per- mitting it to soak in an aqueous solution of the dye. The gelatine coating will absorb considerable of the dye, the depth of coloring being controlled chiefly by 338 COLOR AND ITS APPLICATIONS the concentration of the colored solution and some- what by the period of time the plate is permitted to remain in the bath. If the coloring is too dense, some of it can be washed out by placing the plate in run- ning cold water. It is sometimes necessary to acidu- late the solution slightly or to add ammonia, alcohol, etc., in order completely to dissolve the dyes, but this does not usually interfere with the above process. Better control is obtained by adding an aqueous solution of the dye to a solution of gelatine in warm water and flowing the dyed gelatine on a level plate of glass or other transparent media. This procedure lends itself to accurate reproduction. It is advisable to use a harder variety of gelatine, which can be pur- chased from chemical supply houses. From four to six per cent of gelatine (by weight) in water is found satisfactory. The gelatine is permitted to soak in cold water for an hour or more ; then the vessel con- taining it is placed in a basin of water and gently heated. It is advisable not to heat the water above 50 deg. C or more than is necessary to liquefy the gelatine. This solution should be filtered through a coarse cloth free from lint, and the plate should be flowed in a dust-free atmosphere. Sometimes it is well to warm the glass plate before flowing the gelatine. The amount of gelatine solution should be approximately one cubic centimeter to ten square centimeters of area. It is well to permit the plate to dry uniformly until completely hardened. The surface will not be optically plane, but where this is necessary another plate glass may be cemented on top of it with Canada balsam and a moderate pressure should be applied for several days. When dried at a temperature of 40 deg. C, only a day or two is re- quired for the balsam to harden. After the plates COLORED MEDIA 339 are thoroughly dry they can be bound together at the edges with metal strips or gummed paper. A dry- ing cabinet heated by means of carbon incandescent lamps is very safe and convenient, and the tempera- ture can be readily regulated by varying the number of lamps in operation. Gelatine sheets can be made by flowing the gela- tine solution upon a level aluminum plate, from which they are readily removed after drying. Doubtless there are better processes for the, latter procedure used in the manufacture of such sheets. 85. Celluloid. This material is of interest be- t- cause of its use in lacquers and its transparency and durability, which make it a substitute for glass or gelatine films. An undesirable characteristic, how- ever, is its inflammability, although tests indicate that the commercial celluloid is not dangerously explosive. It resists most acids and bases of moderate concentration when cold. Glacial acetic acid rapidly dissolves it, and when this solution is poured into water the nitrocellulose, camphor, and other substances are precipitated. It dissolves in alcohol, the best solvent being camphorated alcohol (10 parts camphor to 100 parts alcohol). Acetone, either the liquid or vapor, dissolves it. Celluloid films can be made by casting or by a continuous process, and can be polished by felt disks or rollers, using powdered pumice stone, soap, or polishing oil. Celluloid takes up dye very well from a solution of the coloring in alcohol. The colors for staining should act like mordants, or their application should be similar; that is they should penetrate deeply into celluloid, thus coloring the mass. The usual solvents are alcohol, acetone, acetic acid, and amyl acetate. 340 COLOR AND ITS APPLICATIONS In staining celluloid it is first moistened by a soften- ing agent in which the aniline dyes are mixed; then on dipping the celluloid into such a solution the dye penetrates the mass. Celluloid is readily colored by the foregoing methods, but can also be colored by means of mineral dyes, though if transparency is desired, which is the condition considered most important here, these colorings are not as satisfactory, although they pro- vide permanent colors. A solution of indigo in sul- phuric acid and neutralized by potassium hydroxide produces a blue dye. Another method which fur- nishes a more satisfactory blue results in the pro- duction of Prussian blue. The celluloid is immersed in a bath of ferric chloride, and after drying is dipped into a bath of potassium ferrocyanide. To color celluloid green, it is dipped into a solution of verdigris and ammonium chloride. To color it yellow it is immersed in a solution of lead nitrate and then dipped into a solution of neutral potassium chromate. Solu- tions of chrysoidine, auramine, and many aniline dyes in alcohol are satisfactory. To color celluloid red it may be dipped first in a dilute solution of nitric acid, then immersed in an ammoniacal solution of carmine. Color will be readily absorbed by celluloid if its sur- face is first sandblasted. 86. Phosphorescent materials. A variety of phosphorescent materials are available from chemical supply houses, in varying degrees of purity and of various colors. These have their place in colored effects, especially for demonstration purposes. They have been used in theatrical productions, but the greatest drawback is the difficulty of obtaining an illuminant emitting rays of short wave-lengths (which are the most effective in exciting phosphorescence) COLORED MEDIA 341 in sufficient intensities. The bare carbon arc and the quartz mercury arc are the most intense excitants for this purpose among artificial light sources. Lumi- nous calcium sulphide, sometimes known as Balmain's paint, is cheap and active and emits phosphorescent light of fairly long duration. It forms the basis of several cheap though not highly satisfactory phos- phorescent paints. Phosphorescent oil paints can be made by using pure linseed oil instead of the varnish which is ordi- narily used in phosphorescent paints. For artists' paints the varnish should be replaced by pure poppy oil. Phosphorescent material can be applied to cloth and paper by omitting the varnish, mixing the powder in water, and applying this paste in a convenient manner. For applying to glass or porcelain, the varnish is replaced by Japanese wax in a slightly greater quantity, and olive oil is added. These mix- tures can be fired successfully when air is excluded. Water glass (sodium silicate) is a satisfactory pro- tecting agent for such applications. 87. Miscellaneous Notes. For purely decorative effects of a temporary nature some of the colored metallic salts that crystallize when the solvent is evaporated are quite useful. For instance, if a sat- urated solution of potassium bichromate be added to a rather concentrated aqueous solution of gelatine, and this mixture be flowed while hot upon a level plate glass, on cooling it forms a yellow diffusing filter of crystalline structure. Such screens do not have a wide application; nevertheless they can be used for temporary decorative purposes. A weak solution of potassium bichromate can be used in gelatine without crystallizing or drying. The greenish tinge of this yellow can be overcome, if desirable, 342 COLOR AND ITS APPLICATIONS by an addition of a slight quantity of a dilute solu- tion of a red or pink dye. The air brush is a useful instrument for the appli- cation of liquid colorings, especially when the pig- ments do not readily dissolve in lacquers. Lamps and other objects can be readily colored by immer- sion in a colored lacquer, but this is not a very satis- factory procedure when the coloring matter is merely held in suspension. By means of an air brush any colored solution can be readily applied to an object with a fair degree of uniformity. Perhaps the most discouraging factor in the production of colored lighting effects is the lack of permanent blue and blue-green pigments that will readily dissolve in a satisfactory lacquer. Prussian blue and cobalt-blue are quite permanent, but insoluble in common lac- quers. These can be successfully applied by means of an air brush when they are held in suspension in a lacquer of thin varnish. By occasionally diverting the flow of air through the liquid such insoluble pig- ments can be kept in suspension in the binding solu- tion. For this class of work a small motor operated from two or three dry cells or a small transformer and equipped with a vertical stirring rod is exceedingly useful. Pigments can be readily tested for durability by placing them on strips of glass and partially covering them with glass. These should be exposed to sun- light or to the radiation from an arc lamp, keeping part of the pigment covered. The exposed portions should include both the unprotected portion and that protected by the cover glass. Another convenient method, depending of course upon the final uses to which the pigments or lacquers are to be put, is found in applying them directly to incandescent lamp bulbs. COLORED MEDIA 343 The lamps should be dyed in pairs, and one should be preserved while the other be operated on normal or slightly above normal voltage. If the pigments are eventually to be exposed to the weather, the tests should be made out of doors. These are a few data that have arisen in ex- perimental work in the study and application of the science of color and in the production of various color effects which may prove helpful to those interested in color. See next chapter. REFERENCES M. Toch, Materials for Permanent Painting, 1911; Chemistry and Technology of Mixed Paints. E. J. Parry and J. H. Coste, The Chemistry of Pigments. F. S. Hyde, Solvents, Oils, Gums, and Waxes. C. H. Hall, Chemistry of Paint and Paint Vehicles. W. R. Mott, Paint and Dye Testing, Trans. Amer. Electrochem. Soc. 1915. CHAPTER XVII CERTAIN PHYSICAL ASPECTS AND DATA 88. A perusal of the literature on colored media and a general acquaintance with color industries has led to the conclusion that the chemistry of such substances greatly dominates the physics in color-technology. In fact, much of the physics of color is so little used in some of these activities that it is either not generally under- stood by color-technologists or its value is underestimated. Spectral analyses the quantitative determinations of the spectral characteristics of colored materials pro- vide the foundations for many important aspects of color- technology and without such data some work is con- ducted more or less blindly. With such data and those derived from less analytical methods, many interesting facts of color-technology can be bared and various factors can be determined which are unapproachable from the viewpoint of chemistry or from ordinary visual inspection.. Of the various methods of analyzing color, that of the spectrophotometer is the most analytical and it provides data of far greater usefulness hi the physics of color than the data which are yielded by any of the other methods. By this method the reflection- (or transmission-) factors of the coloring media are de- termined for radiant energy of all wave-lengths in the visible spectrum. When these are plotted we have the spectral reflection (or transmission) curves for the visible spectrum. By the same method the spectral character of an illuminant may be obtained. By multi- 344 CERTAIN PHYSICAL ASPECTS AND DATA 345 plying the relative energy-values of the various wave- lengths of any illuminant by the corresponding visi- bilities of radiation, the spectral luminosity-distribution curves are obtained for the given illuminant. These latter will vary with the illuminant and are often of greater importance than the spectral energy-distribution curves from a visual viewpoint. It is obvious that by multiplying corresponding spectral values, the spectral energy-distribution and luminosity-distribution curves of any colored medium maybe readily obtained for any illu- minant. Such data and their uses will be presented later. Owing to the indefiniteness and limitations of the data yielded by most of these so-called colorimetric methods and the difficulties attending the use of the monochromatic colorimeter at present, this chapter will be confined almost entirely to spectrophotometric data and their uses. Many instances arise when the degree of absorption for ultra-violet and infra-red rays is of interest. The former can be determined readily by spectrophotography and the latter by means of such energy-measuring instruments as the bolometer or ther- mopile. 