LIBRARY OF THE UNIVERSITY OF CALIFORNIA. ClMS COLOUR COLOUE AN Elementary flfcanual for Stubents BY A. H. CHURCH M M.A. OXON., F.C.S., F.I.C. PHOFESSOR OF CHEMISTKY IN THE ROYAL ACADEMY OF AKTS, LONDON WITH SIX COLOURED PLATES EIGHTH THOUSAND CASSELL AND COMPANY, LIMITED LONDON, PARIS, NEW YORK & MELBOURNE. MCMV ALL, BIGHTS RESERVED CONTENTS. PAGE Bibliographical Notes - - - - ix List of Illustrations xi CHAPTER I. Colour a Sensation Connection of Physiology and Optics with Colour Luminous Bodies Illuminated Bodies The Undu- latory Theory of Light Emission, Reflection, Transmission, Absorption, Refraction, and Dispersion of Light 1 CHAPTER II. Composition and Analysis of Light The Solar Spectrum Different Colours differently Refracted White Light always . Compound Coloured Light often Simple Recomposition of White Light The Rainbow Sound and Light Compared and Contrasted 15 CHAPTER III. Production of Colour by Absorption, by Diffraction, by Polarisa- tion Selective Absorption and Selective Reflection Pro- duction of Colour by Loss of Colour Hue affected by Thickness of MediumInterference of Light-waves Colours of Thin Films 26 CHAPTER IV. Opalescence and Turbid Media Cloud, Fog, and Mist Fluo- rescence Phosphorescence Calorescence Incandescence Coloured Flames The Unity of the Solar Spectrum - - 38 CHAPTER V. The Constants of Colour Purity, Luminosity, and Hue of Colours Shades, Tints, and Tones Broken ColoursClas- sification and Nomenclature of Colours 50 CHAPTER VI. Mutual Relations of Colours Complementary Colours Young's Theory of Three Primary Colour-Sensations Theories of Helmboltz, Clerk - Maxwell, Von Bezold, Rosenstiehl, Roechlin, and others Abnormal Perception of Colour* Colour-Blindness 64 Vlll CONTEXTS. PAGU CHAPTER VII. Brewster's Theory of Three Primary Colours, or the Red, Yellow, Blue Theory Secondary Colours Tertiary Colours Mixtures of Coloured Lights Mixtures of Coloured Pig- ments Colours Mixed by Rotation 80 CHAPTER VIII. The Chromatic Circle Colours of Pigments The Laws of Con- trastContrasts of Tone Contrasts of Colour Simulta- neous Contrast Successive Contrast and the Negative . Image 90 CHAPTER IX. Persistence of Retinal Impressions Irradiation The Colour of the Lens of the Eye Partial and General Visual Fatigue The Development and Cultivation of the Sense of Colour - 107 CHAPTER X. Description of Certain Colours The Contact and Separation of Colours Colours with White, Grey, and Black Double and Triple Combinations of Colour Complex Colour-Com- binations - Chromatic Equivalents 113 CHAPTER XI. The Small Interval and Gradated Colours Harmonies of Analogy Harmonies of Contrast Harmonies of Seriation Harmonies of Change Distribution, Balance and Quality of Colour Throbbing Colours Decorative and Pictorial Colour Colour as Modified by Painting-Media, by Grounds and by Brush-work 130 CHAPTER XII. Modification of Colour by Illumination Diffused Daylight Light of the Sky and Clouds Sunlight A Dominant Coloured Light Artificial Lights Two Lights - - - 146 CHAPTER XIII. Surface and Structure Modify Colour Colours of Metals- Barton's Buttons and Iridescence Damascening and Plating Enamelling on Gold and Silver Bronzing, Patinating, and Lacquering Japanese Alloys - - - 158 CHAPTER XIV. Colours of Glass, Earthenware, and Porcelain Colours of Gems, Marbles and other Minerals Colours of Mineral Pigments Colours of Plants, Flowers, Woods, and Vegetable Fibres -The Coal-tar Dyes Colours of Animals and Animal Pro- ducts BIBLIOGRAPHICAL NOTES. BENSON, W. Principles of the Science of Colour (London : 1868). BENSON, W. Manual of the Science of Colour (London : 1871). BEZOLD, W. VON. Theory of Colour (Boston, U.S.A. : 1876). BRUCKE, E. Des Couleurs, traduit par J. Schutzenberger (Paris : 1866). CHEVREUL, M. E. Contraste Simultane des Couleurs (Paris: 1839). FIELD, G. Chromatography (London : 1835;. FIELD, G. Chromatography, Modernised by J. S. Taylor (London : 1885). HELMHOLTZ, H. Popular Lectures on Scientific Subjects ; 1st Series, pp. 197 to -3 16 (London : 1873). 2nd Series, pp. 73 to 138 (London: 1881). PORTAL, BARON P. DE. Essay on Symbolic Colours (1845). RIDGWAY, R. Nomenclature of Colours for Naturalists (Boston, U.S.A.: 1887). ROOD, 0. N. Modern Chromatics (London: 1879). SHARPE, E. Four Letters on Colour in Churches (Birmingham : 1871). STOKES, G. G. Light: the Burnett Lectures for 1884-5-6 (London: 18,87). WRIGHT, LEWIS. Light : a Course of Experimental Optics (London: 1882). The titles of several treatises, pamphlets, and essays on Colour and on its industrial and pictorial applications will be found in the " List of Works on Painting " in the National Art Library at South Kensington. Of this list, compiled by R. H. Soden-Smith, the second edition was issued in 1883. Many of the most important scientific researches on Colour have appeared in the Transactions of Societies, in the Philosophical Magazine, and in other scientific periodicals. The references to the titles of such papers and memoirs, and to the volumes in which they were published, may be learnt from the Royal Society's "Index of Scientific Papers,'* under the names of the several authors Sir D. Brewster, J. H., Gladstone, J. C. Maxwell, G. G. Stokes, G. Wilson, &c. LIST OF ILLUSTRATIONS. 1. Ruf faction of Light by the Prism 2. Analysis of Light by the Prism 3. Monochromatic Light not susceptible of Prismatic Anplysis 4. The Prismatic Spectrum 5. The Normal Spectrum - 6. Recomposition of White Light by Reflection 7. Recomposition of White Light by Two Prisms 8. Recomposition of White Light by Newton's Disc - 9. Change of Hue with Thickness of Coloured Medium - 10. Interference of Light 11. Apparatus for Coloured Flames 12. Production of Tints by Rotating Sectors 13. The Chromatic Tetrahedron - 14. The Three Colour-Sensations Illustrated 15. Diagram for Mingling Coloured Lights - 16. Yellow and Blue Combined - 17. Lambert's Method of Combining Colours 18. Transmission of Green Light through Glasses Combined ... .... 19. Rotation- and Palette-Mixtures of Pigments Contrasted - 20. The Chromatic Circle in its Simplest Form - 21. The Improved Chromatic Circle ------ 22. True and False Complementaries Contrasted 23. Advancing and Retiring Colours - 24. Contrast of Tone - .... 25. Contrast of Tone - 26. Contrast of Colours with White, Grey, and Black coloured 27. Simultaneous Contrast coloured 28. Simultaneous Contrast between Allied Colours ... 29. Lustre Produced in Binocular Vision - - - coloured 30. The Separation of Related Hues / - - - coloured 31. The Gradation from Yellow to Green 32. Chromatic Analysis of Hues between Yellow and Green 33. The Gradation from Yellow to Violet - 34. Chromatic Analysis of Hues between Yellow and Violet 35. Interchange of Tones of Two Colours - 36. Counterchange of Two Colours ------ 37. Repeated Reflections from a Metal PAGE 12 15 17 18 IS 21 21 22 30 - 35 47 - 56 - 61 coloured Front is. - 70 - 80 - 81 Blue and Yellow - 83 89 - 90 - 91 coloured coloured 93 96 97 98 99 100 101 96 117 136 137 138 139 139 140 160 ""OF TrtF UNIVERSITY COLOUR. CHAPTER I. COLOUR A SENSATION CONNECTION OF PHYSIOLOGY AND OPTICS WITH COLOUR LUMINOUS BODIES ILLUMI- NATED BODIES THE UNDULATORY THEORY OF LIGHT EMISSION, REFLECTION, TRANSMISSION, ABSORPTION, REFRACTION, DISPERSION OF LIGHT. 1. CERTAIN waves or vibrations which affect the fibres or rods of the optic nerve of the eye are translated by the brain into colour. Such excitation of the optic nerve may be brought about by pressure on the eye-ball, by an electric discharge, by internal causes, and, pre- eminently and generally, by light. Colour is, in fact, an internal sensation, and has no external and objective existence. As, however, it originates, in all the cases considered in the present volume, in the impact, on the optic nerve, of that force or energy or "mode of motion" which we call light, the study of some of the elementary facts of optical science may very suitably precede the consideration of the laws of colour and their applications. 2. Everything that we can see is visible because it y is either luminous or illuminated. In other words, visible objects are seen because they either emit light or reflect light. Examples of luminous bodies are afforded by a candle-flame, a glowing piece of charcoal, the sun. From these sources of light luminous rays are sent out ; these rays are the lines along which the light is propa- gated. The term beam of light is usually applied to a large group of such rays, the term pencil designating a 2 COLOUR. CChap. I. smaller group in both cases the constituent rays being parallel to each other. Fiona such luminous bodies as are near the eye the rays emitted are divergent : those rays which reach the eye from the sun and other distant bright objects may be regarded as practically parallel. Divergent rays passing through a bi-convex lens emerge parallel; conversely, parallel rays transmitted through such a lens are rendered convergent ; the point at which they meet is the principal focus of the lens. The forms of highly luminous bodies can be clearly seen only when much of the light which they emit is cut off by a special contrivance, such as is afforded by a piece of dark green glass or of " smoked " glass. In this way it is quite easy to see the shape and to watch the changes of the carbon- pencils in the electric arc lamp, intense and dazzling as is its light. 3. The light emitted from bodies travels in straight lines, and causes, when obstructed by opaque materials, the production of shadows. The form and sharpness of shadows is influenced not only by the shape and by the relative size of the opaque body which casts the shadow, but by the form and by the degree of luminosity of the luminous body, the light of which is intercepted. Thus a brilliant luminous point gives a sharply-defined shadow, while a large luminous surface, on the other hand, pro- duces a shadow which is surrounded by a paler and less definite one, which goes by the name of a penumbra. If we place an opaque screen, A, midway between a small source of light and a second screen, B, it will be found that the light falling on screen A will produce a shadow four times the area of A upon the screen B. This ex- periment likewise illustrates the law of the intensity of light at different distances from its source. The light which falls on A has to cover at double the distance (at B) four times the area, arid consequently has no more than one-fourth the intensity, or, in other words, the intensity of light varies inversely as the square of the distance from the luminous point or source. Chap. I.J ILLUMINATED BODIES. 3 4. So far we have spoken of self-luminous bodies ; something must now be said of illuminated bodies. They shine by borrowed light. They are marked out and dis- tinguished from one another by the different amounts and qualities of the light which they reflect, and also, as we shall presently see, by the manner in which they reflect it. A piece of black cloth on a white earthenware plate reflects but a very small proportion of the light which falls upon it ; the plate, on the contrary, reflects a very large proportion. Had the black cloth possessed no power whatever of reflecting light, it would have been invisible ; black velvet, which reflects less light even than black cloth, sometimes produces on the eye the effect of absolute blackness that is, of an empty and dark space. Similarly, a sheet of perfectly clean and perfectly polished plate glass may appear lustrous and visible enough if the light which falls upon it is sent back to the eye ; but if we are so placed in front of the glass that the regularly-reflected rays of light escape us, it ceases to be visible, and we may, perchance, stretch out our hand to take something from behind the glass, wholly unconscious of its presence. But it is possible to render a piece of polished and transparent colourless glass per- manently visible. Crush it to powder, and then, in whatever direction the light falls upon its particles, the surfaces of those particles will turn back or reflect some of the light-rays, and so render the particles visible ; the clear glass has become in a measure opaque. Thus, too, transparent water, when broken up into numerous fine particles, as in a cloud, has acquired, with its opacity, the property of irregularly reflecting light. A dense cloud, which appears nearly black when between the observer's eye and the sun, owing to the considerable degree of completeness with which it intercepts the light, may become brilliantly white when the sun's rays fall upon its constituent particles, for the light which cannot get through the cloud is continually reflected to and from the surfaces of its minute parts, and so illuminates 4 COLOUR. iCLap. I. it. Thus it happens tha,t the lower part of a cloud seen against a background of dark mountain may appear white, while the upper part of the same cloud seen against a luminous sky may appear a dull grey. The lessening of reflection, on the other hand, diminishes visibility. The numerous small reflections which occur from and between the surfaces of the felted fibres in a piece of white paper may be greatly lessened by wetting or oiling the paper, when it becomes less opaque, and at the same time greyer and clearer ; to this cause the transluceiicy of tracing-paper and tracing-cloth is due. 5. We said before that illuminated bodies differ, not only in the amount, but in the quality of the light which they reflect. Now, one of the chief differences as to quality of light is its difference in colour. Powdered vermilion reflects to tho eye a good deal of the light which falls upon it ; this light, however, is chiefly red light, not unmixed with light of other hues, and accom- panied by no inconsiderable amount of white light. A stick of red sealing-wax (coloured by vermilion) shows in some positions a bar of white reflected light in the direc- tion of its length, while in other positions we see only the red light reflected from the particles at its surface, and from the particles at a slight depth below that sur- face. Why this light happens to be red in the case of vermilion we shall discuss in another chapter ; we con- tent ourselves here with pointing out that while the reflection from a polished surface is regular, that from a roltgh surface is irregular, and that from a coloured sur- face coloured. A polished plane metallic surface affords an example of the first kind of reflection, a piece of chalk of the second j the reflection differs in kind as well as in ' degree. So great is the difference in effect produced by regular reflection on the one hand and irregular reflection on the other, that it may be confidently affirmed that, if an illuminated polished body could be found which was wholly incapable of irregularly reflecting any part of the light falling upon it, that body would be wholly invisible. Chap. I.] IRREGULAR REFLECTION. ft We may therefore say that we discern bodies by the aid of the light which they irregularly reflect or scatter j a perfectly regular reflection gives, on the contrary, an image of the source of light, not of the object illuminated. Before we further explain the peculiarities of the scatter- ing or irregular reflection of light, the great law of reflec- tion should be stated. It is this : " The angle which an incident ray of light makes with a perpendicular to the reflecting surface is equal to the angle which the re- flected ray makes with that perpendicular : " in other words, the angle of incidence and the angle of reflection are equal. Of course, if a ray fall perpendicularly on the reflecting surface, it is reflected back along the same path. This perpendicular is called the " normal/' and from it, and not from the plane of the reflecting surface, angles of reflection are measured. It should be added that both the incident and the reflected rays are in the same plane, which is perpendicular to the reflecting surface. 6. Now it might be imagined that, in cases of irre- gular or scattered reflection, the law just stated of equal angles could hardly be applicable. But in reality, when the rays of parallel light from the sun strike upon a rough, that is, an unpolished surface, say of a piece of white paper, they are incident at all imaginable angles with the minute surfaces of the hollows and ridges which make up the reflecting substance, and such of them as are reflected obey the law, but are reflected in a countless number of different directions. 7. The study of the irregular reflection of light shown by clouds and vapours and dust leads to a very important conclusion. A beam or pencil of light travers- ing a perfectly clear medium, whether a vacuum, or a colourless gas like air, or a colourless liquid, is itself invisible. The sunbeam, passing through a hole in a shutter into an otherwise dark room, reveals itself only when motes and dust are suspended in the air. A beam of light from the electric lamp is not seen as it traverses 6 , COLOUR. [Chap. I. pure water in a jar. Light is invisible : such is the con- clusion to which we are forced by countless experiments and observations ; but it may become at any moment visible by the interposition in its path of particles which can irregularly reflect it. Particles of dust, particles of water, particles of smoke, reveal the invisible light, not, strictly speaking, by making it visible, but by becoming visible themselves in its path. So in liquids as in air, suspended particles reveal the path of the passing rays. By adding a solution in alcohol of mastic, or a few drops of milk, or a little sodium thiosulphate solution followed by some hydrochloric acid, to a vessel of clear water through which a beam of light is passing, the invisible rays are revealed by the "scattering" which they suffer when they impinge upon the minute particles of sus- pended matter in the water. 8. Perhaps the question may now be asked, " What is light ? what is the nature of the rays which are emitted by self-luminous bodies,, and reflected by those which are illuminated 1 " The most reasonable answer which can be given involves a very large assumption. But we are warranted in assuming, at least provisionally, the truth of a theory which serves to explain all the diverse and complex phenomena of light, even if that theory demands some admissions which are hard to make and difficult to apprehend. The theory in question is called the Wave Theory, or the Undulatory Theory of Light. It involves two primary assumptions : namely, a motion and a something moved. What is the kind of motion, and what is moved] or rather, in what medium does the motion occur 1 The undulatory theory of light assumes the existence, throughout all space and throughout ail matter, of an almost infinitely thin and elastic medium, called the luminiferous or light-bearing ether. It must be supposed that this ether is everywhere, universally present, without break in its continuity, in solids, liquids, and gases, and not to be excluded even from a vacuum. It cannot be material in the exact sense in which we r Cliap. I.] THE LUJJI^IIJEROUS ETHER. 7 apply that term to the elements of the chemist, but to account for some of the phenomena of light we are com- pelled to endow the medium which conveys it with some at least of the properties of matter. But our older notions of the nature of the elements have been so shaken by recent researches, that some physicists have hazarded the f\ suggestion that matter itself may be nothing more than vortices in this ether ! Whatever the ether may be, the movement of this ether (or rather, a particular kind of movement in this ether) is light. This movement is in waves, the undulations of the particles of the ether being across the direction in which the light is propagated. Ordinary light consists, then, of vibrations in all azimuths, but all perpendicular to the path of the ray, and is sup- posed to originate in the following manner : The par- ticles or molecules of a luminous body are in a state of disturbance, a state of intensely rapid motion. This motion of the molecules is communicated to the ether, and sets it in vibration, and is propagated in all directions in the form of spherical waves. Reaching the retina of the eye, this fine motion of the ether is translated by the brain into the sensation which we call light. This is a broad and general statement, for, as we shall explain more fully further on, not all the vibrations set up in the ether by luminous bodies are thus capable of translation into light. For the waves of the ether are of different lengths, and it is only those which measure from about 3" 3 JoiT f an i ncD > on the one hand, to about yj-oo- f an inch, on the other hand, which are capable of exciting the sense of sight. Or, indicating these wave-lengths by their duration in time, the visible waves vary between the ^Q- and -J^ of a billionth of a second. The longest and slowest of these waves gives the sensation of red, the shortest and quickest that of violet. Longer waves than the longest above-mentioned do not excite vision, but are manifested as heat; shorter waves than the shortest above-mentioned do not excite vision, but cause chemical change, and are known as actinic. The relations of heat, 8 COLOUR. [Chap. I. light, and actinism to each other will be discussed in Chapters II. and III., at least so far as they are connected with the primary subject of the present volume, namely, the nature and causes of colour. 9. Thus far we have considered very briefly, it is true the emission and the reflection of light, as well as the undulatory theory. A few words must now be said as to the meaning of the terms transmission, absorption, refraction, and dispersion of light. Bodies are said to be transparent when they permit light to pass so freely as to allow objects to be perfectly discerned through them ; / translucent, when they allow light to pass less perfectly, so that objects on the other side of them cannot be clearly v/ distinguished ; opaque, when the light is wholly shut off. But, in point of fact, 110 bodies are perfectly transparent or perfectly opaque. The most colourless and flawless polished glass cuts off some rays, while some substances, such as metals, which are commonly regarded as quite opaque, become transparent, or at least translucent, when reduced to a state of great tenuity in the form of thin leaves. Thus, the sun may be conveniently viewed through a plate of glass which has been coated on one side with a thin film of pure silver, the light which passes through the metal appearing of a blue colour, while the light transmitted through a piece of gold-leaf is bluish- green. 10. In addition to this, it may be remarked that different transparent bodies, which appear to allow light bo pass through them with equal and perfect facility, do, in reality, arrest the passage of some of the constituent rays, if only the transmitted pencil of light be critically examined by appropriate optical methods. Take the case of water. A pencil of white light, made up, we will assume, of 1,000 rays, perpendicularly strikes the surface of some water two or three inches deep, and contained in a suitable vessel ; eighteen rays will then be reflected towards the original luminous source, while 982 will find their way through the water unchanged. But increase Chap. I.] ABSORPTION OP LIGIIT. the thickness of the layer of water to something like two feet, and this free transmission will no longer occur. The number of the emergent rays will be distinctly reduced, and their quality will be altered in one respect : that of colour. The eye will perceive that they are bluish, not white, the explanation lying in this fact, that all the constituent elements of white light have not been equally transmitted through the thick layer of water, some of the colour-producing waves being quenched or absorbed, leaving a large residuum which passes but which shows in its bluish hue the suppression that has taken place. The same phenomenon, in a much more obvious form, is observed in the case of coloured liquids, such, for instance, as an aqueous solution of sulphindigotic acid or of car- rninate of ammonia. In the former case a considerable proportion of the large waves of red light is suppressed, the transmitted rays being chiefly blue, while in the latter case the transmitted waves are those which pro- duce the sensation of red, the intercepted those giving green and blue. Still the production of colour is not always the result of the imperfect , transmission of light through a so-called transparent medium. For instance, there are several apparently colourless liquids, such as alcohol, benzene, and an aqueous solution of didymium salts, in which the imperfect transmission of certain light-waves is so distributed amongst them as not to affect the hue of the transmitted beam, although it lessens its intensity. How this occurs will be explained further on (see Chapter II.). 11. In the preceding paragraph we have given some examples of the absorption of light, the liquids mentioned " filtering " the rays, permitting some to pass and " strain- ing off " others, colour being in very many cases the result. Solids, such as coloured glass and coloured gems, constantly produce similar results. One example will suffice to illustrate this point. We will take the case of the very beautiful mineral known as lapis-lazuli, from which the pigment genuine ultramarine is prepared. A 10 COLOUR. [Chap. I, very thin slice of lapis-lazuli appears transparent under the microscope. White light cannot be transmitted in its entirety through it. The white light does not become blue by traversing the lapis, but is decomposed in its passage, a very large number of its constituent vibrations being in some way intercepted, quenched, or absorbed, while the remainder, which escape absorption, on their emergence produce the sensation which we call blue ; this is a case common enough of selective absorption. Let us proceed a step further. If we reduce the lapis to an impalpably fine powder, it remains blue, but becomes apparently opaque. In reality, however, it retains a certain degree of translucency. White light falling upon this powder is in part reflected unchanged, but a portion plunges into the surface particles and suffers the absorp- tion of its longer red and green waves, while the residue emerges as blue light, and is irregularly reflected from the general surface of the powder. Such selective absorption is the main cause of the colour of pigments. 1 2. It has been stated before that when a beam of white light falls perpendicularly upon the surface of water more than 8 per cent, of the rays pursue a straight course through the water, the direction of the beam being unaffected by its passage from the rarer medium, air, into the denser medium, water. But the result is altogether different when the incidence of the beam is oblique. Not only is a much larger proportion of the incident light reflected from the surface of the water, but the rays which penetrate that medium are bent down towards the perpendicular, or, as this change of direction is called, refracted. The more oblique the incidence of the beam, that is, the larger the angle it makes with the perpendicular, the more strongly is it refracted. However, whatever the angle, refraction always obeys what is known as the " law of sines," the sine of the angle of incidence of the beam in air bearing to the sine of the angle of refraction in water the in- variable ratio of 4 : 3. Expressed as a decimal fraction Chap. I.] REFRACTION OP LIGHT. 11 this ratio, which is identical for all angles, becomes 1 -335, which value is called the " refractive index " for water air being assumed in this, as in all cases not specially excepted, to have a refractive index of 1. A familiar example of refraction is the case of a stick par- tially immersed in a vessel of water, which appears broken at the surface of immersion when placed obliquely. But it may reasonably be asked, " How is it, if re- fraction in a denser medium is towards the perpendicular, or dowmvards, that the immersed part of a stick seems to be bent upwards 1 " It is because the rays from the immersed portion of the stick, before they reach the eye, have to pass from the denser medium of the water to the rarer medium of air, and in consequence, the effect of the previously described refraction is precisely reversed. Another familiar instance of refraction is afforded by placing a coin in an opaque bowl, so that it cannot be seen by an eye above and to the side of the edge of the vessel : on pouring water into the vessel, the image of the coin becomes, as it were, lifted up into view by refraction. The explanation of refraction on the undulatory theory involves the conception of a certain degree of hindering in the motions of the ether by the denser medium. One side of the wave-front of a beam of light striking obliquely the surface of water, must meet that surface before the other side, and must be first hindered or retarded by it. The beam swings round, the other side reaches the denser medium, and then the whole beam proceeds on its path but more slowly and in a new direction. 13. We are now in a position to consider the mode of action on light of the most important of all optical contrivances for studying the nature of light and of colour : that is, the prism. We know that a beam of light in passing from air into water, or glass, or other relatively dense medium, is refracted towards the per- pendicular ; conversely, in passing out of glass or water into air, the reverse refraction occurs, and to a precisely equal extent. If, therefore, a beam of light enters 12 COLOUR. [Chap. I. obliquely a piece of glass, the faces of which are parallel, the refraction towards the perpendicular on. entering the glass will be exactly compensated by the refraction away from the perpendicular on leaving the second parallel surface of the glass, so that the emergent beam will necessarily be parallel with the incident beam, though not continuous therewith, for it has been deflected in the glass. But suppose we employ a prism of glass instead of a flat plate, then the beam is permanently refracted on emergence. The prism, so extensively employed in optical experiments, is a wedge-shaped piece of some transparent and colourless substance, generally of flint glass, but sometimes of other kinds of glass, or even quartz or other transparent minerals : it is an indis- pensable instrument in the study of colour. The angle enclosed by two oblique sides of this prism is called the v refracting /angle. If we place the prism so that this angle, A (Fig. 1), is below, and the opposite side of the prism horizontal, then a beam of light, R, from above falling on to one of the oblique sides, I, will be refracted towards the perpendicular or normal, N, of that side in passing from air through the glass, and on emerging into air from the second surface, E, will be again deflected, but this time in the opposite direction, namely, from the perpendicular or normal, n, to that surface, and therefore in an upward direction, r. The new direction of the emergent beam will deviate from the original direction Chap. I.] PRISMATIC ANALYSIS OP LIGHT. 