CHAP, in.] 



SIGHT. 



95 



already said, it is difficult in these matters of sensation, to 

 distinguish between peripheral, retinal, and central, cerebral 

 events ; we may accordingly extend the above view to the whole 

 visual apparatus, central as well as peripheral, and suppose that 

 when rays of a certain wave-length fall upon the retina, they in 

 some way or other, in some part or other of the 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 



R O Y G B V 



FIG. 147. DIAGRAM TO ILLUSTRATE BERING'S THEORY OF COLOUR VISION. 



The lines R.O.Y.G.B.V. indicate, as in Fig. 146, the position on the spectrum of 

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



The line r.g., which includes a space, shaded vertically, is intended to represent 

 the effect of rays of different wave-lengths on the red-green visual substance. 

 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 >/.b. similarly represents the behaviour of the yellow-blue substance, 

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



The line to. similarly indicates the white-black substance, unshaded, katabolic 

 (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 



