CHAP, in.] SIGHT. 131 



would give rise to white, that is to a colourless grey. And a 

 similar psychological explanation has been given of the other cases 

 of simultaneous contrast. Such an explanation is in itself not 

 very convincing ; and against it may be urged the fact that in the 

 cases quoted the predominance of the primary colour over the field 

 is not necessary for the effect ; and yet the interpretation is based on 

 this. Moreover an experiment may be so arranged that a marked 

 effect of simultaneous contrast should present itself in the vision 

 of one eye but not of the other ; now we can hardly imagine that, 

 when both eyes are being used, we can interpret in different ways 

 the sensations derived through the two eyes. 



Hering's theory on the other hand offers a direct physiological 

 explanation of the effect ; it supposes that when one part of the 

 retina is stimulated, the neighbouring portions of the field of vision 

 are affected at the same time in a manner which may be roughly 

 but only roughly compared to electric induction, so that they undergo 

 changes antagonistic or complementary to those going on in the 

 part of the field of vision corresponding to the portion of the retina 

 actually stimulated. Thus in- the case of the grey patch on the 

 green field, the anabolism of the red-green substance in the green 

 field surrounding the grey patch leads to a certain amount of 

 katabolic action of the red-green substance within the grey patch, 

 and so gives rise to a red sensation. In a similar way the bright 

 corona seen round the black negative image developed by shutting 

 the eyes after staring at a white patch on a black ground is due 

 to the anabolism in the black negative image inducing katabolism 

 of white-black substance, that is a sensation of (white) light, in its 

 immediate surroundings. It will of course be understood that the 

 theory does not maintain that the effect is necessarily produced in 

 the retina itself; it may be developed in the visual centres, or in 

 them and in the retina together. We must not go into further 

 details, but we may add that many of the details of these effects 

 of simultaneous contrast are more easily explained on Hering's 

 theory when the possibility of such an inductive action is admitted 

 than by any psychological hypothesis. 



We have, contrary to our wont, dwelt so long on two contend- 

 ing theories, and have here renewed our discussion of them in con- 

 nection with their effects of contrast, partly because of the intrinsic 

 interest of the matter, but also, and not least, because an attempt to 

 decide between the two views opens up important lines of thought 

 and leads to considerations which must have great influence on our 

 conceptions not only of visual sensations but also of all sensations 

 and indeed of nervous processes in general. We have not at- 

 tempted anything like a full discussion of the subject ; we have 

 only ventured to indicate some of the leading criticisms which 

 may be made on each theory ; and so far as we are aware no 

 crucial test between the two has as yet been brought forward. 

 We may now leave the matter with the remark that while the 