13 in degree according to (1) the density of the glass, (2) the angle made by the oblique sides of the prism, (3) the position of the prism. The last factor in the amount of deflection is of much importance in working with the prism, and it is always desirable to secure such a position for it that the incident and emergent rays make equal angles with their respective surfaces ; this is known as the position of minimum deviation. It should be added here that for many purposes hollow prisms may be substituted for those of one material ; they are generally filled with carbon disulphide, Sonstadt's solution of potassio-'iodide of mercury, or some other liquid having a higher re- fractive index than that of flint glass. 14. Commonly something more happens to the beam of light which has traversed, as just described, the prism than a mere deflection. If the light be simple, if its waves be of one measure only, it will be simply deflected ; if, as is nearly always the case, the light be compound, if its waves be of different lengths, then they will be differently affected by the prism. It will retard the short waves more than the long ones, and so we shall find that these short waves are more refracted. The more refrangible rays are, then, the short violet rays, the less refrangible rays are the longer red rays. In every case, then, where a luminous body emits rays of various degrees of refrangibility, these rays can be separated from each other by means of the prism. As solar light consists of an enormous number, of rays having different refrangibilities (in consequence of their differing wave-lengths), it may be decomposed, analysed, or split up into an enormous number of rays, the wave- length of each of which belongs to a particular component. Not all these different vibrations can excite vision and the sense of colour, as we have already learnt, but the heat-rays, whose vibrations are even longer than those of the red rays, as well as the actinic rays, whose vibrations are still shorter than those of the violet rays, obey the same law as the visible rays. It must not, however, be 14 COLOUR. [Chap. L forgotten that the material of the prism, though prac- tically transparent to true light-rays, is generally very far from being equally transparent, or " dia-actinic," to the chemical rays, or dia-thermic to the heat-rays. 15. When sunlight is examined by means of the prism, it will be found, if the necessary care be taken to insure a pure and long spectrum, or ordered sequence of differently refracted rays, that there are gaps in its continuity that there are blank spaces to which no ray corresponds. The light of the electric arc, on the other hand," when analysed by the prism, presents no such breaks, consisting as it does of an unbroken series of rays having every possible wave-length within its range. Most burning metals and glowing vapours emit fewer rays, so that their light, however intense, is of much simpler constitution than that of the electric arc or of the sun, being made up in some cases of very few elements, a red ray in the case of lithium being, for example, followed at a certain interval by an orange ray, then by one of a blue colour, and finally by a violet one. In other words, the light of burning lithium (at the temperature of the electric arc) is made up of four rays having different wave-lengths, and therefore different refrangibilities. Consequently the prism, in refracting these rays differently, separates or disperses them widely, and enables us to observe them apart from each other. Such dispersion is, then, the general accompaniment of the refraction of compound light, although it does not always happen that substances having the highest refractive index possess also the highest power of dispersion. 15 CHAPTER H. COMPOSITION AND ANALYSIS OF LIGHT THE SOLAR SPECTRUM DIFFERENT COLOURS DIFFERENTLY RE- FRACTED WHITE LIGHT ALWAYS COMPOUND, CO- LOURED LIGHT OFTEN SIMPLE RE-COMPOSITION OF WHITE LIGHT THE RAINBOW SOUND AND LIGHT COMPARED AND CONTRASTED. 16. THE compound nature of the sun's light was first demonstrated by Kepler, and a century and a half later was more thoroughly investigated by Sir Isaac Newton. His primary experiment may be easily re- peated, and a beam of the solar rays analysed into its Fig. 2. constituent parts. The arrangement shown in Fig. 2 may be adopted. Through a small circular hole, or, preferably, a narrow slit in the shutter of a darkened room, allow a beam of sunlight, s, to fall obliquely upon the face, I, of a glass prism, so arranged that its base, P, is uppermost and horizontal. The beam will be refracted and dispersed, as described already in 13 and 14. If the prism have a refracting angle of about 60, a vertical 16 COLOUR. [Chap II. band of rainbow colours will be produced, and may be examined on a screen of white card-board placed at a distance of five yards or so from the prism. This band is the solar spectrum. It consists of a very large num- ber of different hues, amongst which we at first single out three as the most conspicuous : namely, red, green, and blue-violet. Looking a little more closely at this coloured band, we find no difficulty in adding orange, greenish-yellow, greenish-blue, and violet to the list. A pure yellow can hardly be recognised, for this hue occupies an exceedingly narrow space in the spectrum of sunlight. Nothing at all approaching the hue of indigo can be dis- cerned, although, from the time of Newton onwards, the name indigo has been generally applied to the coloured light which separates the blue from the violet. But aa the pigment indigo in its purest state is a dull blue having a greenish cast, its place in the spectrum would certainly not be on the violet side of the pure blue, but rather on the side of the green. For our present purpose it will be sufficient to enumerate six colours. Beginning at that end of the spectrum which is nearest to the spot s', which the solar beam, s, would have reached had there been no. prism to bend it out of its original path, we find that the sequence- of the spectral colours is as follows : red, orange, yellow, green, blue, and violet. Now the operation by which these coloured rays have been separated from whife light would lead one to predict that they cannot be further decomposed that they cannot be themselves amenable to prismatic analysis, or be separated into other kinds of colour. This anticipation is realised by actual trial. For if, as is shown in Fig. 3, one of the colours, v, of the spectrum be allowed to pass through a hole in the screen, E, on which the band of coloured light was first received, it will be found to remain unaltered after having been transmitted through a second prism, B. The ray will be refracted, of course, but it will remain violet, as seen on screen H. 1 7. The celebrated chemist Wollaston first described, Chap. II.] THE SOLAR SPECTRUM. 17 in 1802, the existence of gaps represented by dark lines in the spectrum of the sun. Twelve years afterwards Fraunhofer further investigated this point, and mapped out no less than 576 of these dark lines, assigning to the most prominent amongst them the letters of the alphabet. These lines, or vacant spaces, in the solar spectrum have been traced to the absorptive power for certain rays possessed by the gaseous envelopes which surround the incandescent body of the sun. From our present point of view they are of chief interest to us as furnishing a ready means of fixing and identifying the variously Fig. 3. coloured tracts of the spectrum. We may say, for example, that the Fraunhofer line, D, occupies very nearly the middle of the orange space in the spectrum, while the pure blue begins at the line F. We are indebted to several distinguished investigators, such as Angstrom, Vierordt, Listing, and Rood, for elaborate chromatic measurements of this character. Professor Ogden Rood, in his most valuable work called " Modern Chromatics," gives two diagrams, reproduced in Figs. 4 and 5, in which the coloured spaces of the solar spectrum are mapped out by means of ten of the Fraunhofer lines, A, a, B, c, D, E, b, F, G, and H. The space from A to F being divided into 1,000 parts, it is thus possible to assign numerical values to the spaces occupied by a selected number of colours. The names assigned to these colours, owing to the vagueness which seems to be in- herent in such a nomenclature, may lack precision, but c Red 149 Keel-Orange 45 Orange 16 Orange- Yellou 201 Yellow 10 { Green -Yellow & Yellow-Green Green & Blue-Green 103 $reen'sh-Blue Blue & Blue-Violet 811 Violet 194 Red 330 Orange-Red 104- Orange 25 Orange-Yellow 26 Yellow 73 Green-Yellow & Yellow-Green Green 87 Blue-Green 16 Greenish-Blu' 51 Blue-Violet , 117 Violet 60 ' Fig. 4. Fig. 5. Chap. II.] THE NORMAL SPECTRUM. 1JT it is so easy for any one studying the subject of colour to examine for himself the actual appearances presented by the spectrum that the uncertainty of colour-names is here of little moment. The two figures we give differ, it will be observed, in one important particular, namely, the relative spaces occupied by the several hues. This- difference is due to the different means employed to produce the two spectra represented. Fig. 4 represents a spectrum obtained by the use of a flint-glass prism, while Fig. 5 was drawn from a spectrum obtained by the employment of what is called a " diffraction grating," that is, a plate of glass silvered on the back, and having ruled on the front an immense number (nearly 19,000 to the inch) of very fine parallel equi-distant lines. This plate, ruled by Rutherford, gave to Professor Rood, whose results we here produce, a magnificent spectrum of great length and remarkable purity. This spectrum may be regarded as normal, for in it the colours between the red and the yellow are not crowded together more than the differences in their respective wave-lengths warrant, as is the case with the ordinary spectrum obtained by the use of a prism, owing to the unequal dispersive power of glass for different rays. Another difference between the two spectra seems at first sight to be more serious. The luminosity of the different parts of the two spectra is differently distributed, and consequently, as is pointed out in 119, the hue of corresponding parts will not be identical ; for a very luminous red tends towards orange, and a very dull blue towards violet. It should be added here that a part only of the visible spectrum is represented in Figs. 4- and 5, for a dull red verging on brown or chocolate may be detected beyond the line A, while a dull lavender-grey extends beyond the line H at the violet end. 1 8. We have seen that the white light of the sun is compounded of an almost innumerable series of coloured elements, a large number of which can be separately examined by covering up all the rest of the 20 COLOUR. [Chap. II. spectrum, and viewing the selected portions individually through a narrow slit. All white light from other sources will be found to be compound, although there are many -cases in which not more than three, or even two, colours can be separated from it. Coloured light, on the other hand, may often be simple, not admitting of analysis into rays having different wave-lengths ; such is the nature of every one of the coloured lights of the spectrum. But coloured lights may be, and often are, compound, sometimes consisting of two, and often of many more differently-coloured elements, although the eye recognises but a single colour in the complex ray. A striking instance is afforded by yellow. There is an elementary yellow in the solar spectrum lying on the green or more refrangible side of the line D. By no contrivance can we optically decompose this spectral yellow, to which belongs a definite wave-length. But there are many compound yellow lights lights which give us, as the sum of the simultaneous visual impression of their several components, a sensation of yellow not to be distinguished by the brain from the simple yellow of the spectrum. Such a compound yellow may be formed by throwing on the same portion of a screen a part of the red light and a part of the green light of a pure spectrum. Similarly there is a pure and simple blue in the spectrum, but a blue indistinguishable from this in hue may be obtained by mingling green and violet light. 19. It follows, from what has been advanced in this chapter, that white light admits of re- composition by the re-union of its separated constituents. Divide a solar spectrum obtained with a slit and a prism into two parts, equal or unequal, by mirrors or lenses, and throw the light from each part on to the same spot on a screen : a white image of the slit will be the result. Similarly reflect, by means of a set of mirrors, the separated red, orange, green-blue, green, blue, blue-violet, and violet rays of a spectrum on to the same spot, and white light Clip. II. j RECOMPOSITION OF WHITE LIGHT. 21 will be re-formed : the apparatus is shown in Fig. 6. Or again, we may by a second ) prism, in a reversed posi- tion, re-join the colours separated by a first prism, and exactly undo its effect, again producing white. If a space be left between the contiguous sides of the two prisms, a black card may be inserted more or less Fig. . deeply between them, and we may thus cut off a part of the rays separated by prism number one then the light emerging from the second prism will have been de- prived of some of its coloured elements, and will no longer be white. Fig. 7 illustrates the ar- rangement of the prisms and the path of the beam, s, as it enters the first prism, and as it leaves the second prism as the re-constituted white beam, E. 20. Another mode of re-uniting colours so as to form white "was suggested by Newton. A disc (Fig. 8), so mounted upon a multiplying wheel as to admit of Fig. 22 COLOUR. [Chap. II. rapid rotation, is painted in radial sectors with such opaque pigments as afford the nearest approximations to the principal colours of the spectrum. It is not neces- sary to employ a large number of pigments ; three, indeed, will suffice, namely, scarlet-vermilion, emerald-green, and ultramarine-blue (the last having been mixed with a little zinc-white). The surface occupied by the vermilion should be about one-sixth larger than that painted with the artificial ultramarine ; the emerald - green must occupy an area interme- diate in size between that of the red and that of the blue the exact propor- tions have to be deter- mined by experiment for the particular specimens of pigments employed. It is a good plan to repeat this triple group of co- loured sectors three or four times on the disc. If a larger number of colours be employed, including orange, yellow, blue-green, and violet, the same final result will be obtained when, on rapidly rotating the disc, the successive impressions of the coloured sectors are mingled on the retina of the eye. A perfect white is not indeed produced, because all the coloured sectors combined., being severally parts only of whiteness, must present a total luminosity much less than that of a white disc. Thus, six coloured sectors cannot offer more light to the eye than a small fraction of that furnished by a white disc of corresponding size. We must be satisfied with a neutral grey, a mixture of white and black, for even a white disc with a low illumi- nation appears grey. When, too, we remember that a very small proportion of the light falling upon such a coloured disc is reflected from its surface, the dulness of the re- Chap. II.] RECOMFOSITION OF WHITE LIGHT. 23 sultant white must not surprise us. Indeed, the degree of absorption (of parts of the white light) which produces colour in pigments is so great that, with the three pig- ments named above, it amounts to a commixture of 71 J parts of black with 28 J parts of white. In other words, the grey produced by the rotation of such red, green, and blue sectors may be imitated by rotating a disc painted with sectors of black and white, in the proportion of 71 J per cent, of the area with the former, and 28 J per cent, with the latter. In practice it is usual to construct Newton's disc by means of sectors of coloured paper. 21. A form of rotating disc for the re-composition of white light makes use of transmitted instead of re- flected colours. Various transparent pigments mixed with varnish or sectors of coloured gelatines are applied to a disc of glass. White light transmitted through thir parti-coloured disc becomes variously coloured by absorp- tion as it passes through the transparent pigments, and so long as the disc is allowed to revolve slowly no un- usual phenomenon is observed. But on rapidly rotating the disc by any suitable mechanical contrivance, the colours, when a sufficient speed is attained, are mingled on the retina, the coloured images being superposed owing to the persistence of visual impressions, and a neutral whitish-grey is the result. There is still another apparatus for re-combining colours into white ; it is known as Maxwell's colour-top, but this is nothing more than a greatly improved and most ingenious form of Newton's disc. We shall have frequent occasion to refer to this arrangement in succeeding chapters, and to bring forward some of the instructive results as to the mutual relations of colours which may be obtained by its use. 22. It should be noted that, in all the above-described arrangements for re-combining coloured into white light, it is necessary to use sunlight or other practically white light, or else to modify the proportions of the coloured sectors in accordance with the different hue of the 24 COLOUR. [Ctoap. n. tinted light employed. For instance, most of the parti- coloured discs, prepared as described in the present paragraph, even if their coloured sectors are so adjusted as to give a good neutral grey by daylight, produce a de- cidedly reddish-grey by gas or candle-light, although by the electric arc light, and by the light of burning mag- nesium, the desired result is satisfactorily obtained. To use them with a " warm " and orange-hued artificial light, either of two plans may be adopted : increase the amount of blue-green in the disc, or interpose between the eye and the disc a piece of pale blue-green glass or coloured gelatine ; the hue required is very nearly that of the paint known as " viridian," or "emerald oxide of chromium." 23. The grandest natural example of the decom- position of white light into its coloured constituents is furnished by the solar rainbow. The parallel rays of sunlight, falling at certain angles upon the nearly spherical rain-drops, are refracted and reflected within them, and emerge as coloured rays. The gatherings of these separated and nearly parallel rays, in the order of their several refrangibilities, into groups of differently- coloured beams, constitute the bands of the primary bow ; the fainter secondary bow is produced from other solar rays, which suffer not only two internal refractions, and one total internal reflection within the drops, but a second internal reflection, which weakens the brilliancy of the coloured bands, and reverses their sequence. 24. Endeavours have often been made to draw analogies between the sensations of sound and those of colour. Such apparent relationships as have been pointed out are for the most part quite fanciful. The nature of sound-vibrations differs from that of light- vibrations in many important particulars, notably in this point : that while sonorous waves move in the direction of their path, the motions of light-waves are transverse. Nor is there any definite ratio between typical colour in- tervals and the musical intervals of the octave. Thus, Chap. II. J COLOUR AND SOUND. 25 in the case of pairs of our complementary colours, the relation of wave-length which subsists between them is not uniform, but varies considerably ; with some pairs it may be expressed numerically as 1 : 1'2, with others it is 1 : 1*333, or, adopting musical notation, the relation may be said to vary from that existing between a note and its fourth to that between a note and its diminished third. Again, the eye is more appreciative of differences in wave-length in the middle region of the spectrum than it is of differences towards either end ; but the ear does not possess a corresponding peculiarity with regard to musical sounds. While we can hear about eleven octaves, we can see but one octave. The sound-waves which the human ear can recognise range from sixteen vibrations in one second of time to 38,000 in the same period ; the light- waves which the human eye can per- ceive range between 390 millions of millions in one second to 770 millions of millions. When several notes are sounded at once we do not get a sound of medium pitch, but either a discord, or else a consonance or chord- euphony, in which the elementary tones may be distin- guished. But when a number of colours, or even two or three only, strike the eye, we get a medium colour, weakened by whiteness, and not enriched by the simul- taneous action of the chromatic elements upon the retina. Again, and lastly, let us compare and contrast a musical trill or shake on two notes with the successive presenta- tion of two colours to the eye. If the number of vibrations necessary to produce an optical effect in the latter case were commensurate with those required in the case of the trill, the lapse of time needed between the two notes would have to be measured by years ! In fact, colour sensations involve rather the element of space than that of time. The symbolism of colour demands a passing notice, although it scarcely admits cf serious discussion. If we allow that the obscurity and gloom associated with black may reasonably be connected also with sorrow, mourning, and death : if we accept white 26 COLOUR. [Chap. IL as typical of purity and innocence ; still we shall have some difficulty in explaining the use of green as sym- bolising felicity and the Resurrection, or in interpreting blue as involving the ideas of piety and divine con- templation. Readers who are curious as to this subject may be referred to the English translation of Baron P. de Portal's Essay on Symbolic Colours. CHAPTER III. PRODUCTION OF COLOUR BY ABSORPTION, BY DIFFRACTION, BY POLARISATION SELECTIVE ABSORPTION AND SELECTIVE REFLECTION PRODUCTION OF COLOUR BY LOSS OF COLOUR HUE AFFECTED BY THICKNESS OF ABSORBENT MEDIUM INTERFERENCE OF LIGHT-WAVES COLOURS OF THIN FILMS. 25. THE absorption and reflection of light are very closely related, yet there are many coloured bodies which, instead of absorbing some of the coloured component rays of white light, and reflecting others, transmit those rays which they do not reflect. Even a third condition exists, in which a substance reflects some of the coloured com- ponent rays of the incident light, transmits others, and absorbs the remainder. And there are cases in which a further and more complicated action takes place ; for some coloured substances when white light falls upon them not only reflect some of its coloured rays, transmit others, and absorb others, but also alter the refrangibility, and therefore the hue, of some at least of the rays which they allow to pass. We may now briefly discuss and explain the production of colour by these methods. 26. Let us suppose a substance such as a trans- parent crystal of cinnabar (vermilion or mercuric sulphide) or else of cuprite (copper sub- oxide). These bodies appear UNIVERSITY or Chap. III.] SELECTIVE ABSORPTION. 27 red both by transmitted and by reflected light. Of the white light which falls upon them, these substances reflect part unchanged, part plunges to a small depth beneath the surface, and emerges as a reflected beam with a red hue, having lost the other coloured constituent rays by absorption, and the remainder, in passing through the substance, suffers a similar selective absorption, and emerges as a transmitted beam of red light. In many such cases, however, the reflected red and the transmitted red, similar as they may appear to the unaided eye, prove on analysis with the prism to be somewhat differently constituted. The next cases to be considered are those in which the coloured reflected light is entirely different from the transmitted light. A few metals may be cited, along with iron pyrites and such complex substances as magnesium platino-cyanide, potassium permanganate, murexide, indigo, eosin, magenta, quinolin-blue. The yellow colour of metallic gold is due to selective absorp- tion. True, a plate of gold reflects some of the incident white light unchanged, but it quenches in another portion many of the green, blue, and violet rays, and so leaves the residual red, orange, and yellow to produce the warm yellow hue which is so characteristic of the light reflected from gold. It might seem likely that this metal would transmit, when in sufficiently thin leaves, all those coloured rays which it does not reflect. This is true to a great extent. If we coat a thin plate of colourless glass with mastic varnish, and, while this is still tacky, let a leaf of gold adhere to it, we find that it transmits a beautiful green light, which contains a large part of those rays which are not found in the light reflected from a surface of gold, and are entirely absorbed or quenched in a sheet of the metal too thick to be translucent. Solid indigo affords us another and similar example of selective absorption and reflection. If a lump of pure indigo be pressed with an agate burnisher, a copper-coloured streak makes its appearance. So long as the substance of the indigo is not virtually continuous that is, so long as it 28 COLOUR. [Chap. III. exists in the form of minute powdery particles so long it shows no sign of a copper-coloured reflection, but appears blue. Now, the blueness thus seen by reflection is not actually produced in or by simple reflection. The incident light or, rather, a part of it plunges to some depth amongst the blue particles, and passes through them, a chromatic selection being thereby made, so that the light finally reflected to the eye, having been pre- viously deprived by selective absorption of some of its coloured constituents, is blue. Increase the coherence of the blue indigo powder, either by pressure or by the pro- cess of sublimation in which crystals are formed, and then, though the transmitted light, if it can be obtained, will be blue, the reflected light will be copper-coloured. Similarly, eosin (one of the coal-tar dyes) in its solid form reflects a yellow-green light, and transmits a red, while potassium permanganate in crystals reflects a bronze light, and transmits a purple or violet. In these and in many other cases where the coloured reflected light has that peculiar intensity of lustre which ap- proaches the metallic, the coloured rays of the reflected light, if re-united to those of the transmitted light, pro- duce a hue which very nearly approaches white. A very curious instance of the difference in colour between the reflected and the transmitted rays is afforded by the brass-yellow crystals of that very common mineral mundic, or iron pyrites. Their lustre is metallic, and their colour as ordinarily seen is brass-yellow. As this substance, which when compact is quite opaque, is gradually reduced to very fine powder, the hue changes and deepens, until at last we have a material which might almost be termed blue at least, we may call its colour a greyish-blue. Although this subject has not been thoroughly investigated, it would appear that the bluish colour of iron pyrites, when in impalpable powder, is due to the selective absorption exercised upon white light when transmitted through its very minute particles. The phenomena described and discussed in the foregoing; Chap. III.1 PRODUCTION OF COLOUR. 29 paragraph often suffice to explain the great difference in colour between a compact solid and its powder. 