89. Types of Colored Media. Three classes of colored media will be represented and discussed, namely, pigments, dyes, and vitrifiable colors or colored glasses. Pigments are distinguished from dyes by their insolu- bility in their vehicle, while dyes are soluble. This distinction may appear arbitrary, especially hi some cases, however, it is employed to some extent and is a con- venient classification. Pigments may be distinguished from paints in that the latter are pigments in a vehicle or medium. Vitrifiable colors are those which impart color to glass and to similar substances. Among pig- ments are found two general classes; one in which each particle is homogeneous and the other in which 346 COLOR AND ITS APPLICATIONS a colorless base has been colored by depositing coloring matter upon it. Colored media vary in many physical characteristics such as opacity, fineness, and refractive- index, and they may be considered as varying hi 'color- ing power.' The color of a pigment in a finely divided state, whether the particles are separated by air or by a vehicle, is due to innumerable selective reflections from, and transmissions through, the minute particles. If the powdered pigment is given a smooth surface by pres- sure it does not appear as pure in color as when it is loosely packed because in the latter case a greater proportion of the incident radiant energy is able to penetrate more deeply into the body and becomes colored by selective reflections and transmissions. Radiant energy is regularly reflected from even the small sur- faces of the particles of pigment and in those cases where the minute areas of surface are properly oriented, this regularly reflected light does not find its way further into the pigment but is reflected practically unaltered in spectral character as compared to that energy which penetrates further into the mass. Thus, there is always reflected from pigments some radiant energy which is practically unchanged in spectral character which ac- counts partly for the general lack of purity of the colors of pigments. It is seen that the character of the surface is important. Furthermore, the refractive-indices of the pigment and of the vehicle (air in the case of dry powders) are of importance because the amount of light regularly reflected from a surface is dependent upon these refractive-indices. A careful study of the influence of the vehicle upon the color of a paint leads to inter- esting data from this viewpoint alone. Careful observation will reveal the influence of the porosity of a pigment-surface upon its color. An excel- CERTAIN PHYSICAL ASPECTS AND DATA 347 lent example for the purpose of illustration here is the color of a white cotton fabric compared with that of a white silk fabric after both have been soaked in the same dye-solution. In Fig. 130 are shown reproductions of microphotographs of white cotton and silk fabrics as photographed against a black background. It is seen that the silk is more transparent than the cotton fibers; in fact, the cotton fibers are merely translucent as compared with the transparency of silk fibers. The latter permit the radiant energy to penetrate more deeply, hi general, than the cotton fibers; in other words, the cotton fibers by diffuse reflection turn the energy backward before it has penetrated very deeply. For this reason the silk fabric appears of a purer color compared with that of the cotton fabric dyed in the same solution, the result, hi the case of the silk, being similar to that which would have been obtained with the cotton if the latter had been dyed in a more concen- trated solution of the dye. In a manner similar to the case of pigments the solvent appears to have certain influences upon the color of a solution of a dye although this subject has not been thoroughly studied. The substance upon which a dye has been deposited by immersion is also of importance in spectral analysis as is indicated by the case of dyeing cotton and silk fibers, Fig. 130. The transmission- factor of a dye-solution is a simple logarithmic function of the depth of a given solution or of its concentration, but this relation varies with the wave-length, in general in no definite relation between wave-length and spectral transmission-factor. For this reason no simple relation between total transmission and depth or concentration can be established. Such values of total transmission must be determined by direct measurement or by in- tegration, as will be discussed later. Fig. 130. Cotton. Fig. 130. Silk. CERTAIN PHYSICAL ASPECTS AND DATA 349 Colored glasses can be treated much in the same manner as dye-solutions. A given concentration of coloring material in a glass, that is, a given colored glass, apparently obeys the same law relating thickness and transmission-factor for a given wave-length as a dye-solution. However, it is not established that the introduction of various amounts of the coloring material (generally metallic oxides) results in corresponding con- centration as would be true in the case of dyes. In glass there is more or less chemical action and the uncertain conditions of melting make this point difficult to decide. The physics of the process by which glasses are colored by means of metallic compounds is not wholly clear. There are many chemical analogies which are of interest for their parallelism to the colors imparted to glasses by the metals in different states but the reasons for the appearance of the colors cannot be considered as being thoroughly established. Garnett 1 has pre- sented a very interesting discussion of the colors ex- hibited by certain glasses in which metallic oxides had been incorporated. It is a common supposition that the colors of certain glasses, such as gold red glasses, are due to the presence of very minute particles of metal. Solutions of some metals exhibit colors which are often exhibited by colored glasses in which the same metals have been introduced. Siedentopf and Szigmondy, by powerfully illuminating specimens of colored glass and colored colloidal solutions of metals obliquely, or at right-angles to the line of sight, were able to detect the presence of the metallic particles. Garnett's work ex- plained some of their observations. It is commonly considered that metals color glass in two ways, one by being in a state of true solution in the glass and the other by being in a colloidal state. 350 COLOR AND ITS APPLICATIONS An example of the former is copper blue-green glass and of the latter, gold red glass. In dealing with the physics of colored media from the viewpoint of the physicist, one cannot avoid the con- clusion that there is a wide application of physics to color- technology in many directions quite unexplored. 90. Pigments. In presenting data which it is hoped will be of direct use to others, only those colored media have been selected which are thought to be fairly con- stant in composition and representative. The spectral reflection-factors of a group of dry powdered pigments, commonly used in the paint industries and which from general observation appear representative, were de- termined by means of the spectrophotometer and the data are presented in Table XXII. The light was re- flected from a thick layer of the powder, the surface being gently smoothed by means of a sheet of plane glass. Whites and blacks have been omitted but these are by no means always neutral pigments. Whites are very commonly yellowish and blacks (which are only approximately black, varying in reflection-factor from 0.02 to 0.1) are often bluish or reddish. Although these departures from neutrality are not relatively great they are sufficient to be detected by means of the spectropho- tometer. Such small departures are readily detected by painting the inner surface of a box with such a sup- posedly neutral pigment and by viewing a white surface indirectly lighted by means of a light-source inside the box. The visible radiation suffers innumerable re- flections (see # 65) from the walls of the box and that which illuminates the white surface is therefore much more colored than the pigment would appear under di- rect illumination. The spectral reflection-factors of pig- ments are more difficult to obtain than the transmission- factors of dyes in solution because in the former case CERTAIN PHYSICAL ASPECTS AND DATA 351 1 8 % THiTH^t (^ c^ T-H 1 TH O co co oacNTijt^oqioooooooTHo ^2 s 2 8 Sofl't2oSS ooo *' o * Hl0 S 8 O) T^ CO C^ OOOOOCOriO^ THvHCNiOOTHOr-IO 5 s I <* COIOOOCOIOO^OOO^OOCO o 3. 0* TH (M TH TH C*3 TH ^ CO TH CO CO IO O) * TH CO CO THTHrHiIOCOOCOCOOOOCOTH^< OOCOCOCOIOCOOOC4THTHO , H n a? o w -g s q q OOfCOCOCOOCqOOCNCOt-lOCO OOOC4lOrH^1^00C4THTHO S H 3. g OOOa^THCOag(NC4cia IH 1 g g CX3C^T}HCOC-C~Tja> 1 1 1 1 1 I 1 I t 5 | I ^*H^i>9SS a*^3es*o^*33 ^^o^ H^SwrtOOlHOOUOOP 352 COLOR AND ITS APPLICATIONS more or less energy is regularly reflected from the particles directly into the instrument. Care must be taken to avoid placing the pigment surface in such a position with respect to the slit of the instrument and to the light-source that an undue amount of regularly reflected visible radiation enters the instrument. The visible radiation which is thus regularly reflected is B, CMKOffC YflLOI#(t1C6/(/fl) C, A f fl ' A/CAN D, C, f, G, \ L Fig. 131. Pigments. practically unchanged hi spectral character as compared with that which penetrates into the interstices of the pigment and is colored by innumerable transmissions through, and reflections from, the minute particles of pigment. At any angle some of the energy is regularly reflected from the minute portions of the surfaces of the particles which are properly oriented. This accounts partly for the general lack of purity of the colors of 'opaque* pigments. The data of Table XXII are plotted in Figs. 131 and 132. Spectral analyses in the ultra-violet and infra-red CERTAIN PHYSICAL ASPECTS AND DATA 353 regions are often of interest in general color-technology. In the former case spectrophotography is the simplest method of attack although the procedure is a tedious one if high accuracy is desired. It is necessary to es- tablish photographic density and pigment-illumination (or exposure) relations for various wave-lengths in order to obtain the reflection-factors for radiant energy of 0.44M Fig. 132. Pigments. various wave-lengths. Besides this the ordinary pre- cautions of photographic procedure must be taken. Another possible method is that which involves the use of the photo-electric cell. No systematic data on pig- ments in the ultra-violet region have been obtained so none will be presented, although ofttimes it has been necessary to investigate this region for a particular pig- ment. It is well to recognize the importance of such analyses in cases involving ultra-violet light. An ex- cellent example is zinc white which absorbs ultra-violet energy quite freely. 354 COLOR AND ITS APPLICATIONS The investigation of the infra-red region requires a more elaborate apparatus although in many cases where total energy-absorption is of interest this can be obtained rather easily by means of the thermopile or bolometer. In fact, the ordinary radiometer or even the thermometer covered with a pigment yields data which have some uses in practice. Coblentz 2 has published interesting data on the reflection-factors of various substances for infra-red and visible radiation of several wave-lengths. Among the substances which he studied were a number of pigments. The reflection- factors of white pigments for energy of wave-lengths 4.4ju varied from about 0.1 to 0.4 and at 8.8/z and 24/z were considerably lower. These data especially empha- size the localized nature of absorption-bands as, for example, cobalt oxide is a better reflector of long-wave energy than zinc oxide, yet for visible rays it possesses an extremely lower reflection-factor than zinc oxide. Lead oxide is a much more efficient reflector of long-wave energy than zinc oxide, magnesium carbonate and other white pigments. The importance of the infra-red analyses is apparent in many practical activities. Coblentz has pointed out that a pigment which has a low reflection- factor for energy of wave-lengths in the region of 8ju to 9/z is a better house paint in hot climes because it re-radiates maximally in this region where the maximum radiation from bodies of temperatures from 20 to 25 C. is found. If the paint has a high reflection-factor for visible rays it thus minimizes the heating effect of the incident energy. Such a combination is quite desirable in minimizing the heating effect of solar rays. This is merely one example of a vast number of interesting problems which could be met with more intelligence if spectral analyses were available. 91. Some Optical Properties of Pigments. In con- CERTAIN PHYSICAL ASPECTS AND DATA 365 sidering the optical properties of a painted surface it is necessary to distinguish between a pigment and a paint. The former and its vehicle constitute the paint and the optical properties of a painted surface depend not only upon those of the pigment but also upon the vehicle and the surface covered. H. E. Merwin 3 has made interest- ing studies of these properties. Most pigments, with the exception of lakes, consist of minute crystals and the color commonly varies with the direction of the passage of light through a crystal. Therefore, the shape of these crystals influences the value of a pigment. The transmission- (and absorption-) bands of pigment crystals are rather wide and shallow and small grains are more transparent than large ones of the same material. For this reason, a pigment con- sisting of small grains is generally brighter than one of large grains. Small grains may be considered to have diameters of the order of magnitude of IM and large ones of the order of 10/z. Usually the diameter of grains of colored pigments lies between 0.5/z and 10/*. The coloring power of a pigment generally increases as the size of the grain decreases but there is no definite relation covering different substances. For a given amount of pigment it is obvious that the total amount of surface exposed to intercept light increases inversely as the square of the diameter of the grains, although the ability of a grain to alter transmitted light in any direction increases more slowly than the diameter. Merwin considered four classes of colored pigments with regard to their adaptability to the making of tints and shades. They are as follows: a. Colored grams are chiefly of such size that if closely packed in a single layer they would transmit (or diffuse and transmit) a clear tint (say roughly 40 to 60 per cent, white). From 5 to 20 such layers would 356 COLOR AND ITS APPLICATIONS produce a full color. Either clear tints or pure shades can be made from such a pigment. Examples: chrome orange, chrome yellow, verdigris, ultramarine blue. b. Grains are so transparent that white light after traversing many layers of grains