1232 THEORIES OF COLOUR VISION. [BOOK in. 



fundamental sensations are the primary sensations, concerning 

 which we are inquiring. And Hering's theory attempts 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 complementary 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 outcome 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 incidence of light, and that 

 the metabolic changes so induced determine the beginnings of 

 visual impulses and thus of visual sensations. In the metabolism 

 of living substance, we recognize ( 30) two phases, the upward 

 constructive anabolic phase, and the downward destructive 

 katabolic phase ; we may accordingly, in the absence of any 

 distinct leading to the contrary, on the one hand suppose that 

 different rays of light, rays differing 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 substance may give rise to different 

 sensations. We say ' in the absence of distinct leading to the 

 contrary,' because though in our study of muscular contraction 

 we were led to regard the effect of a stimulus as a katabolic one, 

 as of the nature of an explosive decomposition, we cannot take a 

 muscular contraction as the exclusive type of the effect of a 

 stimulus ; and indeed even in the case of muscular tissue we saw, 

 in the instance of the augmentor and inhibitory cardiac nerves, 

 that nervous impulses, in acting as stimuli, might have contrary 

 effects, effects moreover suggesting that the one, augmentor, were 

 associated with katabolic and the other, inhibitory, with anabolic 

 changes. In all probability, we ought to regard the study of 

 sensations as more likely to throw light on the molecular changes 

 involved in muscular contraction, than to take the little we know 

 about the latter as a guide to our views concerning the former. 



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 