27. We will now consider the case of such coloured bodies, whether solid or liquid, as would, in ordinary par- lance, be called transparent. Of course, were they per- fectly transparent to all the waves of white light they would not be coloured at all. It is because they are transparent to some only of such waves, and opaque or partially opaque to others, that the light they transmit is coloured. First of all, let it be clearly understood that white light is not converted into coloured light by passing through a coloured medium. As a general rule;/ it becomes coloured not by change, but by loss. Some- thing coloured is removed from it during transmission, and a coloured residue is left. You cannot turn red light into green light by a piece of green glass ; if the particular kind of green glass chosen is quite opaque to the red light, the latter will be invisible when the green glass is placed between the eye and the luminous source. And when white light passing through green glass yields a green beam, the result is obtained through the absorp- tion by the green glass of the red and of all the other ray, save the green, or, at least, of all the other rays save those which, viewed together, give the sensation of green. A very easy experiment with the spectrum of any white light proves this point. We have only to provide our- selves with a few strips of differently- coloured glass, and to interpose them between the luminous source and the spectrum projected on a screen. Beginning with a red glass, we find that its interposition between the light and the red region of the spectrum does not visibly affect the colour of the latter. Passing on to the orange band, we shall find this becomes less luminous than before, and this darkening effect becomes more conspicuous as we approach the green, where, in all probability, we shall observe that the red medium very nearly destroys all the light. Similarly a blue glass cuts off" little light from the violet and blue portions of the spectrum, a good deal 30 COLOUR. [Chap. III. from the green, and nearly all from the yellow and orange portions, and considerably darkens the red. Were the blue light transmitted by the glass perfectly homogeneous that is, did this medium transmit light of one refrangi- bility only, or rather, did it transmit rays the refrangi- bilities of which differed only so much that the colour sensations they excited in the eye could not be dis- tinguished, then it would blacken every part of the spectrum save the blue. But no such glass exists, all blue media permitting considerable quantities of green (and often of red) rays to pass through them. 28. In continuing our experiments with coloured glasses and strips of thin gelatine (used for cracker- Fig. 9. bonbons), we shall soon come across another phenomenon of great interest. We shall find that transparent-coloured media do not appear of the same colour when the thick- ness through which the light is transmitted is made to increase or diminish. The easiest way to try this ex- periment is to cut half a dozen strips of coloured gelatine, each shorter than the last by a quarter of an inch. Wet the largest piece and lay it on a sheet of colourless glass ; superpose on this number two, and so on to the last and smallest strip. We might expect to find nothing more than a darker tint, a progressive purity or richness in colour, as we proceed from the single layer to the band where six layers are superposed. But this is not all that we see. We find that the hue alters as well as the richness, or comparative freedom from white. In the Chap. III.] CHANGES OF HUE. 31 case of yellow gelatine (or yellow glass), a, in Fig. 9, will of course show the normal hue, but b will not be a deeper yellow : it will be orange-yellow, while c, d, e, and f will verge gradually towards a full orange, or even a red. We learn from this experiment that the single plate not only exercises less absorbent power than two or more plates, but that the absorption differs in kind as well as in degree, the thicker layers cutting off in succession groups of variously-coloured rays which the thinnest layer permitted to pass until nothing but orange-red or red light is transmitted. By placing the compound plate upright in a horizontal solar spectrum, beginning at the red end, the exactness of this conclusion will be demonstrated. The gradual change of hue as well as of tone may also be beautifully seen in many crystals, both natural and artificial. Blue vitriol (copper sulphate) in-\y be readily obtained in large smooth crystals, and jiffords a good example of the peculiar absorption now under discussion. A crystal of this salt transmits in its thinnest parts a bluish-green hue, but gradually, as we look through an increasing thickness of blue vitriol, the blue deepens, and we are no longer able to recognise the slightest greenish cast in the colour. This change is due to the fact that a thin layer of blue vitriol transmits green rays as well as blue, while a thick layer absorbs the former, and still continues to transmit the latter. The majority of coloured solutions exhibit corresponding appearances. Good examples are furnished by solutions of potassium permanganate (Condy's Purple Fluid), potassium bichromate, chromium sesquichloride, chloro- phyll, turacin, colein, and the majority of the so-called " coal-tar colours." The three first-named substances are employed in the form of a watery solution ; the chlorophyll may be examined dissolved in alcohol, and may be prepared by drying nettle or beet-root leaves quickly in warm dry air, crushing them, and then macerating them in strong alcohol. Turacin, the red cupreous pigment from the wings of many species of the 32 COLOUR. [Chap. Ill African plantain-eaters, or Touracos, is best obtained in an available state by soaking a feather in weak ammonia- water. Colein may be extracted with acidulated alcohol from the stems of that well-known ornamental-foliaged plant, Coleus Verschaffeltii ; and the coal-tar dyes may generally be dissolved in alcohol or water. A very good plan for observing the striking differences in hue be- tween thin and thick layers of such solutions as those just mentioned was devised by Professor Stokes. A fine slit, about one-fiftieth of an inch across, between two blackened metallic edges, is adjusted vertically in a blackened piece of board j behind the slit is a source of light, such as a bright flame or a white cloud. Hold a prism, having a refracting angle of 60, against the eye. By adjusting the position of the prism suitably a pure spectrum will be obtained, showing, if solar light be em- ployed, the principal fixed absorption lines. Now, to observe the characteristic absorption exerted upon dif- ferent rays of the visible spectrum by any liquid, ad- just by a clip or elastic band, a test-tube, or flat glass cell, containing the liquid to be examined, behind the slit. ' Begin the observation by using a pale and weak solution, and then gradually increase the strength, noting the formation and development of dark bands or spaces in different parts of the spectrum, and the blotting out of colour after colour. If, as first suggested by Dr. Gladstone, a wedge-shaped trough be used to hold the coloured liquid behind the slit, it may be gradually moved downwards, so as to interpose thicker and thicker layers of the coloured liquid, and thus to produce the same effects as those obtained by gradually increasing the strength of the solution. Examined in a wedge-cell, chlorophyll solution is seen to begin by cutting off or absorbing, when in a thin layer, the violet, much of the blue, and some bands in the red ; a thicker layer cuts off the blue, the yellow, the orange, and part of the green ; and finally, in a still thicker layer, everything is extin- guished save a part of the extreme end. Another liquid, Chap. III.J DIFFRACTION. 33 chromium sesquichloride solution, which is green in thin layers, and reddish in thick, like chlorophyll solution, owes this peculiarity to its transmission, when in thin layers, of very much of the green, a little yellow, blue, and violet, and very much of the red : increasing the thickness of the layer considerably, nothing is trans- mitted save a small part of the green, and a good deal of the red ; everything is absorbed by a very thick layer, except the extreme red. We must not dwell longer on this very attractive topic, but the experimental methods we have described are of importance when we want to learn accurately the effect of diluting any transparent colour which is to be employed for artistic purposes. Thus it will be found that some reds when diluted, instead of becoming pink, pass through orange to orange-yellow ; while some blues, instead of becoming merely paler blues when weakened, become either greenish-blue on the one hand, or violet-blue on the other. It is evident, there- fore, that if a deep tint of a transparent red pigment is found to match the same colour in a natural or artificial object before the painter, it does not at all follow that the paler tones of that pigment will equally well repre- sent the paler tones of the object to be painted. 29. Thus far we have been studying white light and its resolution into colour by means of the prism and of selective absorption : we will now see how the rays of different refrangibility may be separated in another way. Some of the most beautiful phenomena of colour are produced by a modification which light undergoes when it passes the edge of an opaque body, or when it traverses a small opening. Light then turns a corner, just as a water-wave will turn the angle of a wall, or spread itself on the further side of a hole in a plate through which it has passed. This bending of the waves of light has been called diffraction. The source of light, in studying the phenomena of diffraction, should be a luminous or highly-illuminated spot. A silvered bead, or a steel globule, or the focus of rays obtained by the D 34 COLOUR. [Chap. III. action of a lens on a beam entering a dark chamber through a small hole : all these contrivances furnish a suitable illumination. A simple way of producing colours by diffraction is to view a bright light through a perpendicular slit one-eighth of an inch wide in a black card, holding, at the same time, close to the eye, a strip of blackened glass, on which we have previously made a fine perpendicular scratch with a needle. The scratch must correspond with the central part of the slit, which is to be kept at a distance of about three feet from the scratch. We shall see the slit in the centre, but on either side of it will appear a series of spectra, proving that the light-waves which have passed the nearer and finer slit, or scratch, do spread laterally, or are diffracted from its edges. Imagine a perfectly regular series of fine scratches on a piece of glass, and we get a diffraction grating. By it we enforce enormously the luminosity of the spectra, the various interferences and correspondences of the several waves falling at regular intervals. If fine enough, such an assemblage of slits practically cuts out at each point all but one single wave-length, and we get pure spectral colours only. If the source of light be monochromatic, or if a plate, say, of coloured glass be interposed between the light and the grating, we shall see bands of one colour only alternating with bands of black. By using, instead of a scratch, or slit, or regular grating, apertures differing in size, number, and shape, very beautiful chromatic appearances may be developed. These may be obtained by looking at a bright point or narrow line of light through a bird's feather mounted in a card frame, through a glass dusted with lycopodium spores, through a fine wire gauze, very fine cambric, or fine perforated cardboard or zinc. 30. The full explanation of the production of colour by diffraction involves the study of a very complex problem, namely, the interference of light -waves. Though this subject cannot be adequately discussed here, the general principle which underlies all interference actions Chap. IU.1 INTERFERENCE OF LIGUT. 35 may be easily grasped. An example drawn from liquid waves will make the cause of the phenomena clear. Two stones dropped at some distance apart into th& water of a still pond will generate two sets of circular waves. At many points the waves of one set must cross the waves of the other. At some of these points the crests of the two crossing waves will coincide, and the waves will be reinforced ; at other points, the same particle of water is elevated by one wave and depressed by the other there it is at rest. A similar but not precisely identical series of inter-actions occurs, under certain circumstances, with light- waves. Take the case of a film, such as an iridescent glass film, or a soap film so thin as to show colour. Assume that a pencil of white light, s (Fig. 10), ^_ j is incident at an angle j on a thin plate, P, at the Fig. 10. point I ; we know that a large part will be reflected at the same angle towards R. But the rest of the beam enters the film and is refracted towards the normal to F, whence at all angles, save that of total internal reflection, a portion leaves the film finally. But the remainder of the refracted pencil is reflected to E, and thence on leaving the film is refracted to R'. But the pencil E R' has had to traverse the film twice (from i to F, and again from F to E) before it emerges parallel with I E, and so has been retarded, or got behind I R. This retardation affects the different-coloured constituents of white light differently according to their wave-lengths, and more or less suppresses some colours, while it strengthens others. So if the film be thin enough to allow the two parallel emergent rays to be so near together that they can interfere, then colour must be produced. The particular colour at any one point will depend upon the COLOUR. [Chap. IIL thickness of the film, and the fraction or number ot fractions of a wave-length to which it corresponds. Newton's rings, seen between a slightly curved lens and a plate of glass, are produced similarly by interference, and so are the diffraction colours described in the preceding paragraph. \ 31. We have remarked that the colours of thin plates correspond in sequence, as do those of refraction, to the colours of the prismatic spectrum. We have seen that they are produced by the interference of the ray which, having impinged upon the first surface of the thin transparent film, is reflected directly therefrom, with the ray which after refraction and reflection within the film suffers a second and inverse refraction from the first surface, and emerges parallel with and very close to the directly reflected ray. The final result is therefore due to the interference of a ray reflected from the first surface with a ray reflected from the second. A soap bubble may be of such a thickness as to retard the ray reflected from its second surface by half a wave-length, or by an uneven number of half wave-lengths. In such a case it will be found that the bubble is black, or rather dark grey, because the two reflected rays are in complete discordance, and a destruction of light ensues. Then, again, soap bubbles may, and generally do, vary much in the thickness of different parts. As the waves of light differ in length according to their hue, so they will require different thicknesses to produce accordance and discordance. The result of this is that a thickness of film which is competent to extinguish one colour will not extinguish other colours. Thin films of variable and differing thicknesses, illuminated by white light, will therefore display in their different parts variable and different colours. 32. As the colour phenomena observed in soap bubbles are extremely instructive, as well as beautiful, a recipe for making a good soap solution capable of yield- ing long-lived bubbles may prove useful. One hundred Chap. ITL1 POLARISATION OF LIGHT. 37 grains of pure potasskim oleate are to be dissolved in eight ounces of recently boiled and still warm water : to the solution, when complete, six and a half ounces of glycerine are added. The mixture is left to repose, and finally filtered. In warm weather this solution may need strengthening with a few grains of Castile soap. When solution has been effected, the liquid is allowed to cool and then again filtered. Light wire rings, coated with paraffin, or glass cylinders, or paper discs, all well soaped, may be used to carry the bubbles. The solution and all the apparatus should be clean and kept warm, say at a temperature of about 65 70 Fahrenheit. The splendid metallic hues of the feathers of the humming-bird and the peacock and of the wing-cases of certain beetles : the rainbow hues of mother-of-pearl and many shells, of antique glass, of the precious opal, and of imperfectly polished metals all these beautiful pheno- mena are due to interference, and not at least in the majority of instances to any actual colouring matters. We have in these objects minute surface sculpturings, or striae, or veins, or foldings, and it is by reflections and refractions among these microscopic mechanical textures that rays capable of interfering with each other are generated. The "Iris Ornaments/' or "Barton's Buttons," made by Sir John Barton, a former Master of the Mint, exhibit in marvellous perfection the splendid iridescent colours due to diffraction, and consequent interference of light. These buttons of steel are covered with minutely- engraved lines, arranged in stellar and other devices. The colours of tar or oil films upon water, and of lead skimmings, and the magnificent chromatic phenomena presented by many crystals, natural and artificial, in polarised light, are likewise due to interference. 33. The polarisation of light just mentioned de- mands a word or two of explanation. A pencil of ordi- nary light consists of waves vibrating in all azimuths. By several methods it may be resolved (as in the resolu- tion of forces) into two pencils vibrating in two azimuths 38 COLOUR. [Chap. IV. only, at right angles to each other. Such resolution is effected by (1) a doubly-refracting medium, such as Iceland spar ; (2) reflection at a definite angle from a polished surface ; (3) passage through certain crystals, such as tourmalines. In the case of Iceland spar, a pencil of light is split into two rays, one more refracted than the other, and having, therefore, a path of different length to travel, less or more retarded than the other. Here, then, we have two rays (vibrating in planes at right angles to one another) of identical origin, and of nearly identical path, which could interfere, and therefore pro- duce colour, if only they could again be brought to vibrate in the same plane. This may be accomplished in several ways, notably by the introduction of a thin plate of mica between the "polariser" and "analyser." For a full description and explanation of the apparatus and its action, reference must be made to the treatises named in our Bibliographical Notes, p. ix. CHAPTER IY. OPALESCENCE AND TUEBID MEDIA CLOUD, FOG, AND MIST FLUORESCENCE PHOSPHORESCENCE CALORESCENCE INCANDESCENCE COLOURED FLAMES THE UNITY OF THE SOLAR SPECTRUM. 34. WHEN solid particles or liquid globules, which do not dissolve, are suspended in a liquid, provided the particles or globules be sufficiently fine, we observe the phenomena of opalescence. Air or any gas may similarly be rendered opalescent or turbid by the suspension therein of minutely-divided solid or liquid substances. And in the same way minute bubbles of air or gas sus- pended in a liquid medium may render it opalescent. Chap. IV.] OPALESCEXCE. 39 Moreover, a fourth class of examples of the same pheno- menon is furnished by solid transparent bodies, in which very small particles of gas, of solid matter, or of liquid are uniformly intermixed. Provided that the medium itself be colourless, the colour and degree of transparency of the suspended particles are often of little or no account in modifying the appearances presented. Finely-divided yellow sulphur in water, black carbon particles or colour- less water-spheres in air, and opaque white tin binoxido particles in glass, all produce the same result. Excellent illustrations of opalescent materials are furnished by adding a few drops of milk, or a little sodium thiosulphate followed by some hydrochloric acid, to pure water ; by burning a little brown paper in air ; by mingling a little bone-earth or some tin binoxide with molten glass. Fog and mist, the translucent common opal, and the so-called opal glass, are familiar examples of opalescence. Putting aside the cases in which colour is produced by the dif- fraction of light caused by the suspended particles, and the consequent interference of the diffracted rays,- we may affirm that opalescence is caused by the scattering of light by small particles, such particles being small in size when compared with a light-wave. The general action of such excessively small particles is this, that they decompose white light, being incompetent to reflect it in its entirety, and that they scatter more of the blue and violet light which have short wave-lengths than of the orange and red light which have longer wave-lengths. The correlative effect of this reflection of blue and violet light is the transmission of orange and red light that is, of the residual rays which have escaped reflection. So it happens that if we look at a turbid medium, such as milk and water, it appears blue ; if we look through it, it appears orange or red. As the turbidity of a medium increases we have, naturally enough, a diminution of the transmitted light ; but it alters in hue also, any violet and blue which may at first have escaped reflection being first cut off, and then, in succession, the green, thfe 40 COLOUR. [Chap. IV. greenish-yellow, the yellow, the orange. The red alone remains, and even this becomes weaker, and is finally stopped by largely augmenting the number of the minute particles present. No more telling example of this reduction of light and progressive reddening of hue can be cited than that furnished by the setting sun as it approaches the horizon, and sends to our eye beams which continually traverse layers of air in which the fine water- particles are present in ever-increasing number. The colour of its light therefore passes gradually from yellow to orange, from orange to scarlet, and from scarlet to crimson. The long receding lines of lamps in a London street exhibit the same progressive change of hue from the nearest to the farthest visible, owing to this cause. On the other hand, the blueness of the sky and of distant mountains may be traced to the larger quantity of blue rays reflected from deeper layers of a turbid atmosphere, and is the necessary complement of the above-described increasing redness. In the higher regions of the atmos- phere, as among lofty mountains, and also in countries where the air is exceptionally dry, we lose these pic- turesque effects, which soften outlines and enhance the forms of nature with the charms of atmospheric colour. Sometimes the blueness of the light scattered by fine particles is artistically disadvantageous, as in thin washes and touches of Chinese white in water-colours. The cold bluish cast of the greys produced by adding a white to a black pigment is traceable, at least in part, to the same cause. 35. The law which is followed by the chromatic absorption of turbid media was enunciated by Lord Rayleigh, and confirmed experimentally by Genera] Testing and Captain Abney. For any ray and through any thickness the light transmitted varies inversely as the fourth power of the wave-length. So, if we have a wave-length of 6,000 in the red and a wave-length of 4,000 in the violet, then the fourth powers of these wave- lengths will be as 81 : 16, or about 5:1. Consequently, Chap. IV/J FLUORESCENCE. 41 if 4 inches of a turbid medium allowed of this par- ticular red ray to be transmitted, they would allow only (j) 5 , or rather less than one-fourth, of the blue ray to pass. 36. There are certain apparently transparent sub- stances, both coloured and colourless, which exert upon the light falling upon them, or which we attempt to pass through them, a most extraordinary and unexpected effect. We refer to the phenomenon called fluorescence, the true explanation of which was discovered by Pro- fessor G. G. Stokes. A solution in weak sulphuric acidl of quinine sulphate, a piece of canary-yellow uranium,' glass, a crystal of fluor spar, an alcoholic extract of green \ leaves, and a solution of the coal-tar dye called eosiii, j afford instances of fluorescence. The action exerted in fluorescence upon certain of the solar rays and of the rays emitted by burning sulphur or magnesium, and indeed upon certain rays emitted by a large number of luminous sources, is a change of refrangibility. Amongst the rays so affected by some of the above-named substances are those which lie beyond the extreme violet, and which are quite incapable of exciting the sense of vision. These ultra-violet rays of very small wave-lengths have their periods of vibration increased by the quinine solution, and becoming visible, " flnoresce " blue. But this action is not confined to the ultra-violet rays, for there are fluorescent substances which change the refrangibility, and therefore the colour, of rays already visible. And there are instances in which an already visible ray has its colour altered by a change of refrangibility in the opposite direction : namely, by long waves becoming shortened ; naphthalin red and eosin are examples. It is found that the coloured light displayed by fluorescent bodies, and reflected from their particles, when it arises from the altered refi:angibility of visible rays, is always betrayed by the absence of such rays from the light which they transmit. It must not be supposed that in fluorescence the incident light is directly changed in re- franibility, But it first causes a molecular disturbance 42 COLOUR. [Chap. IV. in the fluorescent body, and then this disturbance affects the waves of the ether. 37. The observation of fluorescence may be simply made as follows : Lay a piece of canary-yellow glass or a crystal of fluor spar on a piece of black velvet, and allow a sunbeam, or the beam from an electric arc lamp, to fall upon it. The transparent solid becomes iilled with a splendid coloured light, quite different from that of the medium as viewed by transmitted light. As visible rays are not necessary to, and are often not concerned in, the production of fluorescence, it will be found a good plan in many cases to interpose a piece of deep blue glass in the path of the original beam, or to view the specimens in a room from which the majority of the visible rays have been cut off by means of such blue glass ; the pheno- menon is still clearly produced. But if we interpose a screen of canary -yellow glass, or a cell containing the quinine solution, between the light and the substances examined, no fluorescence occurs, the rays producing it having been stopped by the screen. The flame of a spirit- lamp burning a mixture of alcohol and carbon bisulphide gives but a pale blue light ; but this, notwithstanding its feeble luminosity, excites fluorescence in a high degree. 38. The following list includes a number of highly fluorescent bodies; most of those which occur in the liquid form may be best examined by allowing a slender stream of their solutions to fall into a jar filled with pure water on to which a beam of convergent light (using that term in its widest sense) is directed by means of a lens Substance. Fluor esces. Fluor Spar . . . . . Green, blue, or violet in different specimens. Uranium Glass Yellowish-green. Anthracen Sky-blue. Quinine sulphate in weak sulphuric acid Blue. Fraxin from horse-chestnut bark, dis- solved in weak alkali . . . Blue-green. hap. IV.] PHOSPHORESCENCE. 43 Substance. Fluoresces. ^Esculin from horse-chestnut bark, dis- solved in weak alkali . . . Blue. Chlorophyll in alcohol .... Red. Eosin in alcohol Green. Naphthalin-red, in alcohol . . . Orange-yellow. Fluorescein dissolved in dilute ammonia Green. Cyclopin in dilute ammonia . . . Greenish-blue. Paraffin oil, both the kind used in lamps and that employed for lubricating machinery, is very frequently fluorescent, the lamp-oil showing a blue light, and the machinery oil a green. 39. Canton's phosphorus and Balmain's luminous paint afford instances of the phenomenon known as phosphorescence. Many substances after exposure to the solar rays continue to shine after removal to a dark room. Rubies, some diamonds, spodumene, the sulphides of calcium, strontium, and barium, and a large number of other inorganic substances, natural and artificial, absorb, during exposure to the radiant energy of the sun, or of an electric discharge, some of the rays, and subsequently niit light-rays of altered refrangibility. Exposure to rays of short wave-length has been found to destroy the phosphorescence of an excited surface of Balmain's paint. The nature of the light emitted by a number of phosphorescent sulphides has been examined by Lommel. He found that these bodies exhibit one or more of three maxima of luminosity one of these being situated in the yellow, one in the green, and the third in the blue region of the spectrum. The variety of colours emitted by this class of phosphorescent bodies is traceable to the presence or relative strength of one or more of these maxima. Substances exhibiting a violet-blue colour show all three maxima; in the blue kinds the second and third are present; in the bluish-green the second maximum is conspicuous; in the orange the first maximum only. As the phosphorescence diminishes, the maxima do not always fade simultaneously, and, 44 COLOUR. tcbap. iv. in consequence of this, the hue of the emitted light is modified. Phosphorescence is linked to fluorescence and the phenomena frequently overlap, for it is found that'the duration of fluorescence is often sensibly prolonged after the source of the rays which have excited it has been withdrawn. The luminous phenomena produced by electric discharges in the highly-attenuated atmospheres of the so-called vacuum tubes, are generally described as cases of phosphorescence. Here the indirect cause of the phenomena is a transformation of electric energy, while the immediate is probably, in some measure, the change of heat into light a change which is known as calorescence, and is described in 40. However, some of the most beautiful phenomena of phosphorescence are undoubtedly observed when such phosphorescent bodies as those above named are exposed to an electric discharge in high vacua. Mr. W. Crookes, who has made an immense number of experiments in this way, has noted, amongst many other instances, the following cases of phosphorescence : Diamonds : phosphoresce red, orange, yellow, green, and blue. Rubies : phosphoresce a brilliant crimson. Sapphires : phosphoresce green. Zirconium oxide : phosphoresces bluish-white. Yttrium sulphate: phosphoresces golden-yellow. Samarium and Calcium sulphate : phosphoresces red. 40. Calorescence may be regarded as a variety of fluorescence. When all the rays of a continuous spec- trum are stopped out, save the slowest waves or those of invisible heat, the " dark heat " rays that remain, after having been gathered into a focus, are competent to raise platinum foil to a visible red heat, or even to a yellow or white heat. Thus heat becomes visible as coloured light, which itself, when analysed by the prism, shows all the colours of the rainbow. There is here an increase of refrangibility, while in the normal cases of fluorescence Chap. IV] INCANDESCENCE. 45 first investigated the actinic waves converted into light suffered a reduction of refrangibility. The mode of stopping out the visible rays in calorescence experiments consists in the employment of a cell containing iodine dissolved in carbon disulphide. Through this solution, which is practically opaque to visible light, a beam of invisible heat radiations is transmitted. 41. Incandescence, the glow of a carbon filament, or of a platinum wire through which an electric current is passing, and of vapours and gases when strongly heated, is a term employed to designate phenomena of somewhat variable and complex kinds. Generally, incandescence is produced by the conversion of heat-rays, visible and invisible, into light-rays. When a ball of red-hot copper is allowed to cool slowly in the air in a dark room, it remains visible to some sensitive eyes after it has become generally invisible. Similarly, as we raisd the tempera- ture of a piece of platinum foil by means of dark heat, it will "appear" to some eyes earlier than to others. The first hue it assumes is a kind of dull chocolate-brown, representing the extreme red of the spectrum. This passes through many varieties of red and orange into yellow, and finally, by the addition of light-waves of greater refrangibility, becomes white. Thus heat-waves become light-waves, though it is not necessary that the heat-rays employed be, as in this example, originally dark or non-luminous themselves ; they may be of one wave-length or of many wave-lengths, but their vibrations being accelerated and shortened, they are transformed into visible rays of all periods in the substance of the material body, which thus becomes incandescent even to whiteness, the incandescent body (such as the lime in the oxy- hydrogen blow-pipe) remaining itself unaltered chemi- cally. The successive development of visible rays of different refrangibilities in a heated platinum wire may be beautifully seen by observing the wire through red, yellow, green, blue, and violet glasses, as it approaches full incandescence. 