Project Gutenberg's Light and Colour Theories, by Joseph W. Lovibond This eBook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org. If you are not located in the United States, you'll have to check the laws of the country where you are located before using this ebook. Title: Light and Colour Theories and their relation to light and colour standardization Author: Joseph W. Lovibond Release Date: June 15, 2018 [EBook #57335] Language: English Character set encoding: UTF-8 *** START OF THIS PROJECT GUTENBERG EBOOK LIGHT AND COLOUR THEORIES *** Produced by Chris Curnow, Chris Jordan and the Online Distributed Proofreading Team at http://www.pgdp.net (This file was produced from images generously made available by The Internet Archive) LIGHT AND COLOUR THEORIES TINTOMETER. Form of Instrument for Opaque Observation. Reproductions of some Medals awarded to JOSEPH W. LOVIBOND’S Method of Colour Analysis FOR SCIENTIFIC AND COMMERCIAL PURPOSES. LIGHT AND COLOUR THEORIES and their Relation to Light and Colour Standardization By JOSEPH W. LOVIBOND ILLUSTRATED BY 11 PLATES COLOURED BY HAND Logo London E. & F. N. SPON, Limited, 57 HAYMARKET New York SPON & CHAMBERLAIN, 123 LIBERTY STREET 1915 CONTENTS PAGE. List of Plates vii Purpose ix CHAPTER I. Introduction 1 CHAPTER II. Evolution of the Method 5 CHAPTER III. Evolution of the Unit 9 CHAPTER IV. Derivation of Colour from White Light 11 CHAPTER V. Standard White Light 14 CHAPTER VI. Qualitative Colour Nomenclature 17 CHAPTER VII. Quantitative Colour Nomenclature 20 CHAPTER VIII. The Colour Scales 28 CHAPTER IX. Colour Charts 31 CHAPTER X. Representations of Colour in Space of Three Dimensions 34 CHAPTER XI. The Spectrum in relation to Colour Standardization 36 CHAPTER XII. The Physiological Light Unit 45 APPENDIX I. Colour Education 59 [v] [vi] APPENDIX II. The Possibilities of a Standard Light and Colour Unit 69 APPENDIX III. Dr. Dudley Corbett’s Radiometer 83 Index 89 ERRATA. Plate I. Newton’s Theory. The Indigo line is erroneously placed between the Violet and the Red; it should be between the Blue and the Violet. Page 40.—Fifth line from the bottom, for Fraunhoper read Fraunhofer. To face p. vi., Lovibond, Light and Colour Theories.] LIST OF PLATES TO FACE PAGE Plate I. Six Colour Theories 4 " II. Circles Illustrating Absorption of White Light 11 " III. Diagram Illustrating Analysis of White Light 13 " IV. First System of Charting Colour 31 " V. Second System of Charting Colour 33 " VI. Six Tintometrical Colour Charts 39 " VII. Two Circles 40 " VIII. Absorption Curves of Dyes 76 " IX. Fading Curves of Dyes 78 " X. Comparison Curves of Healthy and Diseased Blood 80 " XI. Specific Colour Curves of Healthy and Diseased Human Blood 82 PURPOSE The purpose of this work is to demonstrate that colour is a determinable property of matter, and to make generally known methods of colour analysis and synthesis which have proved of great practical value in establishing standards of purity in some industries. The purpose is also to show that the methods are thoroughly scientific in theory and practice, and that the results are not likely to be changed by further discoveries. Also that out of the work done a new law has been developed, which the writer calls the Law of Specific Colour Development, meaning that every substance has its own rate of colour development for regularly increasing thicknesses. THE THEORY. Of the six colours in white light—red, orange, yellow, green, blue and violet; Red, Yellow and Blue are regarded as dominants, because they visually hold the associated colours orange, green and violet in subjection. An equivalent unit of pure red, pure yellow and pure blue is adopted, and incorporated into glass. The unit is multiplied to obtain greater intensities, and divided to obtain lesser intensities. The coloured glasses are called absorbents. The red absorbent transmits violet, red and orange, but the red ray alone is [P.R. 1317 [vii] [ix] [x] visible as colour, until the other absorbents are superimposed, and the character of the group of rays changed. In the same way yellow transmits orange and green, and blue transmits green and violet, whilst the yellow and blue alone are visible as colour. Orange, green and violet are here called subordinates, which may be developed as follows:— Or. = R. + Y. Gr. = Y. + B. Vi. = B. + R. Twenty-five years’ experience in the application of the theory and the method to the requirements of practical work, have given no reason for change. Following will be found a list of awards from International Juries and Scientific Societies, also a list of industries in which the writer’s method is giving entire satisfaction. Awards by International Juries. St. Louis 1 Silver Medal. 2 Bronze Medals. Brussels 1 Gold Medal. 3 Silver Medals. Turin Diploma of Honour. 1 Gold Medal. 1 Silver Medal. Awards by Scientific Societies. Sanitary Institute of Great Britain— Bronze Medal for Colourometrical Water Analysis. Royal Sanitary Institute— Bronze Medal for Measuring Smoke Densities. International Congress on School Hygiene— Bronze Medal for Colour Educator. Royal Sanitary Institute— Silver Medal for Colour Educator. Smoke Abatement Society, Sheffield— Diploma for System of Colour Measurement. Royal Sanitary Institute— Bronze Medal for Quantitative Estimation of Colour Blindness. Franco-British Exhibition— Gold Medal for Colour Educator. International Medical Congress— Bronze Medal for Tintometer as Medical requisite. Royal Sanitary Institute— Silver Medal for recent developments. Royal Sanitary Institute— Silver Medal Corbett’s Radiometer. Formal Adoption of Tintometer Standards by— The Petroleum Industry. The Massachusetts Board of Health. The International Association of Leather Trades Chemists. The Inter-states Cotton Seed Oil Association. The Bureau of Engraving and Printing, China. In general use by the following Industries— Brewing and Malting. Tanning. Wine and Spirit Merchants. Dyeing and Printing. Paint, Oil and Varnish Merchants. Millers. Water Works Chemists. Ceramic Works. For estimating per cent. of Carbon in Steel. For estimating per cent. of