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PRINCIPLES 
 
 OF 
 
 THE SCIENCE OF COLOUR 
 
 CONCISELY STATED 
 
 TO AID AND PROMOTE THEIR USEFUL APPLICATION 
 
 IN THE DECORATIVE ARTS. 
 
 By WILLIAM BENSON, Abch t 
 
 LONDON : 
 CHAPMAN & HALL, 198, PICCADILLY. 
 
 1868. 
 
 [the right of translation is reserved]. 
 
 
 Ks 
 
 
To G. G. STOKES, M.A., D.C.L., 
 
 SECRETARY TO THE ROYAL SOCIETY, AND LUCASIAN PROFESSOR 
 OF MATHEMATICS IN THE UNIVERSITY OF CAMBRIDGE, 
 
 THIS LITTLE TREATISE 
 
 i 
 
 IS BY HIS KIND PERMISSION INSCRIBED 
 
 IN ACKNOWLEDGEMENT OF 
 
 SS 
 
 HIS MANY VALUABLE CONTRIBUTIONS TO SCIENCE, 
 
 AND OF 
 
 ASSISTANCE OBLIGINGLY RENDERED 
 
 IN THE AUTHORS INQUIRIES. 
 
 £££££ 
 
PREFACE. 
 
 In the following pages I have noticed those points only which seem essential to the 
 theory and practice of colour, including some which have not hitherto received the 
 attention they deserve. In recommending the use of the prism, an instrument 
 indispensable to those who would learn to judge of colours correctly, I have pointed 
 out how, by a simple experiment with it, we may produce at will all possible hues, 
 in all their tints and shades, with the nearest possible approach to perfection, and 
 present them at once to the eye ; and how, by another, we may see any of these colours 
 in juxtaposition with its perfect complementary, and may also learn to compare the 
 luminosity or brightness of colours differing in hue. 
 
 The beautiful method of finding colours intermediate between any two given 
 colours, by the aid of a slip of polished glass, used by Lambert in his Photometria 
 (Berlin, 1760), but strangely neglected since, is then stated; an easy way to discover 
 the true mean is pointed out; and the difference between the results thus obtained, 
 and the effects of the mixture of pigments, explained. 
 
 What has now for some years, I believe, been admitted by scientific persons as 
 the true doctrine concerning the simple or elementary sensations of colour, is next 
 laid down, instead of the very incorrect theory still generally repeated in popular 
 and elementary treatises. The diagram which I have given of the principal colours, 
 notwithstanding the imperfection in hue and inequality in strength of the pigments 
 used, will I hope speak for itself in commending this doctrine to the eye, and help 
 to overcome the natural bias of many in favour of what they have hitherto assumed 
 to be unquestionably true, though unsupported by a single rational experiment. The 
 necessary double brightness of the secondary colours, when of full strength, on which 
 so much of their beauty depends, and which is neglected in schemes founded on the 
 mixture of pigments, is here particularly noticed ; and the importance of bearing this 
 in mind has induced me to use the term Pink, instead of Purple, for the secondary 
 containing Red and Blue ; especially as the colour commonly understood by the latter 
 contains a great preponderance of Blue, besides being much too dark. 
 
VI. 
 
 PREFACE. 
 
 Observations are added on the qualities of colours, especially in defining and 
 distinguishing what I have called their depth and clearness ; on the reasons why some 
 may excel in one of these qualities, and others in the other; and on the extent of this 
 difference. These may lead to rules concerning the proportions in which colours may 
 neutralize each other, more reliable than those which Field so hastily laid down from 
 his experiments on the superposition of coloured glasses or solutions, the results of 
 which he totally misunderstood, since the thicknesses of the coloured substances 
 upon which he experimented indicated anything rather than the quantities of the 
 respective colours in the transmitted light. 
 
 A complete but very simple system of colours is next proposed, on the true 
 theory of the three elementary colour-sensations, and on the only principle on which 
 all possible combinations of those sensations can be correctly arranged according to 
 their natural relations (a principle which I have since found was suggested more than 
 forty years ago by Sir John Herschel, in his Treatise on Light in the Encyclopoedia 
 Metropolitana). This will, be useful in classifying and grouping different kinds of 
 gradations and contrasts, and in helping the artist to devise endless varieties of 
 beautiful arrangements of colours. 
 
 In the remainder of the work the ocular modifications of colours, their mental 
 effects, and the doctrine of the harmony of colours, are very briefly handled; but on 
 each of these will be found, I think, some new considerations, tending to enlarge, 
 correct, or simplify what has been advanced in previous works on this subject. 
 
 Should this treatise, notwithstanding the many defects I am sensible of, both in 
 itself and in the coloured illustrations which accompany it, prove acceptable to a 
 discerning public, I hope to publish a larger work, already prepared, on the same subject. 
 The plan and contents of that work are appended, and, to those who have not studied 
 this branch of science, will give some idea of what has been done to elucidate it, both 
 in other countries, and in our own. 
 
 147, Albany Steeet, Regent's Park, London, N.W. 
 29th May, 1868. 
 
Vll. 
 
 ADVERTISEMENT TO THE ISSUE OF 1872, 
 
 Since this treatise was published the truth of the theory of the three colour- 
 sensations has been confirmed by further careful observations on the colours of the 
 spectrum made in a new way by J. J. Muller in Germany, which leave no reason to 
 question the conclusions previously arrived at. The nature also of the common 
 peculiarity of colour-vision, called dichromism, has been more clearly elucidated, and 
 the subject much simplified, by Professor J. Clerk Maxwell, who in a lecture at the 
 Boyal Institution (24 March, 1871) showed cause to believe that it is nothing more 
 than a total incapacity for the sensation of red ; so that dichromists may be regarded 
 as seeing all colours just as others would see them, subtracting the red element in 
 each. And when one has learnt correctly the composition of the different colours, 
 that is in what proportions the red, green, and blue sensations are combined in them, 
 one can easily tell what colour any given object appears to such persons, and why the 
 colours of objects to others so different are alike or the same to them. The observa- 
 tions on Dichromism in Chapter XII. therefore require correction in this respect, 
 and so do those made on the same subject in Chapter VIII. of my Manual of the 
 Science of Colour (1871). 
 
 Several persons have taken exception to the use of the term Blue to denote the 
 third colour-sensation. They understand by that term a colour which contains a large 
 admixture of green, and think that Violet would be a preferable term ; but though it 
 is true that Blue in the English language is frequently applied to colours which ap- 
 proach to seagreen, yet it is certainly not incorrect to apply it to the colours of the 
 whole of the more refrangible part of the spectrum to the very end. The term Sea- 
 green has also been objected to, but I have not heard a better term suggested for the 
 colour for which I have used it. I would here mention that in several cases other 
 pigments than those mentioned on page 31 have been used in colouring the illlustra- 
 tions. Ultramarine, or " French Blue," with or without Zinc white, is better for both 
 full blue and light blue than Cobalt, its hue being more distant from green. " Perma- 
 nent Yellow," toned with a little Chrome, makes a yellow as good as that of 
 " King's Yellow," and not liable to change ; a mixture of Scarlet Vermilion with Pose 
 Madder is used for pink-red ; and a mixture of the same with Indian Lake for dark 
 red. Seagreen and light seagreen are represented by mixtures of "Emerald Green" 
 with Cobalt and " Ceruleum "; dark seagreen by a mixture of the same with " Cyanine 
 Blue." 
 
 "West Street, Hertford, 
 2nd Sept., 1872. 
 
EEEATA. 
 
 In page 28, in the middle of the central column, for Light yellow read Light green. 
 
 In page 29, near the foot of first column, for Light red read Light pink. 
 
 In page 30, in the middle of the first column, for Light green read Yellow green. 
 
CONTENTS. 
 
 CHAP. I. 
 
 CHAP. II. 
 
 CHAI\ III. 
 
 CHAP. IV. 
 
 CHAP. V. 
 
 CHAP. VI. 
 
 CHAP. VII. 
 
 CHAP. VIII. 
 
 CHAP. IX. 
 
 CHAP. X. 
 
 CHAP. XL 
 
 CHAP. XII. 
 
 PAGE. 
 
 The nature and cause of colours 1 
 
 The prismatic colours and their continuous combinations 3 
 
 Complementary colours 7 
 
 Intermediate colours 10 
 
 The primary and secondary colours 18 
 
 Qualities of colours 15 
 
 The natural system of colours 18 
 
 Gradations and contrasts of colours 21 
 
 Ocular modifications of colours 33 
 
 Mental effects of colours , 36 
 
 The harmony of colours 38 
 
 Appendix on dichromism and defective colour-vision 42 
 
 LIST OF ILLUSTRATIONS. 
 
 Scheme to show the colours of all possible continuous combinations of the prismatic rays 6 
 
 Schemes to sIioav complementary colours in juxtaposition 8 
 
 Scheme to show the true means between several pairs of colours 11 
 
 Diagram of the eight principal colours 14 
 
 Plate I. The cube of colours to face pa»-e 20 
 
 Plate II. Four series of sections of the cube „ 26 
 
 Coloured Plate representing sections at right angles with the pi-imary axes of the cube ,, ,, 27 
 
 Coloured Plate representing sections at right angles with the secondary axes ,, „ 28 
 
 Coloured Plate representing sections at right angles with the secondary cross axes „ „ 29 
 
 Coloured Plate representing sections at right angles with the tertiary cross axes „ „ 30 
 
 Coloured Plate representing sections at right angles with the tertiary axis of White ... „ „ 31 
 Plate III. \ 
 
 Plate IV. / Modes of combining portions of the principal sections of the cube according to 
 
 Plate V. ( different laws to follow page 32 
 
 Plate VI. / 
 
 [The binder will distinguish the five coloured Plates by the descriptions of their colours on the page 
 which they are to face. The top of each is that edge nearest to which the white circles lie]. 
 

 "Ecrrt Ss rd ■ypaj^a.ra ravra, arfsp pova cry^hv 6u Svvzvrai itoieiv 6t ypa$£i$. *Evia yap ocvtoi nspav- 
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 ^pcop^ara 
 
 " The colours of the rainbow are those which, almost alone, painters cannot make. For they compound some colours; but 
 Scarlet and Green and Violet are not produced by mixture, and these are the colours of the rainbow. 
 
 Akistotle, Meteor ologica, iii., 2. 
 
PRINCIPLES 
 
 OF THE 
 
 SCIENCE OF COLOUR. 
 
 CHAPTER I. 
 
 THE NATUEE AND CAUSE OF COLOURS. 
 
 Colours are merely sensations produced by the action of light on the nervous 
 tissue of the retina, which covers the back of the eye. 
 
 The ordinary white light of the sun contains an infinite variety of different 
 kinds of homogeneous light, differing essentially from each other in nothing but the 
 period of the etherial undulations of which all light consists; and each of these 
 different kinds of light, when it enters the eye so as to fall alone on any part of the 
 retina, excites there a peculiar sensation of colour. 
 
 When several of these different kinds of light fall on the retina together, they 
 produce a sensation which is made up of all those sensations which they would 
 produce separately, if they entered the eye after separate intervals of time, or fell on 
 different parts of the retina; in other words, their colour is the sum of the colours of 
 all the component lights. 
 
 When the intensity of the light is increased or diminished, without altering its 
 nature, or the proportions of its components, the brightness of its colour increases or 
 diminishes. Hence arise all the shades of colours, from the utmost brightness, 
 down to black, the absence of all light. 
 
 The variety of colours in natural objects, illuminated by the same white light of 
 the sun, arises from peculiarities in the constitutions of those objects, causing them to 
 transmit or reflect to the eye some kinds of light in larger proportions than others. 
 
 B 
 
& THE NATURE AND CAUSE OF COLOURS. 
 
 In the vast majority of instances, whatever light is reflected from their surfaces 
 consists of all the ingredients of the incident light in equal proportion, and is 
 therefore of the same colour as the incident light. But, in general, a large part of 
 the incident light enters the body, and of this part some of the ingredients are often 
 more powerfully absorbed, and therefore sooner extinguished than the rest; so that 
 if, after traversing a certain thickness, any of it emerges and reaches the eye, it is no 
 longer of the colour of the incident light. 
 
 In this way not only transparent and pellucid substances, seen by light which 
 has passed through them, but also pigments and most other bodies seen by reflected 
 light, derive their proper colours; for, disregarding the light reflected from the surface, 
 which varies in quantity with the angle of incidence, the colour depends on that 
 reflected from some depth below the surface, so that it has traversed twice a small 
 thickness of the absorbing substance, where part of the original light has been lost. 
 All colours caused by absorption vary in general with the thickness of the substances 
 traversed, in consequence of the unequal rates in which the differently colored rays 
 are absorbed. If all the light that enters is destroyed, the body is of course Black 
 excepting the superficial reflection. If the incident rays of all sorts are absorbed in 
 equal proportion, large or small, the body is Gray or White. 
 
 Superficial reflection is usually small, except at very oblique incidences ; but in 
 some instances, as in silver and other metals, it is so intense that the body is lustrous 
 from that cause alone. There are also bodies which reflect superficially some kinds 
 of light more copiously than others, as gold and copper, and some few non-metallic 
 substances, as compressed Indigo, which have in consequence a sort of metallic 
 appearance. 
 
 The varying colours of finely-striated surfaces, as in mother-of-pearl, and those 
 of thin laminae, as in peacocks' feathers and butterflies' wings, are produced by the 
 mutual interference of luminous undulations, whereby the different kinds of light are 
 alternately strengthened and destroyed in different positions, according to their 
 wave-lengths. 
 
CHAPTER II. 
 
 THE PRISMATIC COLOURS AND THEIR CONTINUOUS COMBINATIONS. 
 
 To see at one view the hues which distinguish the different kinds of homogeneous light, 
 the best way is to cause a beam of the light emitted by the sun or by any other bright 
 luminous object which emits light of all possible wave-lengths, to pass through two 
 inclined faces of a prism of some powerfully refracting medium, such as flint glass, 
 and then to reflect it from a white screen to the eye, or to receive it directly in the 
 eye, all other light being excluded. For the less the wave-length the more is the 
 light, on emerging from the prism, bent out of its original direction ; and thus each 
 different kind occupies a different place on the screen, and enters the eye in a different 
 direction, so as to affect a separate part of the retina, producing the splendid series of 
 colours known as the prismatic spectrum. 
 
 In such a spectrum, therefore, we have an orderly exposition of the colours 
 proper to all the different kinds of homogeneous light. When precautions are used 
 to obtain a tolerably pure spectrum,* these colours are at once seen (except by persons 
 of dichromic or of defective colour vision, the peculiarities of which will be noticed in 
 an appendix) to constitute three conspicuous bands ; the least refrangible rays (or 
 those of the longest waves) being Red; the middle band, Green; and the most 
 refrangible rays (or those of the shortest waves) being Blue. A closer inspection 
 however shows that the colours are not uniform in the bands, but change gradually 
 both in brightness and in hue. 
 
 * If the spectrum is to be reflected from a white ground, then to make it as pure as possible the sun 
 beam should fall on the prism in a narrow line, parallel to one refracting edge, all other light being excluded 
 from the room ; and the face of the prism should be so inclined to the sun beam as to divert the emergent 
 rays as little as possible from their original direction. If the spectrum is to be received directly into the eye, 
 the line of white light to be viewed must be also as narrow as possible without too much diminishing the 
 light, or must be viewed at a distance through a large prism, in which case it must be bordered on both 
 sides with a great breadth of total blackness ; care must be taken also to shade the eye and the prism 
 form all other light. 
 
4 
 
 THE PEISMATIC COLOUES AND THEIR CONTINUOUS COMBINATIONS. 
 
 The series begins with dark Red, which first becomes bright Scarlet and then 
 passes through Orange into still brighter Yellow, and then through Yellow-green into 
 Green ; the Green then changes through a dullish Seagreen into Blue, and this 
 deepens and fades away into darkness through a faint tinge of Violet; the colour of 
 the last rays thus seeming to tend towards the redness of the first.* 
 
 By adding different rays together, additions of their several colours are made. 
 The strongest Red and Green and Blue are obtained by throwing together all the 
 rays in which these colours respectively predominate, and excluding the rest. The 
 strongest Yellow is produced by combining the Red and Green rays, and excluding 
 the Blue; the strongest Sea-green by combining the Green and Blue rays, and 
 excluding the Red ; the strongest Pink by combining the Red and Blue rays, and 
 excluding the Green. When part only of the third band in each case is excluded, 
 the resulting colour is brighter but paler, until when all are included the pure 
 White of the solar light is obtained. 
 