46 COLOUR. LCtap. IV. 42. The light emitted by flames is often a mixture of that derived from incandescent solids, vapours, and gases, with that sent out by substances actually under- going chemical changes, such as oxidation. Frequently it is of very complex constitution, including rays of all refrangibilities between the infra-red and the ultra-violet heat-rays, light-rays, and actinic rays. Frequently it presents a moderately brilliant continuous spectrum, accompanied or overlaid, as it were, by a comparatively brilliant discontinuous spectrum. Sometimes the latter spectrum is all that we can see, and it may be very simple or very complex. It is easy to illustrate this and to produce a flame which, when analysed by a prism, or better, by the spectroscope, shall show not merely an immense number of minute black spaces, like the spectrum of the sun, but broad bands of darkness divided here and there by lines of brilliant colour. The simplest spectrum which we can readily use as an illustration is that of the metal sodium. Dissolve a little common salt in some methylated spirit of wine, and introduce the solution into a spirit-lamp The flame will appear fairly luminous and of a yellow hue, slightly verging on orange. Now place, close to the flame, a narrow slit (about -^ inch broad) in a metallic plate, and look at the slit with the prism. In order to get a pure spectrum of this, or of any other flame, a good spectroscope should be employed. This instrument which consists essentially of a fine adjustable slit, a collimating lens to make the luminous rays parallel, a prism, or chain of prisms, made of highly refractive and dispersive glass, and a telescopic eye-piece gives a succession of clear images of the slit, corre- sponding to the succession of visible rays from one end of the spectrum to the other. But with sodium the vast majority of rays are wanting, and so there is but one part of a spectrum presented to the vision. This corresponds to what is called the black line, D. Really, this line, D, consists of two firm black lines, with several faint lines between them and on either side. In the Chap. IV.] COLOURED FLAMES. 47 flame of sodium, the exact spaces of these black lines of the solar spectrum are occupied by bright orange- yellow lines ; these constitute the ordinary spectrum of sodium, and explain the yellow hue of its very simple light. Why these very same bright lines occur as black lines in the spectrum of the sun is owing to the fact that the light of the body of the sun loses the rays which correspond to these lines by its passage through the solar gaseous enve- lope, which contains sodium vapour, and is found to be opaque to the rays it emits. 43. In trying experiments with coloured flames, in order to study their effects on the ap- pearance of differ- ently coloured ob- jects, or to study their spectra, the contrivance repre- sented in Fig. 1 1 Fig. 11. may be used. A is a Bunsen gas-burner (the top of which may be made of steatite) ; B is a bundle of fine platinum wires bound together by a spiral coil of rather coarser wire of the same metal, and dipping into a small vessel containing a mixture of a solution of the metallic salt to be experi- mented with and pure ammonium chloride. A ball of pumice attached to a bundle of asbestos fibres may be substituted for the arrangement of platinum wires. The following is a list of chemical compounds, chiefly metallic 48 COLOUR. [Gimp. IV. salts, which impart colours of different hues to the flame of burning gas under the circumstances described. Substance. Calcium nitrate Lithium chloride Strontium chlorate or nitrate Sodium chloride Barium chloride or chlorate Boracic acid . Thallium perchloride Copper chloride Indium chloride Potassium chlorate or chloride Colour of Flatru Red. Carmine. Crimson. Orange-yellow Yellow-green. Green. Green. Blue-green. Indigo-blue. Lavender. 44. The above substances give spectra having few or many bright lines in different parts of the spectrum. The number and intensity of these lines differ with the degree of temperature to which the materials are raised, and even in some measure with the nature of the com- pound heated. Consequently, the exact hue of the several flames, the resultant effect that is, as a colour-- sensation of the combined rays of each body differs somewhat with the temperature. One of the most striking and most instructive of all experiments on colour is made by illuminating a bouquet of flowers, an oil painting, or a painted representation of the solar spectrum with the light emitted by any of the flames we have men- tioned. If these coloured objects do not receive the par- ticular coloured rays which they are competent to reflect, and to which their hue in ordinary white light is owing, they become dingy, or even black. The most brilliant blues, violets, and reds show no colour in the yellowish illumination of the sodium flame, and the human face ex- hibits a ghastly yellowish- grey pallor. We shall describe in a subsequent chapter (Chap. XII.) many other important modifications in the appearance of coloured objects, caused by variations in the components of the light with which they are illuminated. But this one experiment with light from a lamp burning spirit containing common salt, Cliap. IV.l UNITY OF THE SOLAR SPECTRUM 49 or from a Bunsen gas-burner encircled at the lower part of its llame with a collar of rock-salt, is amply sufficient to demonstrate the complete dependence of the varied colours of objects upon the presence, in the light by whicl; they are seen, of those particular coloured rays to which they are responsive, and which they have the power of irregularly reflecting. 45. What has been already said in the present chapter as to the changes of refrangibility effected by certain materials in the rays of dark heat, of actual light, and of invisible actinism, will have prepared the reader to accept the true view as to the essential unity of such a spectrum as that of the sun or of incandescent lime or carbon. The notion that the spectrum of the sun is a triple one, consisting of a heat spectrum, a light spectrum, and an actinic spectrum, partly superposed or overlapping each other, cannot be maintained. True, there is a region where the thermal effect is greatest, another where the luminosity reaches its maximum, and a third where the chemical action is most pronounced. But these facts, taken in connection with the changes of refrangibility occurring in calorescence, fluorescence, &c., and with the observation that all the rays, visible and invisible, ther- mal, luminous, and actinic, obey the same laws of reflection, refraction, polarisation, &c., negative the idea of three distinct spectra. The rays from the infra-red to the ultra-violet differ solely in their periods of vibration in the length of their waves. The diverse effects which they produce are due to the specific adaptation of par- ticular rays to produce certain effects. More than this, for one single ray, of perfectly definite wave-length and position in the spectrum, may produce a rise in the thermometer, a sensation of colour, and a chemical change in a sensitive substance. And these effects may, and do, occur without any necessary change of refrangibility in the active ray. Its motion, appropriately transferred from the ether to material molecules, is heat, light, actinism. Of course, innumerable cases occur in which 50 COLOUR. [Chap. V. the motion of a ray is stopped by a material substance, and is commonly said to be quenched or absorbed. It is not lost, however difficult it may sometimes be to trace its effects. Thus, a solution of alum in water is almost impervious to heat-rays, a clear crystal of rock-salt is almost perfectly transparent to them all. The impervious liquid is warmed by the rays it cannot transmit, while the rock-salt remains but little altered in temperature by their free passage. Invisible chemical rays are quenched very largely by glass, but pass freely through quartz : no chemical change occurs in either, but the glass rises in temperature. Again, there are other cases in . which heat-rays and light-rays absorbed or quenched by certain media are transformed into actinism. CHAPTER V. THE CONSTANTS OF COLOUR HUE, PURITY, AND LUMINOSITY OF COLOURS SHADES, TINTS, AND TONES BROKEN COLOURS CLASSIFICATION AND NOMENCLATURE OP COLOURS. 46. THE one characteristic of any colour which first appeals to the eye, and first demands consideration, is its hue : we endeavour to name the colour, be it red or orange, green or blue, violet or purple. When we are dealing with the solar spectrum, or with the continuous spectrum of incandescent carbon or lime, we can identify each colour (that can be separately recognised) by means of its wave-length or refrangibility. We can isolate a small coloured strip in the solar spectrum, and fix its position by a reference to neighbouring fixed lines. We can follow the same course with the several constituents of the bright line spectra of lithium, sodium, &c. ; but when a colour, as it affects the eye, is made up of several Chap. V.] THE CONSTANTS OF COLOUR. 5i rays, or groups of rays when its constitution is complex this method fails. For instance, there is not in any spectrum, a hue which can be called purple there is no wave-length of any single ray which corresponds to the ocular sensation of purple. That sensation may be excited by the combined or simultaneous action on the retina of red and blue waves, or of red and; violet waves ; we have only to mix lights of these colours in proper proportions, and we get purple j but it is clear" that we are unable to fix its spectral position. And this statement leads to another point of importance. Many of the colours which we perceive both in luminous and illuminated bodies, although they may appear identical in hue with particular portions of the spectrum, yet are found, on examination, to be not simple, but compound : not to consist of rays of one refrangibility only, but of a number of rays having different wave-lengths, but pro- ducing on the eye a resultant effect, exactly the same as the corresponding simple ray of the spectrum. Thus, there is a compound yellow, containing red and green waves; a compound blue, containing green and violet waves ; a compound orange, containing red and yellow waves, but each of these three compound hues may be exactly matched by a simple hue in the spectrum. 47. The second constant of colour is purity. A colour is said to be pure when it is unmixed with white. A pure colour is not necessarily a bright colour, for many bright colours contain a large admixture of white. Nor are pure colours always strong, rich, and deep, for there are parts of a pure spectrum where colours may be observed (wholly unmixed with white light, and having, of course, perfectly definite wave-lengths) of so weak a tone as to be recognised with difficulty. In a spectrum of the sun, or of any ordinary white light, as usually obtained by using a single prism, unless special pre- cautions be taken, there will always be some white light. And in pigments and coloured objects the proportion that the white light bears to the coloured will often be much 52 COLOUR. [Chap. V, larger than we expect. The colours of vermilion, emerald- green, and ultramarine are not pure in the sense in which this term is employed to designate one of the three con- stants of colour; for if we compare a strip of paper, painted thickly with vermilion, with the nearest corre- sponding colour of a pure and complete spectrum, we shall find that we can match its hue, but that it looks paler. To make the two colours correspond, we must add white light to the spectral red, to the extent of about one-fifth of the amount of the latter. In other words, we ascertain by this means that the red light reflected from vermilion is not '''pure" red, but contains in one hundred parts about oighty parts of red and twenty parts of white light. Of course, much depends upon the way in which the surface Of the pigment is prepared, and the medium in which it is mixed. A nearly matt surface of vermilion ground in copal varnish, oil, and turpentine reflected seventeen parts of white light to eighty-three of red. 48. It is of importance to remember that the addition of white to colour alters its hue, and does not make it merely paler. The addition of a very small quantity of white light to coloured light (1 part to 360) can be recognised by the eye in the paler tint produced, but it requires a much larger addition to bring about a perceptible change of hue. Increase of brightness also alters the hue, as fully described in 119 and 120. 49. Brightness, or luminosity, is the third colour- constant. It is measured by the total amount of light reflected to the eye, and is therefore independent of purity and of hue. It is sometimes spoken of as "clear- ness." If a given colour be at once perfectly pure and perfectly bright, it is saturated. The comparative bright- ness of the different hues in the solar spectrum depends in a measure upon the thickness of the atmospheric layer through which the sun's light has penetrated previous to its prismatic analysis ; the state of the atmosphere, in respect of dissolved water- vapour, and suspended water and solid particles, also greatly affects this relative brightness. Chap. V.] COLOURS OF THE SPECTRUM. 53 Tims, while the total brightness of sunlight is reduced by the depth of the atmosphere, the brightness of its several constituents is unequally affected, the red rays suffering the least dimunition. Vierordt made a series of observations on the relative brightness of the several main colours of the solar spectrum ; the results he ob- tained have been re-calculated by Kood, for a prismatic spectrum divided into 1,000 parts between the fixed lines A and H. Rood, moreover, has named the several coloured regions, so that with these two data (of spaces and luminosities) he has been able to construct the following Table showing the Amounts of Coloured Light in 1,000 Parts of White Sunlight. Red . Orange-red . Orange . Orange-yellow Yellow . Greenish-yellow Yellowish- green 54 140 80 114 54 206 121 Green and Blue- green 134 Cyan-blue ... 32 Blue . . . .40 Ultramarine and Blue- violet ... 20 Violet 6 These numbers are, then, the product of the respec- tive areas into their corresponding luminosities. And we find, on examining the spaces of the spectrum occupied by individual colours, that some of the narrowest give very high figures, owing to their great brightness. Thus the pure orange-yellow, about the line D, corresponding in area to no more than 1*12 per cent, of the whole spectrum, exceeds in luminosity all other regions. Captain Abney has more recently arrived at a similar result by an original and ingenious photometric method. He found, at an elevation of 8,000 feet, on the Riffel Alp, that the region of intensest luminosity in the spectrum was shifted somewhat farther still beyond the line D on the more refrangible side, when compared with its position as observable at low levels ; the degree of luminosity was also greater in every part of the spectrum save the red. 54 COLOUR. [Chap. V. 50. By means of Maxwell's rotating sectors, Rood has determined the luminosity of several pigments in common use. He mounted, upon a disc painted with the pigment to be tested, a smaller compound disc of black paper, with a white sector, the area of the white sector being adjusted till the grey, produced on rotation by its ocular combination with the black, exactly matched in luminosity the coloured disc which formed the back- ground. A correction having been made for the small quantity of white light reflected by the black paper itself, the following degrees of luminosity were obtained (the author adds four observations of his own, which are distinguished by an asterisk) Substance. Luminosity. * Zinc- white 110 White Paper 100 * Whatman's Paper (not hot-pressed) . 97 Pale Chrome-yellow (water-colour wash) Pale Emerald- green (in thick paste) Cobalt-blue (water-colour wash) . Vermilion (in thick paste) . * Natural Ultramarine Artificial Ultramarine . 80-3 48-6 35-4 25-7 9-1 7-6 Black Paper 5-2 * Lampblack ....... '8 These numbers represent, of course, the comparative luminosities merely of the particular specimens examined, the results varying considerably with different prepara- tions, with the mode of applying, and the thickness of the layers of pigment. Nor must it be supposed that in the case of any coloured pigment the reflected light is " pure." With chrome-yellow, for instance, the light measured is not yellow only, but contains much orange- yellow and greenish-yellow, as well as orange, red, and even green light. But the total ocular effect of all this mixed light is yellow. With black paper, however, and with lampblack, the small quantity of scattered light they send out is nearly, if not quite, white. Chap. V.I TINTS, SHADES, AND GREYS. 55 51. Having acquired a clear conception of the three constants of colour namely, hue, or colour par excellence ; purity, or freedom from white ; and brightness, often called luminosity, or the quantity of optical sensation caused by a given area we are now in a position to de- iine the meaning to be attached to such expressions as tones, tints, and shades, as applied to coloured substances. Tones are estimated by the absolute amount of colour- sensation they excite : they may be grouped into three series for every possible hue or kind of colour, according as these hues are admixed with white, with black, or with both black and white, or grey. Apart from any alteration of hue which may occur by such admixtures, we may affirm that we weaken or reduce a normal colour by the addition of white, producing a scale or series of tones from deep to pale ; that we darken, but do not deepen, a normal colour by the addition of black. Tones belonging to any of the above series are commonly spoken of as shades, but it is better to limit the use of this term to admixtures with black. A scale is a regular series of such tones as those which have been defined above. So each hue admits of three scales. 1. The reduced scale that is, the normal hue mixed\ / with progressive increments of white, thus forming tints. 2. The darkened scale that is, the normal hue mixed with progressive increments of black, thus forming shades. 3. The dulled scale that is, the normal hue mixed with progressive increments of grey, thus forming broken tints (commonly called " greys "). 52. There are two ways of preparing a series of tones belonging to each of the scales, assuming that we are dealing with pure pigments, and not with coloured lights. If we wish to obtain a scale of ten tints of vermilion, we may mix a given weight, say ninety-five parts of that substance, with five parts of zinc-white for the first tint, ninety parts with ten for the second, eighty -five with fifteen for the third, and so on, up to fifty parts with fifty for the tenth tint. Or we may 56 COLOUR. [Chap. V. take a paper disc, painted with vermilion, and add to it, by means of one of Maxwell's graduated white sectors, the above areas, or amounts of white (assuming the circle to be divided into one hundred degrees), each five parts of white added covering up five parts of vermilion. On rotating the compound disc, we shall obtain ten concen- tric rings, the outermost being like that of vermilion when mixed with white to the extent of five per cent., and the innermost being the palest tint of the scale, and Fig. 12. containing fifty parts of white in the hundred : the com- pound disc, when at rest, is shown in Fig. 12. A similar sector of black will yield ten corresponding shades of vermilion : a grey sector will yield a " broken " scale of ten tones. But by neither of the processes do we ob- tain scales of the exact normal hue from which we start, for this hue alters with each increase of luminosity (as in adding white), or by each diminution of luminosity (as in adding black). But we do not now dwell on this matter further, as it will be fully studied in Chapter XII., and has been already touched upon in 48. 53. Many attempts have been made to classify colours, including under that designation not only all hues, with their shades, tints, and broken tints, but also Chap. V.] CLASSIFICATION OF COLOU RS. 57 white a balanced or neutralised compound of two or more hues and black the negative correlative of light and colour. In its simplest form the scheme of classifica- tion may be tabulated thus : DAEKNESS . . . Black. ( n 7 7 J White ; Colourless . . LIGHT ^ [Coloured . . | ieS T i nts and Shades of Hues. The real difficulty begins when we attempt the classi- fication of hues : that is, of colours proper. Where can we find standards of comparison for all colours in respect of the three constants of colour hue, purity, luminosity 1 If we start from the solar spectrum, disregarding the absence from it of all the purples, shall we employ the prismatic spectrum, or that obtained by diffraction 1 ? In either case we shall not thus obtain a constant standard of luminosity, since the brightness of the various spectral hues differs enormously. What the spectrum does fur- nish us with is a series of well-defined hues, of which the exact positions and wave-lengths are ascertainable, and to which reference can at any time be made. It is even possible to compare the hues of a very large number of pigments with these fixed spectral hues, and to determine thereby their position in the normal spectrum. Thus, Professor Rood has ascertained the position (and con- sequently the wave-lengths) of the hues of several im- portant pigments. Adopting the normal spectrum and dividing that part of which which lies between the lines A and H into 1,000 parts, he finds the position of vermilion to be at 387, that of emerald-green at 648, of cobalt-blue at 770, of lapis-lazuli at 785, and of artificial ultramarine at 857. 54. The difficulties in the way of classifying colours are augmented by the very great number of hues, with their shades and tints, possessed of varying degrees of 58 COLOUR. [Chap. V. luminosity, which the human eye is competent to dis- tinguish. From experiments, in which small quantities of one colourful light were added to another or to white, Aubert calculated that fractional quantities of light, varying from y^ to -3^, produced recognisable dif- ferences, and that a thousand hues could be distinguished in the solar spectrum. Add to these the hues produced by gradual increments or decrements in luminosity and the whole series of purples, and we reach a grand total of colours which must be measured by hundreds of thousands. 55. These high figures alone preclude the possibility of assigning names to any hues but those which are well known, and are separated by considerable intervals of wave-length. The dearth of well-defined colour-names in English, and the want of flexibility in the language as to the coinage of new designations, makes the question of colour nomenclature a most difficult one. Moreover, there are some words expressive of colour which are so vaguely assigned, in common parlance, that when we hear, for instance, of " purple " we are not sure whether a rich blue, or a red- violet, or even a deep red is meant. Chev- reul's graduated series of chromatic circles, containing a set of typical hues, with their progressive admixtures with white and with black, requires very considerable correc- tions as to many of its chromatic intervals, and as to its assumption with regard to the relations between yellow, green, and blue, before it can be even provisionally accepted as affording a basis for colour nomenclature. Kadde's Colour Chart, with its 882 hues, though nominally based on the colours of the solar spectrum, is essentially that of Chevreul. It lacks precision as to the places in the spectrum from which the normal hues are taken, while its attempted realisation by the chromo-lithographic process is very far from being a success. 56. Many attempts have been made to construct colour-charts, both for coloured lights and for coloured materials, such as pigments, by the aid of geometrical chap, v.] BENSON'S COLOUR-CUBE. 59 figures. Two triangular pyramids set base to base, the sphere, the cone, the circle, the cube, and the triangle, have all been employed for this purpose. The triangle, as arranged by Maxwell, with the modifications intro- duced by Rood, yields results as satisfactory as any that can be obtained with a figure of one dimension. Mr. W. Benson's colour-cube presents many advantages over any plane figure ; its defects and limitations will be best understood after a brief description of it has been given. At one solid angle of the cube, black, or the absence of light, is placed ; at the opposite solid angle, white. At the three solid angles nearest to black the full red, green, and blue are respectively placed ; in the corresponding and opposite corners nearest to white the three secondaries occur : namely, sea-green, pink, and yellow. The centre of the figure is occupied by grey : that is, by white of moderate brightness, half-way between black and white. A few examples of the position of particular hues on and within this cube will serve to show the principle under- lying its arrangement. Thus, in the middle of the edge joining red to black occurs "dark red," other shades of red (red of lowered brightness) being found along the same line. Half-way between the solid angles occupied by red and yellow is the normal orange ; half-way be- tween yellow and white is pale yellow : that is, a tint of yellow, mingled with much white. Along the axis which joins the opposite corners red and sea-green, occur various tints of these two colours, in which one or other preponderates over the white which these comple- mentaries produce by their union, except at the middle point, which is also the centre of the cube, where a neutral grey is found. And here it is that we meet with certain defects of this colour-cube. For theoretically, at some point along this axis, where equivalents of red and sea-green coincide, true white ought to be found, and if, as in Mr. Benson's primary assumption, the " intensities," that is, the brightnesses, of the three primaries be equal, then the centre of the cube must be occupied by white ; 60 COLOUR. [Chap. V, but this is not the case.' In this inconsistency, and in the incorrect assumptions that the intervals between the primaries are equal, and that the brightness of the primaries is identical, we see the defects and the limita- tions of this and of similar arrangements. And if wa replace coloured lights by pigments, we shall find that it is impossible to locate the hues, brightnesses, and purities, which they represent anywhere near the points theoreti- cally assigned to them. But after all, this colour-cube furnishes a large number of most interesting and beauti- ful colour arrangements when we examine the hues, situated in sections of the cube taken at right angles with its principal axes, while its defects are inseparable from solid forms bounded by plane surfaces. And there can be 110 question that this mode of classifying colours has conspicuous merits, when regarded from what we may call a qualitative rather than quantitative point of view. A few words concerning the colour-cone of W. von Bezold may be fitly introduced here. Its apex is black, the centre of its base white; along the axis greys of every shade will occur. Normal colours of full bright- ness are located at certain points on the circular boundary of the base, following the precise order of their refrangi- bility, and passing by insensible gradations from one hue to another, purple uniting the violet with the red end of the spectrum. The exact angular position of each of these hues can be determined (see 78), and they can be placed in strict conformity with such determination an arrangement obviously impossible in the case of the cube. Then the lines which pass through the centre of the base will be found to join complementary colours ; in fact, this circular base constitutes a true chromatic circle. Here, however, the theoretical position of the white in the centre is not correct, for it assumes that the neutralisa- tion of all complementaries is effected by their union in equal quantities. On the exterior of the cone, as we ascend, we find the darkened tones or shades of each hue \ the basal plane exhibits, as we proceed towards its centre, . V.] THE COLOUR-TETRAHEDRON. 61 mixtures of each normal hue with increasing quantities of white. A horizontal section of the cone, taken, say, at half its height, will show each hue admixed, as we near the axis, with increasing quantities of grey. The positions of the colours on the surface of the cono are, as we have previously stated, so adjusted that a line meet- ing the axis at right angles, and drawn from any one point on the exterior, will meet, on the opposite surface, the position occupied by the complementary. A tetra- hedron may be substituted for this cone with some sacrifice of accuracy, as the an- gular values of the several hues must be ignored. We assign black, red, green, and blue to the four solid angles of the figure, white to the centre of the face opposite to black, and grey to the geometrical centre of the figure. Somewhere along the edge, joining the red (R) with the green (G), we shall find bright yellow (Y) ; between the green and the blue (B), sea-green (s) ; between the blue and the red, purple or pink (p). We use the above expressions (bright yellow, sea-green, pink) because when we are dealing with colours such as those which are obtained, as we shall presently see, J>y_ the union of two lights, the resultant hue must be brighter, that is, more luminous, than either of the con- stituents must have the combined brightness of both. Now, the only way readily available for expressing this added brightness lies in the use of terms which imply a lighter hue a hue more nearly approaching the bright- ness of white, though not in reality mixed with white. In fact, these combined hues may be said to be much farther on the road towards the brightness of white than Fig. 13. 62 COLOUR. [Chap. 7. are their separate constituents. This statement, with the meanings to be attached to some of the terms just introduced in the present paragraph, such as primary, secondary, and complementary, will be fully explained in the next chapter. 57. A few words may now be said concerning an old attempt at naming colours, which was first made so long ago as 1774 by the mineralogist, Werner. His system included ninety- two terms, arranged in nine groups. He gave names to ten "metallic" colours, to six varieties of white, or very pale tints, to six greys, to five blacks, to fourteen reds, to nine yellows, to thirteen greens, to ten blues, and to nine browns. His classification and his descriptions of the supposed constituents of the several hues will not bear critical examination, but some of his terms, derived from well-known materials natural and artificial, animal, vegetable, and mineral may be adopted with advantage. One may select certain typical substances, the hues of which vary little, and employ them as standards for reference. This plan, which is generally adopted with regard to orange and violet, may be considerably extended with advantage, and we shall follow it in the remaining chapters of the present work. To give an idea of Werner's nomencla- ture, we now cite the ten terms employed to designate the varieties of blue : 1. Blackish-blue : blue mixed with, black. 