828 A MANUAL OF PHYSIOLOGY 



coloured covering spread over the field. The mind does not recog. 

 nise 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 />., 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 of the colour complementary to green that is, rose- 

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

 Hering has severely criticised 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. 



Bering's Theory of Colour Vision. The Young- Helmholtz 

 theory of colour vision has not met with universal accept- 

 ance. The best-known rival theory is that of Hering, who 

 takes his stand upon the fact that certain sensations of light 

 (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 sensations 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 sub- 

 stances. 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. 

 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 contrast receive an easy and natural 

 explanation. 



