422 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



between the two areas is only obtained when white is- indeed mixed 

 with blue for the fatigued area, but on the other hand, the blue 

 light for the unfatigued area is suitably diminished. In general, for 

 reasons already given, an equality in colour tone for blue can only 

 be obtained under exceptional circumstances, when the character 

 of the daylight is specially favourable and the tone of the blue 

 just right." ( 29 ) 



It has been said by Professor William James that John Stuart 

 Mill was in the habit of boldly asserting in general terms the 

 truth of a philosophical theory derived from his father, and then 

 admitting in detail the validity of all objections to it. A trace of 

 this method can be discerned in the passage just quoted. The 

 whole memoir is designed to show that white retuning does not 

 affect colour valency. Experiments apparently in favour of the 

 rival hypothesis are met by other experiments in support of this 

 contention. We are led to suppose that white retuning and colour 

 retuning are independent, and naturally infer that since white 

 retuning does not affect colour valency, colour retuning ought not 

 to affect white valency. This, however, is specifically asserted not 

 to be the case. 



It is not, of course, to be supposed that the two propositions 

 are logically inconsistent, but the independence of the postulated 

 physiological processes is no longer paralleled by the facts of 

 sensational analysis. What is (from the standpoint of sensations) 

 a pure white has a definite colour valency. Pure white (in terms 

 of the physiological process in the black-white substance) is just 

 as hypothetical as anything postulated by the theory of stimuli. 



It will, I think, be clear that in order to cover the facts of 

 retuning, the sensational theory loses some of its original simplicity. 

 It is interesting to notice that the theory becomes more difficult 

 to follow by developing in a direction opposite to that followed 

 by the stimulus theory ; the latter became unsatisfactory, to 

 some, by tending to be too general, the former by multiplying its 

 details. 



I have enlarged upon the question of retuning, a matter not 

 perhaps of very special importance in itself, because, as I hope 

 to have shown, the facts are not quite so easily described by the 

 Hering theory as some of its supporters would lead one to 

 imagine. These very facts have been claimed by eager adherents 

 of both schools as decisively favouring their respective beliefs. 