Ammonia. [xi] For estimating Colours for Anthropological Classifications. For estimating Smoke Densities. For estimating Haemoglobin in the Blood. For estimating Colour of Whale Oil, etc., etc. THE METHOD. The colour composition of any object may be measured by superimposing units of different colours until the colour of the object is matched. A convenient apparatus is furnished for this purpose. The composition of the colour is learned by merely reading the markings on the glasses. It is of course necessary that in the isolation of colour rays, some unit for measuring the intensity of both light and colour be established. As will be explained later, all such units are necessarily arbitrary. In this method the unit has been established by taking the smallest amount of colour easily perceptible to the ordinary vision. This unit or “one” is divided into tenths in the darker shades, and hundredths in the lighter scale. One one-hundredth is the smallest amount of colour measurable by a normal trained vision. When equivalent units in the three colours are superimposed, their equivalent value (not their aggregate value) represents so much white light absorbed. For instance, 2 R. + 2 Y. + 2 B. absorbs two units of white light. When the absorptive power of the colour standards is less than the intensity of the light, associated white light remains. JOSEPH W. LOVIBOND. The Colour Laboratories, Salisbury. December, 1914. Light and Colour Theories CHAPTER I. Introduction. It may at first appear strange that colour, one of the most important indices of value in the Arts, Manufactures, and Natural Products, should have no common nomenclature or reliable standard for reference, the reproduction of a given colour depending for exactitude on the memory of a sensation; whereas this branch of science requires a physical means of recording a colour, with a power of recovery. It remains to be shown that this power of record and recovery is possible, and depends only on the observance of a few simple natural laws easy of application. The study of colour is carried on by two principal methods: the spectroscopic, where the colours are partially separated as a continuous band by a regular variation in their indices of refraction, the colours gradually merging into each other by overlapping in opposite directions; or by absorption, where a colour is developed by absorbing its complementary, and is isolated as a single or complex colour. This latter is nature’s own method. It is necessary to touch on some theoretical differences which exist between Scientists and Artists, as to which are Primary colours, as confusion of this character retards investigation. Scientists adopt Red, Green, and Violet as Primaries, regarding all other colours as mixtures of these; whilst Artists and Colourists adopt Red, Yellow and Blue as the Primaries, and all other colours as made from them. The theory of the Scientists is based on the phenomena developed by mixing coloured lights taken from different parts of the spectrum. This is a method of synthesis, each added colour being a progressive stage towards the complexity of white light. In this case the colour developed is that of the preponderating ray of a complex beam. The theory of the Artists is based on the phenomena developed by mixed pigments. This is a method of analysis, tending towards ray simplicity, each added pigment reducing the complexity of the colour developed by its power of selective absorption. The theoretical differences between the two schools appear to have arisen from supposing that a given colour developed by the two methods should correspond; but considering the differences in their ray composition, this would be impossible, for although both may be describable by one general colour term, as for instance a Red, they would be of two varieties. It remains to be shown that one theory may cover both sets of phenomena. The Red, Green, and Violet theory appears to be based on two principal assumptions: first that there are only three fundamental colours; and second that the rays taken from different colour areas are pure colours. Both assumptions are open to question. In regard to the first, there is no difficulty in isolating six colours; and as to the second, it can be [xii] [1] [2] [3] demonstrated that the colours do overlap in every part, with a double overlapping in the middle colours, and are therefore not simple but complex. PAST THEORIES. In a work of this nature it is unnecessary to deal minutely with the theories which have been adopted from time to time since Newton’s discovery of the continuous spectrum. It will, however, be useful to touch on the principal points where theorists are agreed, and also on some of their points of difference, the latter in order to find, if possible, the causes of their difference. TABLE I. No. of Rays. Primary Colours. Newton (later) 7 Red, Orange, Yellow, Green, Blue, Indigo, Violet Werner 6 Red, Orange, Yellow, Green, Blue, Violet Newton and Helmholtz (early) 5 Red, Yellow, Green, Blue, Violet Hering 4 Red, Yellow, Green, Blue Chevieul, Brewster, Hay, Redgrave, Field 3 Red, Yellow, Blue Young, Helmholtz (later) 3 Red, Green, Violet Note to Plate I. The respective positions of the primaries of each theory in regard to the whole cycle of distinguishable colours are illustrated above, and the primaries of each theory are shown in their several spectrum positions, the spectrum being shown as bent in circular form. The six principal theories of primary colours are given in Table I, and illustrated on Plate I, with the names of the primaries of each theory opposite the names of some of their principal advocates. It should not be forgotten when comparing these wide divergences, that each theory has been the result of experimental evidence, in what was, at the time, and remains up to the present, a new and progressive branch of science. They agree that the spectrum colours are purer than the pigmentary colours, and that by reason of their being referable to wave length positions, they are most adaptable as standards of colour. There has also been common agreement that