STUDIES IN SPECIAL SENSE PHYSIOLOGY 423 



Perhaps the ingenuous reader will conclude that neither claim 

 is admissible. 



Space and the reader's patience do not admit of analysing in 

 detail the further applications of the theory to visual phenomena. 

 I will merely say a few words on the Hering theory of dichromatic 

 systems. 



It was originally thought that the theory covered these facts 

 extremely well, and even in so good a text-book as that of Professor 

 Howell it is suggested that an advantage of the sensational 

 theory is its better description of the facts of partial colour-blind- 

 ness. In reality, it seems very difficult to bring the facts into line 

 with the theory. 



It was once suggested that both types, protanopes and 

 deuteranopes, depended on the non-existence of the red-green 

 substance, the typical distinction being due to varying degrees 

 of macular and lens pigmentation, combined with unequal respon- 

 siveness of the blue-yellow substance. 



We have already seen that the characters of the two groups, 

 protanopes and deuteranopes, cannot be due to differences in 

 pigmentation, and the inconsistency of the colour equation, 

 R + G = 652, is difficult to reconcile with this view. 



Tschermak, a prominent member of Hering's school, seems 

 definitely to abandon this attempt to describe dichromatic vision, (^ 

 and, so far as I know, an adequate expression of the facts in terms 

 of Hering's hypothesis is not yet forthcoming. This is not a 

 fatal objection to the theory, because the sensational analysis of 

 dichromatic vision is extremely difficult, owing to the fact, among 

 others, that colour nomenclature is adapted to trichromatic vision, 

 trichromatics forming the large majority of all civilised races. It 

 must, however, be realised that this hypothesis does not seem likely 

 to advance our knowledge of colour blindness further than that of 

 three components. 



Our general conclusion, therefore, may perhaps be expressed 

 in the following terms. 



The theory associated with the name of Hering is an attempt 

 to arrive at a general conception of the visual processes by an 

 analysis and comparison of sensations. In this way prominence 

 is given to many interesting facts not specially or adequately 

 considered by the theory founded on stimulus relations. If, 

 however, we attempt to build up upon such data an hypothesis 