 The colours produced by successive continuous additions of the prismatic rays, 
 beginning either at the Red or at the Violet end of the spectrum, may be beautifully 
 seen by viewing through a prism the nearer or further edge of a white space against 
 Black. The first set contains the best Red, Orange, and Yellow, which last, by 
 addition of Blue, passes into White; the second contains the best Blue, Seagreen- 
 blue, and Seagreen, which last again passes, by addition of Red, into White. 
 
 The same results are obtained by analysis; for just as all the three bands are 
 obtained by viewing through the prism a bright white line against a black ground, so 
 are the Red and Green bands obtained by viewing, in like manner, a Yellow line, 
 as for example, one coloured with King's Yellow; the Green and Blue bands by 
 viewing a Seagreen line, as one coloured with Verdigris; and the Blue and Red 
 bands by viewing a Pink line, as one coloured with Rose Madder; and lastly, the Red, 
 the Green, or the Blue band, is seen alone, when a Red, a Green, or a Blue line, as 
 for example, one coloured with Vermilion, with Emerald Green, or with Cobalt, is 
 so viewed. 
 
 Since the colours of all natural objects are merely the sensations produced by 
 those of the incident rays which they send to the eye, it is evident that every 
 one of them must be produced by some mixture of prismatic rays, and that 
 such colours must therefore be compositions of the prismatic colours. In every case 
 
 * Newton's celebrated division of the prismatic colours into Red, Orange, Yellow, Green, Blue, Indigo, 
 and Violet, only differs from the above in not specifying the Yellow-green and the Seagreen (the former of 
 which was included in his Yellow, and the latter partly in his Green and partly in his Blue), and in callinc 
 the deep dark Blue, Indigo. 
 
THE PRISMATIC COLOURS AND THEIR CONTINUOUS COMBINATIONS. 
 
 the best colours are produced by rays which belong to some one continuous portion of 
 the spectrum, beginning either at the one extremity or at the other, or at some inter- 
 mediate point, or (in the case of Crimsons, Pinks, and Purples) by two such portions, 
 one at one end, and the other at the other, all thrown together in their full intensity, 
 while the rest of the rays are totally extinguished. It is easy to see that this must 
 be the case, because the colours of the prismatic rays change gradually from end to 
 end of the spectrum; and it is shown by analysing the colours of natural objects 
 with the prism, and comparing the result with the spectrum of white light (which 
 is easily effected by viewing through the prism, over a dark cavity, a stripe of the 
 colour continuous with a stripe of White). Such experiments, however, show 
 ■that the best natural colours are inferior to those which may be produced by 
 artificial combinations of the prismatic rays, since there is no substance which 
 transmits, without diminution, all the rays of any one portion of the spectrum, and 
 totally absorbs or extinguishes all the rest. 
 
 By one simple and beautiful experiment with a prism, it is possible to present to 
 the eye, not only the prismatic colours themselves in their greatest possible purity, 
 merging into darkness, but also the colours of all possible parcels of continuous 
 prismatic rays, forming an ensemble of the loveliest colours which the eye can 
 behold. We have only to produce the spectrum of an angular space of White upon 
 a black ground, meeting an angular space of Black upon a white ground, as in the 
 following diagram, by means of a prism fixed parallel with the plane of the diagram, 
 and at right angles with the medial line dividing the black and white grounds. The 
 prism having its refracting edge directed away from the spectator, and being at such 
 a distance from the object, that a small spot only of White appears amongst the 
 colours to the left of the medial line, and one of Black amongst those to the right, 
 the resulting colours are arranged according to the scheme which accompanies the 
 diagram. 
 
 It is easy to see that the central lozenge-shaped space in the spectrum must 
 contain all the colours spoken of. For the left half of the spectrum is formed by the 
 overlapping of a series of angular spaces of the several prismatic colours, so that the 
 pure colours appear close to the left side of the medial line where the vertices lie, and 
 white in the space covered by them all, while the colours due to all possible different 
 combinations of parcels of the prismatic rays, between the extreme red and violet, fill 
 the intermediate space. The other half on the contrary is made up of the colours 
 due to combinations of two parcels of rays reaching from the extreme rays to the 
 intermediate parcel belonging to the corresponding part in the first half. The 
 predominant colour within the left half of the lozenge is Green, and within the 
 
■';■*"•' .. v-.- ■ ■■ 
 
 & 
 
 6 THE PRISMATIC COLOURS AND THEIR CONTINUOUS COMBINATIONS. 
 
 right half, Pink. The colours on the edges are of course the same in each half, but 
 reversed in position. 
 
 To see this splendid compendium of colours to advantage, and to study it as it 
 deserves, the spectrum should be produced on a very large scale, by making the object 
 very large, and viewing it from a distance proportionately great ; the White in the 
 object should be as brilliant as possible, and the Black should be that of a dark 
 cavity. With a little ingenuity it is possible to compare the colours of flowers, of 
 pigments, or other brightly-coloured objects, with these combinations of the prismatic 
 rays ; the coloured surface being placed in front of the prism so as to show against 
 that part of the spectrum which most nearly resembles it. It is thus capable of being 
 used as a natural standard or exemplar of colours, produceable with perfect truth in 
 every place under the sun, and universally applicable, for every colour in nature must 
 be some shade of a colour included in it. 
 
CHAPTER III. 
 
 COMPLEMENTARY COLOURS. 
 
 If the rays of the Spectrum are collected in any two portions, the two resulting 
 colours will be perfectly complementary to each other, the one containing both in hue 
 and in brightness exactly what the other wants to make up the full White of the 
 original light. Thus, the strongest Red which can be produced by combining the 
 prismatic rays, is perfectly complementary to the strongest Seagreen ; the strongest 
 Green to the strongest Pink ; and the strongest Blue to the strongest Yellow ; and by 
 the extreme application of the same principle, so also are Black and White. 
 
 The prismatic colours and their complementaries may be seen in opposition with 
 each other by viewing through the prism a white line on a black ground, continuous 
 with a black line of the same width on a white ground. In this beautiful and 
 instructive experiment the observer who has not studied the subject before can 
 hardly believe that the pale Seagreen and Yellow which he sees on one side should 
 have power to neutralize (or turn into white) the deep Red and Blue which lie 
 opposite to them. Yet that so it must be, is evident from the consideration that if 
 the black line were filled up by the superposition of the white, that is, if the 
 spectrum of the white line were superimposed on that of the black line, all the 
 colours would merge into one uniform White. 
 
 If the white and black stripe is made of considerable width, then by holding 
 the prism nearer and nearer to it, until White and Black appear in the middle of its 
 spectrum, all the complementary combinations of the prismatic colours, exhibited at 
 one view in the experiment mentioned in the last chapter, may be brought by turns 
 into juxtaposition. But the strongest possible Red and Blue may be still more easily 
 contrasted with their perfect complementaries by viewing through the prism an edge 
 of white against black continuous with an edge of black against white; in which 
 experiment Black, passing through the strongest Blue, Seagreen-blue, and Seagreen, 
 into White, by the continued addition of all the prismatic rays beginning with the 
 Violet, are beautifully opposed to White, passing through pale greenish Yellow, and 
 
8 
 
 COMPLEMENTARY COLOURS. 
 
 then the strongest Yellow, Orange, and Red, into Black, by the continued subtraction 
 of the same rays. 
 
 To aid the performance of these experiments, which are of essential service for 
 acquiring correct ideas of the prismatic colours and their complementaries, the 
 following diagrams are given; and the names of the colours which will appear when 
 they are viewed through a prism (the refracting edge pointing away from the 
 spectator) are stated on each side.* 
 
 Black. 
 
 Dark violet 
 Blue. 
 
 Blue. 
 
 Seagreen. 
 
 Green. 
 Yellow. 
 Eed. 
 Black. 
 
 White. 
 
 Black. 
 
 Light greenish 
 
 
 Yellow. 
 
 Dark violet 
 
 
 Blue. 
 
 Yellow. 
 
 
 Red. 
 
 Full Blue. 
 
 Pink. 
 
 Full Seagreen 
 blue. 
 
 Blue. 
 
 • 
 
 Seagreen. 
 
 Seagreen. 
 
 White. 
 
 White. 
 
 White. 
 
 Light greenish 
 Yellow. 
 
 Full Yellow. 
 
 Full Orange. 
 
 Red. 
 
 Black. 
 
 In the former of these experiments, when the prism is held at a certain distance 
 from the object, the brightest or yellow part of the simple spectrum on the left-hand 
 is seen to be equally luminous with the opposite darkest or blue part of the compound 
 spectrum on the right-hand. Held nearer, this part of the simple spectrum becomes 
 brighter, and the corresponding part of the other darker, and there appears on each 
 side of it a place where the opposite colours are of equal brightness. In the latter 
 experiment this is the case with one pair only of complementary colours— a Yellow- 
 red and a Seagreen-blue ; all the others exhibiting a contrast not in hue only, but in 
 light and shade, or more accurately speaking, in White and Black, as well as in hue. 
 
 * For all such experiments as those recommended in this and the preceding chapter, it is a good plan 
 to cut pieces in the form of the black parts out of a sheet of stiff white paper, and to view through the 
 prism the remainder when strongly illuminated over a dark cavity ; or stiU better, to cut pieces of the form 
 of the white parts out of an opaque black screen, and use a bright white sky for the white. 
 
COMPLEMENTARY COLOURS. 
 
 9 
 
 Such observations as these are useful to aid the eye to distinguish colours that are 
 complementary in hue, whether they differ in brightness or not. 
 
 All the pairs of complementary colours shewn in these experiments are alike in 
 this, that the two added together not only completely neutralize each other, or destroy 
 each other's hue, but also make the full White. Such pairs may be called perfect 
 complementaries ; and it is evident that for every possible colour such a perfect 
 complementary is possible. But as one or both of such a pair of colours may be 
 reduced in intensity, diluted with white or otherwise, without any alteration of hue, 
 it is easy to see that there may be colours whose hues are in their kind complementary, 
 although not equally powerful, so that the lights reflected from equal spaces of the 
 two colours would not, if thrown together, fully neutralize each other; and also that 
 if equally powerful, yet their lights added together may be lighter or darker than the 
 full White. 
 
 When perfect complementaries are equally luminous, each colour is half as 
 bright as the full White ; and when one is darker than this, the other must be lighter 
 in the same proportion. 
 
 c 
 
CHAPTER IV. 
 
 INTERMEDIATE COLOURS. 
 
 To determine correctly the colours which lie between two given colours is a most 
 important point in the theory and practice of colours. There are two ways in 
 which it may be readily effected: — 
 
 (1.) The two colours may be presented to the eye in the required proportions in 
 such rapid succession that the one sensation does not vanish before the other is 
 perceived. This is best done by causing the coloured surfaces to revolve on a 
 circular disc, on which, by means of overlapping segments or other devices, any 
 required combination may be effected. But unless mixtures of two colours in any 
 given unequal quantities are wanted, this is not so convenient a method as the next. 
 
 (2.) Small equal spots of the colours to be mixed may be laid a little distance 
 apart upon a neutral ground, equally illuminated ; and a clean slip of thin polished glass 
 being held vertically between them, the reflection of the one may be made to fall 
 upon that part of the glass through which the other is seen. The more obliquely 
 the light impinges on the glass, the more light is reflected and the less transmitted; so 
 that by lowering the eye from a position high and near to the plane of the glass, to a 
 position opposite to its lower part, every colour intermediate between the colours 
 of the two spots may be seen. 
 
 To find . the position of the eye in which the glass reflects and transmits the 
 lights of the two coloured spots in equal proportions, a black and a white spot may be 
 placed on alternate sides of each of the coloured spots ; the required position will be 
 that in which both of the opposite black and white spots are blended into the same 
 Gray. In this way the true mean between any two colours may be easily found, and 
 compared with any other colour, or with the mean between any two other colours. 
 Thus the colours of Vermilion and Emerald Green compound an Olive-green, which 
 is a shade of Yellow, not far from the mean between King's Yellow and Black: those 
 of Emerald Green and Cobalt Blue, a dark Seagreen, not far from the mean between 
 Verdigris and Black : those of Cobalt Blue and Vermilion, a Purple, not far from the 
 
INTERMEDIATE COLOURS. 
 
 11 
 
 mean between Rose Madder and Black. The colours of Rose Madder and King's 
 Yellow, again, compound a light Red, like that of Vermilion and White: those of 
 King's Yellow and Verdigris, a light Green, like that of Emerald Green and White : 
 those of Verdigris and Rose Madder, a light Blue, like that of Cobalt and White. 
 Disregarding a little inequality in strength, the colours of Vermilion and Verdigris, 
 those of Emerald Green and Rose Madder, and those of Cobalt Blue and King's 
 Yellow, compound neutral Grays, not very far from the mean between Black and 
 White, and are therefore tolerably good examples of complementary colours. 
 
 In the accompanying dia- 
 gram spots of these colours are 
 so arranged that by erecting the 
 glass over the several diameters 
 of the hexagon all their inter- 
 mediate colours may be seen by 
 turns; and the position of the 
 eye in which the true mean in 
 each case appears, is indicated 
 by the reflected and transmit- 
 ted White being equal on the 
 right and left of the compound 
 spot.* 
 
 If it be desired to see at 
 once the whole gradation be- 
 tween two given colours, a stripe of each may be laid so as to cross at an acute 
 angle, and the glass erected at the point of intersection. 
 
 In connexion with this head it should be noticed that a mixture of two pigments 
 does not in general give a colour intermediate between their separate colours, unless 
 all the rays given out by one of the two pigments are also given out by the other, as 
 in the case of a white pigment being mixed with one of any colour, a yellow with a 
 red or a green one, a pink with a red or a blue one, a seagreen with a green or a 
 blue one, or lastly in the case of a pigment of any colour being mixed with a black 
 one. For every pigment being more or less transparent, the kind of light which 
 traverses both most freely is that which determines the resulting colour. Indigo, for 
 instance, absorbs the red rays, but allows some of the green, together with the blue, 
 
 * A mixture of Emerald Green and Cobalt is used for the Seagreen, though inferior to Verdigris, 
 the latter being extremely fugitive. To see the tints and shades of these colours, the glass should bisect the 
 opposite black or white angles. 
 
12 
 
 INTERMEDIATE COLOURS. 
 
 to traverse a very small depth of its substance ; Gamboge absorbs the blue, but is 
 permeable by the green and the red; little therefore beside the green rays can 
 traverse both of these pigments together, and be reflected by the white paper under- 
 neath them to the eye, and the colour of their mixture is a dark Green, instead of a 
 neutral Gray, which the mixture of their colours produces. 
 
 In other cases again the mixture of pigments may present a colour of the same 
 hue with the true mixture of their colours, but darker. Thus by mixing Vermilion and 
 Emerald Green, Emerald Green and Cobalt Blue, or Cobalt Blue and Vermilion, we 
 may produce a series of Olive-greens, of Seagreens, or of Purples, but darker than 
 those produced by the true mixture of the colours of those pigments. This is because 
 each mixed pigment destroys some of the light which would otherwise be passed by 
 the other. But the light-absorbing powers of the different pigments are so various 
 that almost every mixture produces some peculiar divergence from the colours which 
 form the direct gradation between the colours of the unmixed pigments, and it is 
 evident therefore that any system of colours founded on the results of such mixtures 
 must be fallacious. 
 
 When mixtures of three or more colours are required, rotation seems the only 
 available method of determining their colours with accuracy. 
 
CHAPTER V. 
 
 THE PRIMARY AND SECONDARY COLOURS. 
 
 The predominance of the three colours Red, Green, and Blue, which strikes the eye 
 so powerfully in viewing the prismatic spectrum, arises from the circumstance that 
 there is a greater depth of hue in the rays of those three colours than in any other 
 This has been proved by the careful experiments of Mr. Maxwell, recorded in the 
 Transactions of the Royal Society for 1860, by which it appears that the colours of 
 all the prismatic rays which lie between the deepest red and green rays may be 
 produced by mixtures of those latter rays with very nearly the same depth of hue 
 which they have in the spectrum; and that the like is true of the colours of the rays 
 which lie between the deepest green and blue rays. 
 