certain colours are primaries, and that all other colours are mixtures of these, but there has been wide divergence as to their number and even the colours themselves. PLATE I ILLUSTRATING THE POSITION OF THE PRIMARIES IN RELATION TO THE COLOURS REQUIRED TO BE PRODUCED BY THE THEORETICAL MIXTURES AND ALSO IN RELATION TO THE CHROMATIC CIRCLE. To face page 4. CHAPTER II. Evolution of the Method. The writer was formerly a brewer, and this work had its origin in an observation that the finest flavour in beer was always associated with a colour technically called “golden amber,” and that, as the flavour deteriorated, so the colour assumed a reddish hue. It was these variations in tint that suggested the idea of colour standards as a reliable means of reference. The first experiments were made with coloured liquids in test tubes of equal diameter, and by these means some useful information was obtained; but as the liquids soon changed colour, frequent renewals were necessary, and there was always a difficulty and uncertainty in their exact reproduction. To obviate this, glass in different colours was tried, and long rectangular wedges with regularly graded tapers were ground and polished for standards, whilst correspondingly tapered glass vessels were made for the beers. These were arranged to work side by side, and perpendicularly, before two apertures of an optical instrument, which gave a simultaneous view of both. The apertures were provided with a fixed centre line, to facilitate the reading off of comparisons of thickness. There was no difficulty in obtaining glass which approximated to the required colour when used in one thickness only. But as thickness varied, the test no longer held good for both standards, their rates of colour change being different, making the method unreliable.[1] The system about to be described is one of analytical absorption, and has been published from time to time in the form of papers, read before Societies interested in the question of colour standardization; as also in two descriptive works by [4] [Lovibond, Colour Theories. [5] [6] the present writer. The earlier works were necessarily fragmentary, but gathered system as the subject progressed. At an early stage in the investigations it was realized that the handbooks of the period dealt largely with theoretical differences which were of little service to the technical worker. Under these circumstances the writer applied for advice to the late Mr. Browning of the Strand, who gave it as his opinion that no work existed which could be of service to the writer. All that could be done was to go on until something should be arrived at. On this, all theoretical reading was put aside, and the work proceeded on the simple lines of observing, recording, and classifying experimental facts. In working with glass of different colours it was found that some combinations developed colour, whilst other combinations destroyed it. This suggested the probability of a governing natural law; and experimental work was undertaken in the hope of discovering it. The result was the construction of a mechanical scale of colour standards, which are now in use in over one thousand laboratories, and no question of their practical accuracy arises. The principal conditions for ensuring accuracy and constancy of results are embodied in the following code of nine precautions, which have been published for nearly twenty years without being disputed. They may therefore be considered as governing laws, at least for the present. The colour theory adopted for these Governing Laws has grown out of a series of experimental facts capable of demonstration, and is summed up in the following code of nine Laws. Laws 1, 2, and 3 relate to White and Coloured Light, and are as follows:— 1. Normal white light is made up of the six colour rays, Red, Orange, Yellow, Green, Blue and Violet in equal proportions. When these rays are in unequal proportions the light is abnormal and coloured. 2. The particular colour of an abnormal beam is that of the one preponderating ray, if the colour be simple, or of the two preponderating rays if the colour be complex. The depth of colour is in proportion to the preponderance. 3. The rays of a direct light are in a different condition to the same rays after diffusion, and give rise to a different set of colour phenomena. Laws 4, 5, 6, and 7 deal with The Limitations of the Vision to appreciate Colour. 4. The vision is not simultaneously sensitive to more than two colours in the same beam of light. The colour of any other abnormal ray is merged in the luminosity of the beam. 5. The two colours to which the vision is simultaneously sensitive are always adjacent in their spectrum order, Red and Violet being considered adjacent for this purpose. 6. The vision is unable to appreciate colour in an abnormal beam outside certain limits, from two causes: (a) The colour of an abnormal beam may be masked to the vision from excess of luminosity. (b) The luminous intensity of the abnormal beam may be too low to excite definite colour sensations. 7. The vision has a varying rate of appreciation for different colours by time, the lowest being for red. The rate increases in rapidity through the spectrum, until the maximum rate is reached with violet. And since this varying rate necessitates a time limit for critical observations, five seconds has been adopted as the limit, no variations being perceptible in that time. Laws 8 and 9 relate to Colour Constants. 8. The colour of a given substance of a given thickness is constant so long as the substance itself, and the conditions of observation, remain unaltered. 