2. Azure-blue : a very bright, rather reddish blue. 3. Violet-blue : a pure mixture of red and blue. 4. Lavender-blue : violet-blue mixed with ash-grey. 5. Plum-blue : a reddish violet-blue, with brown. 6. Prussian blue ; the purest blue. 7. Smalt-blue : a pure but pale blue. 8. Indigo-blue : a dark blue, with black and green. 9. Duck-blue : blue, with a great deal of green and a little black. 10. Sky-blue : a bright, rather greenish blue. Now there is scarcely a single description amongst these hues which does not challenge criticism, even apart Chap. V.I NOMENCLATURE OF COLOURS. 63 from the general lack of orderly sequence and of pre- cision which they exhibit. Lavender-blue and indigo- blue may perhaps pass muster, but what are we to say to the definition of Prussian blue as the purest blue, when it clearly shows the presence of more than traces of green 1 Lapis-lazuli finds no place in the list, although it was, of course, well known to Werner, and furnishes an admirable standard of the normal blue. In order to furnish an illustration of how the hues of natural substances, belonging to the three kingdoms of nature, may be utilised in naming colours, we may cite a few of the materials which represent the range between red and yellow : 1. Chinese Vermilion. 7. Gold (pure). 2. English Vermilion. 8. Amber. 3. Orange Vermilion. 9. Straw. 4. Eed Lead. 10. Gorse-flowers. 5. Saffron (dry). 11. Lemon-peel. 6. Orange-peel. 12. Sulphur. Quite recently, another attempt at naming and classi- fying colours has been made by R. Ridgway. The small volume which he has prepared is intended primarily for the use of naturalists, but it possesses one feature at least which is likely to be appreciated by many persons interested in decorative and pictorial art. This is a comparative vocabulary of colour-names, giving in paral- lel columns, on nine double pages, the equivalent words in English, Latin, German, French, Spanish, Italian, and Norwegian. The most striking characteristic of the book is, however, a series of coloured plates. Each of these plates has been planned with skill and care, and executed in water-colour pigments of considerable or complete stability. There will certainly be diversity of opinion as to the justness of the application of many of the names to the actual colours given. But to name tints, hues, and shades, instead of merely numbering them, constitutes a step in the right direction. Till an 64 COLOUR. [Chap. VI. " International Standard-Colour Conference " of artists and scientists lias finally agreed upon the names to be given to a couple of hundred different hues, reproduced in onamel, and preserved for reference, like our standards of weight and measure, we must be grateful for any attempt, even though it be but partially successful, in the way of a consistent and complete nomenclature. As an example of Mr. Ridgway's sets of colour-names, we may cite those which he assigns to twenty hues, lying between red and blue, and belonging to the group of purples : 1. Prune. 2. Dahlia. 3. Auricula. 4. Plum. 5. Pansy. 6. Indian Purple. 7. Royal Purple. 8. Aster. 9. Maroon. 10. Violet. 11. Phlox. 12. Pomegranate. 13. Mauve. 14. Magenta. 15. Wine-purple. 16. Lavender. 17. Solferino. 18. Heliotrope. 19. Lilac. 20. Rose. The mere inspection of this list suffices to show that although there may be a judicious selection of colour- names here, there is nothing approaching to a scientific classification of them. CHAPTER VI. THE MUTUAL RELATIONS OF COLOURS COMPLEMENTARY COLOURS YOUNG'S THEORY OF THREE PRIMARY COLOUR-SENSATIONS THEORIES OF HELMHOLTZ, CLERK- MAXWELL, W. VON BEZOLD, ROSENSTIEHL, ROECHLIN, AND OTHERS ABNORMAL PERCEPTION OF COLOUR COLOUR-BLINDNESS. 58. WE have seen ( 1921) that the several colours of the solar spectrum, if by any means they be re- united, reproduce white light. It is not necessary, for this Chap. VI.] COMPLEMENTARY COLOURS. 65 purpose, to divide the colours into seven, or any other particular number of groups, and then combine them ; it will suffice to separate the spectrum into two sections, equal or unequal. And, we may go further, choosing two, three, or more of the colours of the spectrum, and leaving the whole of the remainder, and yet we shall find that, with the few chosen portions, we get white light, diminished, it is true, in brightness, but quite free from colour. The three conditions to be fulfilled in order to reach this result are these : that the brightness, the quantity and the spectral position of the several selected hues shall have certain definite relations. Let us assume that we have chosen three such spectral hues ; then, if we combine two of. them, we shall obtain a colour, which, united with the third hue, will form white. Such pairs of colours, one compound, the other simple, are on this account called "complementary." It is obvious that an immense number of such cornplementaries must exist. Their study is of great importance to artists, and especially to workers in the so-called decorative or ornamental arts, for a knowledge of the strength of con- trast in colour depends upon a right appreciation of the true complementaries. 59. There are many ways of producing, side by side, complementary colours, The Schistoscope of Briicke is perhaps the best instrument for this purpose. It consists essentially of an eye-piece, containing a plano-convex lens, and a rhomb of calc-spar, a diaphragm with a very small square opening, and a Nicol's prism. A large number of very thin laminae of selenite should be pro- vided. One of these, if laid on the diaphragm, will, when the instrument is properly adjusted, produce differently- coloured images of the square opening, lying close together. The colours of the pair of images are always complemen- tary, and if the selenite-films are sufficiently thin, the colours will be bright and saturated. The two images are polarised in opposite planes. The following list includes a few of the more important : 66 COLOUR. [Chap. VL PAIRS OF COMPLEMENTARY COLOURS. Red. . Blue-green. I Yellow-green . Violet. Orange . Turquoise. | Green . . Purple. Yellow . Lapis-lazuli. j Green-blue . Carmine. It is a good plan, when a pair of complementaries has -been obtained, to imitate it as nearly as possible by means of water-colours on white paper. 60. In order to obtain the true complementaries of the darkened and dulled hues, such as chocolate, olive, russet, lavender, &c., so commonly occurring amongst pig- ments, Briicke's Schistoscope is useless.' We may in such cases use Dove's achromatic calc-spar prism. We place a small square piece, say of chocolate-coloured paper, on a background of black velvet : near it we place another piece of paper, painted with a broken blue-green. The prism will enable us to see whether the part of the two images which overlap yields a pure neutral grey : if it does not we must add to the second slip a little more of the pigment which is necessary to complete the trial hue, perhaps a little more green or a little more blue will be needed. A more exact and easier method consists in the employment of Maxwell's rotating sectors. A small combined black-white disc is mounted on the axis, and behind it are placed larger discs of the two pigments which are known to be necessary to make the complemen- tary of the hue under examination. A disc painted with this hue is to be associated with the two others, and the proportions of the three are to be so adjusted, by means of the radial sectors, that on rotation the whole makes a neutral grey, the neutrality of which can be gauged by its correspondence with the central black-white disc, which should contain about 75 per cent, of black Rood, operating in this way, determined that the com- plementary of a dull yellow, like that of brown paste- board, was obtainable by combining forty-five parts of artificial ultramarine with fourteen parts of emerald- green. This mixture equalled forty- one parts of the dark yellow in question, and the whole corresponded on Chap. VI.] PRIMARY COLOUR-SENSATIONS. 67 rotation to a mixture of twenty-four parts of white with seventy-six of black. Certain adjustments haye, of course, in every case to be made, in order that the com- plementaries may have equal brightness. With some pigments, however, such as carmine, vermilion, and Indian yellow, it is not possible to compare complemen- taries of equal brightness and saturation, as the optical constituents of the pigmentary complements of these substances do not correspond to them in brightness and saturation. Thus, we must add a black sector to a vermilion disc to get its true complementary in bright- ness, as well as hue, by rotating together green and blue sectors. White sectors must also be introduced in certain cases of the comparison of pigments, in order to introduce white light into one or other of the pairs of complementaries, and so to equalise their tints. 61. The question as to the true meaning to be attached to the term primary, as applied to colour, may now be discussed. Every ray of differing refrangibility in the visible part of the spectrum is in one sense a, primary colour, for it is simple and excites a definite sensation. But there are many reasons, mainly connected with the structure and functions of the eye, which have led to the selection of certain coloured lights generally three in number as yielding primary colour-sensations. This primariness is then not objective, but subjective in re- spect of human vision. Wiinsch, a German physicist, was the first to select, in 1792, the three so-called pri- maries which are now generally accepted as best fulfilling the conditions of the case. Dr. Thomas Young, in 1802, after first fixing upon other sets of three hues, indepen- dently decided upon the triad of Wiinsch, and propounded a very reasonable theory of the cause of colour- vision. In recent years, Helmholtz, Maxwell, and Rood, as well as many other physicists, have developed the theory of Wiinsch and Young, and have adopted the same, or very nearly the same, triad of primary colour-sensations. These three fundamental hues, or primaries, as we will for 68 COLOUR. [Chap. VL brevity's sake call them, represent three widely separated and very bright colours of the spectrum, The earlier experimenters possessed no means, beyond mere verbal description, of exactly localising the spectral hues they had selected. Maxwell and Helmholtz, and other ob- servers, 011 the contrary, have assigned particular posi- tions, in relation to the solar fixed lines, to the several members of the triads they employ. It must not "be supposed that there is not a measure of arbitrari- ness and some slight variety in the selection of the primaries. Young simply adopted red, green, and violet. Helmholtz first chose a red not far from the end of the spectrum, a full green near the middle, and a violet not far from the more refrangible limit of the spectrum. Max- well adopted a scarlet-red with a tinge of orange like orange-red vermilion, lying in the spectrum one- third of the way towards D, between the line c and D. His green is to be found at one-quarter of the distance from E, between E and F, and resembles emerald-green in hue. For his third member Maxwell selected a blue- violet, midway between F and G, which is fairly imitated by the best sorts of artificial ultramarine. Other investiga- tors have selected for their fundamental green a slightly bluer hue ; others, one more nearly approaching a yellow- green. On examining a map of the spectrum, and experimenting with the spectrum itself, it will be evident that if w greenish- blue- < [12 1,49 ] 41 Gamboge Prussian blue Hook, green ) grey green I 5 Black i [ 20 Vermilion 50 Vermilion 50 Ultramarine ^ Dull > Red-purple violet- < 1 20 Ultramarine Black purple | 5 9 White !23-5 Hook, green 50 Hooker's green ) Yellowish- Brick- 8 Carmine 50 Carmine J flesh red 52-5 Vermilion 16 Black 50 Green (Gamb. & j Pale Dull (50 Carmine Pruss. blue) > reddish- dark 24 Hook, greeu 50 Carmine ) flesh red 26 Black The results in the above table are amply sufficient to show how great a stride towards blackness is frequently made when pigments are mixed together. In one instance, the first in the list, shown in Fig. 19, no less than 52*5 per cent, of black had to be added to the Ohap/VIL] DULNESS OF MIXED PIGMENTS. 89 rotating disc in order to make its hue match the ex- tremely dull brown made by mixing violet carmine and Hooker's green. Artists constantly notice the very marked dulness and muddiness of the hues obtained by mixing pigments, and they frequently and wisely have recourse to the mixture of coloured lights by placing small dots and touches of pigments so close together that A Fig. 19. The colour produced by rotating the A disc was a yellow-grey : the same pigments mixed on the palette gave a brown which needed, for its production by rotation, the additions of black and vermilion shown in B above. the colours they reflect mingle on the retiua. To this stippling of bright colours, as a fine mosaic work, must be attributed in great measure the luminous arid sump- tuous effect of many works of Samuel Palmer, William Hunt, and J. F. Lewis. Another important conclusion to be drawn from this table of Hood's is the frequent impossibility of reproducing, without the aid of a foreign element, by the true process of mixing the coloured lights reflected by the pigments, the exact hue of the palette mixture. 90 CHAPTER VIII. THE CHROMATIC CIRCLE COLOURS OF PIGMENTS THE LAWS OF CONTRAST CONTRASTS OF TONE CONTRASTS OF COLOUR SIMULTANEOUS CONTRAST SUCCESSIVE CONTRAST AND THE NEGATIVE IMAGE. 77. A VERY convenient device for the arrangement of the chief varieties of colour, and for the study of their mutual relations, is afforded by the chromatic circle. In its simplest form it is shown in Fig. 20. Around the cir- cumference, at equidis- tant points, are placed the three primary hues ; three diameters,, drawn from these points, will touch the circumference where the respective complementaries will be found. White is assumed to occur at the centre. RED Purple BLUE /GREEN occur Between the circumfer- ence and the centre will be found various hues in which the complementa- ries do not balance or neutralise one another, but one or other exists in excess. Thus along the diameter RED Blue-green we shall discover in succession those hues in which red preponderates, then white will occur, and then those hues in which blue-green preponderates. But the simplest form of the chromatic circle has several defects. It is based upon the assumption that equal quantities of any two complementaries produce white, and it further assumes that there are equal intervals in the colour series between the six hues embraced in the figure. If we . VIII.] THE CHROMATIC CIRCLE. 91 waive the obligation to rectify the former defect as being of minor practical importance, the angular distances from each other of the several hues on the circumference can be corrected by means of direct experiment. In the case of pigment- colours, to which we purpose almost ex- clusively to confine our attention at present, we may select certain pigments of decided hues representing the Crimson Red-Purpl Purple RED Orange Purple -Violet Turquoise \Orange-Yellow Yellow Greenish Yellow 'Green-Yellow ellowish Green Greenish-ljlu GREEN 'merald Green luish Green Green-Blue Fig. 21. chief colours, ascertain the position in a normal spectrum of the colours they severally present to the eye, and then determine the wave-lengths of those colour rays, and so fix their angular position. The results are by no means absolute, partly owing to difficulties inherent in all work of this kind with pigments, but the approxima- tions we reach will be found very serviceable in studying the questions of complementary colours and of colour- contrasts. One of the most important deductions is that very considerable difference of wave-length, and con- sequently of angular position amongst the various hues 92 COLOUR. [Chap. VIII. between green-blue and blue, is not accompanied by a difference in colour-sensation corresponding to that which occurs amongst the hues of equal intervals between orange and purple. From this it follows that a difference between two bluish-greens hardly perceptible to the eye demands a difference between their complementaries which seems quite disproportionately large. It has been suggested that this peculiarity may in some measure account for the great difficulty frequently experienced in balancing the greens and bluish-greens in a scheme ol colour. 78. We now proceed to give, in Eig. 21, an am- plified and partially corrected chromatic circle. Of the large number of distinguishable hues we have selected no more than twenty, forming ten pairs of complemen- taries. In the table below we give the names of these hues in pairs, and at the same time offer a list of cor- responding pigments which may be taken as in some measure representing them. In the chromatic circle itself we print the primary colours we adopt in large capitals : in the table the normal secondary colours are given in smaller type, and the remaining hues in a still smaller fount. PAIRS OF COMPLEMENTARIES. ( BED .... Madder-red or crimson-vermilion. < GREEN-BLUE . Viridian, the emerald oxide of chromium, ( with a little cobalt. j ORANOE .... Cadmium-yellow, of full orange hue. ( GREENISH-BLUE . Cob alt -green, the " Vert de Cobalt." {ORANGE-YELLOW . Cadmium-yellow, or deep chrome. TURQUOISE. . . Coeruleum, or cobalt-blue with a little emerald green. YELLOW - Lemon-yellow, pale chrome or aureolin. BLUE .... Ultramarine from lapis-lazuli. GREENISH- YELLOW Aureolin with a little viridian. VIOLET- BLUE . . French ultramarine. GREEN- YELLOW . Lemon-yellow with some emerald- green. VIOLET .... French ultramarine with madder- carmine. YELLOWISH-GREEN Lemon-yellow with much emerald- green. PURPLISH- VIOLET . Madder-carmine with French ultramarine. True and False Cornplerrieritari&s Contrasted. Fig. .22. (see page 93.) 'Vmcen.t Brooks, Day &. San.Lt4litK Chap. VIII.] COMPLEMENTARY COLOTp 93 URS; PAIRS OF j GREEN . . . Emerald-green with a little lemon-yellow. j PUEPLE . Madder- carmine, some French ultramarine. ( EMEKALD-GREEN . Emerald-green alone. ( REDDISH-PURPLE . Madder- car mine with a little F. ultramarine. Whether the above pigments are used or any others, such as cobalt-blue, Prussian blue, Indian yellow and carmine, there will always be a difficulty in securing, not merely the proper relationship between the two colours of each pair but the exact or standard hues required both in regard to their colours and their tones. Some aid may be drawn from the observation of the com- plementary colours of polarised light ; but it would be far better if the student could have easy access to a standard set of complementary colours executed in enamels. There would be little difficulty in producing a large number of such sets, which might be suspended in all public libraries, schools of art, and picture-galleries, if not in all school-rooms. We offer, in Fig. 22, a com- parison of the three chief pairs of complementary colours according to the theories of Young and of Brewster. It will probably be concecTed that the three pairs of com- pound discs shown on the left of the diagram offer more complete and satisfactory contrasts than those (Brew- ster's) on the right. If the pigments used had corre- sponded more closely to the normal hues the effects would have appeared still better. 79. Before proceeding further with the description of the uses of our chromatic circle, especially in relation to pigments, it will be advisable to recur to a subject already noticed in 63. Strictly speaking, the relative amount of brightness, that is, luminosity, in each of the two colours in a pair of complementaries must be con- sidered. \ For with red and blue-green the brightness of the latter must have the brightness both of blue and green if it is to form white with its true equivalent of red. 1 Thus a dark blue -green is not the perfect com- plementary of a bright red. When we are dealing with 94 ^, COLOUR. [Chap. VIII. coloured lights this conclusion is obvious, and we find that the blue-green made by uniting blue and green lights is much brighter than the red with which it unites to form white light. With pigments the case is otherwise and so the only way in which, in a diagram or figure, we can make a blue-green pigment approximately neutralise a red pigment is to add a certain amount of white to the former ; in other words, to use a pale tint of it. The same is true of the pair, purple and green, and of the pair, yellow and blue. In the last-named case there is no difficulty in achieving the desired brilliancy of the yellow, for yellow pigments are characterised by quite exceptional brightness ; with purple the case is so dif- ferent that we are obliged to dilute or weaken this com- pound hue until it corresponds to o, kind of pink, a pink a little bluer than rose-madder. This colour is very near that of the rose-acacia, or of almond blossom, or of the flowers of the double peach ; it is perhaps more closely represented by the flame of burning cyanogen. With the six other pairs of complementaries in the circle the difference in . brightness between the two colours will be less, for in each case the total brightness is shared more equally as each colour is a compound one. Thus in the pair violet and yellow-green the violet has the brightness of the full or normal blue with a portion (say about one-third) of the bright- ness of the full red, while the yellow-green has the brightness of the full green with the remainder of the brightness (say two-thirds) of the full red. Assuming unity to represent the brightness of each one of the three primaries, we shall have this equation representing the apportionment of the total brightness : Violet, 1-33 ) ._ w ,., + Yellow-green, 1-67 f ' 80. We now pass on to consider the constitution of those hues which contain grey. They may be con- sidered as primary and secondary colours of low Chap.VIII.1 BROKEN HUES, ^fl 95 luminosity mingled with white ; ^BWh speaking of pigments we regard them as containing both black and white. We have explained how, from an erroneous notion as to their constitution, they came to be called " tertiaries." These constitute "broken" hues, and are the dulled tones of the primaries and secondaries. It is not easy to name them in a way which will prove generally acceptable, but the following list, in which the order of the chromatic circle is followed, may prove of some service : a. Broken red = Maroon. Broken orange = Russet. Broken orange-yellow = JBrown. b. Broken yellow ==. Citrine. c. Broken yellow-green = Olive. d. Broken green = Sage. a. Broken blue-green = Bluish-sage. b. Broken blue = Slate. c. Broken violet = Lavender. d. Broken purple = Plum. Pairs of complementary broken hues are indicated by prefixed italic initials : the complementary of russet is a rather bluish sage-colour ; of brown, a very bluish sage. Colours are often spoken of as either retiring or advancing, as either cold or warm. These distinctions, though in part founded upon the association of particular imes with distant and near objects or with cold or warmth, are susceptible of explanation, in some measure, by means of physiological and physical considerations. Thus the warm colours, which are found in our chromatic circle between the greenish-yellow and the reddish- purple, represent more than two-thirds of the brightness or luminosity of the whole of the chromatic elements of white light, although the proportion of the area they occupy does not exceed one-third, Again, in the case of advancing colours, such as yellow, orange and red, they occur at that part of the spectrum where the rays are less refracted, and are at the same time highly luminous. 96 ^^ COLOUR. [Chap. VIII. The Fig. 23 illu^l^ls some of the curious optical effects produced by different dispositions of some of the chief of these so-called advancing and retiring colours. 81. Complementary colours of full brightness and purity afford the most striking examples of the effect called contrast. When each of a pair of such colours differs as much as possible from its fellow in hue, but is of the same degree of brightness, it is found, while the brightness of both is enhanced, that the hue of both is unchanged by the close neighbourhood or contiguity of the two colours. But if the pair be not truly comple- mentary, or if in brightness or purity one colour differ from the other, then such difference will not be seen exactly as it is, but such dissimilarity as exists, whether it be of one hue, of purity, or of brightness, will be enhanced by juxtaposition. This is the primary law of contrast, which embraces three varieties dependent respectively upon differences as to the three constants of colour, namely, purity, brightness, and hue. If two adjacent colours differ in brightness, that which is the more luminous will increase in brightness, while the less luminous will have its brightness diminished.! If two adjacent colours differ in hue such difference will be increased, each hue tending to change as if it had been mixed with the complementary of the other. In the case of complementaries no increase of difference in hue ig, however, possible. 82. Contrast caused by difference in brightness is commonly called contrast of tone. This kind of contrast may occur alone or it may be associated with contrast of hue and contrast of purity. It will be well to consider first the simplest cases, in which contrast of tone is unaccompanied by other contrasts. But it is impossible to reduce experiments on tone-contrast to their simplest expressions. A third element always comes in, namely, the background on which the pair of tones is placed for examination. Whether this background be black, white, grey, or coloured, it must necessarily differ in some Advourucu-Lg and Returuig Colours. Fig. 23. (see page 96.) Lustre Pro.ciuuoecL Fig 29. (see page 107) TT B R OF THE ( UNIVERSITY } Chap. VIII.] CONTRAST OF TONE. one direction from one or from both the trial pieces, and will therefore itself produce a contrast. To minimise the complication thus introduced we may try our first experiment for producing the phenomena of tone-contrast in three ways, using three backgrounds with identical trial pieces on each. We first take two strips of pale grey paper, A and A' in Fig. 24, and place them a few inches apart on a large sheet of paper in a good light. We then prepare two similar strips of a considerably darker shade of grey, B and B', and place them, as shown in the figure, one alongside of A and the other the same distance from B as A' is from A. Upon steadily looking Pale Dark Fig. 24. at the four strips for a short time it will be seen that A close to B appears lighter than A', which lies at some distance, while B appears correspondingly darker than B'. The effect of contrast in enhancing differences of tone may be further studied thus : Make such openings, five in number, in a piece of card, as will serve to divide each of the stripes A and B into three portions. When viewed through this card, held between the trial pieces and the eyo, it will be found that the two contiguous portions of the strips are most contrasted in tone, and the others less so in proportion to their distance from the line of con- tact. The experiment should now be repeated with a background of black velvet, and again with a background of grey paper lighter in tone than either of the strips. But the effect of contrast of tone is still better seen when a series of toned strips is placed in contiguity. In such a case the effect on all the strips save the end ones is that of H 98 COLOUR. [Chap. VIII. double contrast. For the second strip or second tone has one side of it made apparently darker by reason of the con- tiguity of the lighter tone of strip, while the other side seems lighter, owing to the contiguity of the darker tone of strip 3. The general results of these double contrasts is that the whole series or scale of tones presents the appearance of a number of hollows, although, in fact, the apparent hollows or concavities are perfectly flat areas of uniform shade. The effect of this experiment is approxi- mately represented in Fig. 25, where the real flatness of each tone of the six may be verified by covering up all Fig. 25. the others by a card. This diagram of contrast of tone may be made in a more effective way by dividing a strip of cardboard into several equal sections say six by faint pencil lines, and then giving all six a light uniform wash of Indian ink. When this is dry five sections receive a second similar wash. Afterwards the same operation is repeated until the third section has received three washes, the fourth four, the fifth five, and the sixth section six. In carrying out this process, all sections, except those which are being tinted, should be hid from view, for without this precaution it is difficult to secure the perfect flatness of each tint. If a series of pieces of grey paper, of the same hue but of different tones, is ob- tainable, it may be used in the construction of the same figure. The pieces should be of the same size, and should be pasted close together on a slip of cardboard ; strips of grey glass or gelatine may also be used in such a way as Contrast of Colours with White, Grey, & Black. Fig. 26. (see pages 99. Sc 111.) Vincent Brooks,Day &.SonLtf itK SunultGuneons Contrast. Fig. 27. (see page 100.) Chap. VIII.] SIMULTANEOUS CONTRAST. 99 to present at one end one thickness of the material and at the other end six or more thicknesses. It is scarcely necessary to state that the tones of any one colour may be employed, instead of grey, to illustrate this kind of simultaneous contrast, but its characteristic effect is not seen unless the contrasting tones differ considerably in intensity, increase by regular gradations and are in close contiguity or absolute contact. If tones of a colour, whether tints or shades, be used, there is generally, how- ever, a complication introduced, owing to the difficulty of getting a series of such tones which shall be identical in hue (see Chapter iii., 28). It is evident that the phenomenon of simultaneous contrast of tone largely affects the chiaroscuro of all drawings in black and white and in monochrome. An example is afforded in the case of a drawing in Indian ink, where light touches will be observed to enrich pale washes at the same time that dark touches serve to re- lieve the heaviness of dark washes. 