Manual of the science of colour, on the true theory of the colour-sensations, and the natural system : with a coloured frontispiece and other illustrations

Introduction

What the Science of Colour includes ; its uses and advantages. Common misconceptions, their causes and consequences. Proofs and authorre - : é , ities. The nature of light, and the action of the eye.

Chapter I. The Principal Colours, And How To See Them

1, Colour-sensations, simple andcompound. 2. Hues, tints, and shades of colour. 3, Excitement of colour-sensations. 4. Cause of variety of hue. 4. Separation of mixed lights. 6. What light gives a complete spectrum. 7. How to obtain the spectrum (figs. 1 and 2). 8. The prismatic colours. 9. What are the primary colours, and how to see them. 10. What are the secondary colours, and how to see them. 11. How to see the primaries and secondaries in contrast (fig. 3). 12. How to see some of the best combinations of prismatic colours in contrast (fig. 4). 13. How to see at once all the best combinations of prismatic colours (fig. 5). 14. Value of such observations. 15. The best standard colours.

Chapter II. The Colours Of Objects Self-luminous Or Illuminated

16. Colours of incandescent solids. 17. Colours of incandescent vapours. 18, Effects of different kinds of illumination. 19, Causes of colours of non-luminous bodies, 20. Colours from transmission of light. 21. Effect of increasing the thickness of a translucent body. 22. Effect of superposing translucent bodies. 28. Colours frominternal reflection. 24. Effect of mixing pigments. 25. Effect of thickening a pigment. 26. Inferiority of colours from internal reflection. 27. Effect of form and texture of surface. 28. Colours from superficial reflection. 29. Colours from metallic superficial reflection, 30. Effect of repeating reflections. 31. Colours from reflection from small particles. 32. Colours from interference. 83. Colours from fluorescence. 34. How to test the colours of objects.

Chapter III. Intermediate Colours, And Means Between Two Or More Colours

35. How to find intermediate colours. 386. Howto find the mean of two colours (fig. 6). 37. How to see whether colours are complementary. 38. How to find the mean of several colours. 39, How to compare means of different sets of colours. 40. Truth of results, 41. Means between principal colours, 42. Shades of tints. 43. How to ascertain the real hues of obscure colours.

Chapter IV. The Natural System Of Colours And Its Use

44. Construction of the colour-cube (fig. 7 and frontispiece). 45. Rectilineal gradations. 46. Plane gradations.’ 47. Complementary plane gradations. 48. Cognate plane gradations. 49. Complementary and cognate rectilineal gradations. 50. Limits of colours in depth and clearness. 91. Utility of the natural system, iv

Chapter V. Ocular Modifications Of Colours; The Sensibility Of The Eye

52. Variable sensibility of the eye 53. Law of variation. 64. Simultaneous contrast. 55. Effects of simultaneous contrast. 56. Accommodation of the eye to light. 57. Sensibility of the eye under a given illumination. 58. Number of distinguishable colours. 59. Effect of very feeble light. 60. Effect of dazzling light.

Chapter VI. The Harmony Of Colour

61. Harmony the source of beauty. 62. Principles of harmony. 63. Rule I. Each colour-sensation should be excited. 64. Rule II. Each colour-sensation should be relieved. 65. Rule III. Kindred colours associate well. 66. Rule IV. Light colours with dark associate well. 67. Rule V. Gradations should be true and regular. 68. Rule VI. Ends of gradations should be equally striking. 69. Rule VII, Contrasting colours should be equally striking. 70. ¢we VIII. Gradations contrasts and single colours should correspond. 71. Rule IX. Colours and groups of colours should occupy appropriate breadths. 72. Rule X. Variety of colour and treatment may increase beauty. 78. Conditions restrictive of rules. 74. Necessary compromise of principles,

Chapter VII. Mental Effects Of Colours And Colour-compositions

75. Impressiveness of colour. 76. Liveliness of colour, 77. Warmth of colour. 78. Effects of colours in colour-compositions. 79. Effects of modes of composition. 80. Imaginary mental effects.

