218 COLOUR VISION 



observers, e.g., Schenck, who regard monochromatic vision as divided 

 into various types, some of which are due to modifications in the cones 

 and must therefore be correlated in some such manner as that suggested 

 with the three-components theory. 



Difficulty is experienced when the attempt is made to explain the 

 phenomena of peripheral vision and of induction by the theory. It is 

 at once clear that neither can be explained as evidence of any reduction 

 system. Fick^ suggested that the reaction-values of the components 

 were not the same in all parts of the retina. The suggestion is inadequate ; 

 on the one hand because it implies a simple reduction system and this 

 fails to account for the phenomena, or on the other hand because it is 

 too general to be of practical value. Schenck's attempt to correlate 

 the facts with a modification of the trichromatic theory will be described 

 later. The relationship of normal and deuteranopic peripheral colour 

 vision to central colour vision approximates more closely to a reduction 

 system than the corresponding relationship in the protanopic system. 

 In all, however, the modifications produced by areal effects negative so 

 simple an explanation and afford some evidence in favour of v. Kries' 

 theory of zones. 



The greatest difficulty, however, is experienced in explaining the 

 facts of induction. In general terms it is not difficult to conceive a 

 diminution in response of the components in one direction, associated 

 with an increase in another, after previous stimulation. We might thus 

 account for the increased response to the complementary after stimula- 

 tion with a given light. Indeed, such a view falls in well with other 

 physiological findings, so admirably elaborated in Sherrington's work. 

 If all spectral lights act upon all three components, then the increased 

 response to the complementary after previous stimulation with a colour 

 can be explained, and this was the view adopted by v. Helmholtz. It 

 lands us, however, on the horns of a dilemma, for the facts of dichro- 

 matic vision — and the same is true of trichromatic {vide infra, Chap, ii) — 

 show that lights of greater wave-length than about 550 /x/x do not act 

 at all upon the V component, since no standard blue has to be mixed with 

 the standard red in order to match colours in this part of the spectrum. 

 Yet the saturation of yellow (589 /x/x) is undoubtedly increased by 

 previous stimulation with the complementary blue. v. Kries has shown 

 that this effect is not to be explained by any alteration of the intrinsic 

 light of the eye, but is caused by a quantitative change in the response 

 to the secondary light. If yellow light acts only on the red and green 



1 Arch. f. d. ges. Physiol XLvn. 274, 1890. 