83. The simplest instances of simultaneous contrast of colour are afforded by the contiguity or contact of a single colour with black, grey, or white. The coloured illustration (Fig. 26) shows how very differently the same normal and full hue of red, of yellow, of green, of blue and of violet appears to the eye when it is seen on colourless grounds of different degrees of brightness. All colours seem brighter on a black ground and darker on a white ground : the effect of a grey ground depends upon the relations subsisting between the tone of the grey and the tones of the several colours. With the grey used in our diagram the yellow alone is much affected, the change being in the direction of an apparent increase of brightness. The alterations suffered by various colours in contact with neutral grounds may be best studied by means of coloured strips or discs of paper of rather small size placed upon rather large surfaces of white, grey, or black paper, and viewed in a mode- rately bright light. Such experiments should always be 400 COLOUR. Lchap. VIH. supplemented by a converse series, in which discs or strips of white, grey, and black paper are placed upon self- coloured grounds. In the latter series we often obtain -very marked examples of simultaneous contrast. The brightness of a white ground is, however, so much greatel* : than that of most pigmentary colours placed upon it, and the brightness of black so much less, that in neither case do we fulfil the exact conditions necessary for obtaining the full effect of simultaneous contrast of hue. What we need for this purpose is a very moderate and not fatiguing luminosity of the neutral element in the com- bination. Pure black paper does not send enough light to the eye to stimulate that part of the retina on which its image falls, white paper (which reflects at least twenty-five times more light than black paper) sends too much. A dark grey tint answers well. In the diagram (Fig. 27) we notice that the grey disc, which is really identical in every respect in all six sections, appears tinctured with bluish-green on the red ground, with blue on -the yellow ground, with purple on the green ground, with green on the purple ground, and with yellow on the blue ground. To avoid the effect of adjacent colours the diagram *should be inspected through a card having au opening just large enough to allow one only of its six divisions to be visible at the same time. In obedience to the law of contrast of colour enunciated in 81, we shall find that each of the grey discs is tinged with colour which is complementary to that of the ground on which it is placed. The same result is observed when black figures are printed on coloured grounds, but in that case the arrangement should be viewed through a thin piece of white tissue paper. With grey discs on coloured grounds the complementary colour produced is strongest in the case of red, orange, and yellow grounds, where the grey disc is rather darker than these in tone ; the reverse is true with green, blue, violet, and purple grounds. 84. We will now consider the rather more complex .Chap. VIII.] SIMULTANEOUS CONTRAST. case of the association of two colours. In order to avoid the introduction of a third element, either of tone or colour, we will dispense with backgrounds by placing a small coloured disc or square upon a large surface of another colour. Two colours not far removed in hue -may be employed in the first experiment. Upon a , ground of turquoise blue place a square of paper painted with French ultramarine j repeat the colours but reverse the arrangement, and let the two combinations be viewed in a medium light. The ultramarine on the turquoise ground will incline still more than before towards violet, Fig. 28. while the turquoise on the ultramarine ground will acquire a still more greenish tinge than it had before. Each acquires something more of the complementary of the other, they become more different in hue than they were before, If, also, one be purer (frejer from white) than the other it will become still purer while the less pure hue will become still less pure. In the special example before us the turquoise will become paler, and the ultramarine deeper or more saturated. Simultaneous contrast in all such cases of course affects the large area as well as the small, but it is usually difficult to detect it on an extensive surface except where it touches the s^maller piece of paper of different hue. For this reason we duplicate the experiment as described above. The usual way of trying this experiment involves a back- ground of white, grey, or black, the coloured strips being 102 COLOUR. [Chap. VIII, placed on colourless paper, as shown on Fig. 28, where c and c' are the coeruleum strips, and u u' those painted with ultramarine ; c' and u' are affected in tone only by the ground, while c and u suffer the change of hue pre- viously described owing to their contact. 85. The changes in the apparent hues of pairs of colours in juxtaposition due to simultaneous contrast can be studied not only by means of the colour-arrangement described in the preceding section, but by the use of Max- well's rotating discs, which have the advantage of enabling us to adjust the brightness and purity of the comparison- hues, in whatever direction we may wish, by adding radial sectors of white or black, or both. By direct ex- periments of this kind, and by previous knowledge of the true complementaries, we are enabled to tabulate the results of simultaneous contrast when any pair of differ- ing hues is placed in contact. The results confirm in every particular the red-green-blue theory, and can neither be predicted nor explained on the red-yellow-blue theory. The following list includes a large number of the most important cases of such contrasts : Pairs of Colours. Change due to Simultaneous Contrast. ( RED with inclines to purple. ( ORANGE yellow. ( RED with purple. ( YELLOW green, j RED with becomes more brilliant. ( BLUE-GREEN ,, brilliant. J RED with inclines to orange. I BLUE green, j RED with orange. | VIOLET blue. | RED with orange. } PURPLE blue. ] ORANGE with red. | YELLOW ^ green. I ORANGE with ,, red. /GREEN blue-green. j ORANGE-YELLOW with becomes more brilliant. ) TURQUOISE ,. brilliant. Chap. VIII.] SUCCESSIVE CONTRAST. 103 Pairs of Colours. Change due to Simultaneous Contrast. J ORANGE with inclines to yellow. ( VIOLET blue. j ORANGE with yellow. j PURPLE blue. | YELLOW with orange. ( GREEN blue-green. I YELLOW with orange. ( TURQUOISE ,, blue. _ | YELLOW with becomes more brilliant. ' BLUE brilliant. GREEN with inclines to yellow-green. BLUE violet. GREEN with yellow-green. l VIOLET purple. \ GREEN with becomes more brilliant. PURPLE brilliant. BLUE with inclines to green. VIOLET ,, purple. VIOLET with blue. PURPLE red. It roust not be imagined that the changes enumerated in the above table are at all equal to one another in amount. We have, indeed, always some change, but it varies much in the case of different pairs. When the chromatic interval (on the colour-circle) is small then the change of hue, in virtue of simultaneous contrast, is large ; when the interval is large the change of hue is slight, but it is accompanied by change of bright- ness : when the interval is as large as possible there is no change of hue, but the brightness of both hues is in- creased. 86. Successive contrast may be observed when we tire one set of retinal fibrils by gazing for some time on a surface of very decided colour and brightness. After- wards, on looking at a colourless surface of white, grey, or black, it will be found to be tinctured with the com- plementary of the first colour.- Thus, if a piece of paper painted with vermilion be first stared at the nerve fibrils which respond to the red waves of light will be much more fatigued than those which respond to green and 104 COLOUR [Chap. VIII. blue light by the continuous call on their perception of red, and, in consequence, any surface afterwards looked at will appear to be tinctured with the complementary of red, namely, blue-green. Even if the second surface viewed be coloured, its proper colour will be affected by the occurrence of this "negative image" due to successive contrast. Looking at a series of pieces of the same scarlet cloth in succession the last piece will appear less saturated than the first, for its hue will be mingled with its complementary bluish-green. But the eye may be rested and its power of appreciating red may be restored by gazing upon a bluish-green surface for some time. One mode of producing the phenomenon of successive contrast consists in placing a square of paper of decided colour and marked with a central black spot upon a sheet of grey paper, and attentively regarding the black spot for twenty ^or thirty seconds. The piece of coloured paper which we will assume to be. green is then suddenly withdrawn, by pulling a thread previously attached to it : a pink or pale-purple after-image will be observed to occupy the space before occupied by the green square. This phenomenon may be thus explained. The green light from this square has fatigued the green nerve- fibrils of the retina to a much greater extent than the red or blue fibrils. When, therefore, the light of low luminosity from the grey paper reaches the eye (after the removal of the green square) its green element meets with little or no response from the green fibrils, while, on fche other hand, the unimpaired sensibility of the blue and red fibrils of the retina responds perfectly to the corre- sponding rays of the reflected light, and produces a resultant image of mixed red and blue hues, that is, a pink or purple. It is unnecessary to describe in detail the colour-phenomena produced by this kind of successive contrast, for they can be predicted by means of a know- ledge of complementary colours such as is afforded by qur chromatic circle (Fig. 21). The following cases of successive contrast may be Chap. VIII.] SUCCESSIVE CONTRAST. 105 specially noted as involving considerations not wholly embraced by the law of complementary colours. If the eye has attentively regarded a series of pieces of paper or cloth coloured red, but having a rather low degree of luminosity (or mingled with some black), the subsequent view of bright red paper or cloth will result in very slight alteration of its hue or tone, as the red nerve-fibrils will not have been sufficiently tired to allow of the pro- duction of the complementary after-image. They may, however, be just so slightly fatigued as to rendtr the bright red last seen rather less brilliant than it would otherwise have been. Reverse the experiment, and look at dark red after having seen bright red, and then suc- cessive contrast occurs. So also a black (or grey) square on scarlet paper, if suddenly withdrawn after twenty seconds spent in observing it, gives a successive contrast- image of the square, which appears of an intense red on a dull red ground, because the part of the retina pre- viously occupied by the image of the black square is perfectly fresh and completely attuned to the perception of reel, while the other portions of the retina have their red fibrils tired out for the time. 87. The table included in 85 will serve to show the directions in which the hues of colours apparently change when the eye has first seen one colour and then looks at another. Thus, if the eye has first seen red and then looks at yellow, the latter inclines towards greenish- yellow, acquiring, as in the case of simultaneous contrast, a hue tinctured with the complementary of that first seen. Another mode of observing such phenomena involves the successive and separate use of each eye. Close the right eye and then look steadily with the left at a sheet of red paper. When the red appears rather duller than at first, owing to the special sort of fatigue it induces in the retina, look immediately, still with the left eye, on a sheet of purple paper. This, receiving the complementary of red, namely, bluish-green, will appear bluer than be- fore, in fact, a kind of violet. To verify this observation 106 COLOUR. [Chap. VIH it is only necessary, after having closed the left eye, to open the right, and to look with it on the sheet of purple paper. The purple will be perceived differently now, and so far from inclining towards violet will actually appear modified in the contrary direction, becoming redder instead of more bluish. These experiments re- quire some care and several repetitions, and will some- time fail, because different individuals possess very dif- ferent powers of appreciating slight variations in hue, and of recording their impressions. One eye, also, will not infrequently be found to differ from its fellow in many important particulars. 88. When one colour is presented to one eye and another colour is simultaneously presented to the other, some curious phenomena occur. If a sheet of paper, painted with ultramarine, is viewed with the right eye at the same time that a similar surface of lemon-yellow is seen by the right eye, we get a fluctuating effect, some- times, though rarely, seeing a grey- white produced by the union of these two complementaiies, sometimes seeing both colours at once, as if one were shining through the other; added to this there is a lustrous or polished appearance of the surfaces which is quite unlike any phenomenon to which we have already drawn attention in these pages. If an arrangement be prepared (of the form usually adopted for stereoscope slides), consisting of two discs, one disc having a series of wave-bands of cobalt-blue, alternating with bands of rather pale viridian, the other, intended for the other eye, being made up of a reversed series of bands of rather pale viridian and cobalt-blue, we shall see, on viewing the two figures in the stereoscope, the several phenomena above-described, sometimes the blue and the green uniting into sea-green and sometimes remaining for a few seconds separate. There will also be the same lustrous appearance as of the shimmer of waves on a summer sea. It is probable that phenomena of this kind, due to binocular vision, occur very frequently in nature ; they are, of course, incapable Chap. IX.] PERSISTENCE OF IMAGES. 107 of direct reproduction by the art of the painter in whose work the same hues are seen by both eyes. Fig. 29 (facing p. 96) shows the mode in which the colours (in this case, blue and yellow) are to be distributed in the arrangement above described. 89. There is one remarkable case of simultaneous contrast to which no reference has yet been made. If the same object, in a darkened room, be made to cast two shadows, owing to the presence of two lights, the shadows will differ in colour if the lights so differ. The shadow of a rod cast by candle-light, when viewed side by side with the same shadow as cast by daylight, appears blue, the complementary colour to that of the yellowish light on either side of it : the shadow cast by daylight will then take on a greyish-yellow tinge, owing to simul- taneous contrast. Phenomena of this order constantly occur in nature where shadows, cast by two sources of light of different hue, will be found to have acquired complementary hues ; so, too, a beam of daylight, finding its way into a room illuminated by the orange-yellow light of candle-flames will acquire a bluish tinge. CHAPTER IX. PERSISTENCE OF RETINAL IMPRESSIONS IRRADIATION THE COLOUR OF THE LENS OF THE EYE PARTIAL AND GENERAL VISUAL FATIGUE THE DEVELOPMENT AND CULTIVATION OF THE SENSE OF COLOUR. 90. AFTER the eye has seen a coloured object for a few seconds it retains for a time the original impression correct in colour, but (at least after the lapse of about one-fiftieth of a second) reduced in force ; this is the positive image. Then succeeds a second or negative image of the object, tinctured more or less strongly 108 COLOUR. [Chap. IX. with a hue complementary to that of the original. The duration of each image depends upon the brightness, the hue, and the length of time the original object has been viewed, By making a small circular aperture near the edge of a large black disc capable of rapid rotation and properly adjusted against a rather smaller circular opening in the shutter of a darkened room, we may determine with some degree of accuracy the duration of images which differ in luminosity and hue. Glasses of different colours may be placed in the opening of the disc and the number of revolutions of the disc may be recorded. In order that the retinal impression of such a coloured spot should be a continuous ring of light the spot must complete its circular path in about thirteen- hundredths of a second, but the minimum time differs slightly with different colours. A piece of glowing char- coal whirled rapidly at the end of a string in the air affords a familiar example of this persistence of the retinal image. But this effect, in the case of highly luminous bodies, is complicated by another appearance. If a piece of charcoal, no thicker than one's finger, be lighted at one end, and then plunged into oxygen, it will actually appear to swell as the combustion becomes more intense and the light brighter. A spiral of platinum wire, heated to whiteness by a galvanic current, not only has its apparent diameter enormously increased, but the separate turns of the spiral seem to approach, and even to coalesce, if not originally too distant. The crescent of the moon appears, for the same reason, to belong to a much larger sphere than the dimmer remainder of the disc which it clasps. All these are either subjective .ocular effects due to what in optical language is called " spherical aberration," or are to be traced to the want of perfect transparency in the humours of the eye a scattered light, of varying degrees of brightness, always surrounding the definite images of highly-luminous and of strongly illuminated objects upon the retina. The result of this nebulous border about such images is to Chap. IX;] YELLOWING OF THE LENS. 109 increase their apparent size, but it is often imperceptible under the ordinary conditions of moderate illumination. Much of the peculiar indefmiteiiess and mystery which impart considerable beauty to flames of different kinds, to strongly illuminated clouds and surfaces of water, arid to the intense reflected lights of metallic ornaments, is due, in part, at least, to irradiation. Even the coloured borders which surround the edges of coloured objects may- be traced to irradiation. A rim of bluish-green light appears by the margin of a red wafer placed on grey or white paper, owing to the extension of the image of the red wafer on the retina beyond its geometrical image. The rim is bluish-green because the image of the nebulous border becomes tinctured with the complementary of red. 91. Another kind of ocular modification of colours is of a quite different kind, and does not occur in the normal and healthy eye. It was shown by Leibreich that the yellow colour which tinges the lens of the eye in advancing years produces remarkable effects upon the appreciation of blue and of bluish colours. This aged condition of the eye is not always strongly marked, but occasionally, as in the case of the painter Mulready, it has produced very curious results. The later pictures of this artist are often spoken of as too cold, too blue, or too purple. If they are examined through a piece of glass tinted of a pale sherry yellow colour they assume a natural appearance, and exhibit the same harmonious and agree- able system of colouring which Mulready adopted in his earlier works. Leibreich pointed out that we have an excellent illustration of how Mulready saw his own works with the naked eye in his later years, if we do but look through a yellow glass at a picture of his, in the South Kensington Museum, called " The Young Brother." Without the interposition of such a glass it is far too blue to be satisfactory ; with the glass it closely resem bles, in its scheme of chromatic effect, an earlier and more accurately-coloured work by the same hand in the same collection, but painted twenty-one years before, when the 110 COLOUR. [Ch&p. IX. lenses of the artist's eyes were normal. Even when they became yellow the objects of external nature were scarcely modified in hue in consequence of the ocular change, for the yellow medium could cut off but a very small proportion of the blue in the intense light and colour of the day, while the eye, through the constant presence of this yellowness, became less appreciative of yellow, and more appreciative of its complementary, blue. But with pictures the circumstances were entirely differ- ent. The light reflected from pigments is so small in quantity and so low in intensity, in comparison with that of external nature, that a yellow lens will seriously interfere with the blues and bluish hues which paints represent it will very largely intercept them. So the painter will try to set this right by strengthening his blues, and increasing their proportions in his mixed colours. To his yellow lenses his pictures then become right in harmony and key of colour. To a normal lens they are too blue, his reds, too, being changed, becoming purple or violet, and his greens likewise tending towards blue-green. If, then, we look at one of Mulready's later pictures through a sherry-yellow glass, or even through a thick layer of mastic varnish, which in course of time has yellowed, it recovers in a great degree its proper hue, and appears to us as it appeared to him ; but if, on the other hand, we look at one of his early pictures through the same media, we see that it is altered in nearly every respect for the worse. No wonder, then, that the artist himself became more and more dissatisfied with his earlier colouring as his lenses grew yellower. 92. We have already explained how subjective colours due to simultaneous and successive contrast arise from partial visual fatigue, certain portions of the retina losing for a time the fulness of their power of perceiving one or other of the three elements of colour. An exces- sive illumination of white light produces fatigue in all the three sets of nerve-fibrils, red, green, and blue ; even the constant strain on the vision caused by a long Chap. IX. J DEVELOPMENT OF THE COLOUR SENSE. Ill examination of drawings in black and white causes some- thing of the same sort of weariness, and so does a day spent in a picture-gallery. In these cases the visual fatigue being general does not produce the subjective chromatic phenomena which has been described in Chapter VIII. But it is of importance as tending to render the eye less appreciative generally of delicate differences in tone and in hue. It is certain that the sensations of light and colour always involve the expendi- ture of a certain amount of vital energy, and are attended by certain physiological changes. Time is required for the restoration of the organs of vision to their equilibrium. Under ordinary conditions of moderate exercise of the power of appreciating light, shade, and colour the inter- vals of non-exercise during the day and the hours of sleep at night more than suffice to bring back to their normal condition both the physical apparatus "and the nervous sensibility of the eye. 93. Endeavours have been made to show that in olden times the appreciation of colour, as distinct from that of light and shade, was very imperfect. The main argument is founded upon the limited vocabulary for various colours possessed by the oldest writers, and the vague usage of such terms as they employ. But when we examine attentively such ancient works of decorative art in colour as remain to us, particularly those of Eastern origin ; when we recognise how our colour language even in the present day fails in fulness and precision ; when we find that peoples who are commonly accounted uncultured or savage continually use agreeable colour- combinations which we would fain imitate ; when we observe children in the present day habitually noticing none but the most vivid hues ; when we discover that a keen sense of colour belongs to many groups of the lower animals, not only to mammals but to birds, fishes, and even insects, then we have very good reasons to doubt that the sense for colour in the human eye and brain is a development of the last few thousand years. THE DIVERSITY V ^ ;! ' ' ; ...K ^ ill 2 . COLOUR. [Chap. IX The experimental proof, obtained by Rood, that the amount of time necessary for vision is the same, namely, one forty-billionth of a second, whether colour or merely light and shade be recognised, tends to show that the human sense for colour is as ancient as the human sense for tone. . 94. But it does not admit of doubt that individual sensibility to colour admits of large variations, and that it is susceptible of immense improvement. This cultiva- tion of the sense of colour is, however, rather psychologi- cal than physiological, rather mental than physical. It .is not that the organ of vision is improved, but our power of interpreting and co-ordinating the sensations which it transmits to the brain. And it is here that the effects of association come most prominently, though often uncon- sciously, into play. We try to trace out the causes of the vast numbers of colour-sensations which we are continually receiving, but we constantly find that the cold methods of analysis fail to explain the mental appreciation with which we regard the astounding fertility of nature in its gifts of colour. We shall endeavour farther on to demonstrate how greatly our pleasure in colour depends upon an infinitude of most minute variations of tone and hue, which, by their suggestion of the wealth, variety and vastness of nature, and by their association with scenes and circumstances of enjoyment and delight, enrich our appreciation of the sensation of colour in a way which no mere optical demonstration of chromatic phenomena can ever completely trace. Still, experiment and analysis are serviceable tools in the hands of the artist who seeks to reproduce, to modify, or to develop, in tangible form, the happy combinations and arrangements of colour with which his mental recollec- tion is stored. But how rarely do we find even the most strenuous and cultivated painter or colour- designer to be quite free from an occasional crudity or weakness in his use of colour and of tone. 113 CHAPTER X. DESCRIPTION OF CERTAIN COLOURS THE CONTACT AND SEPARATION OF COLOURS COLOURS WITH WHITE, GREY, AND BLACK DOUBLE AND TRIPLE COMBINA- TIONS OF COLOUR COMPLEX COLOUR-COMBINATIONS CHROMATIC EQUIVALENTS. 95. THE discussion of colour-combinations may ap- propriately follow the account which has been given of the very important phenomena of contrast. But as we shall^have to deal chiefly with a certain number of hues of very decided character, it may be serviceable to gather under a few headings some descriptive remarks concerning these hues, following so far as may be the order in which the colours succeed one another in the spectrum. We shall thus acquire some knowledge of the chief characteristics of the chromatic elements with which our groupings or assortments are built. Red. Red, when of low luminosity or mingled with much black, appears of a chocolate hue. The normal red is approximately represented by crimson or Chinese vermilion, or by scarlet vermilion washed over with madder carmine. Madder carmine itself, and ordinary or cochineal carmine, verge slightly upon purple, that is, contain some blue. The normal red is less bright or luminous than yellow, but it is warmer and more retiring. The majority of our red pigments do not correspond so nearly to the normal red as mercuric iodide, which has been called " geranium colour." It is, however, from its fugitive or changeable character, wholly unfitted for use as a paint. If we take a stick of red sealing-wax, which is coloured by vermilion, it will reveal, on examination by the prism, the presence, in the light scattered from its surface, of all the rays from red up to the line D in the I 114 COLO till. [Chap. X. orange-yellow. Even the flame of a burning lithium salt shows an orange element. Red glass coloured by copper sub-oxide does not transmit unmixed red rays, but many orange rays as well. Two or three thicknesses of it do, however, transmit a purer red beam. Orange. This colour passes, according to its pro- gression towards green, from orange-red to orange-yellow. In brightness its yellower hues come very near to the most luminous and advancing of all the spectral colours, yellow. It is seen in tolerable perfection in the pigment known as cadmium yellow, the sulphide of cadmium, and in the skin of a richly-coloured ripe orange. To make a good bright orange colour by a mixture of pigments it is essential that the yellow pigment used should incline to orange rather than to green, and the red pigment to orange rather than to crimson or purple. If the contrary be the case, and a greenish-yellow pigment be mixed with a red, or a yellow pigment with a red inclining to purple, a large amount of grey is produced by the con- siderable absorption which takes place that is, the quenching of several chromatic elements, so that a muddy 'or dulled tone of orange results. By dilution with white pigments some orange pigments yield hues having a somewhat buff colour. Yellow. This is the most luminous of all the colours of the solar spectrum, where, however, it occupies an ex- ceedingly narrow space. The brightness of most yellow pigments, such as lemon yellow and chrome yellow, is proportionately much higher than that of red, green, and blue pigments. They reflect to the eye a good deal of the light lying on either side of the yellow, but the resultant visual impression such light produces is that of yellow. All the methods of obtaining yellow from the combination of the light from red and green pigments yield a hue which is disappointing in luminosity, and would be called a greyish-yellow. Some transparent yellow pigments verge towards green in their lighter tints, and towards orange in their deeper tints. Yellow 13 Chap. X.] PURPLE DESCRIBED. 115 regarded as an " advancing " colour. Mixed with grey it yields citrine, with black, dull yellowish-green, or olive. Green. In the spectrum it will be seen that much of the so-called green light is tinged with yellow, and much towards the more refrangible end, with blue. Yellowish- green is often observed in the budding foliage of trees in spring, bluish-green is not an infrequent hue of the ocean, especially near the shore ; it has been called sea- green and aquamarine. Emerald green is not a pure typical green, but contains a decided trace of blue ; it reflects more white light than vermilion. Its brightness is not particularly great, but the visual fatigue which it causes is very marked, in fact, all tolerably strong greens tire the green nerve fibrils sooner than reds tire the red fibrils and blues the blue. It is probably on this accoimt that strong greens are often so disagreeable in chromatic combinations, requiring unusual skill to bring them into a pleasant colour scheme. It is cold, too, as well as intense. Green paints mixed with white and black pro- duce sage greens, which are always bluer than the appearance of the green used would lead one to pre- dict. Blue. Blue acts less strongly upon the retinal nerves than violet, which itself is less energetic than green. It is a retiring and cool colour. Genuine ultramarine from lapis-lazuli is probably the purest blue pigment. Artificial ultramarine generally inclines towards violet, though different preparations of it vary considerably in hue. Cobalt blue reflects much green and violet light, Prussian blue, indigo, and cceruleum contain a good deal of green. On mixing any of the last three pigments with madder carmine, in order to form a violet or purple, a large absorption of light occurs, and a dull or greyish- purple or violet is the result. Blues mixed with white and black produce slate colour. Purple. There is no sound pigment which exactly re- presents purple, but a mixture of two of the aniline dyes, magenta and mauve, makes a tolerable representative 116 COLOUR. IChap. X. of it. Its paler tints are approximately near in hue, but a trifle bluer than the flowers of the peach and the almond. A mixture of genuine ultramarine and madder carmine may be used in painting to represent purple and also violet, the ultramarine being laid on a white ground, and then glazed with the madder pigment. All purples and violets lose much of their blue when viewed by the light of gas or candles, becoming redder and generally duller. Purple belongs to that region of the chromatic circle in which the warm hues are situated, but is much less warm than red. Vermilion and cobalt blue produce by admixture on the palette a very dull purple, owing to the orange in the former pigment and the green in the latter. 