still contains a good deal (20 per cent, or more) white. Such a pigment can be used in making clear tints but not pure shades. Ex- amples: barium yellow, basic copper carbonate, stron- tium yellow. c. A single layer of grains absorbs several per cent, of the characteristic hue, and other hues almost com- pletely. Pure shades and dull tints may be made from such a pigment. Examples : vermilion, scarlet chromate, Harrison red, chrome green. d. Single grains absorb several per cent, of the characteristic hue and even several layers of grains do not absorb other hues completely. When darkened by a black pigment dull shades result, and when lightened by a white pigment dull tints are formed. Examples: Naples yellow, some Dutch pinks and yellow ochres. In the last two classes diffusing power determines to some extent and absorbing power to a greater extent, what range of pure shades can be obtained. Vehicles when dried have refractive-indices in the neighborhood of 1.5 and this indicates that the amount of light regalarly reflected from a smooth surface of a vehicle is about four per cent. A substance to be most effective as a pigment should have a high refractive- index for the hue it most freely transmits. The re- fractive-index varies considerably in the neighborhood of an absorption-band, being greater on the long-wave side than on the short-wave side. This is a reason for the greater refractive-indices usually exhibited by yellow, orange, and red pigments than by blue and violet. Of course, the refractive-index of a lake is largely deter- CERTAIN PHYSICAL ASPECTS AND DATA 357 mined by the base and is usually comparatively low. If the refractive-index of a pigment closely matches that of the vehicle, the former will diffuse very little light. Such a pigment would ordinarily be mixed with one of higher refractive-index which will diffuse the light. Obviously, a black pigment to appear black in a dried vehicle should have the same refractive-index as that of the vehicle and it must absorb all the light incident upon it. In the dry state surrounded by air the pigment particles will reflect some light regardless of their absorbing power. Even when the refractive- indices of pigment and vehicle are equal, there is re- flected directly from the surface of the paint about 4 per cent, of the incident light. To overcome this, light-traps such as possessed by a velvet may be pro- vided. Ivory black is an excellent black because its refractive-index is nearly the same as that of oil or varnish. From the foregoing consideration it is obvious that a high refractive-index is essential to a white pigment. The grains should be fine and there should be no selec- tive scattering of light of various wave-lengths. The burning vapor of metallic zinc produces very fine grains of zinc oxide. These are less than I/* in diameter. Most of the zinc oxides contain enough fine grains less than IM in diameter to give a bluish tint to paints by virtue of the selective scattering of light of the shorter wave- lengths. 92. Some Applications of Spectral Analyses of Pig- ments. The chief use of data derived from such spectral analyses is that of establishing the spectral character of the pigment. The general value of such data needs no defense, for it is the actual foundation of the pigment as a coloring material. Its purity is thus estab- lished; its influence in color-mixture may be predicted; 358 COLOR AND ITS APPLICATIONS the purity or desirability of a color resulting from various mixtures of pigments whose spectral analyses are avail- able may be predetermined; and in many ways such data are useful. It is quite beyond the scope of a single chapter to discuss all the physical uses of such data, besides it is the intention to confine the discussion chiefly to aspects which are likely to be less commonly appreciated. For the latter purposes other data such as the spectral energy-distribution in illuminants and the visibility of radiation of various wave-lengths are necessary, therefore Table XXIII is presented. The relative energy-values at various wave-lengths are given for four illuminants which represent nearly the extremes commonly encountered from the viewpoint of color. In the last column are presented the visibility data 4 standardized in the 1918 report of the Nomenclature and Standards Committee of the Illuminating Engineer- ing Society. There is no exact agreement as yet among investigators regarding the visibility of radiation of different wave-lengths, however, the data are sufficiently well established for the present purpose. On multi- plying each ordinate of a spectral energy distribution curve of an illuminant, pigment, dye, etc., by the cor- responding value of visibility, the resultant data yield the spectral luminosity-distribution of the illuminant, pigment, dye, etc. Thus, from the spectral energy and visibility data the relative spectral luminosity-values can be determined. On integrating the areas of the spectral luminosity curves, the relative total luminosity-values of colored media and of illuminants can be obtained and by dividing the area of one of the former by the area of one of the latter, the reflection-factor of the particular colored medium is obtained for the particular illuminant. Thus by computation, the reflection-factors of colored media can be obtained without any of the difficulties and CERTAIN PHYSICAL ASPECTS AND DATA 359 TABLE XXIII Spectral Energy-Distribution in Common Illuminants and the Visibility of Radiation Wave- length Blue sky Noon sun Tungsten (vacuum) Incandescent Lamp 7.9 lumens Tungsten (gas-filled) Incandescent Lamp 22 lumens Visibility of radiation* Relative to that at 556MM Absolute (Lumens per watt) watt watt 0.40/i 170 67 9 15 0.0004 0.0000006 .41 177 72 95 16.5 .0012 .0000018 .42 181 75 10.5 19 .0040 .0000060 .43 185 79 12 23 .0116 .000017 .44 186 83 15 26.5 .023 .000034 .45 187 84.3 16.7 30 038 0.000057 .46 185 88 20 33.7 .060 .000090 .47 180 91 23.5 38 .091 .000136 .48 173 92 27 42.6 .139 .000208 .49 162 92.5 32.7 47 .208 .000312 .50 157 95 37 5 52 323 0.00048 .51 146 96 42.6 56 5 .484 .00073 .52 140 97 49 62 .670 .00100 .53 132 98 54.9 67 .836 .00125 .54 127 99 62.1 72.5 .942 .00142 .55 120 99 68.6 78 993 0.00149 .56 115 100 76 83 .996 .00149 .57 108 100 83.4 88 .952 .00143 .58 104 101 91 94 .870 .00130 .59 100 100 100 100 .757 .00114 .60 97 100 108 105 631 0.00095 .61 93 100 117 111 .503 .00075 .62 90 99 126 116 .380 .00057 .63 87 98.5 136 121.5 .262 .00039 .64 85 98 146 126 .170 .00025 .65 82 97.1 157 131 0.103 0.000154 .66 80 96 167 135 .059 .000089 .67 77 95.5 179 140 .030 .000045 .68' 76 94 189 143 .016 .000024 .69 72.5 93.5 202 147.5 .0081 .0000122 .70 71 91.7 212 151 0.0041 0.0000061 .71 69.6 90 223 153.5 .0021 .0000031 .72 68 88 235 156 .0010 .0000015 * Standardized in 1918 Report of Committee on Nomenclature and Standards of I.E.S. uncertainties of color-photometry, for these have been involved in the determination of the visibility data. Such computations are found to yield results quite in 360 COLOR AND ITS APPLICATIONS agreement with those obtained by direct measurement of reflection- (or transmission-) factor. In fact, this method appeals very strongly to the author, especially because the spectral analyses should be available for many other reasons so that reflection- and transmission- factors would be by-products. The spectral luminosity-distributions of the visible radiation reflected from pigments whose spectral re- sf- - CHROMC rf C - - - fuami 1 6 1 x\ Q44M Fig. 133. Pigments. flection-factor distributions are shown in Figs. 131 and 132 and in Table XXII are presented in Figs. 133 and 134. These may also be considered as the spectral reflected- energy distributions for an imaginary illuminant of uniform spectral energy-distribution. Incidentally, the light from the noonday sun approaches this ideal fairly closely as seen by Table XXIII for in this table the energy-values of this ideal illuminant would be 100 for all wave-lengths in order to be directly comparable with the other illuminants. CERTAIN PHYSICAL ASPECTS AND DATA 361 93. Re flection- factor of Pigments. In order to cover the general case more accurately much of the fore- going discussion will be expressed mathematically but, for the sake of clearness, reference will be made to these various curves in Fig. 135 for a specific case. / = Spectral energy-distribution of an illuminant (tungsten fila- ment at 7.9 lumens per watt). J - fl/itr J//VWX A"- BufiHr J/^/v/Vx* L- fHOIJN REO //- COBALT Buse M JO J Letter* J\ V K\ LI P Fig. 134. Pigments. = Energy-value of the illuminant at any wave-length, \. = Visibility curve. = Visibility-value for energy of wave-length, A. = Spectral luminosity-distribution of illuminant /. = Spectral reflection-factor distri- bution of a pigment (light chrome yellow). 362 COLOR AND ITS APPLICATIONS R\ = Reflection-factor of the pigment for energy of wave-length, X. LP = Spectral luminosity-distribution of radiant energy reflected by the pigment. For X = 0.52 M , ad = 7 X , af = KX, ae = # x , ac = K X J X , and ab = R\K X J\. 150 100 50 7 4 (XOjj 44 46 v j60 64 .66 7* Fig. 135. Analysis of a pigment. K^J x d\ = Area enclosed by L/, which is propor- tional to the total luminous flux , received by the surface between limits Xi and X 2 , hence is equal to CE where C is a constant of proportionality. If the total is desired, the limits, Xi and X 2 , are respectively the limits of the visible spectrum which for most practical cases may be taken as 0.4^ and 0.7ju. R\K\J\d\ = Area enclosed by L Pj which is pro- portional to the total luminous flux, E' ', reflected by the surface (pigment P) and is equal to CE' . CRETAIN PHYSICAL ASPECTS AND DATA 363 If energy is of interest instead of luminosity, K\ is eliminated and the limiting wave-lengths Xi and X 2 are given the desired values. - = - = R = the reflection-factor of the /K J d X pigment P for the illuminant /, and Xi and X 2 are respectively the wave-lengths at the limits of the visible spectrum. These limits could be expressed as and oo without changing the result because beyond the visible spectrum K\ is zero. Many useful data can be obtained by such computa- tions when the spectral energy-distributions of pigments and of illuminants are available. These computations can be made for a sufficient number of wave-lengths throughout the spectrum and the relative values of the integrals can be obtained by means of a planimeter from the plotted curves or more readily by summating the computed values. Similar computations have been made for the group of pigments already introduced for four illuminants including the ideal having a uniform spectral energy- distribution. These values are presented in Table XXIV and Fig. 136. The values are given to the third decimal place not with the belief that the absolute values are determined with such accuracy but to show the differ- ences as accurately as possible obtained by this method of computation. The relative values are perhaps accurate to the third place. It is seen that the reflection-factor for a given pigment is not constant (see # 42) but varies with the illuminant. This is a point not gener- ally appreciated and inasmuch as this difference exists the suggestion is made that, for general purposes, reflection-factors be given for an illuminant of uni- 364 COLOR AND ITS APPLICATIONS IS 5 a w ti <* bo OOOC*t-CSOO 1 1 a> s^> S o> I I Eosine Naph hol S Cobalt ammon Cobalt nitrate 366 COLOR AND ITS APPLICATIONS I- o* 5 I o 3 * CO OOC~C0t-Ti< O> OOOO500OO 3. O 8 g I oo o ot^t-^iaeoiQoo co 10 oso> t>oqooqo>oo>o> <*>* CERTAIN PHYSICAL ASPECTS AND DATA 367 t ^ O CO O O 00 ^ O ' 10 C* T-4 CO d ^ : g : <=> 1 co c* el * co o* C9 iH O O O O ! * . ^ : q ::::::::: : q 1 g :::::::::: - S o i g ::::::::: g ; : 1 S ::::::::: g * * i s 3 $5 ::::: g g : a : : ^ O d 8 : " q : i : . q q ': q ! : 8 s c^S^g : : :3SSS S g ' S i o" 229JSS ' ^SE:S5 CO O3 CO d ^^ CO t~l ^? CO CO q ? 1 CQiot^^OO *iOO^THC5 g 5J 00 C5 i d B S S S S S S B S S S q S 3 S 1 ^g^^JSS :wS : o S S 1 ea T4i \Q CO * * CO 00 ^i* d CO W rH : ^ . PQ "o PQ ; *o d g d d d I I S 1 d a 1 $ d d o y El fib E' K H 9 s fi B IT bfibfibfl M. M. 1 V ! i r 1 1 1 1 V f f s 1 J f ! i 1 i I f 1 1 I 1 1 1 1 i I i 1 1 1 Aniline green Napht Neptune green . . . . | 0? *S 1 -g & & fc O 368 COLOR AND ITS APPLICATIONS form spectral energy-distribution. In cases of direct measurement of these factors the clear noon-day sun sufficiently approaches the ideal as will be shown shortly. In cases where other illuminants are used these should be specified. The measurement of reflection-factor directly is by no means standardized and in the case of colored pig- Fig. 136. Pigments. ments this measurement is attended with many diffi- culties such as the distribution of luminous flux upon the surface, its angular position with respect to the photometer, color-photometry, etc. A discussion of this has been presented elsewhere. 