i6 



NATURE 



[Nov. 6, 1879 



The luminosity of a pure colour will thus be = J, and if 

 mixed with an equal quantity of white, it will be " 



that of pure white being 1. 



After these preliminary explanations the author pro- 

 ceeds to develop the chief features of his theory of colour- 

 sensation, adopting the principles previously laid down as 

 applicable to black and white only. He expresses the 

 chief points in two propositions : — 



1. There are six fundamental sensations of the visual 

 substance, arranged in three pairs. These are — 



Black and white. 

 Blue and yellow. 

 Green and red. 



2. Each of these pairs corresponds to a dissimilation 

 process and an assimilation process of a special kind, so 

 that the visual substance is subject to chemical change, or 

 change of matter, in a threefold way. 



The three kinds of change may be either connected or 

 independent ; the latter is the simplest supposition, and 

 the author finds it convenient to assume that there are 

 three different components of the visual substance, which 

 he considers as corresponding with the three pairs of sen- 

 sation, and which may, in short, be spoken of as the black- 

 white, the blue-yellow, and the red-green substances 

 respectively. 



These three substances are not all present in equal 

 quantities ; the black-white one is much more richly pro- 

 vided in the visual organs than either of the others, and 

 the latter are not present in equal amount. 



As in the black-white substance, both dissimilation and 

 assimilation go on, the former corresponding to the white 

 and the latter to the black sensation ; so the same pro- 

 cesses take place in the two other substances, but with 

 much less activity, whence the weight of the six funda- 

 mental sensations is very unequal ; relatively great in the 

 black and white ; relatively less in the four colours. The 

 author does not venture to pronounce which of a pair of 

 colours corresponds to the D and which to the A action. 



All rays of the visible spectrum Jiave a dissimilating 

 action on the black-white substam e, but the different rays in 

 different degrees. On the blue-yellow or the red-green 

 substance certain rays alone have a D action, certain 

 others an A action, and certain others no action at all. 

 Thus each of the three substances has, so to speak, its 

 own particular sensation-spectrum ; and in the actual 

 impression of the spectrum these three overlap or intersect 

 each other. 



The spectrum of the black-white substance is the 

 brightest in the yellow, and diminishes on both sides. 



The spectrum of the blue-yellow substance consists of 

 two parts, one yellow and one blue, which are separated 

 by a spot which is lightless for this substance ; this is 

 the place of the pure green. 



The spectrum of the red-green substance consists of 

 three parts, one green in the middle, and one red at each 

 end, giving two spots which are lightless for this spectrum, 

 i.e., the place of the pure yellow and that of the pure blue. 



Thus the total spectrum of the visual substance has 

 three distinguished physiological points, where, in addi- 

 tion to the white, only one fundamental colour is visible, 

 viz., yellow, green, or blue. The real red is very small in 

 the spectrum, for the spectral red contains much yellow. 

 The first part of the spectrum from red to pure yellow is 

 thus a mixture of white, red, and yellow; the second, 

 from yellow to green, is a mixture of white, yellow, and 

 green ; the third, from green to blue, of white, green, and 

 blue ; the fourth, from blue to the end, of white, blue and 

 red. In the pure yellow, green, and blue, only these 

 colours mixed with white are seen. 



Mixed light appears colourless, when it acts, on the 

 blue-yelloiv or the red-green substance, with equal D 



and A power, for then both effects neutralise each othen 

 and the action on the black-white substance alone appears. 

 For this reason two objective kinds of light, which, when 

 mixed, give white, are not complementary but antagonistic; 

 they do not produce the white by their combination, but 

 merely destroy each other and leave visible the white 

 which was already there. 



The excitability 'Errcgbarkcii) may vary, doubly, on 

 each of the three substances in the same manner as 

 before explained, so that one arrd the same objective 

 light-mixture may not only appear lighter or darker, but 

 varying in colour according to the proportions present of 

 the six kinds of excitability, giving various conditions 

 [Stimmungeii) of the visual organ. 



Suppose the eye to have been long at rest, so as to be 

 in what has been called the neutral condition (the D and 

 A-actions being equal in regard to all three of the visual 

 substances), the total sensation will comprise the two 

 opposite colour-sensations of each colour-substance, as 

 well as the white and black sensation, but the latter will 

 be much the stronger, and more predominant, as each of 

 the colour-sensations will neutralise its opposite one, i.e., 

 the red and green will neutralise each other, and so will 

 the yellow and blue. This neutralising effect, however, 

 does not take place in regard to the black-white substance, 

 because the assimilation causing the black sensation is 

 not produced by any direct rays ; if there were any solar 

 rays causing A-action in the black-white substance, then 

 solar light might, under certain conditions, be invisible, 

 like an equal mixture of blue and yellow, or red and green. 

 Hence the resulting total sensation, when the eye is at rest, 

 is the mixture of white and black, called neutral gray. 



Now if, in this condition, we allow the light of any one 

 of the simple spectral colours, say green, to fall on the 

 eye, it strengthens the already present green sensation, 

 and also the white one. The result is a mixed sensation, 

 formed of green, with a considerable mixture of white, 

 and also with some mixture of black, already existing in 

 the neutral gray. Thus it is that even the spectral colours 

 never appear pure, but always obviously contain white 

 and black impurity. 



Pigmental colours are still more impure, as they reflect 

 mixed light, in which only certain rays are more or less 

 weakened. To the colour-action of those rays which are 

 hereby unneutralised by antagonistic rays, is added not 

 only the action of such rays on the black-white substance, 

 but also the action of the whole remaining neutral-mixed 

 light which only acts on the black-white sensation. 



Mixed light which gives us a beautiful colour-impression 

 may, if colourless light be added, appear entirely or nearly 

 colourless, because the original colour-sensation is already 

 mixed largely with colourless light. Even the spectral 

 colours quickly lose their force and become pale and 

 whitish, when the vision is steadfastly fixed for a time on 

 one of the dark lines, for the excitability of the colour- 

 substance is quickly lowered, while that of the black-white 

 sensation retains its power. 



In general the conditions are very unfavourable, in 

 ordinary vision, for the powerful production of colour; for 

 both on the light and the dark parts the colour-sensations 

 are rendered strongly impure, if not almost entirely sup- 

 pressed by the black-white action. In the darkened eye, 

 in which the latter has less power, the colour-conditions 

 are more favourable, and hence the after-images are often 

 highly coloured. The most favourable conditions are 

 where the black-white substance has been fatigued by 

 dazzling light, and hence the after-images in such cases 

 often show colours almost more powerful than even those 

 of the spectrum, where, in the original objects, scarcely 

 any colour was appreciable. 



It must be always borne in mind that every visual 

 sensation, however it may appear, is really a mixture of 

 all the six fundamental sensations. That one of the six, 

 which has relatively the greatest weight, gives the 