Chapter VIII. Peculiar And Defective Colour-vision

81. Dichromism. 82. Defective sensibility toredlight. 83, Dichromism with defective sensibility. 84. Effect of colour-designs in such cases,

ILLUSTRATIONS.

Two coloured diagrams of the colour-cube, in explanation of the natural system of colours . : " Fr ontispiece. Two diagrams, showing how to obtain the prismatic spectrum ps6;

Three diagrams, for exhibiting the prismatic colours and

their regular compounds when viewed through a prism, facing p. 10. A diagram, showing how to find intermediate colours by a

slip of glass 5 ° ° 4 ° - p. 22.

A diagram of the cube of colours . , : ° : : p. 28,

INTRODUCTION.

Tue Science of Colour teaches the nature and causes of colours; their distinctions; their relations to each other ; their classification; the mental effects that attend them, and the causes and laws of their harmony. It includes the modifications of colour arising from the varying sensibility of the eye, and also the peculiarities of colour-vision which are found to exist in different persons. It enables the eye to distinguish and recognize the different hues in their various tints and shades, and to acquire a correct judgment about the real constituents of colours, upon which their effect in any composition depends; it improves and directs the natural taste, and aids the student in devising as well as appreciating good compositions of colour.

The whole subject is full of interest and beauty, and an acquaintance with it adds extremely to the pleasure to be derived from contemplating natural objects and scenery, as well as pictures and other works of art; and it has one advantage, perhaps, above any other branch of natural philosophy ; it teaches us in a very remarkable way to distinguish between sensations and their causes, and not to judge hastily according to the appearance of things. And as the chief difficulty of the science seems to arise from this, it is proper in the Introduction to call the student’s attention especially to it, and to guard against those misconceptions which might otherwise hinder his progress.

First, then, the science teaches us that colours are purel and simply sensations, excited by the action of light on the nervous tissue of the retina which forms a screen at the back of the eye; and a little observation and reflection is enough to convince us that they must be so, since they are only seen by the aid of light, and vary with different kinds of light. But when we look at an object, we see so clearly the colour on its surface, at a distance from us, and that colour under the vi ordinary light of day is so constantly the same, that we can hardly imagine it anything but some inherent quality of the surface, belonging to the object, just as much as the form which we see. This is therefore the notion which unreflecting persons usually have about colours, insomuch that it is not always easy to make them understand the difference between colours (properly so called) and the pigments or colouring matters which go by the same names.

But suppose one has advanced beyond this primitive stage, and knows that colours are caused by the light that enters the eye; and suppose one has learnt also that there are innumerable different kinds of light, each distinguished by some difference of colour, and that a difference of colour in two objects certainly indicates some difference in the kinds of light which they send to the eye; what then is more natural than to infer that colour is an indication of some distinctive quality residing in the light? Even eminent philosophers have hastily assumed it to be so; and some have supposed that there must be as many different species of light as there are distinct elementary sensations of colour, which is inconsistent with what is known of the nature of light. And it is still more common for those who have not studied the science of colour, to think that the colours of simple or homogeneous light (light that is all of one kind) are themselves simple or homogeneous. Yet our science tells us that all the colours of homogeneous lights may be more or less compound sensations, and most of them certainly are so; and that mixtures of different kinds of light may have the same colours. For the colours of even certain kinds of homogeneous light may be imitated by mixtures of other kinds; and again the very same kinds of light may have different colours, not only in different eyes, but also under different circumstances in the same eye.

Hence we must adhere, contrary to all natural prejudices, to the doctrine that colours are sensations, simple or compound ; and it must be clearly understood that when their names are used as adjectives to describe any light, as red light, yellow light, white light, they mean only that the light, whatever it is, has the property of exciting in our eyes the sensation of the colour red, yellow, white, or whatever it may be. In like manner, when the same terms are applied to pigments or other material objects, as red paint, yellow solution, white paper, they must be understood to mean that the lights which the objects

Vil send to the eye, under the ordinary illumination of daylight, produce those same sensations.

