Chap, hi.] SIGHT. 935 



prolonged or intense excitation of the retina, such as rarely 

 occurs in ordinary vision, still the effect must intervene, even if 

 to a slight extent only, in our daily sight, and proportionately 

 contribute to the discrepancy between the perception and the 

 object. 



§ 585. The phenomena of ' simultaneous ' and ' successive 

 contrast ' are farther of interest in relation to the theory of 

 colour vision. The mere occurrence of the negative images can 

 be explained as a result of exhaustion on either hypothesis of 

 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 vary- 

 ing 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 pri- 

 mary 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 

 sensations 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. 



The Young-Helmholtz theoiy does not explain so readily as 

 does Hering's theory 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 sashes and dark panes. Looking at a bright red 

 spot gives rise to a green after-image, and so with other colours. 

 Moreover, the eyes being still shut, there may be a series of 

 after-images ; 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 pos- 