5 In Table XXIV the relative reflection-factors of each pigment by itself for the four illuminants are presented, that for the uniform energy-spectrum being taken as unity. This gives a better idea of the magnitude of the variation of the reflection-factor with the spectral char- acter of the illuminant. These values are plotted in Fig. 137 and as would be expected, the red and yellow CERTAIN PHYSICAL ASPECTS AND DATA 369 pigments show relatively greater reflection-factors for tungsten light than for blue sky-light with the values for sunlight (circles) lying between. It is interesting to note the proximity of the circles to unity which repre- sents the relative value of reflection-factor in each case for the ideal illuminant having an uniform spectral energy-distribution. The effect of the illuminant upon the appearance of RVO o VOON ~S ---- Slue St Fig. 137. Pigments. the color is shown in Fig. 138 using, for example, ultra- marine blue whose spectral energy-distribution is shown. The spectral luminosity-distributions of this pigment for the different illuminants have been computed for equal total amount of reflected light (enclosed areas equal). Thus an idea of the appearances of the color can be formed or conversely the reason for these differ- ent appearances under the three illuminants is manifest. Incidentally, it is seen that the pigment is of a purer color under blue skylight than under either of the other illuminants. The wave-length of maximum luminosity is 0.495/z and 0.54/z respectively for the skylight and tungsten light. This wave-length of maximum lumi- 370 COLOR AND ITS APPLICATIONS nosity is not necessarily the dominant hue of the color as analyzed by the eye or by the monochromatic color- imeter although these are often nearly coincident. 94. Spectral Analyses of Dye-solutions. The mix- ture of dyes is governed by the same subtractive principles of color-mixture as the mixture of pigments although the greater number of dyes and the more (MOM 40 JK. ffif .60 Jt* Fig. 138. Ultramarine. exacting or delicate applications of dyes in industries, in the making of accurate filters, etc., make their spectral analyses of perhaps more importance than in the case of pigments. Certainly, a knowledge of the spectral characteristics of dyes, as in the case of pigments, makes for an ease and certainty in making and in visualizing mixtures which cannot be enjoyed without such data. It is beyond the scope of this section to present a com- plete discussion of the usefulness of spectral analyses CERTAIN PHYSICAL ASPECTS AND DATA 371 of dyes or to present the spectral analyses of all the dyes available; however, a few representative analyses of dyes most common and perhaps most reproducible should be of value. These are presented in the following tables roughly classified as to color. The highest ac- curacy is not claimed for these data because it does not appear worth the effort necessary because there is no indication that these dyes are in general constant in spectral characteristics as obtained from tune to tune in the market. For the same reason it has not been considered necessary to give values of concentration. From the data presented in the tables it is possible to obtain an idea of the spectral characteristic of a given dye-solution for any depth of the particular concen- tration employed and also for any relative value of con- centration. In other words, from the data in the tables and the discussion which follows it is possible to be guided in the selection of dyes for many purposes. For the study of a dye-solution throughout an entire range of depth and concentration by the method de- scribed later, the spectral analysis should be obtained as accurately as possible. In all cases where not indicated otherwise, the solvent was distilled water. The dyes were obtained from various well-known com- mercial sources. Among the solutions will be found a few solutions of metallic salts which are incorporated for their usefulness as filters. All data have been cor- rected for surface reflections and for the absorption of the glass cell by the method of substitution. In Table XXV are presented the spectral analyses of a number of dye-solutions commonly classed as red although many are purple. The sharpness of the absorp- tion- or transmission-bands is readily visualized from the data although it is of advantage to plot the data in many cases. There are some excellently sharp bands 372 COLOR AND ITS APPLICATIONS shown, for example, that of eosine of moderate con- centration. In some cases spectral analyses for two con- centrations have been presented. In Table XXVI spectral analyses of a number of yellows are presented. It is noteworthy that there is no known dye which transmits only a narrow region near spectral yellow. The value of sharp absorption-bands is seen when a fairly monochromatic filter is desired. For instance a yellowish green dye with a sharp cut-off on the long-wave side combined with a greenish yellow dye with a sharp cut-off on the short-wave side will yield a fairly monochromatic green filter. Some of the dyes fluoresce which from the point of view of color alone is of considerable interest. Fluorescein and uranine are among the many which fluoresce strikingly. It is interesting to study these by projecting a spectrum upon their upper liquid surface and by viewing the result both from above and from the side. The spectral analyses of potassium bichromate and cobalt chromate are in- cluded. Among the greens in Table XXVII are a number of dichroics. In fact, a very common characteristic of green dyes is the exhibition of dichromatism. This can readily be ascertained by noting the energy-spectrum or spectral transmission characteristic of one of these dyes. If the transmission-factor for red, say 0.7^, is in any one case greater than that for any wave-length in the other regions of the spectrum (in the green for so-called green dyes) the solution at great depths or concentrations will appear red and therefore will be dichroic. Naphthol green is an excellent yellowish green dye. Among the greens presented, malachite, saurgriin, methylengriin, and neptune green exhibit dichromatism. One of the most annoying features of dyes is the extreme rarity of pure blue dyes. Nearly all blue dyes, CERTAIN PHYSICAL ASPECTS AND DATA 373 Table XXVIII, transmit the extreme red rays quite freely and the scarcity of blue-green dyes which are not dichroic makes it difficult often to find a combination which transmits only the violet rays. In extremely high concentrations or great depths some blue dyes effectually absorb most of the extreme red rays. In Table XXIX are presented a number of spectral analyses grouped under the common name of purple for the purpose of classification. An interesting case is that of ethyl violet in gelatine both wet and dry. After the dyed gelatine, which was flowed on clear glass, had set, and while still wet the spectral analysis was made. The sample was then allowed to dry and another spectral analysis was made. On plotting these data a decided difference in the spectral transmission curves is seen as indicated by the numerical data. The wet specimen is decidedly more reddish than when dry and an actual shift in the absorption-band takes place on drying. Although not definitely established this may be ex- plained as due to a difference in the refractive-index of the solvent in the two cases. The data are corrected for reflections from the gelatine and glass surfaces. In Table XXX are presented spectral analyses of dyed gelatine filters before and after fading by exposure to solar radiation. Such data are of special interest in many cases and it appears of interest to make a thorough study of the fading of dyes with the aid of spectral analy- ses. Certainly no great amount of information is avail- able regarding the relation of the spectral character of radiation to the spectral deterioration of dyes or the relation of either of these to the chemical composition. Incidentally, the testing of dyes under illuminants containing ultra-violet rays of extremely short wave- lengths which are practically absent in solar radiation at the earth's surface or in artificial illuminants as com- 374 COLOR AND ITS APPLICATIONS 1 -. .a Bl t- I 2 00 OJ O 00 T* t- T* t- Ol CO O1 01 TH IO TH 10 TH O> I> * I ; 5 S C- CD Tfl O OJ : g g g g : 3. ci S TH TH Tj O O TH 00 ^J< TH 2 TH O s 10 o : g S S S : : S : ^ : 2 TH g 01 TH 05 ; ; 10 co oo T< o 10 o o 1 TH : S : S : CO 01 01 CO O O g ::::: : : S : g - . 1 00 c* : : : : : |J 10 T*t 'CO TH TH O o , .. 1 oo ; CD g g g : : g : S g 5 : 8 : c> I 00 g 5 S S S : S ol : ol : o 1 TH 10 TH O 8 SCO t- 01 CO O5 rH O O t- IO 00 O Tt< IO O1 O ^ TH rH O IO * s 8 co g g OJ CO CO J t~ o> oo co oo oo co iH 10 TH CO TH t- TH 1 8 i-H I- 0> IO CO 00 TH TH O t- CD iH CO O IO O) ^> O) O) 00 vH 01 CD eo t- 01 oo eo o 3. g 8 g O 00 00 OJ CD 00 10 t- oo ^j o 01 ^j TH t- CO 00 CO O5 ^ 'oT :::_:: 1 1 Turnbull's bli Victoria blau. Prussian blue 5 g 3 1 s 1 t t | | - 1 j I 1 1 1 IP 1 21 IIIII J j CERTAIN PHYSICAL ASPECTS AND DATA 375 85 c* o> 8 CO 10 o> o> J^fcJSSSSXJSrJiSSe^w^tow OSOSOSOS^iOSOSrHOSOSOSrHOSOSCO rH OS OS O OS OS 00 8 3 8 8 S g 3 5 8 OS rH *: n . : *# o : co q -1 : => <=> o o s 3 S > 8? . .a :.a M -M .a '-& -5 -2 : iS w 1 1 > > Magenta I I s < 5J *i *i fl -O I 1 t o CERTAIN PHYSICAL ASPECTS AND DATA 377 monly encountered is open to criticism. Spectral analy- sis has not been sufficiently utilized in permanency tests to warrant all the conclusions which have been drawn in this matter although some excellent work has been done. 6 Mott has shown that the results with the ( snow- white ' flame arc in dye-fading are practically the same as those obtained in daylight. He states that the white flame arc at 25 amperes affords light at a distance of two feet more intense than summer sunlight. By focussing the image of a quartz mercury arc by means of a quartz lens, an intense illumination rich in ultra-violet rays may be obtained. The large incandescent lamps may also be used with success. 95. Applications of Spectral Analyses of Dyes. The uses for spectral analyses of dyes are manifold, as in the case of any class of colored media. In general, they provide a physical basis for systematic color-mixture be- sides providing the necessary information for choosing dyes for many purposes. In many aspects of color- technology only the integral or subjective color is finally of interest but the author cannot refrain from empha- sizing that even in such cases an intimate knowledge of colored media and their mixture cannot be attained without spectral analyses and that the combination of dyes becomes systematic with such data available. With spectrophotometric apparatus well maintained, a complete spectral analysis can be made in about an hour although there is much room for improvement in such apparatus which will result in the saving of time. However, this is not a serious matter because for a given coloring material only one anlaysis need be made, as will be shown later, to provide information for all degrees of concentration or depth of solution. The author has available hundreds of spectral analyses which, after once obtained, are a perpetual source of information. 378 COLOR AND ITS APPLICATIONS 96. Laws Pertaining to Colored Solutions. In order to simplify the study of coloring media, especially dyes and colored glasses, several simplifications have been made. These are based on theory and have been con- firmed by experiment on a few typical specimens. In order to develop this procedure it is necessary to revert to some of the established laws. Lambert first stated that all layers of equal thickness of a transparent medium absorb equal fractions of the radiant energy which enters them. This is true for homogeneous or monochromatic radiation, but cannot be applied to the total absorption of radiant energy of many wave-lengths or of extended spectral character. It follows from Lambert's law that if the thickness of the absorbing medium increases in arithmetical progression the radiation transmitted should decrease in geometrical progression. Let J be the intensity of radiation of a given wave- length entering a layer dl, then On integrating this we obtain, where J is the original intensity, J the intensity after traversing a thickness d, and k is a constant depending upon the substance and upon the wave-length of the radiant energy. Various terms have been applied to this factor such as absorption-index. In logarithmic form this equation is expressed as, Log T = log T\ = - k\d log e = - e^d Jo where 7\ is the transmission-factor for energy of wave- length X, and the subscripts, X, indicate the factors which CERTAIN PHYSICAL ASPECTS AND DATA 379 vary with the wave-length. Beer deduced the law that the absorption is the same function of the concentration of a dispersing absorbing substance as of the thickness of a single substance which may be expressed thus : / = J Ax cd or Tx = Ax cd or log Tx = cd log Ax where c is the concentration, A is the transmission- coefficient or transmissivity and the other symbols repre- sent the same factors as in the foregoing equations. The validity of Beer's law has been questioned by some and it appears that there is some doubt as to its validity in such cases as colloidal solutions. This law appears to hold when the absorbing power of a molecule is un- influenced by the proximity of other molecules. Ob- viously, if any change takes place in the condition of the dispersed substance on altering the concentration the law will not hold. Incidentally, there is work to be done on the validity of this law hi the cases of 'colloidal' glasses. Lambert's law appears to be firmly established. In so far as the foregoing laws are valid (and it ap- pears that this is true for all practical purposes such as described in this chapter) for a given solution long T\ is proportional to c?, and for a given depth, or containing cell, log TX is proportional to c. By the use of coordinate paper having a logarithmic scale along one axis and a uniform scale along the other, a great deal of interesting data can be obtained from one spectral analysis. By means of the foregoing mathematical relations the spectral analyses of colored solutions (and colored glasses) of any thickness and concentration can be obtained from two determinations of spectral character which may be reduced to a single determination. Such a method has been found exceedingly practicable in preliminary reconnoitering in search of combinations of dyes for filters, in