936 AFTER-IMAGES AND CONTRAST. [Book hi. 



itive and negative images, each gradually becoming fainter and 

 more obscure. These and similar phenomena are more or less 

 satisfactorily explained on Hering's theory as the results of 

 rhythmic oscillations between katabolic and anabolic changes ; 

 on the other theory we have to have recourse to psycholog- 

 ical explanations. This is especially the case with the phe- 

 nomena of simultaneous contrast. In the case for instance of 

 the grey patch seen as pink in the midst of a green field, it is 

 argued that the patch does not actually excite a sensation of 

 pink but that we think it is pink because we attribute the green- 

 ness of the whole field to the covering tissue paper, and seeing 

 the patch shine through this judge the patch to be reflecting- 

 just those rays, namely pink, which mixing with the green 

 would give rise to white, that is to a colourless grey. Hering's 

 theory on the other hand offers a direct physiological explana- 

 tion 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 sub- 

 stance 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. 



§ 586. We have seen (§ 553) that visual sensations may be 

 produced in other ways than by light falling on the retina. In 

 such cases the effect which is produced upon our consciousness 

 is wholly misleading. A mechanical or electrical stimulation 

 of the retina may give rise to a visual sensation identical with 

 that which would be produced by the rays from a flash of light 

 falling upon a part of the retina. In both cases we should have 

 a perception of a flash of light occurring in a certain part of the 

 field of vision; and so far as the perception itself is concerned 

 we could not distinguish between the latter which is a real and 

 the former which is a false perception. 



Not only single and simple sensations, but also complex 

 groups of sensations may be excited by other means than that of 

 light falling on the retina, and we may thus experience varied 

 and intricate perceptions which have no objective reality at all. 

 Many people when they close their eyes at night, or indeed at 

 other times, see images of faces or other objects; and though 

 under such circumstances it is easy to recognize the subjective 

 origin of the perception, that conclusion is reached by reasoning 

 upon the circumstances, and not because the perception itself 

 differs in character from a like perception caused by looking at 

 an external object. In such cases it is probable that some 