96. We may now enter upon the discussion of chromatic combinations. Of these, the simplest con- sist of two tints, or shades or broken tones of the same hue. If these pass by insensible gradations into one another we have in reality a very complex arrange- ment, which, from its soft and tender character is, when appropriately used, agreeable to the eye. A useful modification of this arrangement is obtained by tinting a colour into white on the one side, and shading it into black or breaking it with grey on the other. Or we may associate a single pale tint of a colour with a single dark- ened or dulled tone of the same colour. This arrangement, though constantly useful in pictorial art, produces a certain vague or confusing effect in decorative art, unless the two tones be separated by a line of black, white, or gold. The general and most conspicuous effect of such use of a dividing line or edging of white is an apparent increase of the saturation of both the tones thus sepa- rated ; black, on the other hand, increases the brightness but diminishes the purity (freedom from white) of both the tones. In the consideration of the specific effects of the association of white, grey, or black with a single colour, we follow the order in which the colours succeed each other in the spectrum, adding purple at the end. X CM OF I Ihe Separation of Related Rwzs. Fig. 30. (see page 131.) "\5hcnt Bro oks, Day & SarLLt d Wv Chap. X.] COLOURS WITH WHITE. 117 The coloured diagram (Fig. 26) illustrates some of the observations here recorded. 1. RED. Red with white becomes deeper, more satu- rated or purer, and less bright. The combination, as to intensity of contrast, is similar to that of green with white, being less than that of blue, violet, or purple with white, but more marked than that of orange or yellow with white. Red with grey, when the latter is moderately pale, becomes brighter and less saturated. Red with black becomes brighter and less saturated, sometimes acquiring an orange tinge. 2. ORANGE. Orange with white is rendered deeper, and perhaps a trifle more reddish. The contrast of tone between orange and white is much greater than that between yellow and white, the combination is con- sequently more effective. Orange with grey, when the latter is pale, is deepened and reddened. With dark tones of grey orange becomes lighter. Orange with black becomes brighter and slightly yellower. 3. YELLOW. Yellow with white is rendered deeper, less bright, and less advancing, acquiring a slight greenish hue. The lighter the tone of the yellow the less pleasing is the combination. Yellow with grey is rendered brighter and perhaps slightly orange. The combination is satisfactory when the grey is rather dark. Yellow with black is rendered paler, brighter, and more advancing. The combination affords the most intense contrast of tone next to that of white with black. The blackness of the black is modified by acquiring a slight bluish hue which enriches it. 4. GREEN. Green with white becomes deeper and purer; the combination is capable of yielding very beautiful effects. Green with grey becomes deeper only when the grey 118 COLOUR. [Chap. X. is pale ; if the grey be at all dark it acquires a purplish tinge. Green with black is rendered brighter and paler, while the black suffers, being tinged witli a reddish or purplish hue. 5. BLUE. Blue with white constitutes a generally pleasing combination. The contrast of tone is very decided when the blue is at once pure and bright. The effect of strongly illuminated white clouds in deepening the tone of the blue of the sky bordering them is a good example of one of the chief characteristics of this com- bination ; under such conditions the white often assumes a slightly yellowish tint. Blue with grey. Grey, if pale, deepens and purifies blue ; the combination, though necessarily cold, is often most serviceable in pictorial as well as in ornamental art. Blue with black. This combination is less agreeable than that of blue with grey, or of violet with black, especially when the tone of the blue is deep. Light tones of blue are made still paler, but broken tones, more saturated, by contiguity with black. 6. VIOLET. Violet with white affords a strong con- trast of tone ; the combination is an agreeable one, resembling that of blue with white. Violet with grey. The distinctive hue of violet makes itself felt strongly in this combination, which is a quiet and agreeable one. Violet with black gives but a slight contrast of tone when the violet is pure. The black acquires a rusty brown hue, which reduces its depth. 7. PUEPLE. Purple with white affords a good con- trast of tone. Pale purples and rosy tints form agreeable combinations with white. Purple with grey resembles in effect the combination of violet with grey; the grey, if of moderate area, becomes decidedly greenish. Purple with black is rarely a satisfactory combina- tion ; the black acquires a greenish hue. Chap. X.] MUTUAL INFLUENCE OF TINTS. 119 97. From what lias been said in the preceding paragraphs it will be seen that the general effect of white upon a colour is to increase its purity, to deepen its tone, and to emphasise its hue. Grey varies in its effect according to its depth of tone ; black usually increases the brightness but lowers the apparent purity of a colour. But these effects are generally complicated by the changes of hue brought about by chromatic contrast the black, grey, or white, if small in area, becoming, when associated with a colour, often slightly tinctured with its comple- mentary. A further difficulty in ascertaining the exact effect upon any colour of black, grey, or white arises from the almost necessary introduction of a third chromatic element as a background. For whether we employ a background of the colour itself, or of white, grey, or black, as the case may be, we cause an enormous disproportion in the area of the two associated members when the background is sufficiently extensive to exclude from our purview any other background, as of white paper or of a dark space. 98. When two tints of the same hue are associated there is always an increase in their apparent difference of tone. When two shades, or two broken tones, of the same hue are similarly associated, the same effect is produced. Often, also, there occurs, in each class of these combinations, a chromatic difference. For a pale tint of a colour frequently differs in hue from a deep tint of the same, the effect of contiguity being similar to that of increased or diminished illumination, a light tint of any ordinary blue having a violet cast becoming less violet, and a deep tint of the same colour verging more strongly upon violet. For details concerning changes of hue, reference may be made to 119. When a broken tone of one colour is associated with a pale or pure tone of the same, the broken tone becomes still more mixed with grey, and often acquires a slight suspicion of the complementary colour ; at the same time the pale or the pure tone acquires additional purity. 120 COLOUR. [Chap. X. 99. Colours of different hues associated in pairs belong to three categories : those in which the difference in hue is small, those in which it is considerable, and those in which it is as large as possible (the complemen : taries). Pairs belonging to the first category require special treatment, and will be discussed in 106, under the heading "the small interval and gradation." We now address ourselves to the subject of dyads or pairs of colours in which the difference of hue is very decided. We cannot consider this matter from a purely scientific point of view, for we are unable in many instances to discover why certain pairs of hues are pleasing, and others unsatisfactory or even offensive. Association, habit, one's surroundings, with other obscure causes, are all factors, unconsciously employed, it may be, in the formation of our judgment. The subject has been investigated by Chevreul, Briicke, Rood, and other experi- menters, and we are indebted to their researches for the majority of the opinions gathered into the following tables. An asterisk attached to the name of a colour indicates that the mixture of grey or black with it improves the effect of its association. It may be further remarked that, in many cases where two colours of full depth yield a bad or unsatisfactory assortment, the reduction of the tone of one of them, by a considerable addition of white, often makes the combination agreeable. Normal red with, violet blue blue-green green green-yellow yellow* . Scarlet with violet ,, turquoise blue ,, yellow . Orange-red with violet purple . blue bad. excellent, good, but strong, good, but hard, fair, unpleasing. bad. good, good, unpleasing. good. fair. excellent Chap. X.] PAIRS OF COLOURS. 121 Orange-red with turquoise . good. , blue-green unpleasing. , yellow-green . fair. Orange with purple .... bad. , violet .... good. , blue .... good, but strong. , turquoise . . . good. , blue-green good. green .... fair. Orange-yellow with purple good. violet . excellent. blue . good. turquoise 5? . fair. blue-green . moderate. green . bad. Yellow with violet .... excellent. ,, . purple .... good. normal red poor. .,, turquoise moderate. ,, blue-green* . bad. green* .... bad. Greenish-yellow with purple good. violet excellent. scarlet strong and hard. orange-red fair. turquoise . bad. , normal blue good. Yellowish-green with normal red . good, but hard. purple . difficult. ,, blue-green bad. blue good. Normal green with purple . strong, but hard. scarlet . difficult. orange-red hard. turquoise * . bad. Blue-g -een with purple fair. violet good. blue bad. green bad. yellowish green bad. turquoise bad. 100. The above list comprises fifty-four only of the very numerous combination, in pairs, of some of the 122 COLOUR. [Chap. X. decided hues which we have named and described in preceding chapters. It is assumed that in our experi- ments on their chromatic effects, pleasing or otherwise, we have been using coloured materials, which neither by any peculiarity of texture, nor quality, nor design, are capable of improving the results. Cloth and paper are suitable ; silk, velvet, glass, and enamel, for various reasons, give results which are complicated by the introduction of new elements. Pairs, in these latter materials, in consequence of the presence of lustre, translucency, or ft throbbing " hues, in varying degrees, will often become quite acceptable, while in prosaic cloth, or paper, they are just the reverse. For the same reason the colours which we so constantly see happily associated in nature must not be assumed to be always susceptible of successful reproduc- tion in the studio or the workshop. The brightness of natural objects, their soft and infinite gradations of hue and tone, their intricate variations of substance and texture, their beautiful forms and their delightful associations, all unite in producing a complex harmony which generally defies complete analysis. Take as an example the blue of the sky seen amidst the green foliage of a forest tree. This is not a mere crude opposition of blue and yellowish-green. The blue deepens and becomes a more decided blue from the horizon towards the zenith ; the leaf-green is deeper than the blue, but is not one hue, but many hues. One leaf, or a part thereof, will reveal the fact that the light it transmits is coloured differently from the light it reflects. One leaf, or a part, will be brightly illuminated by much white, grey, or bluish light, from the clouds or the sky ; another will show its proper hue and tone ; another will exhibit a deep shadow. One leaf will have its contour wholly dark, or partly dark and partly light ; and then will have its surface exquisitely toned from pale to deep green. Add to all these enrich- ments the variations introduced by means of the forms and groupings of individual leaves, to say nothing about stems and branches, and it becomes evident that green Chap. X.J USE OF BROKEN HUES. 123 foliage against a blue sky is not to be reproduced or re- presented by a piece of green paper laid upon a piece of blue paper. Yet such analysis as we have attempted to make above, of the constitution of this natural contrast, may teach us how to amend our first crude impression as to its nature. We may separate our pair of hues by outlines of white, or grey, or gold ; we may make one hue paler than the other ; we may cause both to palpitate with minute variations of tone and of hue. 101. The dyads, or pairs of colours described in our list, do not comprise many of those intermediate hues and dulled or broken hues of which ornamental art constantly makes such admirable use. No mention has been made of garnet-red, salmon-pink, and rose-grey ; of amber, straw, fawn, and citron; of lavender, lilac, plum, and puce. All of these hues, to adopt the conventional terms we have previously explained, are to be regarded as compounded of two primaries, in proportions other than in which these elements occur in the so-called secondary colours, or in those hues for which we have found a place in our chromatic circle } all, or nearly all of them, also contain some grey. There are two methods by which we may construct pairs, into which these hues shall enter. The simpler of these methods consists in preparing a large number of these rarer hues by means of strips of paper painted with water-colours, or by means of a large selection of what are known in the shops of fancy stationers as "surface-papers." These strips are arranged and re-arranged in twos, with or without separating lines of black, grey, white, or gold, until agreeable combinations are obtained. The other method, which has a more limited range of usefulness, involves the use of Maxwell's rotating discs. If, for instance, we want to learn what kind of pale blue will yield the strongest possible contrast with a certain deep amber colour, which we desire to employ in this associa- tion, we take discs of amber, white, blue, and green, and mount them together on the axis. Then by means of 124 COLOUR. [Chnp. X the radial slits in the discs, we adjust them all so, as to obtain, on rotation, a neutral grey. Then we note the relative areas of the white, green, and blue sectors used, withdraw the amber sector and replace it with an equal grey sector of proper tone. On rotating this compound disc, we obtain a pale greyish-turquoise, which is the exact complementary of the amber. In this example, as in all others, we have the opportunity of ascer- taining approximately beforehand what colours the several discs must possess, by referring to the chromatic circle and its pairs of complementaries. 102. And here it will be well to state that every hue has in fact an immense number of complementaries. Yellow is complementary to blue, but you may mingle the yellow, or the blue, or both, with white, with grey, or with black, and yet the mixtures will be complemen- tary. The use of such modified complementaries is often far preferable to that of hues in which both brightness and purity are at their highest available maximum point. Nor, indeed, must it be supposed that pairs of colours exhibit their highest beauty when they are complemen- tary \ the large interval on the chromatic circle, say at least of 80, more often of 90, or above, yields very many beautiful pairs. Colours differing by a smaller interval than this generally suffer by harmful contrast which causes the hues to look dull and poor, while a considerably greater difference than this produces, in many cases, too strong and crude a combination by the excess of helpful contrast which a near approach to the complementaries brings into action. The merits of the small interval, as we shall presently show, rest upon a different basis. That contrasts may be too complete, and may be, in consequence of their strength, trying and hard, is exemplified by three pairs : Black with white ; green with purple ; red with blue-green. The first pair is distinguished by the highest possible difference of tone ; the other pairs by a very moderate Chap. X.] TRIPLE COLOUR-COMBINATIONS. 125 difference of tone, but the largest difference of hue ; they need the greatest judgment in their mode of employment if they are to be introduced into ornamental work. For reasons which it is difficult to formulate, the strong colour contrast, yellow with blue, is much more easily managed ; but in this case there is a great difference of brightness in the pair; an approximately equal brightness in a pair of strongly contrasted colours is generally unpleasant. 103. The study of triple colour combinations is surrounded with peculiar difficulties, the moment we leave the triads in which two hues are associated with white, grey, or black only. Perhaps, however, a few words con- cerning these latter triads may help in clearing the way. Thus, if we wish to separate and emphasise two nearly related colours, such as deep violet and deep blue, both of which are cool and retiring, black will prove to be much inferior for this purpose to white. Deep tones of violet and blue approach so closely in their measure of brightness, to black, that the latter effects little towards their separation, while it is itself injured by contact with them, acquiring a rusty hue. But white, on the other hand, though it deepens these colours, renders them purer, and, by acquiring a faint tinge of their comple- mentaries, yellow or orange (in obedience to the law of simultaneous contrast), causes their differences to appear more distinctly. Still, there is a triple combination, which is slightly preferable to that of blue, white, violet ; it is formed by the substitution of grey for white. The contrast of tone becomes less violent, and the whole effect is undoubtedly more agreeable. 104. Many pairs of hues forming agreeable com- binations are injured by the addition of a third hue, but all the poor and bad dyads may by this means be im- proved. Thus, while greenish-yellow with normal blue forms a good pair, and the addition of violet spoils it, it will be found that violet improves the effect of the pair, greenish- yellow and greenish-blue. Generally speaking, good triads may be arranged by taking three colours 126 COLOUR. [Chap. X. which are situated from 90 to 140 or 150 apart on the chromatic circle. It is, however, well to remember that, as a rule, two of the hues should belong to the " warm " group, for triads in which two of the colours are " cold " are more difficult of management, and are less generally esteemed, though valuable in certain schemes of chromatic decoration. Of the modes of collocating the colours of a triad, and of their relative proportions, we speak in a subsequent chapter, here we merely give a short list of such arrangements, derived partly from ex- periment, and partly from the study of specimens of de- corative art in textiles, in wall-papers and wall-decora- tions and in pottery. IX. X. XI. XII. ( Terra -cotta. < Maroon. ( Sage-green. Yellow. Violet. Yellowish-green. !' Normal green. Orange. Turquoise. ( Amber. < Blue (pale). / Crimson. ( Normal red. I. \ Yellow. ( Normal blue. ( Purple-red. II. 1 Yellow. ( Greenish-blue. ( Orange. III. \ Normal green. ( Violet. | Orange. IV. -I Normal green. ( Purple- violet. {Amber. Cream. Blue (medium depth) . ( Normal red. VI. 1 Gold. ( Normal blue. ( Leaf -green. VII. < Puce (deep). ( Eose-grey. {Leaf-green. Violet. Salmon. By the addition to these triads of white, grey, or black, or by the introduction of one or more tints, shades, or ( Maroon XIII. < Bronze-yellow. ( Olive-green (dark). ( Apricot. XIV. -j Crimson. ( Gold-brown (pale). ( Flesh-red. XV. \ Normal blue. ( Olive-green (pale). Chap. X.] COMPLEX COLOUR-GROUPS. 127 broken tones of the fundamental hues, we reach complex colour-combinations which would require a far more elaborate study than we can accord them in this place. But apart from the generally useful addition of a contour or boundary-line, it is not to be imagined that in purely decorative work great complexity and extensive multi- plication of hues is commonly advantageous. Directly we are able to use choice and fine materials, such as silk or enamel, which give us what artists call " quality " of colour, we discover that two or three hues are frequently more effective than half-a-dozen. The beautiful sixteenth- century velvets of Italy may be cited as examples of such happy simplicity in combination. By beauty of pattern, by varying depths in the pile, and by contrast of surface and of texture, such simple dyads as yellow- green with medium violet, pale olive-green with deep indigo, leaf-green with deep blue, and pale leaf-green with deep amber, the most beautiful effects are produced. However, it may not prove useless if we cite a few ex- amples of complex colour-combinations (hexads), which are capable of yielding delightful results when their elements are properly collocated, and are used in due pro- portions. ( Full bluish-green Medium yellowish- I. ej]sivej)ower, and a more complete transparency than water possesses ; the emerald exhibits two distinct hues in the same specimen, and there is in consequence a play of colour in this gem for which it would be idle to search in any paint ; the ruby, too, exhibits a vibration of hue between purple-red and crimson-red, which gives it a charm superior to any dye. But, further, these depreciators of gems insist upon their being cut, if cut at all, in a way which is usually fatal to the develop- ment of those qualities upon which the beauty of precious stones depends. This method is known as cutting en cabochon, or tallow-topped, and is, as a rule, appropriate to those stones only which are not transparent opals, cats' eyes, moonstones and chrysoprases. When ap- plied to transparent gems, it prevents the full play of light and colour proper to them, internal reflection is im- perfect and the marvellous dispersive power often present does not show its effect in producing the so-called " tire " of the stone. Analysed by means of a prism, the colour of gems is often found to differ from that of the nearest approach in artificial paste, that is, glass, that can be manufactured as an imitation of them. The minute internal fissures, to which the splendid play of colours in sphene and in the precious opal of Hungary, Mexico and Queensland is due, cannot be imitated. The opal, when polished, has its beauty enhanced by being set in a border of small brilliant cut diamonds, which form, with its soft milkiness and variegated splendour, a delicate yet effective contrast, by reason of their perfect transparency, their whiteness and their almost metallic surface lustre. The peculiarities of the " star-stones, " such as the star- sapphire and the star-ruby, like those of the opal, have not been reproduced artificially. These varieties of crystal- Used alumina are translucent, not transparent, and owe their beauty to their intimate structure. \ This is properly Chap. XIV.] PLEIOCHROISM. 175 and fully developed only when one of these crystals is cut across its principal axis and left with its summit en cabochon. Then a six-rayed star makes its appearance. This star is best seen in sunlight, or by the light of a bright flame, or in the focus of a condensing lens. It is due to the symmetrically-disposed layers of which the crystal is built. The less transparent varieties of red garnet, when cut as carbuncles, occasionally show a star, but it has only four rays, owing to the simpler crystalline structure of this stone. Amongst other chatoyant stones having a play of light upon, or rather within them, the moonstone, a variety of one of the species of felspar, is the most familiar. Its light is more diffused than that of the asterias, or star-stone, and has a pearly sheen. Moonstones may in some cases be used to replace pearls in jewellery, and may be associated effectively with dark- coloured clear amethysts. The stones called cats' eyes belong to the class of chatoyant stones. There are three entirely distinct kinds, one of these, the hardest and most precious, is the chrysoberyl. It is yellow, yellowish- green, or brown, and shows a pale bluish line of light when properly cut. This effect is wholly due to the optical structure of the crystal. In one of the commoner sorts of cats' eyes there are fine parallel lines of asbestos which catch and reflect the incident light, These fibres are embedded in quartz. In the African crocidolite, or tiger-stone, which constitutes a third kind of cats' eye, we have a silicious matrix crowded with parallel fibres of a ferruginous substance ; it reflects a deep golden or brown light. Bluish and reddish varieties of crocidolite also occur, and exhibit the same phenomenon. 141. To one property possessed by many precious stones we have already referred when describing the two- fold hue of the emerald and the ruby. This property is called pleiochroism. A crystal which is straw yellow in one direction may be ultramarine blue in another, for a beam of white light is affected differently according to the direction in which it traverses the crystal. By 176 COLOUR. [Chap. XIV means of a small instrument called the dichroiscope the presence or absence of this property may be readily ascertained. But with the unassisted eye it is easily detected in many stones which are or may be used in jewellery. Amongst these we name the chrysoberyl, the green, pink, and brown tourmaline, the iolite and the amethyst ; of the dichroism of the emerald and the ruby mention has already been made. There is no doubt that much of the chromatic effect of these stones is due to their dichroismj In the annexed table are given the twin-colours (as we may call them) of some of these gems, as seen in cut specimens without the aid of any optical apparatus : Name of Gem. General Colour. Twin Colours. Euby Crimson Pure red Purplish-red. Sapphire Blue Pure blue Greenish-blue. Emerald Green Pure green Yellowish-green. Tourmaline Pink Eose red Salmon. )> Leaf-green Bluish-green Yellowish-green. Brown Orange-brown Greenish-yellow. Topaz" Sherry-yellow Straw-yellow Pink. Chrysoberyl Amher Golden brown Greenish-yellow. Iolite Lavender Pale buff Violet-blue. Amethyst Purple Eeddish purple Violet-purple. 142. Amongst precious stones which lack the pro- perty of pleiochroism, and which, therefore, may be termed monochroic, the garnet and the spinel may be cited as examples. Individual specimens of garnet, belonging in most cases to different varieties of the same species of this mineral, range in colour from crimson to red and amber ; there are even garnets as green as the emerald. The hues exhibited by the spinel, a single well-defined mineral species, are even more varied, for they include nearly if not quite all the colours of the solar spectrum, as well as the rose pinks and purples, which cannot be obtained by the direct prismatic analysis of sunlight. Were we to attempt to describe the most beautiful combinations or associations of the above- named and of other precious stones amongst themselves, Chap. XIV.] PRECIOUS STONES. 177 or with gold, pearls, and enamels, we should have to do little more than repeat the suggestions as to colour- harmonies already offered in Chapters X. and XL Yet there are two considerations not hitherto taken into account which should always influence our methods of arranging.. pjrecious_ stones. We refer to their association according to certairTqualities of surface and of substance. Stones fashioned with curved sufaces may be in general agreeably combined with those which are faceted ; stones having a waxy lustre look well when grouped with those having a resinous or adamantine polish ; opalescent and translucent stones harmonise pleasantly with those which are transparent. In gengral, it is best to avoid putting together those precious stones which have salient proper- ties or aspects liable to come into hostile competition with each other. The annexed tabular statement of such properties presents them in a concise form and in regular sequence, and may be used in devising appro- priate associations of gems. It is quoted from the author's "Handbook of Precious Stones," to which reference may be made for detailed information as to the particular kinds of gems which exhibit the several characteristics named : SURFACE . . . SUBSTANCE . . < Form . Lustre Light Colour 1. Plane. 2. Curved. f 3. Metallic. 4. Adamantine. 5. Resinous. 6. Vitreous. 7. Waxy. 8. Pearly. 9. Silky. 10. 'Transparent. "11. Translucent. 12. Opalescent. 13. Chatoyant. 14. Opaque. 15. Prismatic. 16. Monochroic. 17. Pleiochroic. 18. Fluorescent, 178 COLOUR. [Chap. XIV. 143. A few words may now be introduced concerning the colours of the commoner sorts of ornamental and tinted minerals, ranging from lapis-lazuli and agate down to building stones. There are two points of special im- portance connected with the employment of such materials. In the first place, it is very difficult and sometimes impossible to associate satisfactorily marbles and similar natural materials with tiles and the arti- ficial products of the kiln. There is some approach to transluceiicy in marble. "With this the dull, dry, opaque surface of unglazed pottery contrasts unpleasantly, while glazed tiles are coarsely artificial in their gloss, the" un- evenness of which competes unsuccessfully with the level surface of polished marble. When, however, the marbles and the tiles are reduced to pieces of very small dimensions, as in the old Roman tessellated pavements, the association of natural and artificial products is quite legitimate, for the breaking up of the large areas of the materials obliterates the offensive contrast of their qualities. In some of the fine pavements at Woodchester and Cirencester in Gloucestershire the happiest effects are produced by the association of tessellse of white, buff, grey, cream, yellow and chocolate-coloured stones, and brown Purbeck marble, with other tessellse of yellow, red and black pottery, and even with pieces of ruby -red glass. As, however, in the case of such pavements, all the substances used received the same final polish after the laying of the pavement, the contrasts of surface are greatly mitigated. The other point for consideration in the employment of coloured marbles relates only to those which have decided mottlings and veinings of colour; these do not admit of sculptured ornament. Your sur- face decoration in relief will clash with nature's previous decoration in colour. The wildness and almost infinite variations of the natural tones of brown in a piece of Derbyshire alabaster are broken up and spoilt when its surface is diapered with a conventional carved ornament ; the natural picturesqueness and the artificial decoration Chap. XIV.] COLOUKS OF MARBLES. 