the development of colored glasses, 380 COLOR AND ITS APPLICATIONS and in the study of many problems arising in color- technology. Some examples will suffice to illustrate the uses of this scheme in practice. Assume a solution of methyl- engriin of either known or unknown concentration. A cell of a known thickness is filled with the solution and a spectral analysis is made. For such a purpose a fairly low concentration or small depth is chosen so that radiations of all wave-lengths which are of interest are appreciably transmitted. On logarithmic paper as pre- viously described, a plot is made as shown in Fig. 139, the transmission-factors from the spectral analysis being plotted on the logarithmic scale vertically above the arbitrarily selected point on the abscissa axis in this case taken as unity. The abscissae scale may represent either concentration or depth and may be either a relative or an absolute scale. Straight lines are drawn through the points to a common point on the ordinate axis representing complete transparency or unity on this logarithmic scale. This is the common point if correc- tions have been made for surface reflections in the cell or from the glass surfaces in the case of a colored glass. If these corrections have not been made, the common point usually will be near 0.92 on the * transmission axis' if two surface reflections must be accounted for. Each straight line represents the relation of log T\ and depth or concentration for a certain wave-length. By extend- ing these lines the spectral characteristic of any depth or concentration may be read from the corresponding vertical line. If the original spectral analysis has been made with care such a simple plot yields a vast amount of data. 97. Dichromatism. --Methylengriin has been chosen in Fig. 139 because it also illustrates the inter- esting case of dichromatism so commonly exhibited by CERTAIN PHYSICAL ASPECTS AND DATA 381 dyes. It is seen that the slope of the line for 0.72^ is less than any of the others. This is proof that the dye is a dichroic. Some lines are very steep which indicates a large value of the extinction coefficient A for radiation of these wave-lengths. From the plot it is seen that this dye, in solutions of high concentration or of great depth, will not be green but will be red. Another interesting plot, of a similar nature but in- H L 1* .03 .0 \\v \\ \ \ \\N \\ Fig. 139. Methylengriin solution. cludhig relative luminosity-values instead of transmis- sion-factors is shown in Fig. 140 for rosazeine. The spectral transmission-factors for the spectral analysis used were multiplied by the visibility of radiation in each case and plotted vertically above the point desig- nated by unity on the concentration or depth scale. Instead of drawing straight lines representing various wave-lengths to a common point on the ordinate axis, each line is drawn to a point of this axis corresponding to the relative visibility of radiation of the particular 382 COLOR AND ITS APPLICATIONS wave-length. The ordinate axis is now a logarithmic scale of relative luminosity. By extending these straight lines a graphical picture of spectral luminosity of the dye-solution is obtained for any concentration of depth. It is seen that this solution in great depth or high concentration becomes deep red because the slopes of the lines become less with increasing wave-length 1.0 - 06 \ \ CONCNTFf>tT/O/V OR &EPT/1 Fig. 140. Rosazeine. after the absorption-band of a weak solution or small depth is passed. Incidentally, it will be noted that the slope of line 0.44/* is less than that of 0.58^ which shows that in low concentrations or in relatively small depths of a higher concentration the solution is purple, that is, it has an absorption-band somewhere between 0.44// and 0.60/z. Only a few wave-lengths have been used for the sake of clearness. 98. Complete Representation of the Graphical Method. In reality the schemes illustrated in Figs. 139 CERTAIN PHYSICAL ASPECTS AND DATA 383 and 140 are only completely illustrated by means of a solid of which, for example, Fig. 139 represents a pro- jection upon the face of the solid bounded by the logarith- mic 'transmission' scale and the concentration or depth scale. A model of this tri-dimensional diagram can be Fig. 141. Graphical representation of laws of spectral transmission. easily made and should be instructive. An attempt is made in Fig. 141 to illustrate the relations between transmission-factor, wave-length and concentration or thickness. For this purpose the spectral analysis of a thin piece of gold red glass was chosen. Many of the cross-section lines have been omitted for the sake of clearness. The scales are designated and the thickness 384 COLOR AND ITS APPLICATIONS of the specimen of gold ruby glass is assumed to be 2 units on the relative thickness scale. In plane 2 repre- sented by the dash-dot vertical rectangle, the spectral transmission is shown in the dash-dot curve a 2 b 2 c 2 . For the limiting case of zero thickness, this curve be- comes a straight line, T = 1, which is the top edge of the foremost rectangle, plane 0. Several points of the 'master' curve in plane 2, were taken for the purpose of illustrating the determination of the spectral char- acteristic of the glass at another thickness. In this example, thickness 5 units is taken and its spectral trans- mission is shown by the dotted curve hi plane 5, the farthest vertical rectangle. This curve is obtained by drawing straight lines in the 'wave-length' planes from the wave-lengths on the upper front scale through the points on the 'master' curve in plane 2 of corresponding wave-lengths. Thus where a given straight line inter- sects the various thickness planes, the transmission- factors for that wave-length are found. For example, & 2 is a point on the 'master' curve in plane 2 and its value as read from the transmission-scale is the trans- mission-factor of this specimen of thickness, 2 units, for radiation of wave-length 0.52^. A straight line drawn through this wave-length on T = 1 and through & 2 (always remaining in the particular wave-length plane) when prolonged intersects plane 5 at b b which is the transmission-factor for 0.52/z for a specimen of the same glass of 5 units thickness. Other points, a b , c 5 , etc., are found in the same manner. These straight lines are the same as those shown in Fig. 148 ; in fact, Fig. 148 would be seen on viewing the solid, Fig. 141, from the righthand side. A model of this solid made of wires and painted to represent the spectral colors should be instructive. In Fig. 140 luminosity-values were treated in a CERTAIN PHYSICAL ASPECTS AND DATA 385 manner similar to the transmission-values in Fig. 139. These also can be completely represented by a solid in a manner similar to that shown in Fig. 141 excepting that the vertical scale must represent logarithms of luminosity. In the limiting case of zero thickness the curve will not be the upper foremost horizontal line but will be the spectral luminosity-curve of radiation and will lie in the foremost vertical plane. On vie whig such a solid in projection from the proper side, Fig. 149 will be seen if the same gold ruby specimen be taken as an example. It appears unnecessary to illustrate this possibility since the general procedure, should be under- stood from the foregoing. In case the analysis is to be made for a particular illuminant the limiting curve in the foremost vertical plane will be the luminosity curve of the illuminant. One of the points which is emphasized in dealing with colored media in the foregoing manner is that the spectral transmission- and reflection-factors are never zero but are merely relatively low for some wave-lengths as compared with others. This is often forgotten when spectral analyses are made with instruments because when the luminosity falls below the threshold the trans- mission-factor is considered to be zero; however, the threshold depends upon the intensity of illumination or upon the brightness of the light-source. 99. Spectral Analyses of Glasses. In the develop- ment of colored glasses for the variety of practical appli- cations, the spectral analyses are extremely valuable and often essential. By means of such data these color- ing elements can be mixed computationally to obtain the desired spectral characteristic. From very meagre data on the chemical composition from one melt, fairly definite strides toward realization may be made in succeeding melts. Of course, there are chemical con- 386 COLOR AND ITS APPLICATIONS siderations which sometimes alter the predictions based on computation; however, such a procedure forms a most definite working basis. In the combination of glasses for special filters, lighting effects, etc., the com- putational method often saves time and provides definite data. Sometimes only the subjective color is desired but even in these cases spectral analyses of elemental colorings provide the basis for manipulating the avail- able verifiable colored media in a manner analogous to the combination of pigments. In the manufacture of colored glass there is a limited number of coloring materials available and when the glass must be limited to one general composition, such as soda lime, for example, the colors which are possible of attainment are further limited. However, by combin- ing various coloring materials, the variety of colored glasses can be enormously extended to meet the require- ments of science and art. In this chapter the spectral analyses of a few funda- mental colored glasses will be presented and also the results of a few simple combinations. The record num- ber of the specimen is placed before the symbol of the coloring metal such as 37 Se. If different relative thick- nesses of the specimen are presented, a number is placed before the designation proper as 10(37 Se) indicates 10 units of thickness (or of concentration); CS indicates lime soda glass; PS, lead soda; BS, barium soda; P, lead, etc. 100. Red. Selenium, copper, and gold are com- monly used for producing red glasses. In Fig. 142 are shown the spectral analyses of a number of selenium glasses. It is seen that some of these are yellow in appearance, varying from this to a deep red. The com- position of the mix is sometimes of considerable influence upon the final color. Specimen 14 Se shown in relative CERTAIN PHYSICAL ASPECTS AND DATA 387 thicknesses, 10, 20, and 34 was of unknown composition but the coloring element was selenium. This is a re- markable specimen. Cobalt blue glass (Fig. 146) trans- mits a deep red band, so a combination of dense cobalt blue and selenium glasses isolates a deep red band as seen in 6 Co + 14 Se, Fig. 142. By computations similar to those presented hi the .46 it8 .50 Jg .34 J6 J8 .60 At j64 46 Ad JO Fig. 142. Red Glasses. case of pigments (substituting transmission-factors, 7\, for reflection-factors R\) the efficiency of such a com- bination in transmitting only a deep red band can be compared with that of a very dense selenium, gold, or copper glass. Unfortunately, at the ends of the visible spectrum the visibility data are least accurate; however, such relative comparisons by computation are depend- able. Incidentally, Hyde, Cady, and Forsythe 7 have determined the visibility at the extreme red end of the visible spectrum with great care and Hartmann 8 at the blue end. 388 COLOR AND ITS APPLICATIONS In Fig. 143 is shown the spectral characteristic of a copper red glass, 4 Cu, and in Fig. 144, the spectral analyses of gold glasses are presented. Gold produces a beautiful pink in low concentrations (or hi thin layers) and deep red in the higher ones (or in thicker layers). The absorption-band is seen to be near 0.53/4 for the more transparent glasses and it is interesting to note glass F ?*- r^3> Jt>4 M M .70 ,7Z Fig. 143. Copper, sulphur, uranium, and chromium glasses. 35 Au, a lead gold, which shows a shift in the absorption- band to 0.50/1. This glass was reheated several times in bringing out the color which is decidedly more ruddy and it appears that there is a different state of division of the metallic particles perhaps as to size. As the concentration or thickness increases (glass 5 Au, which is shown for three thicknesses) the blue band gradually disappears; however, the transmission does not closely approach monochromatism. In Fig. 144 are also shown the results of combining cobalt and gold glasses of dif- ferent thicknesses (or concentrations), with the resulting transmission confined to the deep red region. CERTAIN PHYSICAL ASPECTS AND DATA 389 101. Yellow. Carbon, sulphur, uranium, and silver are among those elements which, when introduced into glass under proper chemical conditions, produce yellow glasses of varying color. No single element isolates spectral yellow. In Fig. 145 are shown the spectral analyses of carbon yellow glasses, 15 C and 44 C, and of combinations of carbon yellow and light cobalt blue I 11 \ \ JO 34 56 J9 .60 .6Z jC>4 .66 AQ JO tt&VC' LCNGTH Fig. 144. Gold and cobalt glasses. glasses. It is known that X-rays, ultra-violet and visible rays will cause some clear glasses to become colored. In Fig. 145 is also shown the spectral characteristic of a glass X, which though originally clear was colored a muddy yellow throughout the mass by X-rays. It is interesting to observe the action of X-rays in discoloring glass, for it is easy at times to observe the progress of the coloring through the thickness of the glass. Pat- terns can be made by this process. In Fig. 143 are shown the spectral characteristics of uranium (11 U) and sul- phur (43 S and 45 S) glasses. The spectral transmissions 390 COLOR AND ITS APPLICATIONS of several uranium samples appear to be kinky in the blue-green region although the exact nature of the curves are not established. 