 Hence there is little reason to doubt that these three are simple, elementary, or 
 primary sensations of colour, and that the hues of all the intervening prismatic 
 colours are compound, caused by some two of these sensations being excited at once 
 in excess of the third. For nothing can be more probable than that each simple 
 sensation is produced with greatest power by rays of some particular period of 
 undulation, and with less and less power, the greater the difference of the period ; and 
 if so, it may well be supposed that those prismatic rays which have the greatest depth 
 of colour, that is, the red, green, and blue rays, are those by which the simple 
 sensations are severally most strongly excited, each in equal excess, or very nearly so, 
 over the other two sensations. 
 
 It may be concluded, then, that the best Red, Green, and Blue, of the solar 
 spectrum give the nearest possible approach to the three primary colours; and there- 
 fore that their complementary colours, Seagreen, Pink, and Yellow, give the nearest 
 possible approach to those binary compounds of the primaries which are usually 
 termed secondary colours. The brightness of each of these last (in its greatest 
 intensity) must be equal to the sum of the brightnesses of the full intensities of their 
 simple components. The Seagreen produced by the green rays added to the blue, 
 must be as bright as the Green and Blue together; the Pink produced by the blue 
 rays added to the red, must be as bright as the Blue and Red together; and the 
 Yellow produced by the red rays added to the green, must be as bright as the Red 
 
14 
 
 THE PRIMARY AND SECONDARY COLOURS. 
 
 and Green together; White being of course as bright as the sum of the Red, Green, 
 and Blue, which compound it. 
 
 These conclusions differ greatly from the theory which has for long been 
 commonly assumed to be true, that Red, Yellow, and Blue, are the primary, and 
 Orange, Green, and Purple, the Secondary colours. But that theory is entirely 
 subverted not only by the researches above mentioned on the prismatic colours, but 
 by all scientific experiments on the mixture of colours, which show that Red and 
 Green, Yellow and Purple, Blue and Orange, are not complementary to each other. 
 
 In the accompanying diagram an attempt is made to represent, though very 
 imperfectly, the true primary and secondary colours by the aid of those permanent 
 pigments which seem most suitable. 
 
 The lower part of the diagram is intended to show 
 the primary colours in equal strength, in three overlap- 
 ping circles upon Black, forming the binary compounds 
 where they overlap in pairs, and White where the 
 
 dffi^fet ■ \ tliree overla P- The upper part, on the contrary, 
 
 ^| \ exhibits the effect of taking away the same three 
 
 simple colours from White, in three overlapping 
 circles ; leaving a secondary colour where one only 
 is taken away, a primary colour where two are 
 taken away, and Black where all are removed- 
 If the representation were perfectly correct, the two 
 parts would be perfectly complementary to each 
 other, so that adding the light reflected from the one, 
 to that reflected from the other, would reduce the 
 whole to pure White ; and mixing the one with the 
 other would produce a uniform Gray, the mean 
 between Black and full White. 
 
 The eight colours (using that term in its larger 
 sense to include Black and White), which are intended 
 to be represented in this diagram, may be termed the 
 principal colours; and it may well be supposed that each of them would form as 
 striking a contrast with its complementary, as full White does to Black, were it 
 possible to present them to the eye with perfect purity : in practice, however, there 
 is far more difference between Black and White than between the best Red and 
 Seagreen, Green and Pink, or Blue and Yellow, that we can procure. The cause of 
 this will be stated in the next chapter. 
 
CHAPTER VI. 
 
 QUALITIES OF COLOURS. 
 
 Besides the brightness of a colour, which depends of course on the quantity of its 
 light, and its richness (or strength of hue), which depends upon the quantity of the 
 unneutralized part of its light, there are two qualities which have the greatest effect 
 in striking the eye, and produce the manifest superiority of some colours over others. 
 These are perhaps best expressed by the terms " depth " and " clearness." 
 
 The depth of a colour depends upon the ratio of its richness (as above denned) 
 to its brightness. Perfection in this quality would require the absence from its 
 composition of one at least of the three simple sensations. Thus the deeper a Black 
 is, the less does it excite any of those sensations; the deeper a Red, a Green, or a Blue, 
 the less does it excite two of them ; and the deeper any colour whose hue is com- 
 pounded of two of them, the less does it excite the remaining one, in proportion to 
 the whole brightness of the colour. 
 
 The clearness of a colour, on the contrary, depends on the ratio of its richness 
 to its darkness (that is, to its defect from the brightness of full White). Perfect 
 clearness therefore would require the presence of at least one of the three simple 
 sensations in its full intensity. Thus the clearer a White is, the more powerfully 
 does it excite all the three sensations ; the clearer a Seagreen, a Pink, or a Yellow, 
 the more powerfully does it excite two of them ; and the clearer any colour in which 
 Red, Green, or Blue predominates, the more powerfully does it excite the one pre- 
 dominant sensation. 
 
 Darkness and brightness, as well as those terms by which the shades and tints of 
 colours (or their mixtures with Black and White) may be more strictly expressed, 
 are therefore quite distinct from depth and clearness. 
 
 The terms " purity " and " fulness " are often applied to colours to express indis- 
 criminately their depth and clearness ; but it is evident that they can apply strictly, the 
 one only to colours perfect in depth, and the other only to colours perfect in clearness ; 
 and it will appear by what follows, that excepting Black and White, such perfect 
 colours cannot be found. 
 
16 
 
 QUALITIES OF COLOURS. 
 
 Accurate experiments on the prismatic colours have shown that they are of very 
 unequal depth. Some of the dark blue rays have power to neutralize the yellow of 
 the brightest part of the spectrum, which is ten times as luminous; so that a very 
 large proportion of the brightness of the latter must be regarded as due to White, 
 even if the Blue of the former be perfectly pure, which there is no reason to believe, 
 for in all probability none of the prismatic colours are perfectly pure, but every ray 
 excites more or less all the three simple sensations * 
 
 The deepest of all colours that are possible (next to Black itself), are the Violet 
 and Blue of the more refrangible rays of the spectrum; the next are the Eeds of the 
 least refrangible, and the Purples, Dark Pinks, and Crimsons, produced by combining 
 the rays of both ends of the spectrum. Next come the Greens of the best green 
 rays, then the Seagreens, and lastly the Yellows, the least deep of all the prismatic 
 colours. With respect to Blue and Red, and their compounds, the deepest colour 
 must be dark; not having one sixth of the brightness of full White. With Green 
 there is no difference in the possible depth up to a brightness of one half of that of full 
 White; with Seagreen there is not much difference up to a brightness of two thirds, 
 and none with Yellow up to a brightness of perhaps three fourths. 
 
 Whatever is true of any colours as to depth, must evidently be true of their 
 perfect complementaries as to clearness ; hence the Yellows are the clearest of all 
 colours next to White; then the Seagreens, the Greens, the Pinks, the Reds, and 
 lastly the Blues. Thus the best light Blues and Reds that can be obtained are no 
 better than tints made by diluting the best dark Blues and Reds with White; while 
 the best dark Yellows and Seagreens are but shades of the light ones. Greens and 
 Pinks hold an intermediate rank in both these respects. 
 
 The colours of all natural bodies, though all more or less inferior in depth to 
 those of combinations of continuous portions of the prismatic rays, agree generally in 
 these properties. Among pigments the deepest are Blues, Reds, and their intermediate 
 hues, and the deepest of these are dark, such as Intense Blue, Ultramarine, French 
 Blue, Smalt, Carmine, and Crimson Lake. The clearest are Yellows, Seagreens, and 
 Greens, and the clearest of these are light, as Lemon Yellow, King s Yellow, Verdigris, 
 Emerald-Green ; though there are also tolerably deep Greens, as that called Viridian, 
 or Veronese Green, and clear Pinks, as Rose Madder. In light Blues we find nothing 
 better than tints of Ultramarine or Cobalt ; in dark Yellow hardly anything better 
 than such Olive-greens as may be made by mixing Gamboge with Lampblack. 
 
 * See Helmholtz's Experiments on the Prismatic Colours, Poggendorff's Annalen, xciv., as well as 
 Maxwell's above referred to; also Fraunhofer's curve of light intensity in the spectrum. From a careful 
 comparison of the last two, the results stated in the next paragraph are derived. 
 
QUALITIES OF COLOURS. 
 
 17 
 
 These circumstances have led to the common opinion that Blue and Purple have 
 the closest affinity to Black, Yellow and Green to White. They account for the fact 
 that of all contrasts in hue between perfectly complementary colours, the strongest are 
 those between dark Blues or Purples, and light Yellows or Greens; and the weakest 
 are those between dark Yellows or Greens, and light Blues or Pinks ; and also that 
 complementary colours of about the middle degree of brightness form far weaker 
 contrasts than do those which are nearer to Black with those that approach to White, 
 and that the strongest of all contrasts is that between Black and White; not simply 
 because there is the greatest difference in brightness between these two, but because 
 Black can be obtained in greater depth, and White in greater clearness, than any other 
 colour. 
 
 The preponderance of contrasts in Black and White, which hence arises, is so 
 great that in a feeble light all distinction of colour is lost long before objects become 
 invisible ; and for the same reason intelligible representations can be made of almost 
 all objects in light and shade alone, without any attempt to imitate their peculiar hues. 
 The same circumstance is doubtless of great value in giving a measure of unity or 
 simplicity of effect to every scene ; and the endless variety of colour in nature becomes 
 an embellishment which charms the eye without perceptibly hindering its appreciation 
 of form. 
 
 It is a singular fact that the colour of a surface may vary in depth under 
 different degrees of the same light. A red which in a bright light is brighter than a 
 blue, in a dull light becomes darker; for while the former approaches rapidly to 
 Black, the latter differs less and less from the appearance of White in diminishing 
 light. Yet no change takes place in the hues, or in the relative strength of the hues 
 of the colours of different surfaces, when the intensity of the light is changed ; for 
 their powers of mutual neutralization are unaltered, and a surface that is white in a 
 dull light, continues so in a bright one. It would seem therefore that the red rays 
 lose depth of colour when their intensity increases, their brightness increasing faster 
 than their redness, as if by the addition of White; while the contrary effect takes place 
 with the blue rays. These changes must arise from some difference in the action of 
 strong and weak lights of the same kind on the retina; for they come without 
 altering the illumination of the coloured surfaces, if the quantity of light admitted 
 into the eye is altered, as by nearly closing the eyelids. 
 
 D 
 
CHAPTER VII. 
 
 THE NATURAL SYSTEM OF COLOURS. 
 
 A scheme by which colours may be arranged according to their natural affinity, 
 and their relationship exhibited by their places in a complete system, will be a useful 
 aid in forming and retaining a correct idea of the whole assemblage of colours, and 
 in devising new and beautiful arrangements. 
 
 The first attempt at such a scheme was made by the astronomer Mayer (1758). 
 Supposing that all colours could be produced by mixtures of Red, Yellow, and Blue, 
 he placed those three colours, with that degree of brightness in which they appear 
 clearest, at the corners of an equilateral triangle, within which all their mixtures of 
 equal brightness were disposed in order. The gradations between all these colours 
 and Black and White fill two pyramids, whose apices are Black and White on the 
 opposite sides of the triangle. In this way, with true primaries, all mixtures equalling 
 a full primary in brightness might be correctly arranged, together with all their 
 shades and tints; but no place is provided for the full secondaries, and many other 
 combinations of greater brightness than a full primary. 
 
 Otto Runge, of Hamburgh (1810), proposed a colour-sphere; Black and White 
 occupying the poles, Gray the centre, and Red, Orange, Yellow, Green, Blue, and 
 Purple, six equidistant points on the equator. All the deepest colours would be 
 found on the hemisphere whose pole was blaok, and all the clearest on the opposite 
 hemisphere, and the gradations between these outer colours and Gray, in the 
 respective radii. This system, though more complete than the former, is less correct 
 in this respect, that many direct gradations, such as those between the full primaries 
 and their adjacent secondaries, and between them and White and Black, would not be 
 represented by straight lines ; it also disregards the brightness of colours in placing 
 the full primaries and secondaries at equal distances from Black and White. 
 
 Chevreul, in his well-known system, comprises the same colours, with less 
 symmetry, in a hemisphere ; his " Normal Colours," the best of each hue, forming the 
 circumference, and White the centre. The shades or " higher tones " of the Normal 
 Colours occupy the surface, up to Black at the summit; their tints or "lower tones," the 
 base, and all other gradations to White, the radii. 
 
 But to apply the ordinary method of representing geometrically in their true 
 relations all the combinations that can be formed out of three independent variable 
 quantities, a point must bo taken to represent zero or Black, the absence of all light, 
 
THE NATURAL SYSTEM OF COLOURS. 
 
 19 
 
 and three lines drawn from it at right angles with each other, in which, and in 
 all parallel co-ordinates, Red Green and Blue respectively must be supposed to 
 increase in intensity from nothing upwards. Those intensities of Eed Green and 
 Blue which taken together constitute White must be supposed to be equal, and 
 will be represented by equal distances in the three rectangular directions. The 
 outer extremities of such three equal lines will therefore be the places of full Red, 
 full Green, and full Blue, in some given intensity of White; and the lines themselves 
 will contain the gradations from Black up to those three colours. If then the cube, of 
 which the same lines would form three edges, be completed, it would obviously 
 contain a place for every possible combination of Red Green and Blue, from Black, 
 in which all three are nil, up to the White in which all three are of full intensity; 
 and the number of distinct combinations would of course be the cube of whatever 
 number of steps are taken from Black to a full primary, both included. 
 
 The corner of the cube opposite to Black would be full White: the corners 
 opposite to Red, Green, and Blue, would be Seagreen, Pink, and Yellow. The 
 central point would be a medium Gray. The three sides which adjoin to the corner 
 of Black would respectively contain all those colours in which there is no Red, no 
 Green, and no Blue ; while the opposite three which adjoin to the corner of White, 
 would contain all those which have full Red, full Green, and full Blue. Thus the six 
 sides may be distinguished by the primary which is absent, or fully present, in each; 
 and the twelve edges, being lines of which each is common to two sides, by the two 
 primaries of which each contains nothing or all. 
 
 There are thirteen principal diameters or axes in the cube, and they may be 
 divided into three classes. The three which join the middle points of the opposite 
 sides may be called primary axes, because on them there is a change in one of the 
 primaries only. The six which join the middle points of the opposite edges may be 
 called secondary axes, because on them there is an equal change in two of the 
 primaries, either in the same or in contrary directions. The four which join the 
 opposite corners in like manner may be called tertiary axes, because there is an equal 
 change of all the three primaries, either all in the same direction, or two in the same 
 and one in the contrary direction. Those axes along which one colour diminishes 
 while another increases in intensity, may be distinguished as cross axes. In like 
 manner there are thirteen principal medial planes, being those which bisect the cube 
 at right angles to the thirteen axes. The advantage of distinguishing these principal 
 lines and planes in the cube of colours will soon be seen in the facility with 
 which the principal gradations, contrasts, and harmonious arrangements of colour 
 may, by means of them, be classified and remembered. 
 
20 
 
 THE NATURAL SYSTEM OF COLOURS. 
 
 In the accompanying plate the primary axes and lines parallel to them are 
 shown by full lines ; the secondary axes and lines parallel to them, by broken lines ; 
 the tertiary axes by dotted lines. The arrow-heads indicate the directions in which 
 the cube may be conceived to increase with an increase of light. The different sides, 
 edges, and corners, with their connected axes and medial planes, are as follows : — 
 
 Z G B S, side of no Red ;— R YPW, side of full Red ; 
 
 s r 1 , axis of Red ; — r m 3 m 4 s 1 , medial plane of Red. 
 ZBEP, side of no Green ;—G SY¥, side of full Green ; 
 
 p g 1 , axis of Green ; — g m 2 m 5 p 1 , medial plane of Green. 
 Z R G Y, side of no Blue ;— B PS¥, side of full Blue ; 
 
 y b 1 , axis of Blue ; — b m 1 m 6 y 1 , medial plane of Blue. 
 
 Z R, edge of no Green and no Blue ; — S W, edge of full Green and full Blue ; 
 
 r s 1 , axis of Seagreen ; — G Y B P, medial plane of the sum of Green and Blue. 
 Z G, edge of no Blue and no Red ;— P W, edge of full Blue and full Red; 
 
 g p 1 , axis of Pink ; — B S R Y, medial plane of the sum of Blue and Red. 
 Z B, edge of no Red and no Green ; — Y W, edge of full Red and full Green ; 
 
 b j\ axis of Yellow; — R P G S, medial plane of the sum of Red and Yellow. 
 
