96 THEORIES OF COLOUR VISION. [BOOK in. 



visual substance, of such a nature that so long as its metabolism is 

 normal, katabolic and anabolic changes being in equilibrium, we 

 experience no sensation, but that when katabolic changes 

 (changes of dissimilation is Hering's own term), are increased, we 

 experience a sensation of (fundamental) red, and when anabolic 

 changes (changes of assimilation) are increased 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 antago- 

 nistic. Further these substances are of such a kind that while the 

 white-black substance is influenced in the same way though to dif- 

 ferent 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. 147). Thus in the part of the spectrum which 

 we call red, the rays promote a large katabolism of the red-green 

 substance with comparatively slight effect on the yellow-blue 

 substance , hence our sensation of red. In that part of the spec- 

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

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

 does not lead to an excess of either katabolism or anabolism, this 

 substance being neutral to them ; hence our sensation of yellow. 

 The green rays, again, promote anabolism of the red-green sub- 

 stance, 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 spectrum, 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 kata- 

 bolism 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 dis- 

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

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

 holtz theory, the secondary result of the mixture of primary 

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

 the range of the visible spectrum) give rise to the sensation of 