102. Green. Iron imparts a green color to glass varying from a bluish to a yellowish green, depending upon the ingredients of the glass. The importance of manganese in glass is as a decolorizing agent, its color M JO JR Fig. 145. Yellow glasses and combinations with cobalt glass. in proper concentrations being roughly complementary to that of iron commonly present in sand. Chromium imparts a yellowish green color to glass as seen in glass 53 Cr, Fig. 143. This glass has a maximum transmission at about 0.56/z and by the addition of copper blue-green (glasses 2 Cu and 8 Cu, Fig. 143) this maximum can be shifted toward the shorter wave-lengths depending upon the proportions of the coloring elements. Glass 21 Cu, called signal blue-green, is evidently a copper glass. Glass 36 Cr is a dense chromium green. In order to compare the actual colors under a given il- CERTAIN PHYSICAL ASPECTS AND DATA 391 luminant it is well to reduce these curves to luminosity values. If monochromatism is desired it is often ad- visable to combine two glasses which transmit a narrow region in common. 103. Blue. Cobalt is the most common element used to impart a blue co or to glass. Its greatest dis- advantage (although sometimes an advantage) is its Fig. 146. Cobalt glasses. transmission of a deep red band as shown in 6 Co and 7 Co, Fig. 146. This red transmission can be utilized in isolating the deep red as shown by combining cobalt and selenium or other red glasses, for example, 1 Co + 14 Se. An excellent blue glass can be made by combining cobalt with copper blue-green, for the latter effectively absorbs the deep red. The spectral characteristic of such a combination is shown in 9 Cu + 6 Co, Fig. 146. 104. Purple Nickel produces a purple color in glass and also manganese but the latter is not an efficient purple because its absorption-band is not sharp. Its 392 COLOR AND ITS APPLICATIONS chief use is to neutralize the green tint due to the pres- ence of iron in the ingredients of glass mixes. The spectral characteristic of a glass containing iron is shown in 41 Fe, Fig. 147. It is seen that a manganese glass of proper density is approximately complementary in color to the iron glass. Although the manganese neutralizes the iron in color, the transmission-factor of the resultant ja j6o MfrtT LfHGTM JO JZ Fig. 147. Manganese and iron glasses. glass may be seriously reduced. Manganese, though a useful element in glass manufacture, cannot be con- sidered important as a coloring element from the view- point of colored glasses in general. In X, Fig. 147, is shown the spectral characteristic of an originally clear glass which has been colored a deep purplish hue by exposure to X-rays. Undoubtedly this coloring is due to an effect upon the manganese present in the clear glass. This effect is commonly observed in lamp globes and window glass exposed to strong sunlight. In the former cases it is a very serious defect of glass manu- CERTAIN PHYSICAL ASPECTS AND DATA 393 facture because the author 13 has observed such globes whose transmission has been reduced as much as 50 per cent, after long exposure to intense solar radiation or to that emitted by an arc lamp. It would be far better in such cases as street-lighting glassware to eliminate the manganese and to endure the unneutralized greenish hue of the iron which is unavoidably present. *3 S-i $* \\ \\ \\ \ Fig. 148. Gold ruby glass. 105. Use of Spectral Analyses of Glasses. The ap- plications for spectral analyses of colored glasses have been fairly well covered in the discussions of pigments and dyes, for the same general procedures can be applied to colored glasses. The concentration is not so definite as in the case of dyes because, owing to the high temper- ature at which glass melts and to chemical action, the concentration of coloring material in the final glass can- not always be predicted from the amount of coloring metal added to the mix. Some of the metals such as cobalt and copper, under standardized conditions of 394 COLOR AND ITS APPLICATIONS melting, appear to produce concentrations of coloring material proportional to the amounts of the oxides added to the mix but in some cases there is doubt as to this proportionality. There is need for systematic study in .VI .008 J006 .004 .003 .00* \ \ \ \ \\ \ \ \ \ Fig. 149. Gold ruby glass. this direction. In the case of the red glasses, for example, gold ruby, which in ordinary manufacture assumes its red color on reheating, the manipulation has consider- able effect upon the density of the color. After a colored glass has been obtained it is possible to procure from a single spectral analysis the integral transmission-factor for any illuminant, the spectral characteristics of other CERTAIN PHYSICAL ASPECTS AND DATA 395 thicknesses, and those of combinations of these thick- nesses with other colored glasses as already outlined. For the sake of further exemplification, in Fig. 148 are shown the straight-line relations between thickness and transmission-factor (for entering radiation) for several wave-lengths for various thicknesses of a gold ruby glass. The relations between luminosity and r I* .03 O.S90 0.&* CL65S Q666 OF GCJSS /Af Fig. 150. Test of ' straight-line ' law. thickness for this glass are shown in Fig. 149 for various wave-lengths. Fig. 148 is a diagram of what would be seen if the solid represented in Fig. 141 were viewed from the right-hand side. At this point it is of interest to show the approximation of experimental results to the relations between spectral character and thickness as predicted by theory. This is shown in Fig. 150 for one of the glasses made during the development of an ' artificial-daylight ' glass some years ago. The method of graphical analysis was tested because of the desire for a simple method and the sped- 396 COLOR AND ITS APPLICATIONS men was ground and polished in five thicknesses. The circles show the verification of the theory. In this case correction had not been made for surface reflection so the straight lines must be drawn to a point near 0.92 on the transmission-axis. Incidentally, it is of interest to note that previous to the adoption of this method, samples of melts were ground hi the form of thin wedges and spectral analyses were made at various thicknesses. It is seen that the graphical method enormously reduces the amount of work in order to obtain the data neces- sary for such studies. In developing a colored glass for a specific purpose, various factors are considered such as the illuminant to be used and the result to be obtained. From these an ideal spectral transmission-curve is determined and by means of a few spectral analyses of different colored glasses, bearing in mind the chemical considerations if a mixture is finally necessary, various combinations can be made with the aid of the graphical method. Often the ultra-violet and infra-red spectral trans- missions are of interest and these are made hi the man- ner already described. The data on a coloring element is not considered to be sufficiently complete for record if the ultra-violet transmission is not studied at least qualitatively and in some cases the infra-red transmis- sion is investigated. 106. Influence of Temperature on Transmission of Colored Glasses. Hyde, Cady, and Forsythe 9 in study- ing red pyrometer-glasses, noted the influence of temper- ature on the transmission characteristic of a red glass and investigated this influence for temperatures from 20 to 80 C. The transmission-factor of the red glass was found to be appreciably less for various wave-lengths at the higher temperatures than at the lower temper- atures. CERTAIN PHYSICAL ASPECTS AND DATA 397 It appeared of interest to ascertain how generally the transmission-factors of colored glasses were affected by temperature. 10 In a preliminary study it did not appear worth while to investigate this question spectro- photometrically; therefore, only the transmission-factor for total visible radiation was considered. However, an idea of the change in spectral transmission is gained through the change in color of the specimen as its temper- ature is altered. It is hoped that at a later date a careful study of this phenomenon can be made spectrophoto- metrically and in parallel with chemical investigations. In order to eliminate the annoyance of large color- differences in determining the transmission-factors at different temperatures, a given specimen was cut into two pieces and one was kept at a temperature of 30 C., while the temperature of the other was altered gradually from this temperature to 350 C. The transmission- factor of a colored glass, of course, varies with the illuminant so that such a value is indefinite unless the illuminant is specified. In this account it appears suffi- cient to state that the illuminants used were gas-filled Mazda lamps operating at normal voltage. A continuous check on the constancy of the light-sources and of the transmission-factors of the optical paths was made possible by removing the two colored glasses from the optical paths momentarily without altering the temper- ature conditions. The relative transmission-factors of the two pieces of the given specimen were measured throughout the range of temperature indicated and the results for ten commercial specimens are given in the diagram and in Table XXXI. No color-difference was encountered during the meas- urements except that due to a change in the spectral transmission characteristic of the heated specimen. This color-difference became very marked for specimens 398 COLOR AND ITS APPLICATIONS Tjl 00 CO Cl O v-4 *~t iH <0 00 Is 8 S3 d ^ o & & | 8 ^ 00 CO lO *- Jj O o rt 88 888 8888 ST D ^ J *rt *'; r' i b/> ^ i 4 1 1 1 1 a g P > f* PQ o >^ -3 o d 1 ^ 8 1 1 | 3 '> , 1 i 1 1 s f X M H K A A f 1 u i i 1 * 1 if 1 1 ,2 Q iS 111 6 !il ill S3 S I 1 ill 1 1 u u o c5 o S 1 1 o o s I rH C4 CO ^< IO CO fr- 00 05 O CERTAIN PHYSICAL ASPECTS AND DATA 399 5 and 10. The transmission-factor of the hotter piece is given in terms of that of the colder piece of the same specimen at the various temperatures that is, the transmission-factors as given are relative values and not absolute. The color of a specimen at the highest temper- ature is given as compared with that of a piece of the same specimen at 30 C., the change being sufficient to be readily described in terms of our ordinary indefinite terminology. All the glasses excepting the two contain- ing cobalt decreased in transmission-factor as the tem- perature increased, and in some cases this decrease in transmission-factor was very large. The curves ob- tained by plotting temperature and relative transmission- factor are, in general, approximately straight lines in- dicating that throughout this range of temperature the transmission-factor changes approximately proportionally with the temperature for the specimens used. Owing to the relatively slight change in hue in the red end of the spectrum, the red glasses 1 and 9 did not change ap- preciably in color when heated, notwithstanding large decreases in their transmission-factors. This preliminary study indicates an interesting field for careful research which might throw more light upon the question, 'How are glasses colored?' It will be noted that the highest temperature studied is below that at which the glass becomes self-luminous or plastic. It will be of interest to carry this investigation close to the melting point. The results obtained are of interest both theoretically and practically. 107. Ultra-violet Transmission. Another interesting fact reported by the author n is the increase in trans- mission of clear glass for certain ultra-violet rays by the addition of cobalt. In other words, the range of transmission extended further in the ultra-violet region in the case of the cobalt glass than in the case of the 400 COLOR AND ITS APPLICATIONS clear glass, the slight amount of cobalt in the former being the only difference in the compositions of the two glasses. Absolam 12 has presented the data in Table XXXII which indicate the wave-lengths where complete absorp- tion commences; that is, in each case the wave-length indicating the longest one of the region of practically complete absorption. He used an arc between copper poles and a quartz spectrograph. TABLE xxxii Natural blue rock salt Beyond 225/z/i Natural rock salt colored by cathode rays. ...... . , " 225 Natural rock salt colored blue by cathode rays . . . '. . ..-> " 225 Sylvite white " 225 Chile saltpetre, ordinary white variety . . . . ....;/. 351 " violet 325 Fluorspar, colored deep violet by cathode rays Beyond 225 Diamond, yellow 320 Diamond, blue V 315 Kunzite 305 Garnet 402 Zircon, (hyacinth) red-brown 262 Zircon, decolorized by heat . . .,. . . . . .... 244 Zircon, green ". 402 Zircon, yellow. 402 Topaz, pale yellow. .......:...... 262 Topaz, dark yellow . . . . '. . . . . 229 Topaz, pale pink-brown 262 Topaz, blue 296 Emerald 320 Ruby 300 Tourmaline, green 351 " green-yellow 300 " pink 306 Spinel, blue . 402 " Purple 325 " Pink . . . 300 Kyanite, blue 320 Beryle, blue 327 Cordierite, blue-purple 325 Cairngorm 326 Ordinary clear glass is practically opaque beyond 300/z/z although clear glasses vary considerably in trans- CERTAIN PHYSICAL ASPECTS AND DATA 401 parency to ultra-violet depending upon the content of silica and other ingredients. In general, the color of a substance is no indication of its transparency to invisible radiation. 