406 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



and ingloriously over the student's imagination : it is time to 

 remember that they are pure abstractions, no more essential to 

 the theory just sketched than the employment in algebra of the 

 letter x to denote an unknown quantity is an essential procedure 

 in that science. To avoid this source of misunderstanding, I shall 

 use Prof. v. Kries' term, Components, and speak of the theory as 

 the " Three Components Hypothesis." 



Another mistake is illustrated by the following quotation from 

 a paper by Miss Calkins : ( l6 ) 



" From the point of view of a psychological analysis of our 

 conscious sensations of colour, the postulates of this theory are 

 not in accordance with the facts of observation ; for, even granting 

 that violet is a simple fundamental colour sensation (which many 

 observers regard as complex), it can hardly be denied that yellow 

 is just as well characterised and definite a sensation as red or 

 green. Yellow looks yellow, and does not seem at all like a mixture 

 of red and green, or indeed any other colour mixture." 



Objections of this type are simply irrelevant. The theory is only 

 an attempt to express physiological processes, of admittedly hypo- 

 thetical character, in terms of experimental facts ; l it has nothing, 

 to do with a psychological analysis of sensations. Whether such 

 analysis be possible may be doubted ; in any case it is and can be 

 no part of our present inquiry, which is only concerned with visual 

 sensations in so far as the latter are signs of the existence of a 

 physiological reaction. 



Having reduced the hypothesis to its simplest terms, let us 

 apply it to the facts reviewed in the former sections of this article. 

 Abnormal trichromatic systems are satisfactorily described if we 

 suppose one of the visual components to be defined in a peculiar 

 way. Thus, if a component A is normally defined as / a (x, y, z), 

 in these cases it is some other function, / aj (x, y, z), say, of the 

 stimulus values. 



Turning to dichromatic systems, we have seen that, from the 

 experimental standpoint, they are reduction forms of a normal 

 system. Theoretically considered, the simplest reduction we could 

 imagine would be the absence or ineffectiveness of one of the three 

 normal components. Thus, if we take R, G, V (for the sake of 

 clearness) as the normal components, in the absence of R all 

 sensations (sensation being used with the meaning above defined) 

 1 i.e. in terms of stimulus values. 



