CHAP. HI.] SIGHT. 1267 



colour vision. According to the Young-Helmholtz theory when 

 the coloured patch is looked at, one of the three primary colour 

 sensations is much exhausted, and the other two less so, in 

 varying proportions, according to the exact nature of the colour 

 of the patch ; and the less exhausted sensations become prominent 

 in the after-image. Thus, the red patch exhausts the red primary 

 sensation, and the negative image is made up chiefly of green and 

 blue sensations, that is, appears to be greenish-blue, or bluish- 

 green, according to the particular hue or tone of the red. So 

 also the yellow patch exhausts both the red and green sen- 

 sations leaving the blue only to make itself felt. On Hering's 

 hypothesis, we may suppose that, owing to the continued effect of 

 looking at the red patch, the katabolic changes of the red-green 

 substance become less and less, leading to a prominence and 

 indeed to an actual increase of anabolic changes in the same 

 substance ; hence, the sensation of green dominating in the 

 negative image ; and we may suppose that like events occur in 

 the yellow-blue substance. 



So far the facts suit both theories, but Hering's theory offers a 

 more ready explanation than does the rival theory of the fact that 

 it is easier to produce a negative green image after positive 

 exposure to red, or negative blue after positive yellow than, vice 

 versa,, red after green, or yellow after blue ; in other words, 

 that the red and yellow sensations are more readily exhausted 

 than the green and blue sensations, as indeed is shewn by general 

 experience. For all living substances are more prone to katabolic 

 than to anabolic changes, destruction is easier than construction ; 

 and, as we have already seen ( 769), the fact that blue sensations 

 preponderate in dim lights must not be taken as shewing that the 

 blue or green sensations are more readily excited than the yellow 

 or red. Further, several phenomena of colour vision, to some of 

 which we have already alluded, seem explicable on the view that 

 the stock so to speak of red-green substance in the retina is sooner 

 exhausted than the stock of yellow-blue substance, and this in 

 turn than the white-black substance. 



The Young-Helmholtz theory does not explain so readily as 

 does the other why negative images often follow upon positive 

 images without any stimulation of the retina subsequent to the 

 primary one. As we have already said, if a white patch on a 

 black ground be looked at for some time, and the eyes be then 

 shut, a negative image of the spot will be seen on the ground 

 of the 'intrinsic light' of the retina much blacker than the ground, 

 and having in its immediate neighbourhood a sort of bright corona. 

 Conversely a black patch on a white ground will give rise to a 

 patch of exaggerated ' intrinsic light ' in contrast to the blackness 

 of the rest of the field. So also, if a window be looked at and the 

 eyes then closed, the positive after-image with bright panes and 

 dark sashes gives way to a negative after-image with bright 



F. 81 