108. Compounds Sensitive to Temperature. Experi- ence with the effect of temperature on colored glasses leads to the belief that the same effect would be found with pigments and solutions. In fact, some of these effects have been noticed and it would be of interest to investigate this point systematically. Certain com- pounds change color with change in temperature and they are of practical as well as of scientific interest. The double iodide of mercury and silver is normally a light yellow but its color changes to a deep orange or red at about 50 C. Its color will return to normal on cooling unless it has been overheated. It is prepared from separate aqueous solutions of silver nitrate and potassium iodide. The latter is added to the former until the original precipitate is dissolved. At this point a strong solution of mercuric chloride is added and the precipitate formed is the bright yellow double iodide of mercury and silver. This is filtered, washed, and dried. It may be used as a paint by mixing into a solution of gum arabic. The double iodide of mercury and copper is normally red but changes to black at about 85 C. returning in color to red as it cools. This is prepared in a manner similar to the compound in the preceding paragraph excepting that copper sulphate is substituted for silver nitrate. 109. Transmission of Water and Fog. The se- lective scattering and consequent selective absorption of the atmosphere is well known and is illustrated in Fig. 13. The fine particles of dust and even molecules of gas are responsible for scattering the rays of shorter 402 COLOR AND ITS APPLICATIONS wave-length more than those of longer wave-length. For this reason the setting sun is red ; a cloud of smoke is blue, and the shadow of a puff of smoke is brownish. That this selectivity is dependent upon the size of the particles is also apparent. For example, smoke from the tip of a cigar is more bluish than that emanating from the moist end. In the latter case moisture has condensed around the carbon nuclei and these larger particles do not scatter light so selectively. It is also known that water appears various tints of blue and blue-green when it is of great depth and purity. This is especially noticeable when flying over bodies of water although the effect of the color of the bottom and of the suspended matter washed from the shore must be separated from that of the water alone. Or- dinary observations indicate that water selectively transmits rays hi the green region; that is, rays of wave-lengths near the ends of the visible spectrum are transmitted less freely than those near the middle or especially in the green region between 0.5/z and 0.6/z. This seems to have been fairly well established by experiment. Recently Utterback 14 made some determinations of the passage of various colored lights, obtained by means of filters, through artificial fogs produced by expanding saturated air. His results indicate that his fogs were most transparent to light rays of wave-lengths from 0.53/z to 0.59/-1. The transparency decreased rapidly toward the red but not so rapidly toward the blue end of the spectrum. Abbott 15 obtained similar results for water-vapor when there were dust particles hi the air. Bertel using a 'submarine' spectrograph photo- graphed the visible spectrum of the light reaching various depths. His results show the visible spectrum to be CERTAIN PHYSICAL ASPECTS AND DATA 403 rapidly narrowed. The red rays being totally absorbed at depths of 5 to 10 meters; the orange at 20 meters; and the yellow at 100 meters. In the other end of the spectrum little selective absorption appears to take place until a depth of 30 meters is reached. At 1700 meters no light has been detected by any investigator. The range of the spectrograms obtained at various depths are presented in Table XXXIII. TABLE XXXIII Depths in meters Range of wave-lengths 2 303/iM to TOOjuju 10 303 618 20 305 588 30 310 577 40 322 568 50 338 561 60 346 556 70 350 550 80 355 547 90 357 645 100 360 543 200 379 513 300 392 500 400 398 498 Many factors can influence the results obtained so that there is bound to be disagreement. However, water appears to have a definite selective transmission for light of a hue in the neighborhood of green. 110. Color-temperature of Illuminants. The vari- ous colorimeters and the spectrophotometer have been used for the purpose of comparing illuminants and of representing their spectral characteristics respectively. Another method is to compare the integral color of an illuminant (at normal operation of the lamp) with the color of a * black-body' radiator and rating the former in terms of the temperature of the latter when a color- 404 COLOR AND ITS APPLICATIONS match (approximate in some cases) obtains. In Table XXXIV the results obtained by Hyde and Forsythe 16 are presented in terms of absolute temperatures (Kel- vin scale). These values are termed * color temper- atures.' From these data and those on the black-body brightness-temperatures, the mean brightness of each light-source may be computed. TABLE XXXIV Gas flame, fish-tail (coal and water-gas) 1870 deg. K. Hefner 1875 Pentane, 10 c. p. standard 1914 Candle, paraffin 1920 Candle, sperm 1926 Kerosene, cylindrical wick 1915 Kerosene, flat wick . . . ; . . . . 2045 Acetylene, as a whole 2368 Acetylene, one spot 2448 Nernst glower, 2.3 w. p. h. c 2388 Carbon filament, 4.0 w. p. h. c 2070 Treated carbon filament, 3.1 w. p. h. c 2153 Metallized carbon filament, 2.5 w. p. h. c 2183 Osmium filament, 2.0 w. p. h. c 2176 Tantalum filament, 2.0 w. p. h. c 2249 Tungsten filament, 1.25 w. p. h. c 2385 Tungsten filament, 0.9 w. p. h. c 2543 Tungsten filament (gas-filled), 0.5 w. p. h. c. 2900 (approx.) Kingsbury 17 using some of the foregoing values as reference points has made measurements of the color- temperature of commercial gas-burners obtaining values from 1940 to 2118 deg. K, As is to be expected, the color-temperature of a flame is within certain limits dependent upon its shape, size, and position and upon the composition of the gas. This method of rating illuminants yields valuable results. REFERENCES 1. Phil. Trans, of Roy. Soc., A, 203, p. 385. 2. Bull. Bur. Stds., 9, p. 283. 3. Proc. Amer. Soc. Test. Mat., 1916, 17, part n. CERTAIN PHYSICAL ASPECTS AND DATA 405 4. Hyde, Forsythe, and Cady, Astrophys. Jour., 1918, 48, p. 65. 5. Elec. Wld., May 19, 1917. Jour. Frank. Inst., 1918, 186, p. 529. Jour. Opt. Soc., 1919, 2-3, p. 39. 6. Trans. Amer. Electrochem. Soc., 1915. 7. Astrophys. Jour., 1915, 42, p. 285. 8. Astrophys. Jour., 1918, 47, p. 83. 9. Astrophys. Jour., 1915, 42, p. 302. 10. Jour. Amer. Ceramic Soc., 1919, 2, p. 743. 11. Jour. Frank. Inst., 1918, 186, p. 111. 12. Phil. Mag., 1917, 33, p. 452. 13. Gen, Elec. Rev., 1917, 20, p. 671. 14. Trans. I.E.S., 1919. 15. Annals of Astrophys. Obs., 3, p. 214. 16. Jour. Frank. Inst., 1917, 183, p. 353. 17. Jour. Frank. Inst., 1917, 183, p. 781. INDEX TO AUTHORS Abney, 39, 68, 85, 93, 96, 104, 109, 190, 297 Aitken, 326 Alcmaeon, 181 Anaxagoras, 181 Aristotle, 181 Arons, 107 Ashe, 132 Aubert, 127, 143, 190 Babbage, 226 Baily, 223 Baltzell, 318 Becquerel, 214 Bell, 130, 196 Benham, 40 Bloch, 106, 144 Blondel and Rey, 144 Boll, 187 Bradford, 262, 320, 326 Broca and Sulzer, 137, 144 Brown, 223 Briicke, 177 Burch, 180 Busstyn, 148 Byk, 223 Cady, 20 Charpentier, 144 Chevreul, 68, 79, 175, 307, 311 Churchill, 147, 151 Clutsam, 326 Cobb, 123, 131 Cohn, 262, 320, 326 Crookes, 159 Cros, 218 Crova, 159, 197, 229 Dember, 212 Democritus, 181 Diogenes, 181 Donders, 186 Dow, 132, 203 Dufton and Gardner, 229, 241 Ebbinghaus, 189 Ebet, 326 Edridge-Green, 124, 178, 188, 190 Empedocles, 181 Exner, 103 Fabry, 107, 197 Fechner, 39, 121 Ferree, 208 Ferry, 143 Fery and Cheneveau, 197 Fick, 143 Fraunhofer, 18 Garnett, 37 Geissler, 127, 326 Greenwood, 189 Hagen, 85 Hall, 343 Harrison, 285 Hauron, du, 218, 223 Haycraft, 146 Helmholtz, 40, 116, 143, 171, 172, 177, 180 Hering, 172, 177, 178, 184 Hertz, 6 Hewitt, 44 Holmgren, 151 Houston, 199 Hughes, 326 Hussey, 229, 242 Hyde, E. P., 20, 90, 114, 143 Hyde and Forsythe, 212 Hyde, F. S., 343 Ives, F. E., 102, 218, 221, 242 Ives, H. E., 20, 93, 103, 131, 146, 196, 197, 204, 209,' 214, 217, 229, 238, 274, 281, 285, 301, 305, 314, 323 407 40S INDEX TO AUTHORS Ives and Brady, 111, 245 Ives and Kingsbury, 198, 212 Ives and Luckiesh, 127, 202, 242 Javel, 226 Johnson, 223 Joly, 60, 217, 218 Jones, B., 258 Jones, L. A., 96, 98, 127, 237 Jorgensen, 301 Karrer, 199 Kingsbury, 212 Kirchhoff, 14 Kirchhoff and Bunsen, 107 Klein, 180 Kleiner, 143 Knoblauch, 45 Koenig, 11, 101, 102, 181, 189, 199, 210 Koenig and Brodhun, 120 Koenig and Martens, 113 Koenig, ., 223 Koettgen, 229. Kries, v., 145, 183 Kiihne, 187 Ladd-Franklin, 186 Lambert, 65 Lea, 213 Lehmann, 214 Lippmann, 30, 214 Loeser, 132 Lucas, 197 Luckiesh, 20, 53, 76, 88, 99, 130, 133, 138, 143, 146, 152, 154, 158, 196, 205, 207, 229, 239, 243, 245, 256, 260 Luckiesh and Cady, 91, 229, 238 Lumiere, 60, 219 Lummer and Brodhun, 88, 108, 143 MacDonald, 326 MacDougal, 163 Major, 326 Martel, 301 Maxwell, 6, 61, 73, 101, 124 Mayer, 178 Mees, 52, 229 Merrill, 245 Mie, 37 Millar, 204 Miller, 226 Moore, 241, 258 Morris-Airey, 207 Mott, 343 Munsell, 79 Nagel, 180, 189 Nernst, 197 Neuhaus, 214 Newton, 23 Nicati, 190 Nichols, 228 Nichols and Franklin, 229 Nichols and Merritt, 212 Nicol, 32 Nutting, 22, 88, 94, 95, 112, 121, 127, 209 Ostwald, 301 Paget, 219 Parry and Coste, 343 Parsons, 159, 190 Paterson, 307, 311 Paterson and Dudding, 148 Pfund, 212 Pirani, 229 Planck, 14 Plateau, 143 Plato, 181 Porter, 145 Prang, 82 Preston, 22 Priest, 212 Purkinje, 11, 164, 191, 204 Rasch, 197 Rayleigh, 37, 124, 197 Rice, 133 Richtmeyer, 212 Ridgeway, 85, 124 Rimington, 312, 315, 322, 326 Rood, 40, 68, 85 Rowland, 29 Ruchmich, 326 Runge, 78 Ruxton, 82 Ryan, 257 INDEX TO AUTHORS 409 Schanz and Stockhausen, 159 Schwartzchild, 202 Scriabine, 315 Seebeck, 214 Seig and Brown, 212 Sharp and Millar, 243 Shepherd, 221 Simmance and Abady, 64 Snellen, 131 Stammer, 107 Starcke, 223 Stebbins, 212 Steindler, 125 Stefan-Boltzmann, 15 Stevenson, 77 Stuhr, 204 Talbot, 143 Thomson, 37 Thorp, 217 Titchener, 76, 262, 320, 326 Toch, 343 Torda, 212 Townsend, 212 Tschermak, 180 Tyndall, 37 Uhler and Wood, 49 Uhthoff, 133 Valenta, 214 Vanderpoel, 301 Vinci, da, 177 Voege, 159 Vogel, 215, 229 Weideman and Messerschmidt, 143 Wien-Paschen, 14 Wiener, 213, 214 Whitman, 100 Winch, 326 Wollaston, 18 Wood, 22, 47, 215, 291 Wundt, 190 Young, 26, 181 Young-Helmholtz, 74, 101, 103, 172, 181, 186 Zenker, 214 Zimmerman, 62 Zindler, 85 INDEX OF SUBJECTS Aberration, chromatic, 118, 284 Abney's templates, 110 Absorption, 35 by atmosphere, 147 by dust, smoke, 304 selective, 36 spectra, 50, 51 atlas of, 52 of solid dyes and refractive index, 309 of rhodamine reflector, 45 Acetic acid, 334 Acetone, 334 Acetylene, spectrum of, 21 Achromatic lens, 119 Acid violet, 306 Additive disks, papers for making, 63 method of mixing colors, 57 primary colors, 57 Advertising, colored light in, 278 displays, 274 Aesculine, 43, 202 fluorescence of, 43 absorption of ultraviolet by, 43 Affective value of colors, 262 After-images, 170, 284 colored, 171 complementary, 170 duration of, 171 effect of, 170 explanation of, 172 in painting, 282 negative, 170 positive, 170 production of, 170 Ah- brush, 342 Alcohol, 333 Allegheny County Soldiers' Memorial, 257 Amber, 336 glass, 254 Amyl acetate, 334 alcohol, 334 Analysis of color, 86 colored media, 96 color of illuminants by photometer and color niters, 107 by colored solutions, 107 by monochromatic colori- meter, 97 by polarization colorimeter, 108 by trichromatic colorimeter, 105 Aniline dyes, 303, 328 reflection from solid, 309 powdered, 309 Aniline yellow, 57 Anthracene, 43 Appearance of colors affected by duration of stimulus, 163 environment, 163, 307 illuminant, 163, 285, 303, Plate IV intensity of illumination, 163 size and position of retinal image, 163 surface character, 163 retinal adaptation, 163 mercury arc, 166 Arc spectrum, 17, 21 Art and light, 285 Art galleries, 258, 294 artificial daylight in, 244 Artificial daylight, 238 and the colorist, 305 production of, 227, 230, 235 uses for, 234 testing, 306 versus natural light, 226, 227 Artist, aim of, 282 attitude of, 282 411 412 INDEX TO SUBJECTS Artist, position of, 282 photography and the, 283 terminology used by the, 284 Artists' pigments, 328 Art of mobile color, 312 Atmospheric absorption, 147 Auramine, 340 Aurantia, 331 Average daylight, 228 Balmain's paint, 43 Banana oil, 334 Benham disk, 39 Benzene, 334 Benzine, 334 Binocular contrast, 177 Black, absolute, 72 Black body, 14 radiation, 21 Black, bone, 333 ivory, 333 lamp, 333 nigrosine, 333 Black paper, 72 velvet, 72 Blue, cobalt, 298, 329 Prussian, 329 ultramarine, 298, 329 Blue-green, filter, 67 * Bone black, 333 Booth, demonstration, 266 Borax bead, 309 Brightness, spectral sensibility and, 10 contrast, 174, Plate m of colors, 70 effect of illuminant on, 168 Brightness increment, 122 scale, 81 sensibility of retina, 122 Brush, air, 342 Cadmium yellow, 298, 331 Calcium fluoride, 41 Canada balsam, 336 Carbon dioxide tube, Moore, 241 incandescent lamp spectrum, 21 Carmine, 298, 332 Cascade method, 208 Celluloid, uses of, 339 coloring, 340 Changeable colors, 309 Charts, color, 82 Chlorophyl, 43, 310 Chrysoidine, 340 Chromatic aberration, 284 of eye, 118 Chrome yellow, 331 Chromium oxide, 298, 330 Chromoscope, 218 Clouds, selective transmission of, 38 Cobalt blue, 298, 329 glass, 205 Collodion, 336 Colloidal solutions, 37 Color analysis, 86 of illuminants, 97, 105, 107, 108 of media, 96 Color and light, 23 and vision, 117 blindness, tests for, 151 Color box, Maxwell, 161 chart, Prang, 82 Ruxton, 82 codes, 317 cylinder, Chevreul, 79 effects, disappearing and chan- ging, 275, 278 for stage and displays, 272 modern tendencies in, 276 principle of, 272 spectacular, 257 Color harmony, 261 in decoration, 251, 257 hi glasses, 37 in interiors, 261 j in lighting, 224 in north rooms, 261 in south rooms, 252 matching, 