 G Y, edge of full Green and no Blue; — B P, edge of no Green and full Blue ; 
 
 m 4 m 3 , cross axis of Green and Blue ; — Z RSW, medial plane of the difference of Green and Blue. 
 B S, edge of full Blue and no Red ; — R Y, edge of no Blue and full Red ; 
 
 m nr, cross axis of Blue and Red ;— Z G P W, medial plane of the difference of Blue and Red. 
 R P, edge of full Red and no Green ; — G S, edge of no Red and full Green ; 
 
 m 1 m 6 , cross axis of Red and Green ; — Z BY¥, medial plane of the difference of Red and Green. 
 
 Z, corner of Black ; — W, corner of White ; — Z W, axis of White ; 
 
 m 1 m 2 m 4 m G m 5 m 3 , medial plane of the sum of Red, Green, and Blue. 
 
 R, corner of Red; — S, corner of Seagreen; — R S, cross axis of Red and Seagreen; 
 
 g b m 3 p 1 y 1 m 4 , medial plane of the difference of Red and the sum of Green and Blue. 
 G, corner of Green ; — P, corner of Pink; — G P, cross axis of Green and Pink; 
 
 b r m 2 y 1 s 1 m 5 , medial plane of the difference of Green and the sum of Blue and Red. 
 B, corner of Blue ; — Y, corner of Yellow ; — B Y, cross axis of Blue and Yellow ; 
 
 r g m 6 s 1 p 1 m 1 , medial plane of the difference of Blue and the sum of Red and Green. 
 
 It has been shown in the preceding chapters that of the colours which belong to 
 the surface of the complete cube, only Black and White are possible. We have not 
 sufficient data for determining how near to the different parts of the surface our best 
 colours may approach. The possible portion of the cube probably varies somewhat 
 in form and relative magnitude when the full White differs in brightness, and in 
 different states of the eye ; but it always approaches the surface of the cube most 
 nearly on the sides of no Green, and of full Green. 
 
THE. CUBE OF COLOURS 
 
 Z, Blacj-c. 
 r, DcwhRed/ 
 
 ~b , ..... .... i>/?/# 
 
 Centre, Grew 
 
 s', Lujht Seaqr-een 
 yj „ Yellow. 
 
 R. 
 
 Red* 
 
 G, 
 
 &re&v 
 
 B, 
 
 BJm&. 
 
 v', 
 
 liqkt Rid 
 
 Si 
 TV, 
 
 Greew 
 Blue/. 
 
 S, Sea-c/reen/. 
 P. ii«y& 
 Y, Yellow. 
 
 s , Doric Seagrreerv 
 p, „ /fri& 
 
 y, „ Yellow. 
 
 m 1 , /»?,& i?^? 
 m 2 . Yellow R<t& 
 m* Yellow Grem . 
 
 m® Seagreetv Greav 
 mf S'eaare&v Bhi& 
 
 J . W.Io wry fai2p t 
 

 
CHAPTER VIII. 
 
 GRADATIONS AND CONTRASTS OF COLOUR. 
 
 Since the quantity of Eed, Green, and Blue, in the colour proper to any point in the 
 cube of colours, is proportionate to the distance of that point from the sides of No 
 Red, No Green, and No Blue, it follows that every straight line drawn through the 
 cube passes through a uniform gradation of colour, the change in the quantity of 
 each primary being equal at equal distances ; also that all those gradations which are 
 formed by the same changes, will be found in parallel lines. Thus, if we imagine 
 lines drawn in every direction through every point in the cube, they will represent all 
 possible gradations of colour ; and these may be conveniently grouped in two 
 ways : — 
 
 (1.) According to their direction; those parallel to the same line being taken 
 together. 
 
 (2.) According to the point to which they converge; those converging to the 
 same point being taken together. 
 
 Groups of the First Kind. 
 
 The leading parallel groups are those that are parallel to the axes of the cube. 
 The principal gradations in each of these groups are given in the following lists, with 
 the laws of their formation : — 
 
 Primary Gradations. 
 
 1. In the direction of the axis of Red, Red increases, without change in Green 
 and Blue, 
 
 From Black, through Dark Red, . , 
 
 Dark Yellow, . 
 
 ■>■> 
 
 Dark Green, . . 
 Dark Blue, . . 
 
 Green, 
 
 Dark Seagreen, 
 
 Blue, 
 
 Seagreen-Green, 
 Seagreen-Blue, 
 Seagreen,, . . . 
 
 Dark Pink, . . 
 Yellow- Green, 
 
 Gray, 
 
 Pink Blue, . . 
 Light Green, . 
 Light Blue, . . 
 Light Seagreen, 
 
 to Red. 
 
 ,, Yellow-Red. 
 
 n 
 
 Pink-Red. 
 Yellow. 
 
 „ Light Red (axis). 
 ,, Pink. 
 
 „ Light Yellow. 
 „ Light Pink. 
 „ White. 
 

 22 
 
 GRADATIONS AND CONTRASTS OF COLOUR. 
 
 2. In the direction of the 
 Blue and Red, 
 
 From Black, . . . 
 Dark Blue, 
 
 axis of Green, Green increases, without change in 
 
 n 
 
 11 
 >> 
 
 11 
 
 11 
 11 
 
 Dark Red, 
 Blue, . . . 
 Dark Pink, 
 Eed, . . . 
 Pink-Blue, 
 Pink-Red, 
 Pink, . . . 
 
 through Dark Green, . 
 Dark Seagreen 
 Dark Yellow, 
 Seagreen-Blue 
 Gray, .... 
 Yellow-Red . 
 Light Blue, . 
 Light Red, . . 
 Light Pink . 
 
 n 
 » 
 
 n 
 n 
 n 
 n 
 n 
 » 
 
 to Green. 
 
 „ Seagreen-Green.. 
 
 „ Yellow-Green. 
 
 „ Seagreen. 
 
 „ Light Green (axis). 
 
 „ Yellow. 
 
 ,, Light Seagreen. 
 
 ,, Light Yellow. 
 
 „ White. 
 
 3. In the direction of the 
 or Green, 
 
 From Black, . . . 
 Dark Red, . 
 
 axis of Blue, Blue increases, without change in Red 
 
 
 11 
 11 
 11 
 11 
 11 
 11 
 J) 
 11 
 
 Dark Green, 
 Red, .... 
 Dark Yellow, 
 Green, . . . 
 Yellow-Red, 
 Yellow- Green 
 Yellow, . . . 
 
 through Dark Blue, . . . 
 Dark Pink, . . 
 Dark Seagreen, 
 Pink-Red, . . . 
 Gray, 
 
 Seagreen- Green, 
 Light Red, . . . 
 Light Green, . . 
 Light Yellow, . 
 
 to Blue. 
 
 „ Pink-Blue. 
 
 „ Seagreen-Blue. 
 
 „ Pink. 
 
 „ Light Blue (axis). 
 
 ,, Seagreen. 
 
 „ Light Pink. 
 
 ,, Light Seagreen. 
 
 „ White. 
 
 
 Secondary Gradations. 
 
 4. In the direction of the axis of Seagreen, Green and Blue increase equally, 
 without change in Red, 
 
 From Black, through Dark Seagreen, ... to Seagreen. 
 
 ,, Dark Red, ... „ Gray, ,, Light Seagreen (axis). 
 
 ,, Red, ...... „ Light Red, ,, White. 
 
 And from Dark Pink ... to Light Blue. 
 „ Yellow-Red . . „ Light Yellow. 
 „ Pink-Red. . . . ,, Light Pink. 
 
 5. In the direction of the axis of Pink, Blue and Red increase equally, without 
 change in Green, 
 
 Also from Dark Green . . 
 
 . to Seagreen- Green 
 
 ,, Dark Blue . . 
 
 . ,, Seagreen-Blue. 
 
 „ Dark Yellow . 
 
 . „ Light Green. 
 
 From Black, through Dark Pink, 
 
 to Pink. 
 
 „ Dark Green, 
 ,, Green, . . . 
 
 ii 
 
 From Dark Blue to Pink-Blue. 
 
 Dark Red „ Pink-Red. 
 
 , „ Light Blue. 
 
 
 Dark Seagreen 
 
 Gray, „ Light Pink (axis). 
 
 Light Green, . . „ White. 
 
 From Dark Yellow .... to Light Red. 
 
 Seagreen-Green . . ,, Light Seagreen. 
 
 Yellow- Green . . . „ Light Yellow. 
 
 ii 
 ii 
 
GRADATIONS AND CONTRASTS OF COLOUR. 
 
 23 
 
 6. In the direction of the axis of Yellow, Red and Green increase equally, 
 without change in Blue, 
 
 From Black through Dark Yellow, ... to Yellow. 
 
 5) 
 
 From Dark Bed . 
 Dark Green 
 Dark Pink 
 
 n 
 
 Dark Blue, . . . 
 Blue, 
 
 , . to Yellow-Bed. 
 , . ,, Yellow-Green. 
 . „ Light Bed. 
 
 » 
 
 Gray, ,, Light Yellow (axis). 
 
 Light Blue, .... „ "White. 
 
 From Dark Seagreen ... to Light Green. 
 
 Pink-Blue „ Light Pink. 
 
 Seagreen-Blue . . . „ Light Seagreen. 
 
 ii 
 
 Secondary Cross Gradations. 
 
 7. In the direction of the cross axis of Green and Blue, Green increases and 
 Blue decreases, equally, without change in Red, 
 
 From Blue, '. through Dark Seagreen, ... to Green. 
 
 Pink-Blue, ... „ Gray, „ Yellow-Green (axis). 
 
 Light Bed, „ Yellow. 
 
 Pink, 
 
 ii 
 ii 
 
 From Dark Blue to Dark Green. 
 
 ,, Seagreen-Blue . . . „ Seagreen-Green. 
 „ Dark Pink ,, Dark Yellow. 
 
 From Light Blue . 
 Pink-Bed . . 
 
 ii 
 
 ii 
 
 Light Pink 
 
 to Light Green. 
 „ Yellow-Bed. 
 ,, Light Yellow. 
 
 8. In the direction of the cross axis of Blue and Red, Blue increases and Red 
 decreases, equally, without change in Green, 
 
 From Bed, through Dark Pink, to Blue. 
 
 „ Yellow-Bed, ... „ Gray, „ Seagreen-Blue. (axis) 
 
 ii Yellow, „ Light Green, ,, Seagreen. 
 
 From 
 
 j> 
 
 . Dark Bed . . 
 . Pink-Bed . . 
 . Dark Yellow 
 
 to . . . Dark Blue. 
 „ . . , Pink-Blue. 
 „ . . . Dark Seagreen. 
 
 ii 
 ii 
 
 From Light Bed to Light Blue. 
 
 Yellow-Green . . . „ Seagreen-Green. 
 Light Yellow .... „ Light Seagreen. 
 
 9. In the direction of the cross axis of Red and Green, Red increases and Green 
 decreases, equally, without change in Blue, 
 
 From Green, through Dark Yellow, ... to Bed. 
 
 Seagreen-Green, ... „ Gray, „ Pink-Bed (axis). 
 
 Seagreen, „ Light Blue, .... „ Pink. 
 
 ») 
 
 From Dark Green to Dark Bed. 
 
 „ Yellow- Green . . . „ Yellow-Bed. 
 . „ Dark Seagreen . . . „ Dark Pink. 
 
 From Light Green to Light Bed. 
 
 ,, Seagreen-Blue. . . . ,, Pink -Blue. 
 „ Light Seagreen . . . „ Light Pink. 
 
 Tertiary Gradation. 
 
 10. In the direction of the axis of White, Red and Green and Blue increase 
 equally, 
 
 through Gray, ... to White (axis). 
 
 From Black, 
 
 From Dark Bed to Light Bed. 
 
 „ Dark Green . . . „ Light Green. 
 „ Dark Blue .... „ Light Blue. 
 
 From Dark Seagreen ... to Light Seagreen. 
 
 Dark Pink ,, Light Pink. 
 
 Dark Yellow .... ,, Light Yellow. 
 
 ii 
 ii 
 
 ik 
 
24 
 
 GRADATIONS AND CONTRASTS OF COLOUR. 
 
 Tertiary Cross Gradations. 
 
 11. In the direction of the cross axis of Red and Seagreen, Green and Blue 
 increase, and Red decreases, equally, 
 
 From Red, . . . through Gray, 
 . . to Dark Seagreen. 
 
 From Dark Eed . . 
 „ Yellow-Red 
 ,, Pink-Red . . 
 
 „ Light Green 
 „ Light Blue. 
 
 . to Seagreen (axis). 
 From Light Red to Light Seagreen. 
 
 Dark Yellow 
 
 Seagreen-Green. 
 
 ,, Dark Pink „ Seagreen-Blue. 
 
 12. In the direction of the cross axis of Green and Pink, Blue and Red increase, 
 and Green decreases, equally, 
 
 From Green, . . . through Gray, ... to Pink (axis). 
 
 From Dark Green to Dark Pink. 
 
 Yellow-Green . . 
 Seagreen-Green . 
 
 „ Light Red. 
 „ Light Blue. 
 
 From Light Green to Light Pink. 
 
 Pink-Blue. 
 
 >> 
 
 Dark Seagreen 
 Dark Yellow . 
 
 :i 
 
 Pink-Red. 
 
 13. In the direction of the cross axis of Blue and Yellow, Red and Green 
 increase, and Blue decreases, equally, 
 
 From Blue, . . . through Gray, ... to Yellow (axis). 
 
 From Dark Blue to Dark Yellow. 
 
 ,, Pink-Blue ,, Light Red. 
 
 „ Seagreen-Blue . . . „ Light Green. 
 
 From Light Blue to Light Yellow. 
 
 ,, Dark Pink „ Yellow-Red. 
 
 „ Dark Seagreen . . . „ Yellow-Green. 
 
 The mean colours in all the minor gradations may be easily found by reference 
 to the figure of the cube. In those parallel to the tertiary axes they are all 
 intermediate between the nearest corner colour, and the central Gray; and may be 
 designated Dark Gray, Gray Red, Gray Green, Gray Blue, Gray Seagreen, Gray 
 Pink, Gray Yellow, and Light Gray. 
 
 Groups of the Second Kind. 
 The leading convergent groups of gradations are those which tend towards the 
 principal colours. The principal gradations in each of these are given the following 
 lists, with the laws of their formation: — 
 
 In gradations tending to Black, Red 
 
 and Green and Blue decrease in 
 
 proportion to their respective 
 
 quantities, as 
 
 From White .... through Gray. 
 
 „ Dark Seagreen. 
 ,, Dark Pink. 
 
 }> 
 
 » 
 
 Seagreen 
 Pink . . 
 Yellow . 
 Red . . . 
 Green . . 
 Blue . . . 
 
 >> 
 
 5J 
 
 Dark Yellow. 
 Dark Red. 
 Dark Greeri. 
 Dark Blue. 
 
 2. In gradations tending to White, Red 
 and Green and Blue increase in 
 proportion to their respective 
 defects, as 
 
 „ Red 
 
 lllUUg 
 
 n vj i ay • 
 
 Light Red. 
 
 ,, Green .... 
 
 » 
 
 Light Green. 
 
 „ Blue 
 
 >> 
 
 Light Blue. 
 
 ,, Seagreen. . . 
 
 )> 
 
 Light Seagreen 
 
 „ Pink 
 
 » 
 
 Light Pink. 
 
 „ Yellow .... 
 
 57 
 
 Light Yellow. 
 
GRADATIONS AND CONTRASTS OF COLOUR. 
 
 25 
 
 3. In gradations tending to Red, Red 
 increases in proportion to its defect, 
 and Green and Blue decrease 
 in proportion to their quantities, 
 as 
 
 through Gray. 
 
 Dark Yellow. 
 Light Red. 
 Light Pink. 
 Pink- Red. 
 
 From Seagreen 
 
 55 
 
 It 
 11 
 
 Green . 
 White . 
 Blue . . 
 Pink . . 
 Black . 
 Yellow 
 
 ii 
 
 Dark Red. 
 YelloAv-Red. 
 
 . In gradations tending to Green, 
 Green increases in proportion to 
 its defect, and Blue and Red decrease 
 in proportion to their quantities, as 
 
 From Pink through Gray. 
 
 „ Dark Seagreen. 
 
 Blue . . 
 White . 
 Red . . 
 Yellow 
 Black . 
 Seagreen 
 
 ii 
 
 ii 
 ii 
 
 Light Green. 
 Dark Yellow. 
 Yellow-Green. 
 Dark Green. 
 Sea s:reen- Green. 
 