179 are incongruous. The polished plane surface of a piece or slab of mottled or veined marble, or the smooth rounded contour of a shaft, displays the beauties of the material. In a carved capital, on the other hand, nature's designs in colour are disfigured by man's work in light and shade, and the sculpture is itself ruined by the casual way in which the projecting portions are here and there darkened by a rich mottling, and the recessed are brought into prominence by reason of the paleness of that part of the marble in which they are wrought. For it is inconceivable that the carver by any skill can bring his design into coincidence with the tones of the material. Examples of this incongruity between colour and form are only too common. The employment of veined Derbyshire alabaster for the interior sculptural decoration of churches was greatly favoured by the late Sir G. G. Scott. Many a costly reredos in this material illustrates the truth of the opinions we have just ex- pressed. 144. -It is scarcely necessary to say that, in the most ( artistic times and amongst the most artistic peoples, coloured marbles and agates and lapis-lazuli have been employed, wherever available, for decorative purposes. Sometimes a shaft, sometimes a wall-panel or lining, sometimes a tessellated floor, and sometimes a tazza or a vase has been wrought out of these beautiful minerals. Where the natural markings of the marble or other sub- stance have been distinct and rich in effect, large slabs or masses have been used ; where the colour approached uniformity the material has frequently been subdivided into numerous small pieces. In some instances both methods have been employed in a single design, a large mottled and veined panel having been bordered by a rich mosaic of small tessellae. Venetian and Saracenic art alike afford illustrations of such a combination. Examples of the Saracenic or perhaps Coptic style of mosaic will be found in the St. Maurice collection at South Kensington. In one of these, large upright slabs of richly-coloured and 180 COLOUR. [Chap, XIV, veined porphyry and marble are bordered by narrow- bands of marble of less distinct patterning, while these again are surrounded by strips of fine mosaic work, made up of cubes and strips, and geometrically-shaped pieces of fine marble, earthenware, glass and mother-of-pearl. From the great judgment and fine feeling for colour shown in the arrangement of these designs, and from the small size of the pieces employed, we are compelled to admit that the effect of the association of these very diverse materials is entirely satisfactory. Venetian mosaic work, in which coloured marbles and porphyries and lapis-lazuli are associated together with enamels and glass backed with gold leaf, belongs to the same category of beautiful chromatic harmonies. It is greatly to be desired that some of our native marbles and beautifully tinted building stones were more largely employed for interior decoration. If we cannot hope to rival by their means the splendour of Eastern work, or the richness of colour of the combination of red and green porphyry with yellow and white marble seen in the famous Opus Alexandrinum, we could easily achieve a pleasant and soft harmony of delicate hues which would agreeably vary the monotony of our favourite whitewash, or the crudity of our flat and meaningless stretches of paint. But the whole subject of the use of natural and arti- ficial colour in architecture and sculpture is fraught with difficulty. We cannot, however, go far wrong if we inter- fere as little as possible with the picturesqueness of nature's chromatic arrangements whenever they possess a distinctive character : with uncoloured and less interest- ing marbles and stones, the case is certainly different. With pure white marble, having a slight translucency and a beautiful sub-crystalline texture, it often seems better not to interfere by any additions of artificial colouring. The substance appears so thoroughly fitted for the presentation of ideal forms that even the barest suggestion of realistic colour may look like sacrilege, and may easily lapse into vulgarity. This degradation is Chap. XIV.] COLOURS OF PIGMENTS. 181 more particularly liable to occur when the sculpture is placed near the level of the eye and in a good but not very strong light. And here a curious effect of very powerful illumination and of a dim light may be noted. Strongly coloured surfaces are rendered pallid by brilliant sunshine, a clear atmosphere and a deep blue sky, reflect- ing, under such circumstances, an augmented proportion of white light. And when the light is dim, as in the interior of many an Eastern dwelling, the most decided colours lose their staring and obtrusive character, and merge into a harmony of darkened tones. In England we generally have to deal with the effects of a moderate degree of illumination, and in consequence we cannot reckon upon any considerable modification of the decided colours we may employ. In architectural interiors, where poor-looking or inferior materials (whether wood or stone) are employed, widely divergent opinions have been urged as to the mode in which artificial colour should be distributed. If the lines, contours, mouldings and carvings are good, it is argued that they do not need accentuation with colour : if they are weak and poor, colour will but bring out their defects. And a scheme of artificial colouring, if quite independent of archi- tectural forms, has the great drawback of breaking up the structural unity of the work. On the whole it may be concluded that the only safe course is to arrange and calculate beforehand the scheme of form and colour as a united whole. 145. A very important series of pigments is furnished, either directly or indirectly, by minerals. Many of these products are permanent. The compounds of iron, mainly the peroxide and its combinations with water, have always been used extensively in the arts, and supply a great variety of useful colours yellows, reds, maroons, and browns. The colours derived from copper such as chessylite or blue verditer, or blue bice, and malachite or green verditer are liable to change in hue, and sometimes become dark brown from the formation of 182 COLOUR. [Chap. XIV. copper sulphide. The true ultramarine obtained from lapis-lazuli is a superb blue and practically permanent ; the artificial variety, even when properly prepared, is somewhat sensitive to acids and alum. White lead and chrome-yellow are liable to blacken, even when protected by oil, but may be rendered less prone to change if asso- ciated with copal varnish. It will be noted from the above-named examples of pigments of inorganic origin that they vary much in one of their most important qualities, that of permanence. But that question requires a treatise for adequate discussion ; and it is further com- plicated by considerations touching the painting-medium employed and the commixture of pigments with one another. We may, however, with advantage give a selected list of those pigments which are generally avail- able for use in decorative and pictorial art, and which may serve in some measure to represent some of the hues to which attention has been called in the earlier chapters of the present volume. It may be premised that ver- milion, emerald green and flake-white should be excluded from water-colour work, but are admissible in oil- painting. In order to avoid a supplementary list we have introduced, where necessary, two or three pigments of organic origin, which do not properly belong to the present section. Red. Vermilion : Indian, Venetian and light red : preparations of madder. A slight wash of madder-red over vermilion nearly represents the typical spectral red j the chocolate colour seen at the least-refracted end of the solar spectrum may be represented by a mixture of ver- milion and lamp-black. Orange. Cadmium red serves as a connecting link between the red and the orange of the spectrum : cad- mium orange and cadmium yellow come next in order : mixed with black or black and zinc white these pigments yield various hues and tints of buff, fawn, and yellow- brown. Yellow. Lemon yellow makes a fair approach to the Chap. XIV.] COLOUKS OF PIGMENTS. 183 normal yellow. Aureolin is nearly transparent in thin washes : mixed with black it gives peculiar hues of olive- green, sometimes called dark yellow. Green. Emerald green, with a trace of lemon yellow, may be taken to represent the normal green. Yiridian has a bluer hue and is very nearly transparent. The best kinds of vert de cobalt afford hues which may be called blue-green. Blue. True ultramarine is not far from a pure and normal blue. Artificial ultramarine is generally of a rather violet cast, but has been obtained of a greenish and also of a decided violet colour. Prussian blue is transparent, of great saturation, but has a decided tincture of green in it. Violet and Purple. No permanent pigments exist having saturated and pure violet and purple colours. Approximations to these hues may be obtained by glazing genuine ultramarine with more or less madder carmine. Dulled or Broken Colours. Yellow ochre, raw siena, burnt siena, and many other preparations of iron, natural or artificial, may be employed to represent broken hues. Many of them may also be obtained by mixing two distinct and rich pigments together, or by the addition of lamp-black, or of lamp-black and zinc white to them. With the aid of the above-named pigments nearly all the illustrative examples of colours and colour-combina- tions named in the present volume may be prepared. For purples and violets recourse may be had to Hofmaiin's violet, to mauve, to magenta, and to many other coal-tar dyes. But none of these colouring matters are sufficiently stable for any purpose where permanency is requisite. It may here be remarked that while the specific optical qualities of different pigments depend mainly upon their selective absorptive power for rays of different refrangibilities, T3iefe~is another most important charac- teristic which influences, to a greater degree than at first sight might be imagined, the nature of their chromatic 184 COLOUR. [Chap. XIV effect. This consists in the varying degrees of trans- lucency and opacity which they possess. Thus a trans- parent pigment may be much less saturated or intense than an opaque one of the same hue, and yet may produce an equal colour-effect. For the light reflected from a transparent colour has passed tivice through it, and is more free from white light than the light scattered by an opaque pigment. Then, again, the thickness of the layer of an opaque pigment has less influence upon its hue (apart from its tone) than in the case of a trans- parent pigment. The application of transparent pig- ments upon opaque grounds, or painted surfaces, is called glazing by artists, and gives a richness and vibration of hue which cannot be obtained in other ways. Scumbling is the converse of glazing, for in it a thin film of an opaque colour, or of white, is used to cover partially, and thus to modify, the colours which have been pre- viously laid on. It conveys an idea of distance, of atmosphere or of mystery. By ingenious combinations of glazings and scumblings, the peculiarities of texture and surface in such materials as marble, fur and feathers may be imitated. 146. Amongst colours of vegetable origin, those of leaves and flowers first claim attention. The special colouring matter, called chlorophyll, or leaf-green, on which the general hue of foliage depends, has peculiar optical properties, already mentioned in 28 and 37. Both in the solid state and in solution, it shows a red fluorescence ; while a thin layer of its solution, in ether or alcohol, transmits green light, and a thick layer, dark red. Thus it is obvious that the yellowish-green light of various tones and hues reflected by and transmitted by green leaves is of a very complex character. In fact, ordinary foliage, when illuminated by the reddish light of sunset, puts on an orange-red hue because of the power possessed by chlorophyll of reflecting the extreme red of the spectrum, as well as of its yellow and greenish- yellow ; these combined produce an orange-red. A spray Cliap. XIV.] COLOURS OF WOODS. 185 of green foliage laid upon a piece of paper painted with a pigment exactly matching its hue shows the pecu- liarity of the light which it reflects when it is illuminated by a red light, or when it is viewed through a combina- tion of a deep cobalt blue and a deep yellow glass ; the foliage appears red on a black ground. But the chro- matic appearances of leaves are not wholly due to the presence of chlorophyll. There are also present in them other colouring matters, such as erythrophyll, a beautiful crimson colouring matter. To this substance, which abounds in the leaves and stems of that beautiful plant and its many varieties, the Coleus Verschaffeltii, and in the copper-beech, the colours of many flowers and fruits (such as purple grapes) are in great measure due. It also goes under the names of colein and cenolin, but though generally diffused in the vegetable kingdom is not the only red colouring matter present in plants. It is very sensitive to the presence of alkaline and acid substances, becoming blue, violet, or even green by the action of the former and a purer red by the action of the latter. It is almost certain that to these changes of hue of erythrophyll are due many of the varied hues of red, crimson, purple, violet, and even blue flowers. But some at least of the peculiar beauties of floral colours depend upon the structure of the cells within which the vegetable pigments occur. These cells are bounded by walls, often very thin and presenting a soft, glistening aspect, which enhances and varies the colour-effects of their contents. This aspect, though often called crystal- line, in no degree arises from any structure to which this term is applicable. Some very beautiful and com- paratively permanent colouring matters are derived from plants, but these, as a general rule, do not exist ready- formed. As instances in point, we may cite indigo, and the alizarin and purpurin of the madder-root. 147. The colours of woods are usually subdued, but varied. Some of the more richly coloured species con- tain dye-stuffs which are liable to fade on exposure to 186 COLOUR. ["Chap. XIV. light ; the paler kinds, on the contrary, generally deepen in tint after a time. But much of the beauty of wood depends upon texture and lustre, rather than upon very definite colour. The medullary rays which give the so-called silver grain, the annual rings of growth, and the undulations of the fibres, all combine to enhance the beauty of colour in wood. In furniture and the general decorative treatment of wooden construction, much use may be made of the contrasts afforded by peculiarities of texture as well as of colour. One wood, dark in colour, but of lustrous texture, may be introduced in the form of bosses, panels, mouldings and inlays into a framework of an opaque and light wood. So woods having distinct rnottlings and figurings may be happily associated with those which exhibit a more uniform appearance. The colours and grain of woods are often brought out by varnishing and oiling, but these processes have a tendency to check those alterations of hue and tone which often render old specimens of wood-work far more agreeable than new. The fibres of vegetable origin used in the manufacture of textile fabrics are generally nearly white or very pale brown, but they may be dyed or stained of any colour. Usually, colouring matters can be made to adhere permanently to vegetable fibres only by means of a mordant. First of all, a sub- stance such as tin peroxide or alumina, having itself an attraction for colouring matter, is precipitated upon the fibre, and then it is immersed in a dye-bath. The colour- ing matter is withdrawn from the liquid and becomes fixed firmly to the mordanted fibre. The lustre of veget- able fibres is usually not strongly developed and is dimin- ished in the operations of bleaching and dyeing. Linen, the woven fibres of flax, does, however, reflect particularly in some positions much of the light which falls upon it. A pattern may thus be made in which the strands forming the warp contrast in lustre, even when no colour has been added, with the strands of the woof. Under these conditions damasked linen, like silk damask, may Chap. XIV. J COAL-TAR DYES. 187 exhibit a curious optical illusion. If a white warjrand a red woof be combined, it will be noticed that in certain positions the white parts of the fabric assume a bluish-green tint, acquiring, very distinctly, the hue complementary to that of the dyed threads, the effect being enhanced by the difference of lustre dependent upon the way in which the light falls upon the fabric. Similar but more marked effects are seen in fabrics where lustrous silk and dull cotton or wool are associated. And we may here mention the peculiar mingled hues which are produced by the repeated recurrence, at very small intervals, of similarly coloured strands, in a fabric consisting of two or more colours. 148. Under the name of coal-tar dyes an immense number of colouring matters derived indirectly from coal, a vegetable product, have been introduced as dyeing materials. The hues they possess are, for the most part, highly saturated ; they range in colour from the fullest red through every hue of orange, yellow, green, blue, violet and purple. The extreme brightness and satura- tion which these colours generally possess render them difficult of management in chromatic combinations. But by considerable subdivision of the spaces they occupy, by the association with them of abundance of paler tints and dulled tones, and by the occasional reduction of their transparency by the addition of solid white and other opaque pigments, it is possible to use these telling and conspicuous dye-stuffs with satisfactory effects. Many of them are of peculiar interest from a scientific standpoint. Amongst them are three colouring matters which do not merely resemble, but are actually identical with, certain pigments previously known only as obtained directly from plants. We refer to the alizarin and pur- purin of the madder- root, and to indigo. It is also interesting to find that chemists are able, in some cases, to predict what change of hue will be brought about by effecting in a colouring substance of this type what is called a replacement. Thus by introducing one, two 188 COLOUR. [Chap. XI7. or three methyl-groups into the red dye known as magenta its hue becomes more and more modified in the direction of blue, passing through a purple and a violet stage. 149. The colours which adorn animals are distri- buted in a very strange and apparently capricious way, and, in many cases, show no correspondence with the structure of their bodies. These colours arise in great part from the minute sculpturing, reticulation and scoring of the surface, and not from definite colouring matters like those present in plants. The metallic colours of the humming-bird and the peacock must be at- tributed in the main to what may be called the optical structure of the web of the feathers : they are, in fact, interference colours (see -30) relieved against a dark background, which owes its blackness to a black or dark brown pigment. Instances, however, do occur in which an actual pigment or colouring matter exists in, and may be extracted from, coloured feathers. Thus amongst the Touracos] or plantain eaters of Africa there are no less than eleven species which owe their splendid crimson coloration to a definite pigment discovered by the present writer. This pigment is remarkable in many ways, notably in containing as an essential ingredient no less than 8 per cent, of metallic copper. And from other birds several other colouring matters, soluble in alcohol or in soda solution have been extracted. As a rule, these pigments are much more permanent than those of flowers. 150. All animal substances, including leather, vellum, silk, wool and feathers, and even ivory and bone, may be dyed without the intervention of a mordant, for they possess a naturai attraction for colouring matters. As in the case of vegetable tissues, animal fibres differ much in lustre, silk greatly excelling wool in this respect. Thus the coloured light reflected from dyed silk is more saturated or purer than that from dyed wool. There is also more play of light and shade, and even of hue, in Chap. XIV.] LUSTRE OF WOOL AND SILK. 189 silk fabrics than in those of wool, for the fibres of silk can be made to assume parallel positions and to lie in com- pact bundles, and thus are enabled to regularly reflect much white light in some places, and very richly coloured light in others. Woollen fabrics, on the other hand, appear comparatively dead, for the irregularity of their fibres and their low degree of lustre preclude them from producing the same sheen as that of silk. The contrast between the chromatic appearance of the two fibres is well seen and effectively utilised in mixed fabrics where the warp is of one of these materials and the woof of the other ; or the ground may be of wool, and the pattern of silk. But in silk itself the range of lustre is so great, according to the mode of working it up, that strong con- trasts of light and shade may be obtained in fabrics of this fibre unmixed. Very beautiful monochrome designs have been executed in cufc velvet upon a glistening silk ground, the velvet pile reflecting very little white light, and the satin or silk ground a great deal INDEX. Absorption of light, 9 Absorption, Selective, 27 Alabaster, Colour of, 179 Alloys, Colours of, 160 Animals, Colours of, 188 Architecture, Colour in, 181 Atmosphere, Effect of, 149 Balance of colour, 129, 143 Barton's buttons, 37, 164 Beam of light, J Benson, "W.,59, 69, 71 Binocular vision, 106 ; Black polish, 162 Brewster's triad, 80 Bright-line spectra, 48 Brightness, 52 Broken hues, 95, 123 Brush-work, 146 Calorescence, 44 Change, Abrupt, 134 Change, Gradual, 134 Changes of hue, 31 Chlorophyll, 184 Chromatic combinations, 116 Chromatic equivalents, 128, 143 Circle, the Chromatic, 90 Clouds, 3, 149 Coal-tar colours, 187 Colour, a sensation, 1 Colour, Balance of, 143 Colour-blindness, 77 Colour-charts, 58 Colour-classifications, 57 Colour-cone, 60 Colour-counterchange, 140 Colour-cube, 59 Colour, Distribution of, 144 Colour-hexads, 127 Colour-interchange, 139 Colour-names, 63 Colour-proportions, 128 Colour-sensations, 67, 73 Colour- sense, 111 Colour-tetrahedron, 61 Colour-triads, 125 Coloured liquids, 31 Coloured mediums, 153 Colours described, 113 Colours in contact, 102 Colours, Number of, 58 Colours, Pairs of, 121 Colours, Reflection of, 27 . Colours, Symbolism of, 25 Colours, Transmission of, 27 Complementary colours, 65, 92, 124 Constants of colour, 50 Contour-lines, 138, 145 Contrasts, 98, 103, 124 Dark heat, 44 Dark lines of spectrum, 14 Dichromic vision, 78 Diffraction of light, 33 Diffraction spectrum, 19 Dispersion of light, 14 Dispersion, Unequal, 19 Dominant light, 150 Dove's Prism, 66 Dulled tones, 55, 66, 95, 123 Emission of light, 1 Enamels, Indian, 167 Enamels, Translucid, 166 Ether, Luminiferous, 6 Faience, Colours of, 141 Fibrils of retina, 73 Flames, Coloured, 46 Flowers, Colours of, 142 Fluorescence, 41 Foliage, Colour of, 122, 153, 184 Fraunhofer's lines, 17 Fundamental triads, 75 Gaslight, its colour, 154 Glass, Ancient, 169 Glass, Arab, 170 Glass, Colours of, 168 Glass, Painted, 171 Gold, its colour, 27, 160 Gradation, 131, 136 Green, 115, 183 Green, Production of, 82 Grey, 55, 86, 94, 97, 129 Harmonies of analogy, 133 Harmonies of contrast, 133 | Helmholtz's triad, 67 t Hue, 50 Illuminated bodies, 3 ! Illumination affects hue, 147 ; Incandescence, 45 i Indigo, Colour of, 27 i Interference of light, 34 i Interval, the small, 131 192 COLOUR. Irradiation, 107 Irregular reflection, 4 J\panese alloys, 164. Lacquers, 165 Lambert's method, 81 Lapis-lazuli, 9, 54 Law of reflection, 5 Law of sines, 10 Light, Analysis of, 18 Light invisible, 6 Lights, Mixed, 68 Luminosity, 52 Luminosity of paints, 54 Luminous bodies, 1 Marbles, Colours of, 178 Maxwell's colour-discs, 23, 66, 80, 89 Maxwell's triad, 67 Medium affects colour, 145 Metals, Colours of, 159 Minerals, Colours of, 178 Mixture of lights, 87 Mixture of pigments, 87 Mixture on palette, 87 Monochromatic light, 48 Moonlight, 147 Mosaic, 178 Mulready's pictures, 109 Music and colour, 24 Neutrals, Use of, 116, 132 Newton's disc, 22 Ocular fatigue, 103, 110 Opalescence, 38 Orange, 114, 182 Patina, 164 Pencil of lighi, 1 Persistence of image, 107 Phosphorescence, 43 Pigments, 181 Plants, Colours of, 185 Plating, 166 Pleiochroism, 175 Polarisation of light, 37 Porcelain, Colours of, 172 Pottery, Colours of, 172 Precious stones, Colours of, 173 Primary colours, 67 Prismatic spectrum, 18 Prisms, 11, 13 Purity, 51 Purple, 74, 115, 184 Purple, the visual, 76 Pyrites, Colours of, 28 Eainbows, 24 Red, 113, 182 Refraction of light, 10 Refractive index, 11 Refrangibilifcy, 13 Refrangibility, Change of, 49 Ridgway's colour-names, 63 Rood, experiments of, 19, 53, 54, 87, 120, 129, 148 Schistoscope, 65 Sculpture, Colouring of, 180 Secondary colours, 71 Shades, 55 Shadows, 2 Shadows, Colours of, 107, 157 Simultaneous contrast, 98 Soap-bubbles, 36 Spectrum of sim, 16 Steel, Colours, of, 164 Successive contrast, 103 Sunlight compound, 15 SurZace modifies colour, 159 Symbolic colours, 25 Tertiary colours, 85, 95 Tessellse, 178 Texture modifies colour, 159, 163 Thin plates, Colours of, 35 Throbbing colour, 144 Tints, 55 Tints, Mutual influence of, 119 Tone-contrast, 96 Trees, Green of, 122, 153, 184 Translucency, 8 Transmitted light, 29 Transparency, 8 Turacin, 188 Turbid media, 39 Twofold illumination, 156 Undulatory theory, 6 Unity of solar spectrum, 49 Water, its colour, 9 Waves of light, 7 Werner's colour-names, 62 White drapery, 156 Woods, Colours of, 186 Yellow, 114, 182 Yellow and blue, 80 Yellow in spectrum, 16, 20, 73 Yellow light, 155 Yellow, simple and compound, 73 Yellowing of lens, 109 Young's theory, 67 PRINTED BY CASSELt & COMPANY, LIMITED, LA BELLE SATJVAOE, LONDON, E.C, A Selection from Cassell & Company's Publications. 5 G 8 04. 2 A Selection from Casscll & Company^ s Publications. 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With Eight Coloured Plates and other Illustrations by HARRY B. NEIL- SON, is. 6d. ; cloth, 23. Peter Piper's Peepshow. By S. H. HAMER. With Illustrations by H. B. NEILSON and LEWIS BAUMER. is. 6d. ; cloth, 25. Pleasant Work for Busy Fingers. By MAGGIE BROWNE. 23. 6d. Quackles, Junior : Being the Extra- ordinary Adventures of a Duckling. With Four Coloured Plates and other Illustrations by HARRY ROUNTREE. Written by S. H. HAMER. is. 6d. ; cloth, 25. The Foolish Fox, and Other Tales in Prose and Verse. Edited bv S. H. HAMER. With Four Coloured Plates and numerous Illustrations, is. 6d. ; cloth, 23. The Ten Travellers. By S. H. HAMER. With Four Coloured Plates and numerous Illustrations by HARRY B. NEILSON. is. 6d. ; cloth, 2S. The Jungle School ; or, Dr. Jibber- Jabber BurchalTs Academy. By S. H. HAMER. With Illustrations by H. B. NEILSON. is. 6d. ; Cloth, 23. The Old Fairy Tales. With Original Illustrations. Cloth, is. " Tiny Tots " Annual Volume. Bo .rds, is. 4d. Cloth, is. 6d. Topsy Turvy Tales. By S. H. HAMER. With Illustrations by HARRY B. NEILSON. is. 6d. ; Clot'l, 2S. Whys and Other Whys ; or, Curious Creatures and Their Tales. By S. H. HAMER and HARRY B. NEILSON. Paper boards, 23. 6d. Cloth, 33. 6d. A Selection from Cassell & Company's Publications. 15 CASSELL'S SHILLING STORY BOOKS. Interesting Stories. A PAIR OF PRIMROSES. ALL IN A CASTLE FAIR. CLARE LINTON'S FRIEND. DOLLY'S GOLDEN SLIPPERS. FRANK'S LIFE BATTLE. ELLA'S GOLDEN YEAR. HER WILFUL WAY. IN THE DAYS OF KING GEORGE. I SHILLING STORY BOOKS BY EDWARD S. ELLIS. Illustrated. All Illustrated, and containing LITTLE QUEEN MAB. RHODA'S REWARD. THE HEIRESS OF WYVERN COURT. THEIR ROAD TO FORTUNE. THE BRAVEST OF THE BRAVE. To SCHOOL AND AWAY. WON BY GENTLENESS. ASTRAY IN THE FOREST. BEAR CAVERN. RED FEATHER. A Tale of the American Frontier. CAPTURED BY INDIANS. THE BOY HUNTERS OF KENTUCKY. THE DAUGHTER OF THE CHIEFTAIN. WOLF EAR THE INDIAN. CASSELL'S EIGHTEENPENNY STORY BOOKS. Illustrated. 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WANTED A KING ; OR, How MERLE SET THE NURSERY RHYMES TO RIGHTS. BOOKS FOR BOYS AND GIRLS. Fully Illustrated. GULLIVER'S TRAVELS. With up- wards of 100 Illustrations from New Plates. Fine Art Edition, 75. 6d. CASSELL'S ROBINSON CRUSOE. With loo Illustrations. Cloth, 35. 6d. ; gilt edges, 53. THREE AND SIXPENNY STORY BOOKS FOR GIRLS. 8 Coloured Plates in each. By L. T. MEADE. STKANGE ADVENTURES IN DICKY- BIRD LAND. Stories told by Mother Birds to amuse their Chicks, and overheard by R. KEARTON, F.Z.S. With Illustrations from Photographs taken direct from Nature by C. KEARTON. Cloth, 35. 6d. ; cloth gilt, gilt edges, 53. Illustrated. With MERRY GIRLS OF ENGLAND. POLLY: A NEW-FASHIONED GIRL. RED ROSE AND TIGER LILY. THE PALACE BEAUTIFUL. THE REBELLION OF LIL CARR INGTON A' SWEET GIRL GRADUATE. A WORLD OF GIRLS: THE STORY OF A SCHOOL. BASHFUL FIFTEEN. BEYOND THE BLUE MOUNTAINS. A MADCAP. By L. T. Meade. With 8 Illustrations. 35. 6d. BOUND BY A SPELL. By the Hon. Mrs. Greene. 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CASSELL & COMPANY'S COMPLETE CATALOGUE WILL BE SENT POST FREE ON APPLICATION TO CASSELL & COMPANY, LIMITED, La. -Belte Sauva?e t L-ud ? ate Hill, Lo ^ & g A ({ y OFTHC CHUMS. The Paper lor Boys. Weekly, id.; Monthly, 6d. TINY TOTS. For the Very Little Ones. Monthly, id. WORK. Weekly, id. ; Monthly, 6d. BUILDING WORLD. Weekly, id.; Monthly, 6d. THE GARDENER. Weekly, id. 14 DAY USE RETURN TO DESK FROM WHICH BORROWED LOAN DEPT. RENEWALS ONLY TEL. NO. 642-3405 This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. rep o \ 1PT1 ruD & * ' ftFP'n in IT CD o 4 T er Oin rx T. Ktou LD rto21 MUN 7 1! 71 -5PM 22 REG. Gift, MAY 1 5 1879 LD2lA-60m-3,'70 (N5382slO)476-A-32 General Library University of California Berkeley i J y.,,i.gE5!