9. Every definite substance has its own specific rate of colour development for regularly increasing thicknesses. CHAPTER III. Evolution of the Unit. The dimensions of the light and colour unit here adopted, together with the scales of division, were in the first instance physiological, depending entirely on the skill of normal visions for exactitude. The co-relation of equal values in the different colour scales, was secured by an elaborate system of cross-checking, rendered necessary because the establishment of a perfectly colourless neutral tint unit, demanded an exact balance in values of the different colour scales. These scales have stood the test of many years’ work by many observers, and in no case has any alteration been required. The original set is still in use. The first point which required consideration after the want of standard colour scales was realized, was the basis and dimensions of the unit. So far as the writer knew there was no published information bearing on this which could be used as a guide. Several arbitrary scales for specific purposes had already been constructed by selecting a colour depth which could easily be distinguished, calling it a unit, and scaling it by duplicating and subdividing. This course was adopted with a coloured glass which approximately matched Ales and Malt solutions, and another which matched Nesslerized Ammonia solutions. No insuperable difficulty occurred in constructing scales available for quantitative work in these two instances. [7] [8] [9] [10] The intensity of the colour unit for these arbitrary scales, was that which appeared to be most convenient for the purpose required, but the several scales had no common basis. The unit was physiological, and the exactitude of the scales depended entirely on the skill of the vision for discriminating small differences. As the writer’s experimental work progressed, it became evident that red, yellow, and blue were the only colours suitable for systematic work. The superimposition of any two, developed a third colour which apparently had no relation to either. The superimposition of the third glass modified or destroyed all colour and reduced the amount of light. This suggested the idea that if the three colours could be so balanced that the light transmitted was colourless, it would be evidence of equivalence of intensity in the individual colours. The real difficulty was in obtaining this equivalence, because a balance which transmitted a neutral tint by one light developed colour by another. This necessitated the selection of a standard light. The light finally selected was that of a so-called sea fog, away from the contaminating influence of towns. The white fog of Salisbury Plain was used as being most available. It required two years’ work to establish equivalence in the unit. PLATE II NINE CIRCLES ILLUSTRATING THE ANALYSIS OF A BEAM OF WHITE LIGHT INTO THE SIX COMPOSING COLOURS BY THE ABSORPTIVE METHOD To face page 11. CHAPTER IV. Derivation of Colour from White Light. The method of analysing white light into its colour constituents by means of coloured glass absorbents of known intensity and purity, is illustrated by the set of nine circles in Plate II, which demonstrate that colour is developed by the absorption of the complementary colour rays. The ratios of transmission are equal. In this set of illustrations the circles represent light of 20 units luminous intensity, and the absorptive value of the three glass colours is each of 20 units, therefore the whole of the light and colour energies are presumed to be dealt with. In the first set of three circles, A represents a beam of normal white light. B a similar beam as divided into the six colour rays, Red, Orange, Yellow, Green, Blue and Violet in equal proportions, C as wholly absorbed by Red, Yellow, and Blue glasses, each of 20 units colour intensity. Figures 1, 2 and 3 represent the specific action of Red, Yellow, and Blue glass on the white light. Red absorbs Yellow, Green and Blue, transmitting Violet, Red and Orange, developing Red only. Yellow absorbs Blue, Violet, and Red, transmitting Orange, Yellow and Green, developing Yellow only. Blue absorbs Red, Orange and Yellow, transmitting Green, Blue, and Violet, developing Blue only. By this method of development, Red, Yellow or Blue, when seen alone are visually monochromatic, although composite in structure, each containing a group of three rays, the middle ray alone exciting the colour sensation. Circles 4, 5, and 6 illustrate the development of Orange, Green and Violet from the triad groups, by intercepting the light with two glass colours. Circle 4, Red on Yellow, develops Orange by absorbing Yellow, Green, Blue, Violet and Red. Circle 5, Yellow on Blue, develops Green by absorbing Blue, Violet, Red, Orange and Yellow. Circle 6, Blue on Red, develops Violet by absorbing Red, Orange, Yellow, Green and Blue. By this method of demonstration the six colours fall naturally into two groups. The first group includes Red, Yellow, and Blue, whilst the second group includes Orange, Green, and Violet. The colours of the second group, Orange, Green, and Violet, are true monochromes, each being isolated from the light, by the absorption of the five other rays. These illustrations deal with light and colour of 20 units intensity;[2] as the intensity of the light here is exactly equal to the absorptive power of the standards, no free light remains; where the absorptive power of the colour standards is less than the light, associated white light remains; for instance, if only one unit of colour was developed, 19 units associated white light would remain. PLATE III To face page 13. This method of colour development by analytical absorption is further illustrated by Plate III, showing the effect of superimposition of the three colours in their several combinations as intercepting a beam of white light. [Lovibond, Colour Theories. [11] [12] [13] [Lovibond, Colour Theories. Not all lights which appear white to the vision are truly normal white; colour may be masked by excess of luminosity, and only become evident when the luminosity has been reduced, by placing neutral tint standards between the light and the observer. Direct sunlight, and some artificial lights, are instances. (Law 6 (a) page 8.) On the other hand, an abnormal light may be too low for the vision to discriminate colour. This may be observed in nature by the gradual loss of colour in flowers, etc., in the waning intensities of evening light. The order of their disappearance is shown in Chart I. CHAPTER V. Standard White