 7. In gradations tending to Blue, Blue 
 increases in proportion to its defect, 
 and Red and Green decrease in 
 proportion to their quantities, as 
 
 From Yellow .... through Gray. 
 
 Red ,, Dark Pink. 
 
 White .... „ Light Blue. 
 Green ,, Dark Seagreen. 
 
 Seagreen 
 
 Black 
 Pink . 
 
 ii 
 
 Seagreen-Blue. 
 Dark Blue. 
 Pink- Blue 
 
 4. In gradations tending to Seagreen, 
 Blue and Green increase in pro- 
 portion to their defects, and Red 
 decreases in proportion to its 
 
 uuaiiw.LV, a 
 
 through Gray. 
 
 „ Pink. . . . 
 
 ii 
 
 Light Blue. 
 
 „ Black . . . 
 
 n 
 
 Dark Seagreen. 
 
 „ Yellow . . 
 
 a 
 
 Dark Green. 
 
 „ Green . . . 
 
 n 
 
 Seagreen- Green 
 
 „ White . . , 
 
 ii 
 
 Light Seagreen. 
 
 
 ii 
 
 Seagreen-Blue. 
 
 6. In gradations tending to Pink, Blue 
 and Red increase in proportion to 
 their defects, and Green decreases 
 in proportion to its quantity, as 
 
 From Green through Gray. 
 
 Yellow .... 
 Black 
 
 ii 
 ii 
 
 55 
 55 
 
 5) 
 55 
 
 Seagreen . 
 
 n 
 
 55 
 
 11 
 
 Blue . 
 White 
 Red . 
 
 Light Red. 
 Dark Pink. 
 Light Blue. 
 Pink-Blue. 
 Light Pink. 
 Pink-Red. 
 
 8. In gradations tending to Yellow, Red 
 and Green increase in proportion 
 to their defects, and Blue decreases 
 in proportion to its quantity, as 
 
 From Blue through Gray. 
 
 Seagreen. . . 
 
 55 
 55 
 
 )) 
 
 55 
 
 55 
 55 
 
 Black . 
 Pink. . 
 Red . . 
 
 White 
 Green 
 
 55 
 
 Light Green. 
 
 55 
 
 Dark Yellow. 
 
 55 
 
 Light Red. 
 
 55 
 
 Yellow-Red. 
 
 55 
 
 Light Yellow. 
 
 55 
 
 Yellow Green. 
 
 Contrast is the effect of the difference between two colours. The nature of the 
 contrast is determined by the direction of the line between the places of the colours 
 in the cube, and its strength by the length of that line. All contrasts of colours on 
 the same line, or on different lines parallel to the same, are of the same nature, only 
 
 E 
 
26 
 
 GRADATIONS AND CONTRASTS OF COLOUR. 
 
 varying in strength according to the distance between their places. They may 
 therefore be grouped in the same ways as gradations; and the extreme colours of 
 the principal gradations above-mentioned under all the leading groups of gradations, 
 will serve as. examples of the principal contrasts belonging to the corresponding 
 groups of contrasts. Thus the colours of distant points in lines parallel to a primary 
 axis form contrasts in Red, in Green, or in Blue ; parallel to a secondary axis, 
 contrasts in Seagreen, in Pink, in Yellow, and cross contrasts in Green and Blue, 
 in Blue and Red, and in Red and Green; parallel to a tertiary axis, contrasts in 
 Black and White, and cross contrasts in Red and Seagreen, Green and Pink, Blue 
 and Yellow. Colours belonging to opposite points on any two parallel sections of the 
 cube will be contrasts of equal strength and of the same nature : those belonging to a 
 sphere described about any point in the cube, will contrast equally, though all in 
 different ways, with their central colours. 
 
 Plate II. contains four series of sections of the cube, taken through the principal 
 and their intermediate colours (one hundred and twenty-five in all) The places of 
 the former are indicated by dark circles; of the latter by light ones; and a double 
 circle denotes the central Gray. Gradations parallel to any primary, secondary, or 
 tertiary axis, are indicated by full, broken, or dotted lines. The series in Fig. 1 is 
 at right angles with a primary axis, and therefore three distinct sets of this form may 
 be made; that in Fig. 2 is at right angles with a secondary axis, and may therefore 
 have six variations; that in Fig. 3 is at right angles with a tertiary axis, and may 
 have four variations. Fig. 4 gives one of the innumerable series of sections which may 
 be taken across other diameters; in its medial section lies a tertiary axis and one of 
 the three secondary axes perpendicular to it, so that it may have twelve variations. 
 
 Representations of the principal colours and their means (twenty-seven in all), 
 arranged as they would occur in sections at right angles with the several axes of the 
 cube, are all that can be given here. They may serve to illustrate those gradations 
 and contrasts which are formed in the directions of the axes, and also to give some 
 idea of the beauty and variety of regular combinations of colour, each distinguished 
 by its own peculiar effect. The characteristics of the several sections, together with 
 the names of the colours represented in each, are stated in front of the representations. 
 All the thirteen sets contain the same colours arranged in different ways; and by 
 ascertaining which of the axes lie in the middle section of each set, it will be easy 
 to see what groups of parallel gradations and contrasts that set presents. In every 
 set the place of the perfect complementary of any colour will be found by drawing 
 a line from the colour's place to the central Gray, and producing it to the same 
 
Fig. 3 
 
 Plate R. 
 
 Fia. 1 
 
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GRADATIONS AND CONTRASTS OF COLOUR 
 
 27 
 
 distance beyond. The colours of each section should produce if mixed its central 
 colour. Those of each pair of sections equidistant from a medial section, as well as 
 those in each whole set, should perfectly balance each other, producing, if mixed, the 
 central Gray. Those of the three corresponding sections of any of the cognate sets 
 should also neutralize each other.* 
 
 SECTIONS AT EIGHT ANGLES WITH THE PRIMARY AXES 
 
 OF RED OF GREEN 
 
 (1). (2). 
 
 OF BLUE 
 
 (3). 
 
 Colours containing full Red. 
 
 Yellow. Light Yellow. White. 
 
 Yellow-red. Light Red. Light Pink. 
 
 Red. Pink-red. Pink. 
 
 Colours containing half the full Red. 
 
 Yellow-green. Light Green. Light Seagreen 
 Dark Yellow. Gray. Light Blue. 
 
 Dark Red. Dark Pink. Pink-blue. 
 
 Colours containing no Red. 
 
 Green. Seagreen-green. Seagreen. 
 
 Dark Green. Dark Seagreen. Seagreen-blue. 
 
 Black. 
 
 Dark Blue. 
 
 Blue. 
 
 Colours containing full Green. 
 
 Colours containing full Blue. 
 
 Seagreen. Light Seagreen. White. 
 
 Pink. Light Pink. White. 
 
 Seagreen-green. Light Green. Light Yellow. 
 
 Pink-blue. Light Blue. Light Seagreen 
 
 Green. Yellow-green. Yellow. 
 
 Blue. Seagreen-blue. Seagreen. 
 
 i 
 
 Colours containing half the full Green. 
 
 Colours containing half the full Red. 
 
 Seagreen-blue. Light Blue. Light Pink. 
 
 Pink-red. Light Red. Light Yellow. 
 
 Dark Seagreen. Gray. Light Red. 
 
 Dark Pink. Gray. Light Green. 
 
 Dark Green. Dark Yellow. Yellow-red. 
 
 Dark Blue. Dark Seagreen. Seagreen-green. 
 
 Colours containing no Green. 
 
 Colours containing no Blue. 
 
 Blue. Pink-blue. Pink. 
 
 Red. Yellow-red. Yellow. 
 
 Dark Blue. Dark Pink. Pink-red. 
 
 Dark Red. Dark Yellow. Yellow-green. 
 
 Black. Dark Red. Red. 
 
 Black. Dark Green. Green. 
 
 * The representations of the first twelve sets of sections have been prepared alternatively on a white 
 or a black ground : the colours generally show better on the latter, but the effect of the gradations to Black 
 is in great measure lost, and on the whole it is questionable which should be preferred. For the use of those 
 readers who may wish to improve these necessarily imperfect imitations, a list of the pigments which have 
 appeared most eligible is given in connexion with the thirteenth set. It need hardly be mentioned that they 
 should be viewed in pure white light (that of a clear day when the sun is high), an excess of any hue in the 
 incident light altering the colours of the pigments, and destroying their balance. The effect of any single 
 section, or combination of corresponding sections, Is best observed when the rest are covered. 
 
28 
 
 GRADATIONS AND CONTRASTS OF COLOUR. 
 
 SECTIONS AT EIGHT ANGLES WITH THE SECONDARY AXES 
 
 OF SEAGREEN 
 
 (4). 
 
 OF PINK 
 (5). 
 
 OF YELLOW 
 (6). 
 
 Colours containing full Green and full Blue. 
 Seagreen. Light Seagreen. White. 
 
 Colours containing three-fourths the sum of full 
 Green and Blue. 
 
 Seagreen-green. Light Green. Light Yellow. 
 Seagreen-blue. Light Blue. ■ Light Pink. 
 
 Colours containing one-half the sum of full 
 Green and Blue. 
 
 Green. Yellow-green. Yellow. 
 
 Dark Seagreen. Gray. Light Red. 
 
 Blue. Pink-blue. Pink. 
 
 Colours containing one-fourth the sum of full 
 Green and Blue. 
 
 Dark Green. Dark Yellow. Yellow-red. 
 Dark Blue. Dark Pink. Pink-red. 
 
 Colours containing neither Green nor Blue. 
 Black. Dark Red. Red. 
 
 Colours containing full Blue and full Red. 
 Pink. Light Pink. White. 
 
 Colours containing three-fourths the sum of full 
 Blue and Red. 
 
 Pink-blue. 
 Pink-red. 
 
 Light Blue. 
 Light Red. 
 
 Light Seagreen. 
 Light Yellow. 
 
 Colours containing one-half the sum of Jull 
 Blue and Red. 
 
 Blue. Seagreen-blue. Seagreen. 
 
 Dark Pink. Gray. Light Yellow. 
 
 Red. Yellow-red. Yellow. 
 
 Colours containing one-fourth the sum of full 
 Blue and Red. 
 
 Dark Blue. Dark Seagreen. Seagreen-green. 
 
 Dark Red. Dark Yellow. Yellow-green. 
 
 Colours containing neither Blue nor Red. 
 
 Black. 
 
 Dark Green. Green. 
 
 Colours containing full Red and full Green. 
 Yellow. Light Yellow. White. 
 
 Colours containing three-fourths the sum of full 
 Red and Green. 
 
 Yellow-red. Light Red. Light Pink. 
 
 Yellow-green. Light Green. Light Seagreen. 
 
 Colours containing one-half the sum of full 
 Red and Green. 
 
 Red Pink-red. Pink. 
 
 Dark Yellow. Gray. Light Blue. 
 
 Green. Seagreen-green. Seagreen. 
 
 Colours containing one- fourth the sum of full 
 Red and Green. 
 
 Dark Red. Dark Pink. Pink-blue. 
 
 Dark Green. Dark Seagreen. Seagreen-blue 
 
 Colours containing neither Red nor Green. 
 Black. Dark Blue. Blue. 
 
GRADATIONS AND CONTRASTS OF COLOUR. 
 
 29 
 
 SECTIONS AT EIGHT ANGLES WITH THE SECONDARY CROSS AXES 
 
 OF GREEN AND BLUE 
 
 (7). 
 
 OF BLUE AND RED 
 
 (8). 
 
 OF RED AND GREEN 
 
 Colours containing full Green and no Blue. 
 Green. Yellow-green. Yellow. 
 
 Colours co?itaining half the full Green in excess of 
 Blue. 
 
 Seagreen-green. Light Green. Light Yellow. 
 Dark Green. Dark Yellow. Yellow-red. 
 
 Colours containing equal quantities of Green and 
 Blue 
 
 Seagreen. Light Seagreen. White. 
 
 Dark Seagreen. Gray. Light Red. 
 
 Black. Dark Red. Red. 
 
 Colours containing half the full Blue in excess of 
 Green. 
 
 Seagreen-blue. Light Blue. Light Red. 
 
 Dark Blue. Dark Pink. Pink-red. 
 
 Colours containing full Blue and no Green. 
 Blue. Pink-blue. Pink. 
 
 Colours containing full Blue and no Red 
 Blue, Seagreen-blue. Seagreen. 
 
 Colours containing half the full Blue in excess of 
 Red. 
 
 Pink-blue. Light Blue. Light Seagreen. 
 
 Colours containing full Red and no Green. 
 Red. Pink-red. Pink. 
 
 Colours containing half the full Red in excess of 
 Green. 
 
 Yellow-red. Light Red. 
 
 Dark Blue. Dark Seagreen. Seagreen-green. Dark Red. Dark Pink 
 
 Colours containing equal quantities of Blue and 
 Red. 
 
 Pink. 
 
 Light Pink. 
 
 White. 
 
 Dark Pink. 
 
 Gray. 
 
 Light Green 
 
 Black. 
 
 Dark Green. 
 
 Green. 
 
 Colours containing half the full Red in excess of 
 Blue. 
 
 Pink-red. Light Red. Light Yellow. 
 
 Dark Red. Dark Yellow. Yellow-green. 
 
 Colours containing full Red and no Blue. 
 Red. Yellow-red. Yellow. 
 
 Light Pink. 
 Pink-blue. 
 
 Colours containing equal quantities of Red and 
 Green. 
 
 Yellow. Light Yellow. White. 
 
 Dark Yellow. Gray. Light Blue. 
 
 Black. Dark Blue. Blue. 
 
 •Colours containing half the full Green in excess of 
 Red. 
 
 Yellow-green. Light Green. Light Seagreen. 
 Dark Green. Dark Seagreen. Seagreen-blue. 
 
 Colours containing full Green and no Red. 
 
 Green. 
 
 Seagreen-green. Seagreen. 
 
30 
 
 GEADATIONS AND CONTRASTS OF COLOUR. 
 
 SECTIONS AT EIGHT ANGLES WITH THE TERTIARY CROSS AXES 
 
 OF RED AND SEAGREEN 
 (10). 
 
 OF GREEN AND PINK 
 
 (11). 
 
 OF BLUE AND YELLOW 
 
 (12). 
 
 Colour containing full Green and Blue, and no 
 Red 
 
 Seagreen. 
 
 Colours containing three-fourths the sum of full 
 Green and Blue in excess of Red. 
 
 Light Seagreen. 
 Seagreen-blue. Seagreen-green. 
 
 Colours containing half the swn of full Green and 
 Blue in excess of Red. 
 
 White. 
 
 Light Blue. Light Green. 
 
 Blue. Dark Seagreen. Green. 
 
 Colours containing one-fourth the sum of Green 
 and Blue in excess of Red. 
 
 Light Pink. Light Yellow. 
 
 Blue-pink. Gray. Light Green. 
 
 Dark Blue. Dark Green. 
 
 Colours containing equal quantities of Red and of 
 the sum of Green and Blue. 
 
 Pink. Light Red. Yellow. 
 
 Dark Pink. Dark Yellow. 
 
 Black. 
 
 Colours containing one half of full Red in excess 
 of the sum of Gi een and Blue. 
 
 Pink-red. 
 
 Yellow-red. 
 
 Dark Red. 
 
 Colour containing jull Red and neither Green nor 
 Blue. 
 
 Red. 
 
 Colour containing full Blue and Red, and no 
 Green. 
 
 Pink. 
 
 Colours containing three-fourths the sum of full 
 Blue and Red in excess of Green. 
 
 Light Pink. 
 Pink-red. Pink-blue. 
 
 Colours containing half the sum of full Blue and 
 Red in excess of Green. 
 
 White. 
 
 Light Red. Light Blue. 
 
 Red. Dark Pink. Blue. 
 
 Colours containing one-fourth the sum of full Blue 
 and Red in excess of Green. 
 
 Light Yellow. Light Seagreen. 
 
 Yellow-red. Gray. Seagreen-blue. 
 
 Dark Red. Dark-blue. 
 
 Colours containing equal quantities of Green, and 
 of the sum of Blue and Red. 
 
 Yellow. Light Green. Seagreen. 
 
 Dark Yellow. Dark Seagreen. 
 
 Black. 
 
 Colours containing one half of full Green, in 
 excess of the sum of Blue and Red. 
 
 Yellow-green. Seagreen-green. 
 
 Dark Green. 
 
 Colour containing fill Red and Green, and no 
 Blue. 
 
 Yellow. 
 