Again, our science teaches that some colours, such as whit» and yellow, are compound sensations. A perfect white, for example, it tells us, is a combination or mixture of three distinct sensations, namely, red, green, and blue, excited at once in equal force on the same part of the retina. A perfect yellow, it tells us again, is a combination or mixture of two of these, red and green, in equal force. Yet, when we contemplate a white object, we perceive nothing whatever to call up the idea of red, or of green, or of blue, or of any other colour. It seems perfectly homogeneous, simple and pure. And when we look at a yellow surface, it is by no means easy (though perhaps not quite impossible under certain circumstances) to imagine we see a combination or mixture of red and green. Nevertheless, a careful investigation of the matter shows that these really are compound sensations as asserted. For we can compound white light by throwing together or mixing red, green, and blue lights, and yellow light by throwing together or mixing red and green lights.* We can also show that the light given out by the best white and yellow objects contains a mixture of the same componentsf. And we can produce the sensations of white and yellow by mixing the sensations of red, green, and blue together, and those of red and green together, without mixing the lights.}

Hence, strange as it seems, it must be admitted that sensations apparently simple may really be compound; and their components such as cannot be recognized in them.

T'o mention another instance, it is well known that if we mix together two coloured solutions, or two transparent pigments, or superpose two coloured glasses, against a white surface, we produce a third colour. What more natural than to suppose that this new colour is a combination or mixture of the colours of the two solutions, pigments, or glasses separately? Yet, our science says this is only approximately true in certain

* By mixing opaque coloured powders; by reflection from glass (as in Sect. 86); and by actually throwing together beams of light that have passed through coloured glasses.

+ By analysis with the prism (Sect. 34); by analysis with coloured glasses (Sect. 22),

{ This is evidently effected when colours are blended by the method of rotation (Sect. 38); or by blending two separate sensations excited in the two eyes, as when we see double images of two different coloured spots, one of each colour overlapping, eS

' f it y 7 {

Viil cases; in most cases the third colour must differ widely from the colour that would result from the mixture of the two colours. Thus, green has been commonly supposed to be a mixture of yellow and blue, because it is easily produced by a mixture of yellow and blue pigments; yet itis easy to show that the true mixture of those two colours is not green but gray; and it is easy also to explain why green is produced by the mixture of such pigments.

It is equally natural to suppose that by making a coloured solution stronger or thicker, or by laying on a thicl ker wash of a pigment, we increase the strength or intensity of the colour, without other alteration. It has even been proposed to use a combination of hollow glass wedges, filled with coloured solutions, as a means of measuring Take intensity of a colour according to the thicknesses of the solutions which give a colour to match it; and experiments made in this way are the basis of what is commonly called Field’s doctrine of chromatic equivalents. Yet the truth is easily shown that to thicken a solution or transparent pigment materially alters the hue as well as darkens its colour; and that it is absurd in any case to make the thickness of a solution the measure of the strength of its colour.*

Once more, as we notice the brilliant whiteness of the sun, and of other intensely luminous bodies, and also the whiteness of many illuminated objects, as clouds, snow, linen, paper, chalk, far brighter than other colours, and the white superficial reflection that takes place from even dark-coloured objects, quite independently of their proper colours; and as we find, too, that a white illumination is necessary to bring out all the various hues of objects, we easily connect the ideas of whiteness and of light, and infer that other colours are something essentially different from white, and perhaps think, as some have done, that they rather partake of the nature of darkness.

Other phenomena added to these, such as the blueness of the light reflected from very small reflecting particles in a transparent medium (which is only noticed when viewed against a dark background), and the prevailing redness of the

* In truth colouring matters act by destroying some kinds of light faster than others; so that the lights that escape from thick washes of a transparent pigment cannot be the same in quantity or quality; and the light that escapes from two pigments is no more a mixture of the lights that escape from both of them separately, than the grain which falls through both of two different sieves is a mixture of that which falls through each one of them alone (Sections 20-26), light which is transmitted through most bodies imperfectly transparent, have led, not only ancient philosophers, but also ingenious moderns, like Goethe, to suppose that bluish colours are constituted by darkness seen through a white or light medium, and reddish colours by white or light seen through a dark medium.

