288 



Report of the Committee on Colour-Vision. 



to each other. In each pair the one colour is complementary to 

 the other ; white to black, red to green, and yellow to blue. 



Now, in the chemical changes undergone by living substances, 

 we may recognise two main phases, an' upward constructive 

 phase in which matter previously not living becomes living', 

 aud a downward destructive phase in which living matter 

 breaks down into dead or less living matter. Adopting this 

 view we may, on the one hand, suppose that rays of light, 

 differing in their wave-length, may affect the chemical changes 

 of the visual substance in different ways, some promoting con- 

 structive changes (changes of assimilation), others promoting 

 destructive changes (changes of dissimilation) ; and on the other 

 hand, that the different changes in the visual substance may give 

 rise to different sensations. 



We may, for instance, suppose that there exists in the 

 retina a visual substance of such a kind that when rays of light 

 of certain wave-lengths — the longer ones for instance of the red 

 side of the spectrum — fall upon it, dissimilative changes are induced 

 or encouraged, while assimilative changes are similarly promoted 

 by the incidence of rays of other wave-lengths, the snorter ones 

 of the blue side. But, it must be remembered, that in dealings 

 with sensations it is difficult to determine what part of the 

 apparatus causes them ; we may accordingly extend the 

 above view to the whole visual apparatus, central as well as 

 peripheral, and suppose that when rays of a certain wave-length 

 fall upon the retina, they in some way or other, in some part or 

 other of the visual apparatus, induce or promote dissimilative 

 changes and so give rise to a sensation of a certain kind, while 

 rays of another wave-length similarly induce or promote assimila- 

 tive changes and so give rise to a sensation of a different kind. 



The hypothesis of Hering applies this view to the six fundamen- 

 tal sensations spoken of above, and supposes that each of the three 

 pairs is the outcome of a particular set of dissimilative and 

 assimilative changes. It supposes the existence of what we 

 may call a red-green visual substance, of such a nature that so 

 long as dissimilative and assimilative changes are in equilibrium, 

 we experience no sensation, but that when dissimilative changes 

 are increased, we experience a sensation of (fundamental) red^ 

 and when assimilative changes are increased we experience a 

 sensation of (fundamental) green. A similar yellow-blue visual 

 substance is supposed to furnish, through dissimilative changes, 

 a yellow, through assimilative changes a blue sensation ; and a 

 white- black visual substance similarly provides for a dissimilative 

 sensation of white and an assimilative sensation of black. The 

 two members of each pair are therefore not only complementary 

 but also antagonistic. Further, these substances are supposed to be 

 of such a kind that while the white-black substance is influenced in 

 the same way, though in different degrees, by rays along the whole 

 range of the spectrum, the two other substances are differently 

 influenced by rays of different wave-length. Thus, in the part of 