Light. The colour of a substance is determined by the ray composition of the light it reflects, or transmits to the vision, the colour would therefore vary with every change in the ray proportions of the incident light; it follows that constancy in colour measurement can only be obtained by a colourless light. Up to the present diffused daylight is the only light which complies with the condition of ray equality. The absolute equality of the six spectrum colours may be difficult to establish in any light, and their constancy in equivalence under varying light intensities may be open to argument. But, as everyday work is carried on mainly under daylight conditions, and as the vision is the final arbiter for colour work, theoretical questions outside the discriminating power of the vision, need be no bar to the establishment of a working standard white light; and in saying that diffused daylight is normal white, it is only intended to mean: In so far as a normal vision can determine. Apart from any theoretical explanation it is an experimental fact, that the abnormal rays of direct sunlight, and some artificial lights, may be so modified by diffusion as to be available for a limited range of colour work. In the case of diffused north sunlight, when taken from opposite the sun’s meridian, the modification is sufficient to make it available as a standard white light. In the case of artificial lights, their use is, as yet, limited to visual matching (not recording) and arbitrary comparisons. THE BLACK UNIT. Ideal black is total absence of light, and can only be realized as a sensation, in the presence of light, which may however be in contrast or in association. The nearest approach to ideal black by contrast, is to view a hole in a box with a blackened interior, so arranged that no light entering the hole, can be reflected back to the vision: in this way associated light, if not entirely absent, is reduced to a minimum and total darkness is practically realized by the vision in contrast with the surrounding light. Pigmentary black viewed under diffused daylight conditions is always associated with white light, as no substance, however black it may be, absorbs all the impinging light; as examples, the following measurements of three white and three black pigments were made at an angle of 45 degrees with a light intensity of 25 units. This is a true quantitative analysis of the 25 units of white light after reflection from the black pigments. The black units represent the proportion of white light absorbed, whilst the beams reflected from the pigments consist of the colour values developed which are associated with the unabsorbed white light. TABLE II. Lime SulphateBlue BlackLamp BlackIvory WhiteZinc WhiteWhite Lead Standard light units 25·0 Black units (light absorbed) 9·0 9·2 9·2 — ·08 Violet units (colour developed) 2·2 1·4 1·4 — — Blue units (colour developed) 1·0 1·9 ·4 ·01 ·05 Green units (colour developed) — — — — ·07 Associated white light 12·8 12·5 14·0 24·99 24·80 Totals 25·0 25·0 25·0 25·0 25·0 25·0 The analyses demonstrate that black is not itself an active energy analogous to colour, but is a minus quantity distinguishable by contrast with the original light. The reflected beam consists of the colour developed, associated with the residue of unaltered light. Note.—Suitable proportions of Violet and Blue give character and value to black, whilst Orange and Yellow are less pleasing as tending to rustiness. CHAPTER VI. [14] [15] [16] [17] Qualitative Colour Nomenclature. SIMPLE COLOURS. The vision can separate six monochromatic colours from a beam of white light, therefore in practical work six must be dealt with, no matter how they may be theoretically accounted for. They naturally take the accepted spectrum names and symbols already in use. To these are added two other terms, Bk. to signify black, and L. for light; these terms deal with the brightness, or dinginess, of a colour. Simple Terms. Symbols. Red R Orange O Yellow Y Green G Blue B Violet V Black Bk White L COMPLEX COLOURS. The order of the association of simple colours to form complex, is governed by two factors. The first is a physiological limitation of the vision, which is unable to simultaneously distinguish more than two colours, in the same beam of light, this limits the most complex colour to two colour names. The second limitation is one of association, based on the experimental fact, that the particular two must be adjacent in their spectrum order, spectrum red and violet being considered adjacent for this purpose. Under these conditions, any given colour must be either a monochrome, or a bichrome, and all complex colours must be bichromes. Therefore the only possible combinations are as follows:— Red and Orange Orange and Yellow Yellow and Blue Blue and Green Green and Violet Violet and Red The classified order of associating symbols for describing the components of the whole range of distinguishable colours is set out in the following tables:— Monochromes of a Standard Brightness. Monochromes Brighter than Standards. Monochromes Duller than Standards. R. R. L. R. Bk. O. O. L. O. Bk. Y. Y. L. Y. Bk. G. G. L. G. Bk. B. B. L. B. Bk. V. V. L. V. Bk. The separation of the six monochromatic sensations from a point of white light The separation of the six monochromatic sensations from a point of white light, and the formation of binary sensations by the combination of adjacent colours, is graphically illustrated in the above diagram. In order to make the qualitative symbols quantitative it is only necessary to add the numerical unit value to each factor as found by direct experiment. CHAPTER VII. Quantitative Colour Nomenclature. THE GLASS STANDARD SCALES. At an early stage of the investigation, it was found that coloured glass gave better results than coloured solutions, and [18] [19] [20] that Red, Yellow, and Blue, were the only colours suitable for systematic work; it was also found that any colour could be produced by their combination. As already described arbitrary scales were first used in many colours, but were superseded by these three, which, when