But our science utterly contradicts all such notions, constraining us to believe that white only differs from other colours in containing all the simple colour-sensations in equal strength. It makes no difficulty in explaining the remarkable phenomena alluded to above; showing us on the one hand that bodies just heated enough to be luminous give out red light, and when more heated give out other kinds also, and all more and more, so that their light becomes not only brighter, but also whiter; and on the other hand, that bodies which reflect all kinds of light equally, often reflect all very powerfully, so that white objects are commonly bright; whilst the blueness of the sky, and the redness of the sun through a fog, and thousands of other effects, are necessary consequences of the various ways in which matter acts upon light.

Whenever, therefore, the term “light” is used (as it often is) to express a sensation, it must be understood as a general term including each and all of the colour-sensations, and not as denoting any other sensation excited by the action of light, for there is none beside red, green, blue, and their compounds.

The importance of understanding these points is great; since whole theories and systems of colour have been founded upon erroneous conclusions hastily drawn from natural phenomena, correctly observed, but not understood; and it is not easy to deduce reliable and useful rules in colour-designing from such defective principles. Besides this, a most beautiful branch of natural philosophy has been neglected and discredited in consequence of them. Through the rashness of several esteemed writers in advancing crude speculations about colour, and through ignorance of what has been really discovered, many are disinclined to entertain the subject at all. They either think that nothing certain is known about it, or that all theories will prove equally useless in art; or else are wedded to preconceived opinions which they are unwilling to disturb.

Hence a manual which aims at freeing the subject from previous errors and imperfections, should call special attention to the proofs of its doctrines (so far as they are not universally x accepted), and the authorities that support them, as well as the advantages which will attend their adoption.

Now it may be safely asserted that by a little intelligent study of the facts revealed in those simple and beautiful experiments which are described in this treatise,* any person who has the use of his eyes, and believes the grand propositions about light which Newton established “when he first unfolded to the world the meaning of the prismatic spectrum, may very quickly satisfy himself of the truth of the doctrine. And it may be safely asserted, too, that when the matter is once understood, no one who takes interest in the harmony of colour will feel any wish to revert from light to darkness, or regret the few hours of interesting study which enabled him to master a charming science, hitherto generally neglected by those who should profit most by it.

But accurate researches for determining the colour-sensations excited by the prismatic rays are not wanting; and they fully establish the doctrine. Those recently made in this country by Mr. James Clerk Maxwell have just now been confirmed by others made on another plan by Herr J. J. Miiller in Germany.t Experiments by Sir John F. W. Herschel, and by Professor Helmholtz, might also be cited in confirmation. For these investigations have distinctly proved that both white and all possible colours can be produced by mixtures of the best prismatic red, green, and blue, and by mixtures of no other three colours but these; quite contrary to the common red-yellow-blue theory. But that this common theory is utterly wrong is obvious from the simplest experiments, which prove that yellow consists, as before mentioned, of the sensations of red and green excited together, and is exactly complementary to blue. So that if red and blue are simple colours, yellow must be compound.{

* See especially Sections 9-13, 34, 35, 36, 38, 39.

t The true doctrine of the three colour-sensations was clearly stated at the beginning of this century by that great philosopher Thomas Young (Natural Philosophy, Lecture xxxvii.), and has lately been explained in a popular manner by Sir John F. W. Herschel (Essay on Light in ‘‘Good Words” for 1865, which has also been reprinted in his Familiar Lectures), as well as by Mr. Maxwell (Lecture at the Royal Institution, 17 May, 1861). Notwithstanding the seeming difficulty of reconciling it with first appearances, the neglect it has hitherto met from artists is strange, so great and so evident is its superiority, for all the purposes of art, over the conventional system that has been substituted for it.

