CHAP, in.] SIGHT. 903 



we experience a sensation of (fundamental) green. A similar 

 yellow-blue visual substance is supposed to furnish through 

 katabolic changes, a yellow, through anabolic changes a blue 

 sensation ; and a white-black visual substance similarly provides 

 for a katabolic sensation of white and an anabolic sensation of 

 black. The two members of each pair are therefore not only 

 complementary but also antagonistic. Further these substances 

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

 influenced in the same way though to different degrees by rays 

 along the whole range of the spectrum, the two other substances 

 are differently influenced by rays of different wave-length (see 

 Fig. 151). Thus in the part of the spectrum which we call 

 red, the rays promote a large katabolism of the red-green sub- 

 stance with comparatively slight effect on the yellow-blue sub- 

 stance ; hence our sensation of red. In that part of the spectrum 

 which we call yellow the rays effect a large katabolism of the 

 yellow-blue substance but their action on the red-green sub- 

 stance does not lead to an excess of either katabolism or anabo- 

 lism, this substance being neutral to them ; hence our sensation 

 of yellow. The green rays, again, promote anabolism of the 

 red-green substance, leaving the anabolism of the yellow-blue 

 substance equal to its katabolism ; and similarly blue rays cause 

 anabolism of the yellow-blue substance, and leave the red-green 

 substance neutral. Finally at the extreme blue end of the spec- 

 trum, the rays once more provoke katabolism of the red-green 

 substance, and by adding red to blue give violet. When orange 

 rays fall on the retina, there is an excess of katabolism of both 

 the red-green and the yellow-blue substance ; when greenish- 

 blue rays are perceived there is an excess of anabolism of both 

 these substances ; and other intermediate hues correspond to 

 varying degrees of katabolism or anabolism of the several visual 

 substances. 



When all the rays together fall on the retina, the red -green 

 and yellow-blue substance remain in equilibrium, but the white- 

 black substance undergoes great katabolic changes ; and we say 

 the light is white. 



Such are the two main theories of colour vision ; and much 

 may be said in favour of both of them ; at the same time both 

 of them present difficulties. We may perhaps regard as the 

 distinctive feature of Hering's theory the view that white is an 

 independent sensation, and not, as according to the Young- 

 Helmholtz theory, the secondary result of the mixture of pri- 

 mary sensations. In Hering's theory rays of all wave-lengths 

 (within the range of the visible spectrum) give rise to the sen- 

 sation of white, whatever may be the colour sensation produced 

 at the same time ; a fully saturated colour, one wholly unmixed 

 with white, according to this view does not exist. This assump- 

 tion enables us to explain much more readily than does the 



