Nov. 6, 1879] 



NATURE 



'5 



that more than two simple colours can never be evident 

 in the same mixture. 



There is also a natural peculiarity in the capabilities of 

 the various simple colours for combining with each other. 

 Red will combine with yellow or blue, but not with green, 

 i.e., red and green can never be both distinguished in the 

 same combinations. Similarly, yellow will combine with 

 red or green, but not wiih blue. Green will combine with 

 blue or yellow, but not with red ; and blue will combine 

 with red or green, but not with yellow. In other words, 

 we find the natural law that on the one hand Red and 

 Green, and on the other hand Blue and Yellow, are never 

 visible together in the same combination ; they are incap- 

 able of combining together. What the cause of this is it 

 is impossible to say. It is customary in books on physics 

 to say that red and green, or blue and yellow combined, 

 make white ; but this is only true when by red, green, 

 &c, are meant ether vibrations, and not physiological 

 sensations; for to insist that red plus green makes white 

 in the same sense that red plus blue makes violet would 

 be contrary to common observation, for in pure white 

 there is no trace of any colour-sensation whatever. It 

 must, therefore, be something in the natural connection 

 of these colours with our sense of vision that makes these 

 combinations incompatible with each other ; and for the 

 sake of using a short expression for this relation, the author 

 proposes, in consideration of their so-to-speak inimical 

 relations to each other, to call red and green, or blue and 

 yellow opposite, or contrary, or antagonistic colours 

 {Gegeu/arbeu). 1 



The combination of any simple colour-sensation with 

 that of another (not antagonistic) gives a sensation of 

 a different hue {Farbenton), and the hue will vary accord- 

 ing to the proportions of the components. Thus different 

 proportions of red and blue will give different hues of 

 violet, and so on. The whole scale of these may be 

 conveniently expressed by a " colour circle." Divide a 

 circle into four quadrants, and at each point of division 

 put one of the four simple colours, arranging the an- 

 tagonistic ones diametrically opposite to each other. The 

 intermediate portions may then be filled in with compound 

 hues, passing in regular gradations from one of the simple 

 colours to the next adjoining on either side. Such a 



1 The following observations of Sir John Herschel (extracted from his 

 Report on my paper on Colour BLndness, presented to the Royal Society in 

 1856) strikingly en firm the views expressed by Heir Hering: — 



'* It is as necessary to distinguish between our sensations of colour and the 

 qualities of the light producing [hem, as it is to distinguish between bitterness, 

 sweetness, sourness, saltness. &c, and the chemical constitution of the 

 several bodies which we call bitter, sweet, &c. Whatever their views of 

 prismatic analysis or composition might suggest to Wcllaston and Young, I 

 cannot persuade myself that either of them recognised the sensation of 

 greenness as a constituent of the sensati ns they received in viewing chrome 

 yellow or the petal of a marigold on the one hand, and ultra-marine, or the 

 bluesalera.cn the other; or that they could fail to recognise a certain redness 

 in the colour of the violet, which Newton appears to have had in view when he 

 regarded the spectrum as a sort of octave of colour, tracing, in the repetition 

 of redness in the extreme refrangible ray, the commencement of a higher 

 octave too feeble to affect the sight in its superior tones. Speaking of my 

 own sensations I should say that in fresh gTass or the laurel leaf, 1 do not 

 recognise the sensation either of blue or yellow, but something sui generis; 

 while on the other hand I never fail to be sensible of the presence of the red 

 clement in either violet or any of the hues to which the name of purple is 

 indiscriminately given : and my impression in th : s respect is borne out by the 

 similar testimony of persons, good judges of colour, whom I have questioned on 

 the subject 



"It seems impossible to reason on the joint or compound sensation which 

 ought to result from the supra-position in the sensorium of any two or more 

 sensations which we may please to call primary. 



" Declaring red and green to be primary sensations, and yellow a mixture 

 of them [isj a proposition which needs only to be understood to be 

 epudiated— quite as decidedly as that the sensation of greenness is a mixture 



■ the 



. the 



by the asserted 



of the sensations of blueness and yell.wness, and f. 

 complete want of suggestion of the so-called sanpli 

 complex ones. 



