CHAP, ii.] SIGHT. 533 



the colour of the patch; and the less exhausted sensations become 

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

 red 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 tint of the red. On the other hypothesis, 

 we may suppose that, owing to the continued effect of looking at 

 the red patch, dissimilation of the red-green substance becomes 

 less and less, leading to a prominence and indeed to an actual 

 increase of the process of assimilation of the same substance; 

 hence the sensation of green dominating in the negative image. 



Similarly, when the eye, after looking at a coloured patch, is 

 turned to a coloured ground, the effects may easily be explained 

 by reference to the comparative exhaustion of the colour sensations 

 excited by the patch and the ground respectively; if a yellow 

 ground be chosen after looking at a green object, the negative 

 image will appear of a reddish yellow, and so on.' 



The theory of three primary sensations does not so readily 

 explain why negative images should make their appearance with- 

 out any subsequent stimulation of the retina. When the eyes are 

 shut and all access of light, even through the eyelids, carefully 

 avoided, the field of vision is not absolutely dark; there is still a 

 sensation of light, the so-called 'proper light' of the retina. If a 

 white patch on a black ground be looked at for some time, and the 

 eyes then shut, a negative (black) image of the spot will be seen 

 on the ground of the 'proper light' of the retina, having in its 

 immediate neighbourhood a specially bright corona. So also, if a 

 window be looked at and the eyes then closed, the positive after- 

 image with bright panes and dark sashes gives rise to a negative 

 after-image with bright sashes and dark panes ; and similar effects 

 appear with colours. These and similar facts have been largely 

 used in support of the Hering theory. When the eye has been 

 looking at red, and so has caused dissimilation of the red-green 

 substance mere rest, as on shutting the eyes, favours assimilation of 

 the same substance and thus leads to a sensation of green. And 

 the rhythmic oscillations from one colour to its correlative and 

 back again, frequently observed under these conditions and which 

 point to assimilation and dissimilation alternately gaining the 

 upper hand, are not without analogies in other common instances 

 of protoplasmic metabolism. 



