410 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



effects pass off rapidly in the case of artificial red blindness (in 

 ten minutes), more slowly after violet fatigue (in two hours). 



It is, I think, obvious that the state of affairs presented by 

 these experiments is highly complex. We are dealing with a 

 change of responsiveness, analogous to the retiming effects, as they 

 are called, produced in the study of after-images, but of greater 

 magnitude, and in less simple form. Take the experiment quoted 

 as to the apparent hue of a geranium petal after green stimula- 

 tion : this is an ordinary after-image effect, and differs in no way 

 from the results obtained by other workers ; in the case of 

 exposure to stronger green light, the effect is similar. How does 

 this, we may ask, differ from the experiment with intense orange 

 light ? Simply in the fact that " retiming " with orange, red 

 and green both act like orange itself in inducing a negative image. 

 The conclusion that in this case the mechanism involved in the 

 production of orange is compounded of a mechanism yielding a 

 sensation of redness and a mechanism responding with a sensation 

 of greenness is reasonable, and finds some confirmation in a recent 

 experiment carried out by Burch. ( 20 ) 



We know that responsiveness to green is increased, relatively 

 to that for red stimulation, by resting the eye in darkness ; hence, 

 if orange or yellow depends upon a fusion of two physiological 

 processes, one concerned with green, the other with red, then, 

 under conditions of feeble illumination and dark adaptation, the 

 yellow should appear greenish, because the mechanism responding 

 by a sensation of greenness, is more active under these conditions 

 than that associated in the same way with redness. 1 



Burch found this actually to be the case; "the sodium lines 

 appeared pale green when of the minimum visible intensity." 



These results do therefore support a contention that com- 

 ponents in the sense of our theory may possibly have a physio- 

 logical counterpart. I do not think, however, that the view that four 

 components a red, a green, a violet, and a blue exist is proved 

 by Burch's experiments. To prove that any light acts upon only 

 one component it would be necessary to show that after dazzling 

 with, e.g., blue, any mixed light was altered by the subtraction 

 of blue, and that any light not containing blue had that colour 

 added to it. The facts that the condition is transitory, is perhaps 



1 The reader must forgive the apparent confusion between object and subject of 

 vision which is rendered inevitable by the necessity of being concise. 