" From these premises it w^uld seem the easiest possible step I 

 the non-existence of yellow as a primary colour. But this conclusion I am 

 unable to admit in the face of the facts 11) that a yellow ray incaoable of 

 prismatic analysis into green and red, may be shown to exist, bjlh in the 

 spectrum and in flames in which soda is present ; and (2) that neither red nor 

 green, as sensations, are in the remotest degree suggested by that yellow in 

 lis action on the eye. 



"Whether under these circumstances the vision of normal-eyed persons 

 should be termed trichromic or tetrachromic, seems an open question." — 

 Proceedings of the Royal Society, vol. x., 1S59-60, p. 72— W. P. 



circle, if supposed to be divided into very minute grada- 

 tions, will contain all possible hues of colour. And every 

 late, both simple and compound, will have, diametrically 

 opposite to it, its natural antagonistic colour} Now 

 every possible hue of colour may appear in many different 

 states of "purity," or, as it is often called, " saturation." 

 These various states are called by the author " nuances," 

 and they are caused by the mixture with the hue in question 

 of various proportions of black and white, i.e., various 

 degrees of the black-white sensation. Thus red may be 

 mixed with black, white, medium gray, light gray, or dark 

 gray, every grade giving a different nuance of the red 

 hue. 2 The different hues and the different nuances of 

 each taken together will comprise all colour-sensations 

 possible. 



The pure colour-sensations are unknown to us ; what we 

 experience are always nuances containing white or black. 

 The fact that some colours, and some parts of the spectrum, 

 appear so much brighter than others is due, the author 

 asserts, to the fact of the former containing more white. 

 He conceives that if the pure colour-sensations alone 

 could be experienced they would all be of equal brilliancy, 

 and would probably have the same degree of luminosity 

 as the medium gray ; for as each colour-sensation must 

 be considered as an independent one, there is no reason 

 for attributing to any of them the particular effect that we 

 are accustomed to connect with whiteness in particular. 



The varieties of colour-sensation admit of being ex- 

 pressed in a formula. Varieties of hue may be expressed 



by the proportions of each colour they contain ; thus 



Red 

 or — — 



Blue 



of it will = 



Red 

 will express all varieties of violet ; the blueness 



Blue and the redness = Re<L 



Blue + Red, " Blue -f Red. 



The nuance of a hue, or its degree of purity, is ex- 

 pressed by the ratio the weight of the pure colour-sensa- 

 tion bears to the whole weight of the combination ; thus 



the purity of a nuance of Blue = — , „„. — =-. — r. 



* ' Blue + White + Black 



Thus, suppose a red is mixed with double its weight of 



medium gray, then there are equal weights of each sensa*- 



tion, and the purity will be 



° x'x =1 = °-3> 

 i + i + I 3 



For a compound hue, for example, violet, the purity 



Blue + Red 



will be 



For example, 



Blue + Red -}- Black -f White 

 suppose the weight of the blue and red sensations are 4 

 each, the white 1, and the black 3, forming a dark nuance 

 of violet, the purity 



= 4 + 4 = i = . 66 . 



4 + 4+ 1+3 12 

 The author further forms an estimate of the bright- 

 ness or luminosity {Helligkeit) of a nuance by the 

 formula 



White + \ Colour 

 White + Black + Colour' 

 Thus in the former of the above two examples, the 

 luminosity will be 



i+jv _ 1 



In the latter 



1 ■+■ 1 + 1 



_ i + - _ 

 1 + 3 + 8 



12 



colours ape 

 being opposite to 



1 Such a circle has been published by Chevreul, but 

 arranged somewhat differently, the antagonistic I 

 each other— W. P. 



2 In technical language mixtures of a colour w.th white are called tints; 

 with black, shades; but this nomenclature is imperfect, according to the 

 author's v.ew, as the various nuances are considered by him to contain black 

 and white together.— W. P. 