graded into scales of equivalent value, were found to cover all daylight colours. Upon this evidence, scales of red, yellow and blue were constructed of glass slips, each scale being all of one colour, with a regular variation of intensity from 0·01 to 20·0 units, equal units of the three scales being in equivalence with each other. The dimensions of the unit are necessarily arbitrary, but the scales comply with the essentials of a scientific standard, in that the divisions are equal, and the unit recoverable. The equality of the unit divisions in the scales, is demonstrated by a system of cross-checking. The test of colour equivalence has already been described on pages 10 and 28. The power of recovering the unit, is by co-relation to well-known physical colour constants, such as is easily obtained by definite intensities of percentage solutions, of selected pure chemical compounds in distilled water, at standard temperatures. For example, a one per cent. solution of pure crystallized copper sulphate C2SO45H2O at 60° F. when viewed in the optical instrument in a 1-inch stratum, must be matched by a combination of Yellow 1·58 and Blue 1·55. The inch of distilled water itself constitutes very little of this colour; the colour of distilled water is remarkably uniform, and might almost be taken as a colour constant, thus: A 2-foot stratum is matched by Yellow 0·1 and Blue 0·34, a 4- foot stratum by Yellow 1·0 and Blue 1·45. A one per cent. solution of Nickel Sulphate NiSO47H2O, tem. 60° F. in a 2-inch stratum must be matched by 2·2 Blue and 2·0 Yellow units. A one per cent. solution of Potassium Bichromate K2Cr2O, Tem. 60° and in a 2-inch stratum after being dulled by 0·5 neutral tint units must be matched by 34·0 yellow and 9·6 red units. METHOD OF DEVELOPING, MEASURING AND NAMING COLOUR. The single sensation colours, Red, Yellow and Blue, are matchable by a single glass from the corresponding colour scale; the depth of colour is directly indicated by the value of the glass used. The single sensation colours, Orange, Green and Violet, are matchable by a combination of equal units, from two of the standard scales, the depth of colour is directly indicated by the unit value of either of the glasses, thus: 2·0 Blue + 2·0 Red develop 2·0 units Violet. A given neutral grey is matchable by a combination of equal units from the three standard scales, the depth of grey, is directly indicated by the unit value on either of the glasses used, thus:— 3·0 Red + 3·0 yellow + 3·0 blue develop 3·0 units neutral tint. The complex colour sensations, red and yellow oranges, yellow and blue greens, blue and red violets are matchable by unequal glasses from two of the standard scales; the colour developed is not directly indicated by the unit value of the glasses, but is recorded by means of an equation, the first half of which contains the separate values of the glasses used, and the second half the names and the depth of the colours they transmit. For instance— The equation of a colour matched by 17·0 red and 2·6 blue units, is as follows:— Standard Glasses. Colour Developed. Red. Blue. Violet. Red. 17·0 + 2·6 = 2·6 + 14·4 The colour developed is a red violet in these proportions. A colour matched by Standard Glasses. Colour Developed. Red. Yellow. Orange. Red. 10·0 + 3·0 = 3·0 + 7·0 The colour developed is a red orange in these proportions. A colour matched by Standard Glasses. Colour Developed. Yellow. Blue. Green. Yellow. 3·0 + 1·5 = 1·5 + 1·5 The colour developed is a yellow green in these proportions. A colour matched by Standard Glasses. Colour Developed. Blue. Red. Blue. Violet. 6·0 + 1·8 = 4·2 + 1·8 The colour developed is a blue violet in these proportions. The standard glass colours are necessarily of a given brightness, and colours for measurement may be either brighter, or [21] [22] [23] sadder than the standards. A given complex colour of less than glass standard brightness, is matchable by unequal numbers from the three standard scales; the smallest unit value always represents the “black,” or neutral unit factor. The equation is as follows:— A colour matched by Standard Glasses. Colour Developed. Red. Yellow. Blue. Neutral Tint. Green. Blue. 1·0 + 3·0 + 9·0 = 1·0 + 2·0 + 6·0 The colour is a blue green, in the proportion of six to two, saddened by one of neutral tint. A given complex colour of greater brightness than the glass standards, is first dulled by the interception of neutral tint units, until measurable in the manner described above; the intercepting glasses represent the unit value of excess of brightness, and is shown in the equation as light units, for instance— Standard Glasses. Colour Developed. Neutral Tint. Yellow. Blue. Light. Green. Yellow. 1·5 + 7·5 + 0·5 = 1·5 + 0·5 + 7·0 The colour is a yellow green in the proportions of 7·0 of yellow, to 0·5 of green, and 1·5 brighter than the standards. Every daylight colour being thus measurable by a suitable combination of standard glasses, with or without the addition of a Light, or a Neutral Tint factor, it follows that any colour can be described both qualitatively, and quantitatively, in terms of the colour sensations yielded by the standard glasses and their combination. The distinct colour sensations are those, which, by common consent are known as Red, Yellow, Blue, Orange, Green and Violet, and they are yielded by single glasses, or by pairs as already described; all colours therefore fall into the following categories:— A.—Single colour sensations:— 1. Transmitted by single glass standards: Red. Yellow. Blue. 2. Transmitted by equivalent pairs of standard glasses: Orange. Green. Violet. B.—Double colour sensations transmitted by unequal pairs of standard glasses. Red orange, transmitted by unequal units of red and yellow, red preponderating. Yellow orange, transmitted by unequal units of red and yellow, yellow preponderating. Yellow green, transmitted by unequal units of yellow and blue, yellow preponderating. Blue green, transmitted by unequal units of yellow and blue, blue preponderating. Blue violet, transmitted by unequal units of blue and red, blue preponderating. Red violet, transmitted by unequal units of blue and red, red preponderating. C.