 Colours containing three-fourths the sum of full 
 Red and Green in excess of Blue. 
 
 Light Yellow. 
 Yellow-green. Yellow-red. 
 
 Colours containing half the sum of full Red and 
 Green in excess of Blue. 
 
 White. 
 Light Green. Light Red. 
 
 Green. Dark Yellow. Red. i 
 
 Colours containing one-fourth the sum of full Red 
 and Green in excess of Blue. 
 
 Light Seagreen. Light Pink. 
 
 Seagreen-green. Gray. Pink-red. 
 
 Dark Green. Dark Red. 
 
 Colours containing equal quantities of Blue, and of 
 the sum of Red and Green. 
 
 Seagreen. Light Blue. Pink. 
 
 Dark Seagreen. Dark Pink. 
 
 Black. 
 
 Colours containing one half of full Blue, in excess 
 of the sum of Red and Green. 
 
 Seagreen-blue. Pink-blue. 
 
 Dark Blue. 
 
 Colour containing full Green and neither Blue nor Colour containing full Blue and neither Red nor 
 
 Green. 
 
 Red. 
 Green. 
 
 Blue. 
 

 
 
GRADATIONS AND CONTRASTS OF COLOUR. 
 
 31 
 
 SECTIONS AT RIGHT ANGLES 
 
 WITH THE TERTIARY 
 
 AXIS OF WHITE. 
 
 (13). 
 
 The following pigments, sufficiently permanent for ordinary 
 use, seem to give the nearest approach to the colours to which 
 corresponding numbers are prefixed in Table 13, and have 
 been used in this work to represent them. 
 
 Colour containing full Red, Green, and Blue. 
 1. White. 
 
 Colours containing Jive-sixths of the sum of full 
 Red, Green, and Blue. 
 
 2. Light Pink. 
 3. Light Seagreen. 4. Light Yellow. 
 
 Colours containing two-thirds of the sum of full 
 Red, Green, and Blue. 
 
 5. Pink. 
 
 6. Light Blue. 7. Light Red. 
 
 8. Seagreen. 9. Light Green. 10. Yellow. 
 
 Colours containing half the sum of full Red, 
 Green, and Blue. 
 
 11. Pink-blue. 12. Pink-red. 
 
 13. Seagreen-blue. 14. Gray. 15. Yellow-red. 
 
 16. Seagreen-green. 17. Yellow-green. 
 
 Colours containing one-third of the sum of full 
 Red, Green, and Blue. 
 
 18. Blue. 19. Dark Pink. 20. Red. 
 
 21. Dark Seagreen. 22. Dark Yellow. 
 
 23. Green. 
 
 Colours containing one-sixth of the sum of full 
 Red, Green, and Blue. 
 
 24. Dark Blue. 25. Dark Red. 
 
 26. Dark Green. 
 
 Colour containing no Red, Green, or Blue. 
 27. Black. 
 
 1. Chinese "White or 
 Constant White. 
 
 2. Rose Madder powder 
 (light). 
 3. Ceruleum with a little 4. Lemon Yellow or 
 
 Viridian (very light). King's Yellow (light). 
 
 5, Rose Madder powder 
 (full). 
 6. Cobalt powder 
 (light). 
 Ceruleum with a little 9. Emerald Green 
 Viridian (light). (rather light). 
 
 7. Vermilion 
 (light). 
 
 10. King's Yellow 
 (full). 
 
 11. Rose Madder powder 12 Carmine 
 
 with Smalt. . (rather light). 
 
 13. Ceruleum or 14. Blacklead 15. Cadmium, 
 
 Azure. with White. palest hue. 
 
 16. Emerald Green 17- Permanent Yellow 
 
 with Ceruleum with Emerald Green. 
 
 18. Cobalt powder 19. Crimson-lake with 
 (full). a little Smalt. 
 
 21. Ceruleum 
 with Viridian 
 
 23. Emerald Green 
 (full). 
 
 20. Vermilion 
 (full). 
 22. Olive 
 Green. 
 
 24. French 
 
 Blue. 
 
 25. Indian 
 Lake. 
 
 26. Viridian or 
 Veronese Green. 
 
 27. Lampblack 
 (full). 
 
32 
 
 GRADATIONS AND CONTRASTS OP COLOUR. 
 
 The following Plates show methods of combining portions of the sections in 
 Plate II., which may suggest innumerable other ways of grouping colours, according 
 to their natural affinities; and any such group may be readily contrasted with its 
 complementary group, or with its cognate groups. 
 
 Plate III. gives projections of the layers receding from an edge or a corner of the 
 cube. In Fig. 5 the series begins at one of the twelve edges, and ends at the two 
 opposite sides; there may therefore be twelve variations of this form. In Fig. 6 it 
 begins at one of the eight corners, and ends at the three sides opposite ; and there 
 may be eight variations, of which the simplest is that commencing with Black, since 
 the colours of the several layers in that case differ in intensity only. In Fig. 7 the 
 series begins again at one of the corners, but ends at the three opposite edges. This 
 kind also may therefore have eight variations. 
 
 Plate IV. gives developements of equidistant layers about the centre of the 
 cube, taken at right angles in Fig. 8 to the primary, in Fig. 9 to the secondary, and in 
 Fig. 10 to the tertiary axes. Every colour in each of these developements is 
 accompanied with its perfect complementary. All of them may be varied in many 
 ways by choosing different points for the middle of the developement 
 
 Plates V. and VI. give developements of the layers surrounding the axes of the 
 cube. The layers are developed about a primary axis, parallel to two primary medial 
 planes, in Fig. 11; to two secondary medial planes in Fig. 12. They are developed 
 about a secondary axis, parallel to one primary and one secondary medial plane, in 
 Fig. 13; parallel to two tertiary medial planes in Fig. 14. They are developed about 
 a tertiary axis, parallel to three secondary medial planes in Fig. 15, and parallel to 
 three of the medial planes of the fourth series of sections shown in Plate II., in 
 Fig. 16. The diagrams at the foot of Plate VI. give plans of the layers developed in 
 Figs. 11 to 16. Of the first and second kinds three variations may be made; of the 
 third and fourth, six; of the remaining two, four. Each presents a set of gradations 
 of the same class with that in its axis. The most interesting is perhaps that which is 
 formed about the axis of White, in the way indicated in Fig. 15 ; the colours 
 increasing in strength of hue in each successive layer.^ 
 
 * 
 
 * The subject of this chapter has been thus largely elaborated, in the hope that the classification of 
 gradations, contrasts, and natural combinations of colours, may be of practical use in ornamental art, and 
 perhaps lead to improved principles of design. 
 
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CHAPTER IX, 
 
 OCULAR MODIFICATIONS OF COLOUES. 
 
 Sensations excited by light continue for a certain time after their cause has ceased. 
 Thus when a constant light begins to act on any part of the retina, it seems to grow 
 stronger, until in a very short time it attains its full brightness, in consequence of 
 each successive luminous wave adding its effect to the unexpired effect of former 
 waves ; and whenever the light suddenly ceases or moves to a new part of the retina, 
 it leaves an image or spectrum of the luminous object, which gradually fades away. 
 This is easily seen when the eye after steadily viewing something bright is suddenly 
 enveloped in darkness; and it causes the luminous streak that follows such an object 
 when it moves. The duration of this spectrum varies somewhat with the state of 
 the eye and with the brightness of the exciting light; but unless the eye has been 
 strongly excited it is but very short, and in that case the colour of the spectrum does 
 not perceptibly vary from that of the light which produced it. But if the eye has 
 been very strongly excited, the duration of the spectrum is much greater; and with 
 extreme excitement it often changes colour, and sometimes even recurs again after 
 having vanished, as is seen when the eyes from being directed to the disc of the sun 
 are suddenly enveloped in total darkness.* 
 
 This self-action of the nervous tissues, commencing as soon as light impinges 
 upon the retina, and continuing until the spectrum finally vanishes after the light has 
 ceased, seems to be universally attended with a diminution of sensibility to the 
 action of light, which produces a much more sensible effect. The best evidence of 
 this is obtained by suddenly directing the eye, after fixedly contemplating any bright 
 object on a black ground, to a uniform gray surface of moderate brightness. That 
 part of the gray surface whose image occupies the part of the retina which was 
 
 * To see these effects the eyes must he not only suddenly closed, after viewing steadily the luminous 
 object, hut must he shaded from all light, the eyelids being hut imperfectly opaque. If this is not done the 
 effect is complicated with the circumstance mentioned in the next paragraph. 
 
 F 
 
34 
 
 OCULAR MODIFICATIONS OF COLOURS. 
 
 affected by the bright object, appears darker than the rest, and tinged with a hue 
 complementary to that of the object. This (which is usually called the " accidental 
 colour" of the object, and is often, though needlessly, attributed to a reaction in the 
 eye) may always be observed as long as the ocular spectrum produced by the bright 
 object can be seen in the dark; and when that changes colour, the spot to be seen on 
 the gray surface changes also, so as to be always complementary to it in hue. Hence 
 spring all those remarkable modifications of colours which have been elaborately 
 treated of by Chevreul and others, under the designations " simultaneous contrast," 
 and " successive contrast," and the knowledge of which is justly regarded of great 
 practical importance ; for in consequence of them some colours seem mutually 
 deeper or clearer in juxta-position, while others seem duller. The law according to 
 which all these effects are produced (the light not being greater than what the eye 
 can easily bear), and by which we can always tell beforehand what effect will be 
 produced when we look at two given colours in juxta-position, may be expressed 
 as follows : — 
 
 The excitement of any one of the simple sensations of colour on any part of the 
 retina is attended with a proportionate loss of sensibility for that same sensation. 
 
 As no colour (except Black) can be obtained perfectly free from mixture of 
 White, the general effect is to cause each of the colours seen in juxta-position to 
 appear more or less tinged with the complementary of the other, both in hue and in 
 brightness. Thus a colour is deepened by its complementary, dulled by its like, and 
 darkened by a brighter colour, as compared with its appearance when seen against 
 Black. Darker surroundings have less effect, but only against a back-ground of 
 perfect Black, the real colour appears in undiminished brightness. 
 
 By looking at a moderately shaded white surface through a perforation in a 
 strongly coloured paper, the change of the White to the complementary of the colour 
 of the perforated paper may be very clearly seen. The experiment may be varied 
 by putting other coloured surfaces in the place of the white. The like modifications 
 are still more strongly produced on parts of surfaces shaded from the light of a 
 sunbeam that has traversed coloured glass. The same cause makes daylight appear 
 blue, when it enters a chamber illuminated with the yellow light of lamps or candles ; 
 and produces many other phenomena, which an intelligent observer cannot but 
 notice. It is obvious that a perfect acquaintance with the true complementary 
 colours is the key to the practical use of this important principle in the Science 
 
 of Colour. 
 
 The following Table of the mutual modifications of our imperfect representations 
 by pigments of the eight principal colours may help to determine those of their 
 
OCULAR MODIFICATIONS OF COLOURS. 
 
 35 
 
 intermediate colours also. The colours as seen against Gray are taken as the 
 standards with which the modified colours are compared, because that comes nearest 
 to the conditions under which they are commonly seen, and it allows the apparent 
 modification by Black to be stated: though in truth a perfect Black neither modifies 
 nor is modified by any colour.* 
 
 
 COLOUR 
 
 WHEN PLACEI 
 ON 
 
 GRAY. 
 
 ) 
 
 ON 
 
 BLACK. 
 
 OCULi 
 
 ON 
 
 BLUE. 
 
 LR MODIFI 
 
 ON 
 
 GREEN. 
 
 CATIONS 
 
 ON 
 
 RED. 
 
 F THE SA1V 
 
 ON 
 
 SEAGREEN. 
 
 [E WHEN PLACED 
 
 ON ON 
 PINK. YELLOW. 
 
 ON 
 
 WHITE. 
 
 
 
 Black. 
 
 Duller 
 Black. 
 
 Yellowish 
 Black. 
 
 Pinkish 
 Black. 
 
 Seagreenish 
 Black. 
 
 Reddish 
 Black. 
 
 Greenish 
 Black. 
 
 Bluish 
 Black. 
 
 Deeper 
 Black. 
 
 
 
 Blue. 
 
 Lighter 
 Blue. 
 
 Duller 
 Blue. 
 
 Pinkish 
 Blue. 
 
 Seagreenish 
 Blue. 
 
 Duller 
 
 Pinkish 
 Blue. 
 
 Duller 
 
 Seagreenish 
 
 Blue. 
 
 
 
 Deeper 
 Blue. 
 
 Darker 
 Blue. 
 
 
 
 Green. 
 
 Lighter 
 Green. 
 
 Yellowish 
 Green. 
 
 Duller 
 Green. 
 
 Seagreenish 
 Green. 
 
 Duller 
 
 Yellowish 
 
 Green. 
 
 Deeper 
 Green. 
 
 Duller 
 
 Seagreenish 
 
 Green. 
 
 Darker 
 Green. 
 
 
 
 Red. 
 
 Lighter 
 Red. 
 
 Yellowish 
 Red. 
 
 Pinkish 
 Red. . 
 
 Duller 
 Red. 
 
 Deeper 
 Red. 
 
 Duller 
 
 Yellowish 
 Red. 
 
 Duller 
 
 Pinkish 
 
 Red. 
 
 Darker 
 Red. 
 
 
 
 Seagreen. 
 
 Lighter 
 Seagreen. 
 
 Greenish 
 Seagreen. 
 
 Bluish 
 Seagreen. 
 
 Clearer 
 Seagreen. 
 
 Duller 
 Seagreen. 
 
 Duller 
 Greenish 
 Seagreen. 
 
 Duller 
 
 Bluish 
 
 Seagreen. 
 
 Darker 
 Seagreen. 
 
 
 
 Pink. 
 
 Lighter 
 Pink. 
 
 Reddish 
 Pink. 
 
 Clearer 
 Pink. 
 
 Bluish 
 Pink. 
 
 Duller 
 
 Reddish 
 
 Pink. 
 
 Duller 
 Pink. 
 
 Duller 
 Bluish 
 Pink. 
 
 Darker 
 Pink. 
 
 
 
 Yellow. 
 
 Lighter 
 Yellow. 
 
 Clearer 
 Yellow. 
 
 Reddish 
 Yellow. 
 
 Greenish 
 Yellow. 
 
 Duller 
 Reddish 
 bellow. 
 
 Duller 
 
 Greenish 
 
 Yellow. 
 
 Duller 
 Yellow. 
 
 Darker 
 Yellow.. 
 
 
 . 
 
 White. 
 
 Clearer 
 White. 
 
 Yellowish 
 White. 
 
 Pinkish 
 White. 
 
 Seagreenish 
 
 White. 
 
 Reddish 
 White. 
 
 Greenish 
 White. 
 
 Bluish 
 
 White. 
 
 Duller 
 
 White. 
 
 
 * Except that it may sometimes be perceptibly tinged in the contrary way with the direct spectrum 
 of a surrounding bright ground, as the eye glances from that to the Black. 
 
 ■■■^■■HDOOnBBflMV 
 
 QfMaan 
 
CHAPTER X 
 
 MENTAL EFFECTS OF COLOURS. 
 
 The mental effects which attend the three colour-sensations are difficult to define, 
 though always more or less perceptible when we contemplate the colours in which 
 those sensations severally predominate. Words that properly belong to other things 
 are usually employed to describe them, such as exciting, lively, cheerful, warm, 
 advancing, for the effect of Red ; refreshing, gentle, soft, and pleasant, for the effect 
 of Green ; and quiet, sober, cool, retiring, for the effect of Blue. In reality these are 
 perhaps but different degrees of the same effect, and amount to no more than this, 
 that Red is more exhilarating than Green, and Green more than Blue. If so, the 
 fact may well be supposed to arise from the wave-periods and lengths being greater in 
 the red rays than they are in the green, and greater in the green than in the blue ; or 
 from the actual force or vis viva of the waves being greater in the red than in the 
 green, and greater in the green than in the blue, as experiments on the heating power 
 of the prismatic rays have proved; A like difference may be observed in the effect 
 of musical notes of lower or higher pitch. 
 
