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THEORIES OF COLOUR VISION. [Book hi, 



visual apparatus, induce or promote katabolic changes and so 

 give rise to a sensation of a certain kind, while rays of another 

 wave-length similarly induce or promote anabolic changes and 

 so give rise to a sensation of a different kind. 



The theory of Hering, of which we are now speaking, applies 

 this view to the six fundamental sensations, and supposes that 



Fig. 151. Diagram to illustrate Hering's Theory of Colour Vision. 



The lines B.O.Y.G.B. V. indicate, as in Fig. 150, the position on the spectrum 

 of red, orange, yellow, green, blue and violet. 



The line r.g., which indicates a space, shaded vertically, is intended to rep- 

 resent the effect of rays of different wave-lengths on the red-green visual sub- 

 stance. In the red, orange and yellow up to the line Y., the effect is katabolic, 

 one of dissimilation (red sensation) . Y. marks the position of equilibrium ; 

 beyond this the effect is anabolic, one of assimilation (green sensation). Beyond 

 the blue, B. the effect (indicated by a broken line) is represented as once more 

 katabolic. 



The line y.b. similarly represents the behaviour of the yellow-blue substance, 

 shaded horizontally, katabolic (yellow) up to G., anabolic (blue) beyond. 



The line w. similarly indicates the white-black substance, unshaded, kata- 

 bolic (sensation of white) along the whole length of the spectrum. 



each of the three pairs is the outcome of a particular set of 

 katabolic and anabolic changes; these we may provisionally speak 

 of as changes in a distinct visual substance, without attempting 

 to decide whether the changes are retinal or cerebral or both. 

 The theory supposes the existence of what we may call a red- 

 green visual substance, of such a nature that so long as its me- 

 tabolism is normal, katabolic and anabolic changes being in 

 equilibrium, we experience no sensation, but that when katabo- 

 lic 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 



