Chap, hi.] SIGHT. 901 



thus appear to us distinct, and which we may speak of as 

 ' native ' or ' fundamental ' sensations, are white, black, red, 

 yellow, green, blue. Each of these seems to us to have noth- 

 ing in common with any of the others, whereas in all other 

 colours we can recognize a mixture of two or more of these. 



This result of common experience suggests the idea that 

 these fundamental sensations are the primary sensations, con- 

 cerning which we are inquiring. And Hering's theory at- 

 tempts to reconcile, in some such way as follows, the various 

 facts of colour vision with the supposition that we possess 

 these six fundamental sensations. The six sensations readily 

 fall into three pairs, the members of each pair having analogous 

 relations to each other. In each pair the one colour is com- 

 plementary to the other; white to black, red to green, and 

 yellow to blue. 



The little we know about the actual nature of sensations 

 leads us to believe that the nervous processes which are at the 

 bottom of sensations are, like other nervous processes, the out- 

 come of metabolic changes in nervous substance. We shall 

 presently call attention to the view that vision originates in 

 the metabolic changes of a certain substance (or substances) 

 in the retina, that the metabolism of this substance, which has 

 been called visual substance, is especially affected by the inci- 

 dence of light, and that the metabolic changes so induced deter- 

 mine the beginnings of visual impulses and thus of visual 

 sensations. In the metabolism of living substance, we recog- 

 nize (§ 30) two phases, the upward constructive anabolic phase, 

 and the downward destructive katabolic phase ; we may accord- 

 ingly, in the absence of any distinct leading to the contrary, on 

 the one hand suppose that different rays of light, rays differ- 

 ing in their wave-length, may affect the metabolism of the 

 visual substance in different ways, some promoting anabolic, 

 others promoting katabolic changes, and on the other hand 

 that different changes in the metabolism of the visual sub- 

 stance may give rise to different sensations. 



We may therefore regard ourselves as at liberty to suppose 

 that there may exist in the retina a visual substance of such a 

 kind that when rays of light of certain wave-lengths, the longer 

 ones for instance of the red side of the spectrum, fall upon it, 

 katabolic changes are induced or encouraged, while anabolic 

 changes are similarly promoted by the incidence of rays of other 

 wave-lengths, the shorter ones of the blue side. But, as we have 

 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 sup- 

 pose 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 