 To compare fairly the effect of different colours in this respect it seems most 
 reasonable that all should be reduced to such degrees of brightness as to form equal 
 contrasts with Black, which can have no positive effect; so that their places in the 
 cube of colours would be all equally distant from the corner of Black. If, for 
 instance, a set of primary and secondary colours, with those of the intermediate hues, 
 and a neutral Gray, all so related, were arranged according to the average wave- 
 lengths, or the average forces of the rays that produce them, they would stand 
 somewhat as follows: — (1) Red; (2) Darkish Yellow-Red or Orange; (3) Dark 
 Yellow or Olive Green, and Darkish Pink-Red or Crimson; (4) Darkish Yellow- 
 Green ; (5) Green, Dark Pink or Purple, and a Lightish Gray ; (6) Darkish 
 Seagreen-Green ; (7) Dark Seagreen and Darkish Pink-Blue or Mauve; (8) Darkish 
 Seagreen-Blue ; and (9) Blue: and most persons, it is probable, will concur in 
 
MENTAL EFFECTS OF COLOURS. 
 
 37 
 
 thinking that these colours, arranged according to the liveliness of their effect, would 
 stand in much the same order. 
 
 If we would compare in like manner the effects of the principal colours in their 
 full brightness, White, Yellow, Pink, Red, would evidently take the first rank in 
 liveliness as well as force; and then Seagreen, Green, Blue, Black. In mental effect 
 there is thus the nearest affinity between Yellow and White, and between Blue 
 and Black. 
 
 It is also remarkable that there is a more conspicuous variety in the assemblage 
 of colours in which Red predominates, than in those in which Green and Blue 
 predominate ; or, in other words, there is more apparent difference between Red and 
 Blue, and between Red and Green, than there is between Green and Blue. This 
 adds to the vivacity of compositions in which colours allied to Red prevail, and to 
 the quietness of those from which they are excluded; as may be seen in the various 
 arrangements of colours given in Chapter VIII. 
 
CHAPTER XL 
 
 THE HARMONY OF COLOURS. 
 
 If the harmony of colours is proved by the pleasure which they give when viewed in 
 succession, or side by side, and when viewed in combination, then the three simple 
 colours must be considered as eminently harmonious. Each of them separately is 
 beautiful, pleasing the eye especially on its first appearance after darkness, but their 
 beauty is increased and lasts longer when they are presented in couples one after 
 another, as when the Red and the Green, the Green and the Blue, or the Blue and 
 the Red prismatic rays are caused to enter the eye side by side without the rest, and 
 the eye is rapidly directed from one to the other; and they give still greater and more 
 lasting pleasure when all three in like manner are presented side by side or in succession 
 to the eye. When seen in conjunction, again, they are equally beautiful, whether 
 presented in pairs, as in Yellow, Seagreen, or Pink, or altogether, as in pure White; 
 nor is their beauty lost in either case though one is presented in greater strength than 
 another, as in the intermediate hues, and indeed in all colours which are not perfectly 
 neutral, no colour being without some mixture of white. It is also very remarkable 
 that the more nearly neutral the colour is with the same degree of brightness, the longer 
 can the eye contemplate it with pleasure. It would seem as if, there being three kinds 
 of nervous fibres, or three modes of action, one for each simple sensation of colour, 
 alternations of rest and excitement in each of them give pleasure ; for such a 
 supposition sufficiently accounts for the principles of the harmony of colour without 
 assuming that the excitement of one sensation creates a desire for the like excitement 
 in the rest. 
 
 Whether there is or is not any relation between the periods of the vibrations 
 which most powerfully excite the several simple sensations of colour, similar to that 
 which exists between musical notes in the same octave, has not, it seems, yet been 
 conclusively shown. The wave-periods of those rays which appeared in Mr. 
 Maxwell's experiments to present the deepest Red, Green, and Blue, though not 
 
SSHU 
 
 THE HARMONY OF COLOURS. 
 
 39 
 
 formiDg any harmonical series, do not differ very widely from the series 1,1, 1, or 
 that of the musical notes, C, E, and G, the simplest and best of any series of 
 three that can be found within an octave. To form that series the extreme terms 
 should be more distant from the mean. But owing to the weakness of the light near 
 the ends of the spectrum, it seems probable that the relative depth of colour in the 
 Red and Blue rays falling outside of those mentioned above is not accurately determined 
 by the experiments, and it is not easy to persuade the eye that the depth of colour is really 
 less in those outer rays. Hence it may yet be possible that the same relation exists between 
 Red, Green, Blue, in colour, as between do, mi, sol, in music; and it must be admitted 
 that not only the analogy between light and sound, arising from their vibratory nature, 
 but also several circumstances attending the sensations excited by light, make it pro- 
 bable that such a relation does really exist. Moreover there is evidently a greater 
 difference between Red and Green, and between Red and Blue, than there is between 
 Green and Blue; which corresponds with what is true of the notes C and E, C, and 
 G, and E and G. But it does not seem that the sequence of colours in the spectrum 
 can be explained on these principles without supposing that the sensation of Red is 
 also excited in a minor degree by vibrations an octave higher than those of the C, 
 and the sensation of Blue by vibrations an octave lower than those of the G. 
 
 If this idea of the harmonical relations of colour is true, the eye might be com- 
 pared to a musical instrument of three strings, tuned to give out those notes only ; and 
 that all possible variations of colours correspond to sounds which are symphonies of 
 these notes only, sounded in various degrees of intensity. But in any case, since the 
 three simple sensations, as before stated, are beautiful and pleasant to the eye, both 
 separately and simultaneously, in succession or in combination, it seems clear that 
 (strictly speaking) that there can be no discord in colours.* 
 
 There are, however, different degrees of pleasure derived from different successions 
 or combinations; and though any variation is preferable to unbroken uniformity, some 
 are far more agreeable than others. The chief grounds of excellence will perhaps be 
 found in the following points: — 
 
 * Some consider that there are colours which, when seen side by side, produce a disagreeable and even 
 painful impression, and therefore maintain the existence of real discords in colour. But conceive the eye 
 viewing a continuous surface of some uniform brightness and hue ; it is hard to suppose that the introduction 
 of any variety in either quality could then take place without being contemplated with positive pleasure 
 (supposing of course that the eye is not too strongly excited). However great the charm of a good compo-^ 
 sition in colours, and the disappointment felt by the educated eye at the want of that charm in other compo- 
 sitions, I am persuaded that the difference is merely one of degree, and that no two colours produce together 
 a positively unpleasant effect. 
 
40 
 
 THE HAEMONY OF COLOURS. 
 
 (1.) The due balance of the colours in quantity or strength, or the equivalence 
 of the Eed, Green, and Blue contained in the whole composition ; 
 
 (2.) The symmetry, resemblance, or correspondence in nature and extent, of the 
 gradations and contrasts in different parts of the composition; 
 
 (3.) The variety of the colours, and of the gradations and contrasts presented. 
 
 The first of these is carried out with the nearest approach to perfection when 
 every colour is either accompanied with its perfect complementary, or with two or 
 more other colours of such sort that the mean or average of the whole is a neutral 
 Gray : the contrasting colours not being necessarily in juxta-position, but so conveniently 
 arranged as that the eye may readily refresh itself by passing from one to another. If 
 the colours do not quite neutralize each other, an excess of strength in some may be 
 compensated by an increased quantity of others, with the like general effect. But it 
 is not essential that this mean or average, which gives the tone to the whole composi- 
 tion, should be neutral, though it undoubtedly ought to be so for a composition which 
 is to fill the eye for any long continuance. Much of the charm of small compositions, 
 or transient effects, often arises from the predominance of some particular hue. Thus 
 in a landscape, while Red and Green almost universally prevail in the objects below us 
 and immediately around us, they are balanced by the brighter though paler blue light 
 of the heavens and the distance, the tone of the whole being white; while bowers 
 and rocks in nature, and compartments of dwellings in art, which are visited for a 
 limited time, may owe much of their beauty to the prevalence (not too strong) of 
 particular hues in their colours. Small compositions, again, such as flowering plants- 
 in nature, and pictures in art, may have great strength of hue in their general tone, 
 and yet please the more in consequence ; but if this effect is to continue, they should 
 be balanced either by the general tone of the surrounding objects, or by other similar 
 objects of an opposite tone at no great distance from them, to which the eye may turn 
 for relief and fresh excitement. In such compositions, again, unity of effect is aided 
 not only by the general prevalence of some hue, but also by giving prominence to some 
 colour on which the eye will naturally rest more than on the others, and which will 
 perform the same office as the key note in a musical composition. 
 
 The second point would be perfectly attained where colours change in all directions 
 according to different laws of gradation, each of which continues the same in the same 
 direction ; a change which may take place with any degree of slowness down to that 
 which is imperceptible. But the rate of change may be different in different directions ; 
 or it may vary in different places, even from nothing to infinity, in which case spaces 
 presenting a uniform colour would adjoin to others presenting other uniform colours, 
 and there would be contrast and no gradation. But it is not essential that every part 
 
THE HARMONY OF COLOURS. 
 
 41 
 
 of a composition should exhibit the same sort of changes; on the contrary, if there 
 are parts on which the eye is to dwell, and which give a character to the whole, they 
 ought to be marked with peculiar changes in the gradations, or something singular in 
 the contrasts. To this head, chiefly, belongs the beauty arising from evenness of 
 colour and truth of gradation; also that arising from unity of design, which gives a 
 character to the whole composition, as well as that arising from the distribution of 
 light and shade, and of various hues, in such a manner as to make them naturally 
 inhance each other's value, and increase the effect of the whole. In nature the 
 principal gradations and contrasts are made in light and shade ; and the important 
 bearing which the disposition of these has upon the beauty of the scene is evident. 
 
 The third point, the variety of the colours gradations and contrasts presented, 
 inhances the richness and charm of a composition, though it increases the difficulty of 
 securing the satisfactory attainment of the two former points. Variety of colours 
 involves first the distance of the extreme colours from each other, or from the mean 
 or average colours ; and its attainment in the highest degree requires the utmost possible 
 depth and clearness in the extreme colours, as well as the greatest number of changes. 
 Variety of gradations and contrasts involves not only the exhibition of gradations and 
 contrasts between different colours, but also the exhibition of gradations of various 
 degrees of quickness, from that which is imperceptibly slow, or the continuation of 
 uniform colour, to that which is infinitely quick, or the juxta-position of different 
 colours. 
 
 G 
 
CHAPTER XII. 
 
 APPENDIX ON DICHEOMISM AND DEFECTIVE COLOUE-VISION. 
 
 DICHEOMISM. 
 
 There are many persons (far more, at least of the male sex, than is usually 
 supposed), who though they see all the prismatic rays usually visible, can 
 distiuguish in them only two different hues. The rays which to the ordinary 
 trichromic vision are red, as well as those which are green, almost always appear 
 to such persons of the same hue with the yellow rays that lie between them; the 
 seagreen rays appear gray, and the blue and violet appear blue. Hence it is 
 evident that they are capable of only two sensations of colour, which are 
 complementary to each other, and are doubtless identical, or nearly so, with the 
 Yellow and Blue of ordinary eyes. All natural objects are of course in their 
 eyes coloured accordingly. A bright scarlet like that of Eed-lead, and a green, like 
 that of Emerald Green, and even the brilliant red and green signal lights used on 
 railways, are to such persons indistinguishable in hue, and differ from the colour of 
 the clearest yellow pigments in depth alone. Verdigris is indistinguishable from a 
 shade of White, and so also is a certain compound of Red and Blue, if the Red 
 predominates (for the red rays affect eyes of this kind less strongly with the sensation 
 of Yellow than the blue rays with the sensation of Blue, as is evident from the fact 
 that Rose Madder usually appears to them a good pale Blue, and Crimson lake, or 
 even Carmine a dark dull Blue). Good Yellows on the contrary, affect such eyes, 
 in all probability, with a deeper or stronger sensation of Yellow than can be excited 
 in the ordinary eye; and what is white to the one kind of vision is the same to 
 the other, so that in the whole spectrum the collected strengths of Yellow and 
 Blue are equal. 
 
 All the possible variations of colour in this kind of dichromic vision may 
 be comprized within a plane rectangle, whose opposite corners are Black and 
 White, and Blue and Yellow; like that medial section of the cube of colours 
 which is given in Series 9, (page 29). 
 
HHHHHHnHHH 
 
 ™nMmff S8^BHBHI 
 
 APPENDIX ON DICHROMISM AND DEFECTIVE COLOUR- VISION. 
 
 43 
 
 Besides this, there may be another kind of dichromic vision, for Dr. G. Wilson* 
 mentions one person who did not distinguish Chrome-Yellow and Orange from 
 Pink and Crimson, which could hardly be if he distinguished Green and Blue; so 
 that Red and Seagreen were perhaps his only two sensations. 
 
 It may be noticed also that some cases are recorded in which it seems that 
 there was no distinction of hue, and the only difference in the appearance of 
 surfaces consisted in brightness. But these, like the last, do not seem to have 
 been sufficiently investigated; and if they really exist, apart from defective colour- 
 
 vision, are rare. 
 
 DEFECTIVE COLOUR- VISION. 
 
 There are also many persons who appear to be wholly or partially insensible to 
 rays belonging to the less refrangible end of the spectrum. This defect is most 
 common amongst persons of dichromic vision, but is not confined to them; since 
 some who are almost insensible to the red rays in general, are able to distinguish 
 Yellow and Green, and, with a very strong light, Red also. To such persons deep 
 Reds are almost indistinguishable from Black. Candle-light, from the small proportion 
 of Blue rays which it contains, is much darker to them than to others ; and the rays 
 emitted by heated substances become luminous only at higher temperatures. It is 
 easy to see that this defect must alter the apparent colours of all objects, and 
 unless accompanied with some corresponding difference of capacity for another 
 colour-sensation, must make them approach in hue towards Seagreen or Blue. Use, 
 however, makes almost unnoticeable any universally predominant hue; as we still call 
 those surfaces white which reflect all the luminous rays, though when seen by candle- 
 light they are really yellow. 
 
 A less common defect is want of due sensibility to the more refrangible or 
 violet rays; and this has been found to occur in some instances by itself, and in 
 others, conjoined with the former defect. These peculiarities vary much both in 
 extent and character.f 
 
 * Treatise on Colour-blindness (Edinburgh, 1855). 
 
 f See a Memoir by Dr. E. Rose, of Berlin, translated in The Phil. Mag., February, 1866. 
 
 THE END. 
 
Nearly ready for the Press, by the same Author, 
 
 COLOUR-LORE, 
 
 AN INTRODUCTION TO THE SCIENCE OF COLOUR. 
 
 It is the object of this work to recount the principal theories, observations, and discoveries that have been made 
 about colours, from the earliest ages to the present time, and then to discuss more fully several points which hitherto, 
 as the author believes, have been either unnoticed or very imperfectly treated. But since colours are effects produced 
 in the eye by the different rays of light, their study is necessarily connected with that of the nature of those rays, 
 and their action on the eye, touching upon that wonderful chain of causes and effects through which it has pleased 
 Him, who said, Let there be light, to present the image of His glorious creation to our minds, not under the 
 plain garb of a simple light and shade, but adorned with the resplendent beauty and ever-changing charms of a three- 
 fold system of colour ; and to assist the reader to see the true bearing of the discoveries which have been made from 
 time to time about colours, as well as on account of the intrinsic interest of the subject itself, a comprehensive 
 popular outline in logical order of the whole science of etherial waves, and especially of light and its action on the 
 eye, has been added by way of introduction. Thus the work will be comprized under the following divisions : — 
 
 PART I. 
 
 AN OUTLINE OF THE SCIENCE OF LIGHT. 
 
 Chap. I. Elasticities of bodies in general, and the ori- 
 gination and propagation of waves of normal and 
 of transverse vibration in a medium filling soace. 
 Illustrations and examples of such waves. 
 
 Chap. II. The nature of light. The constitution of 
 the ether, both free and in conjunction with matter, 
 necessary to account for the phenomena of light. 
 
 Chap. III. The origination of luminous waves. "What 
 is intended by Rays of light. Distinctions of rays, 
 (1) as to the periods of their waves, attended Avith 
 distinctions of colour ; (2) as to the directions and 
 forms of their vibrations. 
 
 Chap. IV. Diffraction, or the lateral spreading of a 
 luminous wave partially intercepted, and the con- 
 sequent divergence of rays of light. 
 
 Chap. V. Dispersion, or the unequal retardation of 
 waves of longer and shorter periods, in material 
 media. 
 
 Chap. VI. Refraction and Reflection, or the formation 
 of new compound waves when luminous waves im- 
 pinge on the surface of a new medium. The forms 
 of the new wave-surfaces in the two media, and 
 consequent directions of the reflected and refracted 
 rays. Effects of mirrors, and of lenses and prisms ; 
 colours arising from refraction. The refractive and 
 dispersive powers of bodies. 
 
