VISION 1057 



covered with translucent paper, the latter appears as a coloured covering 

 spread over the field. The mind does not recognize that at the grey 

 patch there is any breach of continuity in this covering; it is therefore 

 assumed that the greenish veil extends over this spot too. Now, the 

 grey seen through the translucent white paper is objectively white i.e., 

 sends to the eye the vibrations which together would give the sensation 

 of white light. But with a green veil in front of it, this could only 

 happen if the really grey patch was the colour complementary to green 

 that is, rose-red. The mind, therefore, judges falsely that the patch 

 is red. Hering has severely criticized this theory of Helmholtz as to 

 false judgments; and the weight of evidence certainly seems to be in 

 favour of the view that simultaneous, like successive, contrast is due 

 to the influence of one portion of the retina, or retino-cerebral apparatus, 

 on another. 



Hering's Theory of Colour Vision. The Young- Helmholtz theory 

 of colour vision has not met with universal acceptance. The best- 

 known rival theory is that of Hering, who takes his stand upon the 

 fact that certain visual sensations (red, yellow, green, blue, white, 

 black) do appear to us to be fundamentally distinct from each other, 

 while all the rest are obviously mixtures of these. Accepting these 

 six as primary sensations, he assumes the existence in the visual 

 nervous apparatus of substances of three different kinds, which may 

 be called the black-white, the green-red, and the blue-yellow. Like 

 all other constituents of the body, these substances are broken down 

 and built up again in other words, undergo disassimilation and 

 assimilation, destructive and constructive metabolism. The sensa- 

 tions of black, of green, and of blue he supposes to be associated with 

 the constructive, and the sensations of white, of red, ^and of yellow 

 with the destructive, processes in the three substances. The black- 

 white substance is used up under the influence of all the rays of the 

 spectrum, but in different degrees; the smaller the quantity of light 

 falling on the retina, the more rapidly is it restored, and the more 

 intense is the sensation of black. The green-red substance is built 

 up by green rays, broken down by red. The blue- yellow substance 

 is destroyed by yellow rays, restored by blue. A prominent dif- 

 ference between this and the Young- Helmholtz theory, and, so far 

 as it goes, an advantage, is that Hering's theory attempts to assign 

 a direct objective cause for the visual sensations of white, black, 

 and yellow, as well as for red, green, and blue, instead of making 

 the sensations depend upon the magnitude of the stimulation pro- 

 cess. When any of the visual substances are consumed at one part 

 of the retina, they are supposed to be more rapidly built up in the 

 surrounding parts, and in this way many of the phenomena of 

 simultaneous ^contrast receive an easy and natural explanation. The 

 same is true of the simpler phenomena of after-images or successive 

 contrast. But in applying the theory to the more complicated 

 phenomena difficulties soon emerge, which, to say the least, are not 



less formidable than those connected with the Young- Helmholtz 



6 7 