—Any of the above colours with the addition or subtraction of neutral tint. Neutral tint itself, is transmitted by a combination of equal units of the standard glasses, thus three units red, yellow and blue, when superposed, transmit three units neutral tint. Examples. Three units red, of standard brightness, completely describes a colour matched by a red glass of three units, and is denoted R. 3·0 Three units red saddened by one neutral tint, completely describes a colour matched by a red glass standard of four units red, combined with a blue and yellow of one unit each, and is denoted R. 3·0 + N.T. 1·0 A given red of three units, which is one unit brighter than standards, after having been saddened by one unit each of red, yellow and blue, is matched by three units of red and is correctly described by Red 3·0 + Light 1·0 Three units of violet, of standard brightness, is matched by a red and a blue glass of three units, and is correctly described by [24] [25] [26] V. 3·0 Three units of orange, of standard brightness, is matched by a red and a yellow glass of three units, and is correctly described by O. 3·0 A binary red violet of standard brightness, in which red preponderates by one unit, is matched by four units red, and a blue of three units, and is correctly described by R. 1·0 + V. 3·0 A binary red orange, of standard brightness, in which orange preponderates by three units, is matched by red four and yellow three units, and is correctly described by R. 1·0 + O. 3·0 A red orange, of less than standard brightness by one unit, in which orange preponderates by three units, is matched by a red five, yellow four, blue one, and is correctly described by R. 1·9 + O. 3·0 + N.T. 1·0 A red violet, in which red preponderates by one unit, and is one unit brighter than standard, is first dulled by one unit red, yellow and blue, and then matched by four red and three blue, and is correctly described by R. 1·0 + V. 3·0 + Light 1·0 A red orange, in which red preponderates by one unit, and is one unit brighter than standard, is first dulled by one red, yellow and blue, and then matched by four red, and three yellow, and is correctly described by R. 1·0 + O. 3·0 + Light 1·0 CHAPTER VIII. The Colour Scales. A normal vision under ordinary conditions, has no hesitation in correctly naming the sensations produced by the triad groups red, yellow and blue, or by the single rays orange, green and violet. It can also correctly describe a complex colour sensation, by naming the two associated colours, such as red orange, yellow orange, blue green, blue violet, etc.; but when called upon to decide differences of colour depth, it can only do so by using arbitrary terms of no precise scientific value, such as light, medium, dark, etc. This deficiency is because the vision has in itself no arrangement for the quantitative definition of colour depth. This want can only be met by co-relating colour sensations, to some physical colour constants. This co-relation has now been effected by a series of glass standard colour scales, which are numerically graded for colour depth, the scales themselves being colour constants by co-relation to percentage solutions, of such coloured chemicals, as copper sulphate, nickel sulphate, potassium permanganate, etc. These substances as well as many others, are always available for checking the constancy of the scales, or for recovering the unit if lost. As already mentioned, the system of taper scales proved to be useless for the purpose, not only because the rate of colour increase was never in proportion to the rate of thickness increase, but also because no two substances are equal in this respect, each having a rate specific to itself. The prismatic spectrum colours were not available for several reasons, first as being unsuitable for critical comparisons under daylight conditions, as being too weak except “in camera”; also they were found to be too crowded for the separation of a working area of monochromatic colour, and some corrections would have been necessary for variation in the refractions of different colour rays. This is more fully dealt with under the heading of The Spectrum in relation to Colour Standardization, page 36. THE EQUIVALENCE OF THE COLOUR SCALES. The method employed for obtaining equality of the unit divisions, and colour equivalence between the different scales was as follows:— Two slips of red glass in a light shade were made exactly equal in colour, and considered as initial units; these were then superimposed and matched by a single glass, which was then considered as of two units, this and one of the initial units were superimposed, and matched by a single glass of three colour units, and so on, until a progressive red scale was constructed, ranging in intensity from ·01 to 20· units.[3] The yellow and blue scales, were similarly constructed, taking care that their similar unit values were in colour equivalence with the red units, the test of equivalence being, that when equal units of the three scales were superimposed against a normal white light, a neutral grey was transmitted, in which no trace of colour could be perceived by the common consent of the whole staff of trained observers. [27] [28] [29] [30] The scales were then considered as in colour equivalence with each other. The system of cross-checking was so elaborate, that after the equivalence of the first unit was established, nearly four years was occupied in the work before the scales were passed as satisfactory. It may be urged that the unit is arbitrary, but this applies also to the unit of any other standard scale; it is sufficient that the essentials of a philosophic scale are complied with, in that the divisions are equal, and the unit recoverable. PLATE IV COMPARISON CURVES OF HEALTHY HUMAN BLOOD WITH THE BLOOD OF LOWER ANIMALS. To face page 31. CHAPTER IX. Colour Charts. A colour chart is constructed by placing two colour scales at right angles to each other, with their zeros at the angle. A measured simple colour, finds its position directly on its corresponding colour scale at the point of its measured value. A measured complex colour, finds its position within the angle, at that point where perpendiculars drawn through the two colour values meet. The above statements are complete only for colours of standard brightness, should the colour be brighter or duller than standards, a light factor is necessary, the value of which is furnished by the measurement itself, and must be written in numerals near the colour point. By this method the chart position of even the most complicated colour is indicated by a single point which is determined by the analytical value of the composing factors. Examples. Simple Colour of Standard Brightness. Complex Colour of Standard Brightness. Simple Colour Brighter than Standards. Complex Colour Duller than Standards. 