302 glasses, 308 light, 309 Color-mixing apparatus, 60 disk, 64 Color mixture, 54 Color music, 314 suggested hi Nature, 319 INDEX TO SUBJECTS 413 Color names, 78 notation, 77 of sun altitude, 38 phenomena in painting, 282 Color photography, 213 Lippmann, 214 Wood diffraction process, 215 filter processes of, 218 Joly, 218 Paget, 219 Lumiere, 219 Shepherd, 221 Ives, 221 Kinemacolor, 222 Kodachrome, 222 Color photometry, 191 preference, 260, 320 production of, 23 pyramid, 75 sensation curves, 104 sensations, growth and decay of, 137, 164 produced by colorless stimuli, 39 Color sphere, Runge, 78 terminology, 69 tree, Munsell, 79 triangle, 73 vision theories, 181 wheel, 59 Colored fabrics, appearance of wet, 310 gelatine, 327 glasses, 327 analysis of, 97 for eliminating glare, 154 for protection against ultra- violet, 157 for use with field glasses, 160 for varying contrast, 160 in the industries, 159 tests of, 157 uses for, 151 Colored headlights, 152 lacquers, 327 light in home, 252, 269 lights and colored objects, 273 lights, range of, 148 Colored media, analysis of, 96 papers, 328 Colored papers, reflection co-efficients of, 168 under colored light, 273 Zimmerman, 63 Colored patterns, successive contrast and, 173 photographs, projection of, 218 shadows, 269 surroundings, effect of, 227, 245, 250 Colorimeter, monochromatic, 93 trichromatic, 101 analysis of illuminants by, 97, 105, 107, 242 Coloring materials, 294, 327 Colorless stimuli, color sensations from, 39 Colors, 283 affective value of, 260, 320 and sounds, 312 artistic, 262 changeable, 309 cool, 252 emotive value of, 260, 320 examination of, 307 Fechner, 39 for demonstration, 306 in Nature, 35, 54 monochromatic, 35, 167 of feathers, 30 of fiery opals, 28, 30 of insects, 30 of potassium chlorate, 30 pigment, 35 produced by mixing pigments, 56 purity of spectral, 35 two-component mixtures of, 99 used with music, 317 warm, 252 Complementary colors, 55 filters, 57 hues, 59 spectral hues, 59, 75 Cones, retinal, 120 Congressional library, 257 Continuous spectra, 16 Contrast, binocular, 177 brightness, 174 hue, 174 414 INDEX TO SUBJECTS Contrast, in Nature, 291 in pigments, 291 in paintings, 292 simultaneous, 174, 285, Plate III successive, 173 theories of simultaneous, 177 Copal, 335 Critical frequency, Porter's law of, 145 wave form and, 146 Crova's method of photometry, 197 solution, 197 Crown glass, 86 Crystals, 30, 32 Cyan blue, 221 Cyanine, 37, 303, 306 Cylinder, color, 79 Dammar, 336 Daylight, artificial, 227, 230, 235, 305 average, 228 color of, 38 efficiency of illuminants, 231, 233 testing artificial, 306 uses for artificial, 234 variability of, 228, 304 versus artificial light, 225, 227 Decoration, color in, 251, 257 Defects of color photography, 220 Defining power of eye, 283 Demonstration booth, 266 Dichroic dyes, 303, 306, 308 Dichroism, 37 Diffraction 26 color photography, 216 grating, 26, 29 copies of, 29 Rowland's, 29 spectrum, Plate I Direct-comparison photometry, 203 Disk, Benham, 39 for varying brightness, 71 for varying saturation, 71 Maxwell, 61 sectored, 90 Whitman, 100 Dispersion of glass, 25 prismatic, 23 Displays, 274 Distribution of light on paintings, 291 Durability of pigments, 342 Dyes, aniline, 328 fluorescent, 310 Ear, analytic ability of, 313 comparison of eye and, 313 Edridge-Green theory, 187 Effect, Purkinje, 11 Effects of radiant energy, 7 Efficiency, daylight, 233 lighting, 228 radiant, 13 Electromagnetic theory of light, 6 Electron, 6 Emerald green, 298, 330 Environment, and colors, 303 Emotive value of colors, 320 Eosine, 310 pink, 303 Equality-of-brightness photometry, 203 Erythrosine, 306 Ether, 5, 334 Ethyl alcohol, 334 violet, 37, 57, 63 Extraordinary ray, 33 Eye, 116 a synthetic instrument, 314 chromatic aberration in, 118 as a simple lens, 283 compared with ear, 314 faults of, 283 not analytic, 92, 313 movements, 284 optical constants of, 117 Fabry's solutions, 198 Feathers, color of, 28, 30 Fechner coefficient, 121 colors, 39 law, 121 Fibers, transparency of, 303 Film, celluloid, 339 gelatine, 338 Filters, complementary, 57 for panchromatic plate, 202 for ultraviolet bands, 47, 51 for visible rays, 47 useful, 46 Flashing sign, novel, 279 INDEX TO SUBJECTS 415 Flicker photometer, Whitman-disk, 100 Simmance-Abady, 64 photometry, 203 Flickering lights, 139 Flint glass, 86 Fluorescein, 43, 310 Fluorescence, 41 colors and, 42 effect of solvent on, 45 examination of, 41 excitation of, 42 in color matching, 308, 310 tests of, 310 Fluorescent dyes, 310 reflector, 44, 153 media, 43 Fluorite prism, 26 Fluor spar, 41 Fovea centralis, 184, 307 Fraunhofer lines, 18, 19, Plate I Frosting solution, 337 Gamboge, 298, 331 Gelatine, 327, 334 filters, 269, 337 Glass, color of, 37 crown, 86 dispersion of, 25 flint, 86 prism, 26 transmission of, 26 Glasses, colored, 154, 327 Grain alcohol, 334 Grating, diffraction, 26 spectrum, Plate I Green made by mixing yellow and blue, 299, 330 Growth of color sensations, 137, 142, 164, 207 Gum kauri, 336 Gum water, 295 Hauron color photography, 218 Headlights, green-yellow, 152 Hefner lamp, spectrum of, 21 Hering theory, 184 Heterochromatic photometry, 191 Holmgren test, 151 Houston's solutions, 199 Hue, 70 and the illuminant, 169, 286, Plate IV contrast, 176, Plate HI difference, minimum, 125 sensibility, 124 Huyghen's principle, 5 Iceland spar, 32 Illuminants, brightnesses of colors and, 167 misuse of, 226 simulating old, 253 spectra of, 13 temperature and color of, 13 values and, 167 Illusion of intense illumination, 291 Impressionism, 60 Indian red, 332 yellow, 298, 331 Indigo, 298, 330 Induction, 175 Infra-red, opacity of water to, 42 photography, 47 Insects, color of, 30 Interference, 29 constructive, 3 destructive, 3 Interiors, color in, 251 Iridescent crystals, 28, 29 Irradiation, 179 Isolating spectral lines, 47, 51 Ives, (F. E.) color photography, 221 Ivory black, 333 Joly color photography, 218 Juxtapositional method, 60 Kerosene, 43 Kinemacolor, 222 Kodachrome color photography, 222 Kries (v.) duplicity theory, 183 Lacquers, 336 celluloid, 337 colored, 327 Ladd-Franklin theory, 186 Lakes, 332 416 INDEX TO SUBJECTS Lambert color-mixer, 65 Lamp black, 333 Laws of radiation, 14 Law, Bloch, 144 Blondel and Rey, 144 Porter, 145 Talbot, 143 Legibility of type, 137 Lens, achromatic, 119 simple, 118 Light and Art, 285 color, 23 Light beam, diagram of, 31 Light, 1 definition of, 1 electromagnetic theory of, 6 production, 12 sensation, 7 shade, and color, 282 the soul of art, 285 velocity of, 6 waves, 4 analogies of, 3 and sound waves, 313 white, 9, 38 Lights of short duration, 143 Lighting artist, 285 color in, 224 of art galleries, 258 of paintings, 286, 291 Line spectra, 16 Linseed oil, 334 Lippmann color photography, 214 Lumiere color photography, 219 Luminosity curve of eye, 208 equation for, 211 Macula lutea, 307 Madder pigments, 332 Magenta, 221 Malachite green, 307, 331 Martius yellow, 331 Mastic, 335 Matching of colors, 302 artificial daylight for, 305 Maxwell disks, 61 color triangle, 73 color box, 101 Mercury afrc, spectrum of, 17, 46, 50 Mercury arc, visual acuity and, 131, 136 colors under, 166 Methods of color photometry, 192, 208 limitations of, 193 secondary, 196 Methyl alcohol, 333 violet, 37, 303, 306 Mica, 29 Miscellaneous notes, 341 Mixture of colors, 54 by shadows, 66 two-component, 99 Mobile-color art, 312 development of, 317 future of, 326 instruments for, 321 Monochromatic colors, 35, 167 acuity in, 135 Moore tube, 241 Multiple reflection, 36, 248, 308 Music, development of, 312 evolution of, 318 Musical notation, 78 Naphthol green, 57, 63, 202 yellow, 306, 331 Naphthalin red, 310 Neodymium, 47 Newton's experiment, 23 rings, 30 Nicol prism, 33 Nigrosine, 307, 333 Non-selective brightness control, 114 Normal spectrum, 26, Plate I Notation, color, 77 Novel color effects, 274 Ochres, 332 Old illuminants, simulating, 253 Opal, fiery, 28, 30 solution, 337 Oil film, 29 Ordinary ray, 33 Organic dyes, 43 Overhand method, 310 Painting, after-images hi, 173 color phenomena in, 282 INDEX TO SUBJECTS 417 Painting, artificial daylight for, 286 in artificial light, 287 Paintings, cleaning, 296 hanging, 292 lighting, 291, 294 Paints, 294, 329 phosphorescent, 341 Panama-Pacific Exposition, 267 Papers, colored, 328 yellow vs. white, 226 Paraifin prism, 26 Phloxine, 310 Phosphorescence, 41, 340 Photo-electric cell, 196, 200 Photography, color, 231 infra-red, 47 the artist and, 283 true values in, 201 ultraviolet, 47 Photometry, color, 191, 207 filters for, 108 primary methods of, 192 secondary methods of, 196 Pigments, 169, 294, 328 characteristics of, 298 classes of, 295 contrast by, 291 durability of, 297, 342 limitations of, 291 mixing, 66, 297 purity of, 297, 299 sources of, 296 Pitch prism, 26 Planck's law, 14 Plane of polarization, 31 rotation of, 34 Plane-polarized light, 31 Polarization, 30, 31 by crystals, 32 by reflection, 31 Polarized light, 31 Poppy oil, 334 Potassium bichromate, 306, 331 Preference, color, 260, 320 Primary colors, 66, 67 Primary sensation curves, 182 Printing inks, 328 Prismatic spectrum, 18, Plate I Prisms, 26 Production of light, 12 Prussian blue, 265, 330, 340 Purity of colors, 70 Purkinje effect, 11, 164, 191, 204 reversed, 205 Purple, 74, 167 visual, 187 Pyramid, color, 75, 76 Quartz, dispersion of, 25 polarization by, 32 prism, 26 transparency, 26 Radiant efficiency, 13 energy, 7 Radiation and light sensation, 7 and temperature, 11 from a solid, 8 laws, 14 Rainbow, 7, 24 Range of colored lights, 148 Red, 332 References, 22, 63, 68, 86, 114, 161, 180, 189, 211, 223, 270, 281, 301, 311, 326, 343 Reflection, selective, 36 Reflectometer, 112 Refraction, 23 Refractive index, 25 absorption of dyes and, 309 Resins, 335 solubility of, 336 Resorcin-blue, 310 Retina, brightness sensibility of, 122 color sensibility of, 119, 307 Retinal rivalry, 177 Rhodamine, 202, 303, 306, 310 reflector, 44 Rivalry, retinal, 177 Rock salt prism, 26 Rods, 119 Rose bengal, 310 Rotation of plane of polarization, 34 Sandarac, 335 Saturation of colors, 70 sensibility, 127 Scattered light, 37 418 INDEX TO SUBJECTS Scattered light, colored glasses and, 162 Sectored disk, 90, 114 Seeing, 282 Selective absorption, 35, 38 reflection, 28, 248 scattering, 38 transmission, 35, 38 Sensation curves, primary, 182 Sensibility, brightness, 122 hue, 119, 124 retinal, 120 saturation, 127 Shades, 71 Shadows, colored, 66 daylight, 304 hi painting, 291 Shellac, 336 Shepherd color photography, 221 Shooting glasses, 154 Signaling, 146 lights for, 146, 152 Silver film, 48 Simmance-Abady photometer, 64 Simultaneous Contrast, 174, Plate m instantaneity of, 178 in color matching, 307 hi painting, 285 . Skylight, color of, 38 origin of, 38 spectrum of, 21 natural, 304 artificial, 305 Slit of spectroscope, 24 Smoke, absorption by, 38 Soap bubbles, 30 Solar spectrum, 17, 18 Solutions, Crova, 197 Fabry, 198 Houston, 199 Ives and Kingsbury, 198 Karrer, 199 Solvents, 333 Sounds and colors, 312 Spectra, arc, 17 of gases, 16 of illuminants, 13, 20, 21 representative, 17 Spectra, of solids, 16 ultraviolet, 50, 51 Spectral character, influence of, 167, 286 colors, 35 complementaries, 75 distribution of energy, 20, 21 lines, 19 sensibility of eye, 10 transmission of media, 91 Spectrophotometer, 69, 88 simple, 92 portable, 89 Spectroscope, 86 direct vision, 86 accessories for, 87 comparison, 88 Spectrum analysis, 15 Spectrum of daylight, 17 helium, 17 mercury, 17, 45, 50 of sodium, 48 of tungsten, 17 Spectrum, energy, 8 grating, 26, Plate I normal, 26, Plate I production of, 24 rotating colored disk, 68 visible, 8 total, 8 Specular reflection, 309 Sphere, color, 78 Spherical light waves, 5, 26 Stage, color effects for, 272 Standardization of colors, 84 Standing wave, 3 Stefan-Boltzmann law, 15 Subtractive disks, 63 color-mixing, 54, 298 primary colors, 55 Subjective yellow, 48 Successive contrast, 173 Sunlight, 38, 304 artificial, 239, 305 Surface character, influence of, 36, 169, 302 Surface color, 309 Surroundings, influence of, 245, 304, Plate IH INDEX TO SUBJECTS 419 Talbot's law, 143 Tartrazine, 202, 331 Temperature, color of light and, 9, 13 radiation and, 11 spectrum and, 9 Templates, 109 Terminology, 69 Terra verte, 298, 330 Theory of color vision, 181 Edridge-Green, 187 Hering, 184 v. Kries, 183 Ladd-Franklin, 186 Young-Helmholtz, 181 Thinner, 295 Tints, 71 Tourmaline, 32 Transmission, 35 glass, 26 selective, 36 quartz, 26 Tree, color, 79 Triangle, color, 73, 76 Tri-color method, 73 Tungsten lamps, spectrum of, 21 Turpentine, 335 Venice, 335 Ultramarine blue, 265, 298, 329 Ultraviolet transmission of media, 50 spectra, 50 Uranin, 43, 57, 310 Uranium glass, 42 Uviol blue glass, 42 Value scale, 81 Values, 70, 283 illuminants and, 167, 286 lighting and, 286 Varnish, 295, 335 Vehicles, 295 Velocity of light, 6 Venetian red, 332 Venice turpentine, 335 Vermilion, 298, 332 Visibility of radiation, 209 of point sources, 149 Vision, 278 color and, 116 Visual acuity in colored light, 129, 135 field, 120 luminosity filter, 199 phenomena in painting, 282, 284 in color matching, 302 Visual purple, 187 bleaching, 188 extracting, 177 Visual yellow, 188 Wall covering for paintings, 294 Wave motion, 2 analogies of, 3, 5 Wave theory, 1 Welsbach mantle, spectrum of, 21 Wheel, color, 59 White lead, 333 White light, 9, 38 aesthetic, 265 artificial, 304 standard, 303 subjective, 55, 235 Wien-Paschen law, 15 Wood alcohol, 333 Wood color photography, 215 Wundt colored papers, 63 Yellow pigments, 331 solutions, 48 spot, 307 visual, 188 versus white paper, 226 Young-Helmholtz theory, 101, 181 Young's double slit expt., 26 Zinc chromate, 331 white, 333 -LIBRARY YC '40017 RETURN wHICH BORHOWED LOAN DEPT. 62A-50m-2,'64 (E3494slO)9412A . Genera] Library University of California Berkeley