In several letters to the ‘‘ Builder” (in the latter part of 1868), and in a paper read at the Institute of Architects (Sess. Paper No. 7, 1869), the author himself has endeayoured to expound the proofs and uses of the science of colour. Since then, he xl

Another advantage of learning the true nature and relations of colours will be found in the improvement to be derived from the intelligent study of the rich compositions of colour presented in natural scenery and natural objects, and also in works of art. This will greatly increase the value of those illustrated works on colour-decoration and ornament which have been so abundantly produced of late years. For those who would study these to profit, must not only feel that some combinations of colour are preferable to others; they must also know the reasons why; otherwise they can but make empirical attempts to improve. Now a false theory mystifies the matter, and even if sound rules are hit upon, often hinders the right application of them; but true knowledge will ever improve the judgment, and direct the hand; and if “a thing of beauty is a joy for ever,” it is doubly so when the understanding co-operates with the feeling.

The nature of light itself is perhaps not strictly a part of the science of colour; but it adds much to the interest of the subject to know that the wonderful medium through which we have communication with the utmost parts of the universe, the all-pervading and perfectly elastic ether, receives and transmits in waves of vibration, with inconceivable velocity in every direction, all sorts of shocks, however minute, arising amongst atoms of matter, just as air does with those arising from the tap of a hammer or the ring of a bell; and that these etherial waves, when they are of the kind called waves of transverse vibration, and their time or period falls within certain limits, constitute light ;—the different kinds of light differing essentially in their wave-times alone.

has met with a treatise on colour, prepared at the suggestion of the Directors of the imperial Austrian Museum of Art and Manufacture, by Ernest Briicke, Professor of Physiology in the University of Vienna, and which has been translated into French (Des couleurs au point de vue physique, physiologique, artistique et industriel, par le Dr. Ernest Briicke, Traduit d’allemand par J. Schutzenberger, Paris, 1866). The progress of the science of colour on the continent appears from the fact that in this work the whole subject is treated in a scientific manner, and what is wrong in the common theories is rejected, though no complete system of colour is proposed, Field’s doctrine of chromatic equivalents, which has long held too much authority in England, Professor Briicke declares “‘is false from beginning to end, and owes its origin to an incorrect interpretation of natural phenomena.” Sir J, F. W. Herschel, after his life-long labours for science, may be allowed to speak with authority here. Inaletter to the author, he tersely describes the prevailing opinions about light and colours as ‘fidola, which, from a jargon, have fixed themselves into a doctrine.” It may be long before that doctrine loses its hold on the public; in lectures, and even in new treatises on Natural Philosophy, its truth is still sometimes assumed by persons who ought to know better; but at least those who direct the education of students in art should take care not to fall behind the age, or allow errors repudiated in Germany and France to remain unchallenged in England, xu

Reflection and refraction, with all their complicated and beautiful phenomena, are but natural consequences of such waves meeting with different media—portions of space in which the etherial density and elasticity are modified, so that the waves traverse them with altered velocity. The equally marvellous phenomena of interference, which led to the discovery of the nature of light, and the actual measurement of the lengths of the luminous waves, arise from the action of one wave upon another, when two either concur to increase the disturbance, or oppose each other to quiet it.

How the minute wave-lengths, and still more inconceivably short wave-times, of the different kinds of light have been measured, and what the results of the measurements are, must be left to works on optics to recount. It is enough to note here that their periods are all so short that there is time, even in the fraction of a second during which the sensation excited by a luminous wave continues, for a succession of innumerable separate waves of any kinds of light to follow each other along the same line or ray; so that we may have lights of all kinds mingled together in any proportions, and making any conceivable mixture of their separate colours at the same moment on the same part of the retina.

The student should of course be so far acquainted with the action of the eye, as to know that the light which proceeds from the various objects in the field of view is caused by it to fall regularly upon corresponding parts of the retina, So as to form upon it a picture of the whole scene, like that produced in a camera obscura; and that when the direction of the eye is changed, this picture shifts its position, so that the part of the sensitive screen which was before affected by the light from one object, is now affected by the light from another, or perhaps by none at all. It should be noticed, too, that the sensations excited on corresponding parts of the retinas of the two eyes become blended together by the union of the two branches of the optic nerve.

