130 AFTER-IMAGES AND CONTRAST. [BOOK UK 



sashes and dark panes. On Hering's theory all this is readily 

 intelligible as a mere physiological process. On this theory the 

 retina, or rather the visual apparatus, has to return to equilibrium 

 after every exposure to light of any kind ; the part of the retina 

 giving rise to the sensation of black, whether it be the patch or 

 the general ground, is not in a condition of equilibrium at the 

 moment of shutting the eyes, the white-black substance is here 

 undergoing anabolism in excess ; and, when the light is wholly 

 removed, it passes into equilibrium by katabolic changes, and in 

 doing so developes a sensation, a feeble sensation it may be but still 

 a sensation, of white. The Young-Helmholtz theory cannot offer 

 any such physiological explanation ; black being the effect of the 

 absence of all stimulation from the visual apparatus cannot be 

 followed by any physiological rebound ; and the theory has to seek 

 a psychological explanation. The parts of the visual apparatus 

 which had been stimulated by light, give rise, when the eyes are 

 shut to a sensation of black, and the parts which had not been, 

 stimulated, appear in contrast with these to yield a sensation 

 of light ; but only appear, the effect is a psychological not a 

 physiological one. 



Hering's theory also offers a physiological explanation of the 

 fact that not only in the case of black and white, but also in the 

 case of colours, the negative after-image with its black, green, &c., 

 corresponding to the white, red, &c., of the positive image, may 

 give way to a return of the positive image with all its original 

 features, to be succeeded by a second negative image like the first, 

 and thus often by a whole series of alternate positive and negative 

 images, each gradually becoming fainter and more obscure. For 

 such rhythmic oscillations from one sensation to its complemen- 

 tary or correlative and back again, pointing to katabolism and 

 anabolism alternately gaming the upper hand, are not without 

 analogies in other common instances of the metabolism of living 

 substance. We may of course apply a like hypothesis to the 

 three primary sensations, but the explanation of the phenomena, 

 as thus given, is not so direct as that afforded by Hering's theory, 

 especially when we consider that each occurrence of the negative 

 image of black has to be accounted for on psychological grounds. 



On somewhat the same line of argument, the phenomena of 

 simultaneous contrast may be appealed to in favour of Hering's 

 theory. The explanation of these effects, given by the supporters 

 of the Young-Helmholtz theory is, like that offered for the 

 negative image of black, a psychological one. 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 sensa- 

 tion of pink but that we think it is pink because we attribute the 

 greenness of the whole field to the covering tissue paper, and 

 seeing the patch shine through this judge the patch to be reflect- 

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



