OTHER THEORIES 297 



type. The arguments in favour of the duplicity theory are destructive 

 of this part of Edridge-Green's theory. The presence of Purkinje's 

 phenomenon, a photochromatic interval, and so on, at the fovea is 

 admittedly disputed, and good observers are ranged on each side. 

 The arguments in favour of the presence of visual purple in the fovea 

 are of two kinds, direct observation and entoptic phenomena. So far 

 as the former is concerned the observation has not been confirmed ; 

 if it should be it is most probable that the substance will be found there 

 only in traces and will represent a vestige of the early ancestry of the 

 cones {v. p. 204). In any case the evidence appears to be against the 

 view that visual purple is the sole and indispensable factor in fovea! 

 vision. So far as entoptic phenomena, other than those due to opacities 

 in the dioptric media, are concerned, they are open to the most various 

 explanations. The " intrinsic light of the retina " is often described 

 as a " light chaos," full of waves and currents ; it is no safe foundation 

 for any fundamental theory of vision. The rod-free area of retina subtends 

 at least 1° — 2° and therefore occupies a very definite finite space. That 

 visual purple in solution should be transmitted to the centre of this area 

 with the velocity necessary to account for the facts on the theory is 

 very improbable, if not impossible on physical grounds. We are 

 familiar with the theory of " sensitisers " and much may be. said in 

 support of them, but they are either bodies which travel in the circula- 

 tion and show no such rapidity of action^, or they are bodies produced 

 by molecular change and act in situ. Those referred to in Schenck's 

 theory, for example, belong to the latter category. 



Much of the evidence in support of the " retinal " part of Edridge- 

 Green's theory is founded upon after-images. As has been pointed out in 

 Part I, Section VI, these phenomena belong to some of the most vari- 

 able in the whole range of colour vision. They vary with the previous 

 stimulation of the eye— and Burch has brought forward evidence to 

 show that " fatigue " effects last much longer than has been thought — 

 with a multiplicity of physical and physiological factors which may be 

 easily overlooked or under-estimated, and above all with psychological 

 factors which are almost uncontrollable. They can only be considered 

 valid if they are repeatedly confirmed by different observers, and even 

 then are usually open to a variety of explanations. This fact is well 

 exemplified by McDougall's experiments. Such observations can be 

 appraised only at the value which an uncertain method merits. 

 The " cerebral "• part of Edridge-Green's theory offers a possible 



1 Cf. McT^oucrall, p. 280. 



19—5 



