STUDIES IN SPECIAL SENSE PHYSIOLOGY 411 



attended with some risk, and almost certainly involves psychological 

 complications, render exact observations difficult. Under the 

 circumstances, it is perhaps best to say that, although the experi- 

 ments are perfectly consistent with a component hypothesis of the 

 type discussed, it would be rash, on the strength of them, to make 

 any general statement as to the nature of the components from 

 the physiological standpoint. Theoretically it does not matter 

 whether we adopt three or four components ; the algebraical form 

 of our theory would not be changed, but we should lose the 

 practical advantages of considering normal colour vision to be, 

 experimentally, trichromatic, which would be a serious objection. 



Before finally summarising the case presented, two matters 

 need attention. First, as to monochromatic vision or total colour- 

 blindness. It has been asserted that such a condition cannot be 

 described in terms of the Young-Helmholtz theory. As a matter 

 of fact, the assertion is inaccurate ; symbolically we could cover 

 the facts by supposing that the functions defining the variables 

 are identical thus : A = / (x, y, z) = B = / (x, y, z) = C = / (x, y, z), 

 or, graphically, we can put it that the three valency curves coincide. 

 In any case, the reader will probably see reason to think that 

 monochromatic vision depends upon a mechanism entirely distinct 

 from the precursors of normal foveal vision, and its treatment 

 should be kept separate from that of the phenomena with which 

 we are here concerned. 



In the second place, no reference has been made to Simul- 

 taneous Contrast. The reason is that it seems doubtful whether 

 the phenomena of simultaneous contrast are not of quite a special 

 kind. It is true that the original hypothesis of Helmholtz, which 

 assumed that contrastive effects are dependent upon factors, purely 

 psychological in nature, can hardly be maintained without some 

 modification ; the subject is, however, so complex that its dis- 

 cussion would not be intelligible in the space at our disposal. It 

 must, however, be said that if subsequent work should compel us 

 to assign a i/urely physiological basis to the facts of simultaneous 

 contrast, it will probably be necessary to modify the theory of 

 components in such a way that it will become somewhat more 

 complicated than it is at present. 



Leaving this matter for future consideration, we can say that 

 the Component Hypothesis associated with the names of Young 

 and Helmholtz supposes (1) That colour sensations depend upon 