MANUAL

OF

THE SCIENCE OF COLOUR.

CHAPTER IL

THE PRINCIPAL COLOURS, AND HOW TO SEE THEM.

1. Colour-sensations, simple and compound. The eye is so constituted in the great majority of persons as to be capable of three simple or elementary sensations of colour, which are best described by the terms red, green, and blue.*

The simple sensations are never excited separately, with perfect purity, on any part of the retina, but always accompany each other in greater or less degree, causing the endless variety of colour which we observe.

When all are excited at once with equal intensity on the same part of the retina, the result is the sensation of white.

When one of them is excited in less degree than the others, the two predominant sensations form a compound or mixed sensation, accompanied with a mixture of white proportionate to the degree in which the third is excited. These binary compounds are—

(1) Seagreen, which consists of green and blue in equal strength, and passes through seagreen-green into green, when the blue is weakened, and through seagreen-blue or azure into blue, when the red is weakened.

* As these and all other names of colours are commonly applied to many different hues, it should be observed that the particular hues here intended are those of the deepest prismatic red, green, and blue, as will be explained further on. Cases of peculiar and of defective colour-vision are noticed at the close of this treatise.

B

(2) Pink, which consists of blue and red in equal strength, and passes through pink-blue or puce into blue, when the red is weakened, and through pink-red or cerise into red, when the blue is weakened.

(3) Yellow, which consists of red and green in equal strength, and passes through yellow-red or orange into red, when the green is weakened, and through yellow-green into green, when the red is weakened.

When two of the simple sensations are equally excited in less degree than the third, we have of course the predominant sensation, together with a mixture of white proportionate to the degree in which the other two are excited.

The simpie sensations are cailed primary colours, or primaries ; and their binary compounds, secondary colours, or secondaries.

White, the peerless ternary sensation, though in common language sometimes distinguished from colours, is in truth the chief of all. Black, on the other hand, though no positive colour at all, must yet for convenience sake be included amongst them, as the lowest limit of all colours.

2. Hues, tints, and shades of colour. The absolute excess or predominance of some one or two of the simple sensations over the rest, as above described, is called the hue of a colour; and the greater such excess, the stronger the hue.

The whole series of hues is best remembered by conceiving the six gradations from red to yellow, from yellow to green, from green to seagreen, from seagreen to blue, from blue to pink, and from pink back again to red, arranged in order and in equal strength on the sides of a hexagon, as indicated in the outer row of coloured spots in each of the diagrams on the frontispiece. A similar arrangement is frequently described under the name of the chromatic circle; but the hexagon is the preferable form, because it distinguishes the six points from which new gradations start. White and black, being perfectly neutral, or without any hue, form no part of any chromatic circle.

9 wv

If the deficiency of each sensation in a colour is diminished in proportion, until all the three are brought to the full and equal intensity which they possess in white, the colour changes by a regular gradation into white, without any variation of hue. The lighter colours thus produced are often called tints of their respective hues.

‘The brightness of a colour is the sum of the intensities of all the simple sensations of which it consists. The proportionate diminution of these from the most powerful that the eye is capable of down to the absence of all sensation, produce all the shades of colours, from the brightest down to black.

3. E. acitement of colour-sensations. Colour-sensations may be slightly and promiscuously excited by various means, as by pressure or an electric shock, and even without any external application, as seen in the undefined images that seem to float before the eye in the dark. But the action of light from without is incomparably the most powerful and effective means of producing them ; and it is for the admission of light in proper quantities, and its regular distribution on the surface of the retina, according to the direction from which it comes, that the transparent parts of the eye are designed.

4. Cause of variety of hue. Each of the innumerable different kinds of light, in each degree of intensity, produces a peculiar mixture of the three simple sensations of colour, one or two always predominating more or less over the other.