3· Red. 6· Blue, 10· Violet.7· Yellow, Light 2·Red 6, Orange 5, Black 2. The number of complex colour charts is limited to the six represented in Fig. 1 as lying in their order on a continuous spectrum. The red and violet mixtures having no visible spectrum position are represented in the ultra violet. The ordinates of the charts are made by erecting the overlying red, yellow and blue scales as perpendiculars. FIG. 1. The information to be obtained by charting measured colour is more extensive than appears at first sight, as by varying the character of the co-ordinates, and charting suitable series of measurements, new fields of investigation are opened, thus throwing light on some hitherto obscure questions, of which the following are some instances. SPECIFIC COLOUR. It has sometimes been assumed that colour increase was in direct ratio to intensity increase, but this is never the case, each substance has its own rate, specific to itself. It is conceivable that the colours of two substances may coincide at one point, but as their densities increase, or decrease, their rates of change vary. The term “Specific Colour” is based on the experimental fact, that the colour of a given substance is constant, so long as the substance itself and the conditions of observation, remain unaltered. During experimental work a sufficient number of instances have accumulated to warrant the writer in advancing and using the term “Specific Colour” as describing a new natural law, as rigid in its application as that of “Specific Gravity” or “Specific Heat.” PLATE V ABSORPTION CURVES OF FIVE COLOUR CONSTANTS. To face page 33. When this principle is applied to the measurement of regularly increasing thicknesses, curves of colour changes can be established, which are specific for the substance in question, and afford a certain means of identifying similar substances in future. This is effected by varying the nature of the co-ordinates, making the ordinates to represent the tintometrical scale of colour units irrespective of colour, whilst the abscissae represent the scale of increasing thicknesses. Then by [Lovibond, Colour Theories. [31] [32] [33] [Lovibond, Colour Theories. plotting the separate factors of each measurement according to their unit values, a series of curves is established, specific to the substance in question, and applicable to none other. We have now two systems of charting colour, in the first, the complete sensation is represented by a single point, as in Plate IV. In the second, each factor is represented by a separate point, and by connecting points of similar colours, a series of curves is established which represents a quantitative analysis of the progressive colour development, as in Plate V. CHAPTER X. Representations of Colour in Space of Three Dimensions. The relations of the different colours to one another, and to neutral tint are, perhaps, best represented to the mind by a solid model, or by reference to three co-ordinate axes, as employed in solid geometry (see Fig. 2). FIG. 2. Let the three adjacent edges OR, OB, OY, of the above cube be three axes, along which are measured degrees of Red, Yellow and Blue respectively, starting from the origin O. Every point in space on the positive side of this origin will then represent a conceivable colour, the constituents of which in degrees of red, yellow and blue are measured by the three co-ordinates of the points. Pure reds lie all along the axis OR, pure yellows on the axis OY, and pure blues on the axis OB. All normal oranges, normal greens, and normal violets lie on the diagonals of the faces of the cubes OO1, OG, OV respectively. Pure neutral tints lie on the diagonal ON of the cube, equally inclined to the three principal axes. Red violets will be found on the plane ROB, between OV and OR. Blue violets on the same plane between OV and OB. “Saddened” red violets all within the wedge or open space enclosed by the three planes, whose boundaries are OB, OV, ON. The other colours, red and yellow oranges, blue and yellow greens, pure and saddened, are found in corresponding positions in relation to the other cases.[4] CHAPTER XI. The Spectrum in relation to Colour Standardization. The spectrum has naturally been considered as a suitable source for colour standards, but the power of analysing has disclosed some difficulties, which have yet to be overcome. Concerning the prismatic spectrum, there has always been a difficulty in apportioning the different colours to specific areas, and further, before this spectrum is available for colour standardization, some method of correction for the unequal distribution of colours must be devised. Neither of these difficulties occur in the use of the diffraction spectrum, where the pure colours are apportioned by Professor Rood from A to H in the manner shown in table on next page. Professor Rood further divides the spectrum from A to H into 100 equal divisions, allotting 20 unit divisions of 72,716 wave lengths to the space between each two colour lines. This allots a space of 3,635 W.L. to each unit division, as shown in Table III. TABLE III. Wave Length Position. No. of Wave Lengths from Colour between each. DivisionW.L.K. per Division 760,400 A.Red 760,400 —— 72,717 == 20 3,635 Orange 687,683 —— 72,716 == 20 3,635 [34] [35]...