STUDIES IN SPECIAL SENSE PHYSIOLOGY 407 



are functions of G and V, a condition approximating to that of 

 protanopia. If G is absent, we get a form resembling deuteranopia. 



Under such conditions, not only would the relation betwesn 

 dichromatics and trichromatics be easily intelligible, but further 

 we could determine, from observations on dichromatics, the com- 

 ponents of a normal system. 1 A stimulus inoperative upon a 

 dichromatic eye must act exclusively upon the missing component. 

 We have learned how to find the situation of such a stimulus in 

 the colour diagram (the " Fehlpunkt "), and the two such points 

 obtained for the two systems of dichromatic vision determine the 

 stimulus relations of two visual components of a trichromatic eye. 



All this depends, however, on the assumption that one com- 

 ponent is absent and everything else unchanged in a dichromatic 

 eye. Modern work renders it doubtful whether we may make 

 this assumption. In 1885, when the second edition of his great 

 treatise was being prepared, Helmholtz wrote as follows to Lord 

 Rayleigh : ( 17 ) " I have never doubted that our colour system de- 

 pended on three variables and no more. In regard to colour- 

 blindness, the recent observations of Bonders and of my assistant 

 Dr. A. Koenig, show that this defect cannot be referred simply to 

 the lack of one of the fundamental colours, but that two of the 

 primaries (red and green) appear to acquire a more even distribu- 

 bution in the spectrum, so that now one and now the other makes 

 a more vigorous impression ; in other words, the resulting curve 

 approximates now more to the red and now to the normal green 

 sensation. In addition to this we have every shade of lessened 

 power of discrimination. Consequently different individuals require 

 very different mixtures of lithium and thallium light in order to 

 make up sodium light." 



But if we cannot regard dichromatic vision as differing from 

 the normal merely in the absence of a component, we can, in 

 terms of the fundamental hypothesis, assert that it depends upon 

 a visual system made up of two variables defined by such expres- 

 sions as x= < t (A, B, C), and y = < 2 (A **, C). 



This way of looking at the matter is consistent and logical, but 

 it is not free from objection. Thus we could not deduce from such 

 an expression any definite statements respecting the components 

 of normal systems ; the interpretation of dichromatic systems is, 

 necessarily, expressed in such highly general terms as to be hardly 

 1 Or rather the stimuli which act exclusively upon such components. 