 Chap. VII. Refraction and Reflection continued. The 
 modifications in direction, form, and intensity of the 
 resulting vibrations in the new waves. Colours 
 
 Effects of the 
 
 arising 
 
 o 
 
 arising from reflection in two ways 
 texture of bodies. 
 
 Chap. VIII. Plane polarization and double refraction of 
 light by certain media. 
 
 Chap. IX. Circular polarization and double refraction 
 of light by certain media. 
 
 Chap. X. The interference of waves. Its exhibition in 
 various simple cases of diffraction, reflection, re- 
 fraction, and double refraction. Colours 
 from interference in different cases. 
 
 Chap. XL Absorption, or the gradual extinction of 
 luminous waves by material media, the principal 
 cause of the proper colours of bodies. Fluorescence 
 and other phenomena attending absorption. 
 
 Chap. XII. The action of light on the eye ; the sensa- 
 tions of colours produced by it ; trichromic vision ; 
 the three simple sensations, and their compounds ; 
 dichromic vision ; defective colour - vision ; the 
 blending of the sensations excited in the two eyes ; 
 irradiation ; the continuance of the sensations 
 excited ; the changeableness of those excited by 
 strong lights ; the diminution of sensibility under 
 the action of light. 
 
 Chap. XIII. Recapitulation of the causes of the variety 
 of colours in nature ; the importance of the study 
 of the solar spectrum ; the marvels of light ; what 
 has been learnt by the aid of colour, and what may 
 yet be learnt. 
 
 ■■^^^^■■R* 
 
 Bi^B^^^^BHBHIflBH^^IHBH^HBIHH^IHIHHH^HBHBH 
 
 "V»?pPlH^ -*j. 
 
46 
 
 PART II. 
 
 AN ACCOUNT OF THE PRINCIPAL OPINIONS, OBSERVATIONS, AND DISCOVERIES, ANCIENT AND MODERN, WHICH 
 
 BEAR UPON THE SCIENCE OF COLOUR. 
 
 ■HI 
 
 Chap. I. Opinions of the ancient philosophers, as stated 
 by Plato, Aristotle, Lucretius, Seneca, Pliny, Pto- 
 lemy, and others. What was known to Alhazen, 
 Vitello, Albertus Magnus. Leonardo da Vinci's 
 remarks on colours. The revival of learning and 
 commencement of experimental philosophy. Kepler, 
 De Dominis, Snellius, and Descartes. The theory 
 of Descartes. The old philosophy still maintained ; 
 Sir Kenelm Digby ; Father Kircher ; Zucchius, 
 Fabri, Dechales. The observations and doctrine of 
 Grimaldi. The views of Hooke ; Boyle's experi- 
 ments; his statement of the ignorance of his age 
 about colours. 
 
 Chap. II. Newton's discoveries about light and colour. 
 A summary of his experiments. His recapitulation 
 of what they prove. His observations on the colours 
 of bodies; his refutation of Hooke's opinion. His 
 division of the prismatic colours. His rule for 
 finding the colour of a mixture of any prismatic 
 rays. Explanation of the theory of the rule. His 
 experiments on the colours of thin plates. His 
 measurement of the thicknesses producing the 
 different colours. Deductions from these measure- 
 ments. His queries on the cause of the sensations 
 of colour. 
 
 Chap. III. Huyghen's views on the nature of light ; his 
 explanation of certain phenomena; his notion of the 
 composition of colours. Euler's theory of light and 
 colours. His thoughts on the nature of colours, on 
 their analogy with sound ; and on the way opaque 
 bodies are rendered visible. Mayer's doctrine of 
 three primary colours ; its adoption by Scheffer and 
 others. Lambert's observations on the intensities 
 of natural colours; and on the brightness of sur- 
 faces; more recent results obtained by Seidel and 
 Zollner. Lambert's experiments on the combination 
 of coloured lights ; his table of the quantities of 
 light reflected and transmitted by glass; his remarks 
 on the prismatic colours. Delaval's experiments on 
 the colours of bodies. 
 
 Chap. IV. The elder Herschel's observations on the 
 illuminating powers of the prismatic rays. His 
 discovery of invisible rays beyond the red rays. 
 Hitter's and Wollaston's discovery of invisible rays 
 beyond the violet rays. The importance of the 
 invisible rays in the theory of colour. Wollaston's 
 discovery of dark bands in the solar spectrum. His 
 division of the colours. Young's observations on 
 the same. The difference between Wollaston's and 
 Newton's spectrum. The need of more exact defini- 
 
 tion of the colours. Young's observations on the 
 colours of the simple and mixed prismatic rays. 
 His method of exhibiting the combinations of 
 colours. His theory of the cause of the colour 
 sensations. His discovery of the interference of 
 light ; and measurement of wave-lengths. His 
 suggestion that luminous waves are -transverse. 
 Fresnel's investigations ; proof the Avave theory. 
 
 Chap. V. Fraunhofer's survey of the prismatic solar 
 spectrum. His plan of the dark lines. His curve 
 of intensity of light. Considerations affecting this 
 curve. Leslie's remarks upon it, and description of 
 the prismatic colours. Herschel's description of the 
 spectrum. Fraunhofer's grating spectrum ; his 
 determination of the wave-lengths of the principal 
 lines. Mosotti on the grating spectrum. 
 
 Chap. VI. Brewster's researches on absorption. His 
 theory of three kinds of light, Red, Yellow, Blue.- 
 The objections of other philosophers. The opinion 
 of Professor Stokes ; the result of other experi- 
 ments ; and on the supposed primary colours. The 
 utility of his idea of three overlapping spectra. 
 His view of the cause of the colours of natural 
 bodies. Herschel's experiments on absorption. His 
 remarks on the different colours of different thick- 
 nesses of absorbing media. His suggestion as to 
 the cause of absorption. Baron Von Wrede's theory 
 of the same. Herschel on the green of vegetation; 
 on the different colour sensations. His reasons for 
 not rejecting yellow as a primary colour. His ex- 
 periments on the effects of lowering the intensity of 
 colored rays. Extract from Brewster and Gladstone 
 on the colours of feeble light. Herschel's description 
 of a method of analysing natural colours. 
 
 Chap. VII. First notices of fluorescence by Herschel 
 and Brewster. Stokes' discovery of its nature. Its 
 effect on the apparent colours of bodies. Early theo- 
 ries as to the cause of colours of bodies. Oersted's 
 theory of two kinds of reflection. Its defects. 
 Provost's theory of reflection, supei-ficial and inter- 
 nal. Its defects. His mode of exhibiting the pure 
 colours of bodies. Arago's mode of distinguishing 
 
 O Co 
 
 (he lights coming from a body. Haidinger's dis- 
 covery of complementary colours from reflection 
 and absorption. Stokes' discovery of the opacity of 
 bodies to their colours by reflection. His exposition 
 of the causes of the colours of bodies. The analysis 
 of those of Permanganate of potash. 
 Chap. VIII. Dove's observations on the difference in 
 the relative brightness of coloured surfaces under 
 
 
47 
 
 different degrees of light. Field's remarks on the 
 same subject. Helmholtz' first experiments on the 
 mixture of the prismatic rays. Grassmann's essay on 
 the same subject. Helmholtz' further experiments. 
 His measurement of wave-lengths of complementary 
 rays. His remarks on colours ; on the distribution 
 of complementary colours in the spectrum ; on the 
 difficulty of blending some of them. Supposed defect 
 of sensibility. His omission to notice the proportions 
 in which the complementary rays neutralize each 
 other. His experiments on the relative brightness 
 of the prismatic rays. Different degrees of light. 
 His observations on their different depths of hue. 
 His discussion of Newton's colour-circle. His plan 
 of the relations of the prismatic colours. 
 
 Chap. IX. Maxwell's experiments on mixing colours by 
 rotation; his diagram representing the hues, the 
 purity, and the brightness of the pigments experi- 
 mented on. The uses and defects of the method. 
 His conclusion as to the truth of the theory of three 
 colour-sensations. His experiments on the compo- 
 sition of the prismatic colours; his diagram repre- 
 senting the hue and purity of the colours of the 
 rays experimented on; his deduction that all colours 
 are compounded of three primary colours ; and what 
 those primary colours are. Explanation of the 
 method, and of the results. His diagram represent- 
 ing the intensities of the colours of the same rays. 
 Explanation of the results. His conclusion that no 
 part of the spectrum produces a pure sensation. 
 His observations as to the uniformity of vision in 
 ordinary eyes ; and as to the accuracy with which 
 the eye distinguishes colours. 
 
 Chap. X. The nature of the invisible prismatic rays. 
 Young's experiments and conjectures as to the 
 extra -violet rays, discovered by Ritter and Wol- 
 laston. Discoveries of Arago, Berard, Somerville, 
 Sutherland, Hunt, Becquerel, and Draper, respecting 
 them. Becquerel's conclusion as to their nature. 
 Professor Stokes' discoveries. The extreme extra- 
 violet rays of the electric light. Esselbach's 
 calculations of their wave-lengths. Researches on 
 the extra-red rays discovered by the elder Herschel; 
 discoveries by Seebeck, Berard, Forbes, Herschel, 
 Melloni, Knoblauch, and others, respecting them. 
 Draper on the light emitted by heated bodies. 
 Different views as to the nature of the visible and 
 invisible rays. Experiments by Masson and 
 Jamin ; conclusion that the rays differ in wave- 
 time only. Observations by Franz as to how far 
 they penetrate the eye. Midler's experiments on 
 the heating powers of the different solar rays. His 
 calculation of the wave-lengths of extreme rays. 
 His observations on the diffractive spectrum. 
 Tyndall's experiments on rays emitted by incandes- 
 cent bodies; on the force of the extra-red solar 
 rays ; on the perviousness of the eye, and the 
 
 insensibility of the retina to them ; on their 
 absorption by water and other substances ; on their 
 intensity in the spectrum of the electric light ; his 
 conjectures as to the cause of incandescence and its 
 analogy with fluorescence, in certain cases. The 
 instance of the lime light. 
 Chap. XI. Observations on the colours of the usually 
 invisible rays, by Helmholtz ; and by Brewster and 
 Gladstone. Gladstone on the very refrangible light 
 of incandescent vapour of mercury ; Kirchhoff and 
 Bunsen on the least refrangible red rays of incan- 
 descent rubidium. Chevreul's description of the 
 prismatic colours. General view of the discoveries 
 made respecting the prismatic rays, given in three 
 diagrams; — (1) Of the curves of wave-lengths and 
 of intensity (according to Midler) of all the known 
 solar radiations, with the principal absorption lines ; 
 
 (2) Of the wave-lengths and light intensity in the 
 luminous rays, with the colours of the different 
 parts according to Newton, Wollaston, Fraunhofer, 
 Helmholtz, Maxwell, and Chevreul, and the positions 
 of Helmholtz' complementary rays, and Maxwell's 
 standard colours, and of the principal dark lines; 
 
 (3) Of the curve of wave-lengths and light intensity 
 as calculated by Mosotti, for Fraunhofer's diffraction 
 spectrum. 
 
 Chap. XII. On the ocular modifications of colours. 
 Observations on the persistence of the sensations of 
 colour, and on accidental colours, by Boyle; Newton ; 
 De la Hire; D'Arcy; Jurin ; Buffon; Scherffer ; 
 GCpinus; R. Darwin; Comparetti; Young; Goethe; 
 Brewster; Tortual; Midler; Wheatstone; Plateau; 
 Fechner ; Seguin ; and Melsen. On the effects 
 of simultaneous contrast and successive contrast by 
 Venturi, Brewster, and Chevreul. 
 Chap. XIII. The classification and nomenclature of 
 colours. Mayer's system of the double pyramid on 
 a triangle. Herschel's suggested correction of it. 
 Lambert's endeavour to carry out Mayer's system. 
 Schiffermuller's treatise on colour- systems. Werner's 
 nomenclature, and attempt to define colours. Runge's 
 colour- sphere ; comparison of his system with 
 Mayer's. Methods of arrangement and nomen- 
 clatures adopted by Field and Hay ; by Forbes ; 
 and by Chevreul. 
 Chap. XIV. Dichromism and defective colour-vision. 
 Early notices of such cases in the Philosophical 
 Transactions. Dalton's Memoir. Memoirs by 
 Seebeck, Wartmann, and others. Wilson's Treatise. 
 Pole's Memoir, with Herschel's observations thereon. 
 Maxwell's experiments, and diagram exhibiting the 
 intensity in which the two sensations are excited by 
 the prismatic rays. Gladstone's remarks on defective 
 colour- vision. Memoir by E. Rose. 
 Chap. XV. Concluding remarks. Reasons for omittino- 
 theories founded on the results of mixing pigments 
 or solutions. Field's doctrine of the proportionate 
 
48 
 
 power of colours; his standard of colours. The 
 fallacy of such experiments. The mischief of false 
 theories. Seasons for not detailing certain other 
 theories and experiments. Fanciful analogies 
 between colours and musical notes. Hasty de- 
 
 ductions from unguarded experiments. Recent 
 endeavours to overthrow the Newtonian doctrine. 
 Goethe's' notions about colour. The truth of nature 
 superior to all mistaken theories. 
 
 PART III. 
 
 AN ESSAY ON SEVERAL POINTS IN THE SCIENCE OF COLOUR WHICH SEEM TO REQUIRE FURTHER NOTICE. 
 
 Chap. I. The colours of combinations of continuous 
 parcels of the prismatic rays, the best possible. 
 The value of a correct judgment of colours; the 
 best means of attaining to it. Numerous easy and 
 instructive experiments designed for that purpose. 
 
 Chap. II. The natural system of colours ; the con- 
 struction and parts of the cube of colours. The 
 utility of distinguishing the several parts. 
 
 Chap. III. Gradations of colour : different methods of 
 grouping them. Contrasts of colour: corresponding 
 methods of grouping them. Reasons for classifying 
 
 gradations and contrasts. 
 
 Illustrations of gradations 
 
 Arrangements of colours according to their 
 
 and of contrasts. 
 
 Chap. IV. 
 
 natural relations. Plane sections of the cube. 
 Illustrations of them. Various combinations of 
 parts of them. Suggestiveness of these methods. 
 Endless variety of colour compositions. 
 
 Chap. V. Limits of possible colours. Approximation 
 to the relative intensities in which the three colour- 
 sensations are excited by the pure prismatic rays, 
 and by the combinations of continuous parcels of 
 such rays. Curves showing the results. Tables of 
 
 the limits of the principal colours in strength, depth, 
 and clearness, with different degrees of brightness. 
 The possible portion of the cube of colours ; its 
 variation with different intensities of white light. 
 Explanation thereby of several natural phenomena 
 of colour. 
 Chap. VI. Ocular modifications of colours. The true 
 law according to which they are effected. Various 
 experiments exhibiting them. The utility of a 
 knowledge of them. Tables describing the effects 
 produced. 
 Chap. VII. Suggested analogy between colours and 
 musical notes. The possibility of the three colour- 
 sensations being caused by the excitement of accord- 
 ant vibrations. The manner in which vibrations 
 of given rates only may be excited by the etherial 
 waves, and the succession of colours in the spectrum 
 be produced. Points of resemblance and of difference 
 between the ear and the eye. 
 Chap. VIII. Considerations on the mental effects of 
 different colours. The pleasure afforded by colours. 
 Principles proper to good compositions of colour. 
 
 No pains have been spared to make the proposed work complete and trustworthy, in the hope that it may be an 
 accepted compendium of what is known on a subject in which, interesting and useful as it is, information has now 
 to be sought from a multitude of sources, and much baseless theory has been reiterated. It has been the occupation 
 of those working hours which the author has been able to spare from the calls of business, pupils, and family, for 
 many years ; the books consulted would form a considerable library, but the literature of the subject has not taken 
 more time than those experiments and original researches by which he has been divested of many preconceived 
 notions, and has been led, he believes, nearer to the truth of nature on this beautiful and inviting subject. 
 
 Aware, however, of several deficiencies which he has not been able to supply, and fearing there are many more, 
 he would be much obliged by any kind suggestions or the loan of any work which may seem, from the above 
 abstract, to have escaped due notice. There are several German books which he has not been able to meet with, 
 such as' Lambert's « Farbenpymmide," 1772, and Wunsch's " Versiiche uber die Farben," 1792. Any suggestion 
 from colourists, also, as to the best means of representing the colours intended to be represented in the coloured 
 diagrams in the present treatise, will be thankfully received. 
 
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