io6 4 VISION. 



The seat of after-images is another question of interest. The 

 influence of movement of the eyeball upon after-images, and the fact 

 that pressure on the eyeball hinders the appearance of an after-image 

 (p. 1098), have been supposed to be in favour of their peripheral origin. 

 Rim pier x found that the sensations arising from stimulation of the stump 

 of the optic nerve, after extirpation of the eyeball, were not followed by 

 after-images. On the other hand, Parinaud 2 has argued in favour of a 

 central seat, from the fact that after excitation of one eye, a negative 

 after-image may be seen in the other eye. This, however, is simply due 

 to the difficulty of knowing with which eye we are receiving an im- 

 pression (see p. 1136). Parinaud also states that after-images do not 

 appear to move when the eyeball is mechanically displaced, but this 

 observation is very doubtful. 



Theory of contrast.— The old view of Johannes Mliller, that contrast 

 depended on reciprocal action of retinal areas, was at one time almost 

 completely neglected*. Contrast was referred by Helmholtz to purely 

 psychological factors, and especially to supposed processes of unconscious 

 inference. 



In the case of colour contrast, Helmholtz's mode of explanation may be 

 illustrated by what is usually known as Meyer's experiment, although in its 

 essentials it was described by J. Muller. 3 If a small patch of grey paper is 

 placed on a coloured (green) surface, it becomes tinged with the comple- 

 mentary colour (pink) by contrast. This pink coloration is greatly increased 

 by covering with transparent tissue paper. Helmholtz 4 supposed that, in 

 looking at such a surface, we seem to be looking at a background through a 

 coloured (green) medium, the grey patch appears part of the background, and 

 is regarded as an object seen through the green medium. It is a matter of 

 experience that an object which appears white through a green medium must 

 be pink, and the patch is therefore seen as pink. If a grey patch, correspond- 

 ing in intensity to that under the tissue paper, is held in front of the cover, 

 it is very little, or not at all, coloured by contrast ; and, according to Helm- 

 holtz, this is due to the fact that it is regarded as an independent object, and 

 is therefore not affected in the same way as if it were part of the background. 



The mirror contrast experiment of Ragona Scina, and the colour contrast 

 produced by rotating such a disc as Fig. 385, were explained on the same lines. 

 In the case of the latter, the explanation was somewhat forced, it being sup- 

 posed that the colours of the disc were regarded as variations of a uniform 

 colour, such as might be produced by shadows, coloured reflections, staining by 

 coloured liquids, or soiling by coloured powders. " In order to produce an 

 objectively grey spot on a greenish surface, a reddish pigment would be 

 required." Coloured shadows were regarded by Helmholtz as giving the most 

 convincing evidence in favour of the psychological nature of contrast. On a sur- 

 face illuminated both by daylight and lamplight, a shadow thrown by an object 

 from the yellow lamplight will appear blue by contrast. Helmholtz observed 

 through a tube the border of the coloured shadow, and, when the blue colour 

 had developed, moved the tube, so that the eye was wholly exposed to the 

 shadow. He found that the blue colour persisted, although there was now no 

 contrast inducing light, and disappeared on removing the tube from the eye. 

 It was supposed that the blue colour, produced by the illusion of judgment, 

 persisted till removal of the tube allowed comparison with other colours, and 

 rendered a new judgment possible. 



1 Centralbl. f. d. mcd. JFisscnsch., Berlin, 1882, S. 4. 

 °-Gaz. d. hdp., Paris, 1882, p. 459. 



3 " Handbuch d. Physiol.," 1837, Bd. ii. S. 369. 



4 "Handbuch d. physiol. Optik," 1867. S. 407. 



