276 Dr Rivers, On Binocular Colour-mixture. [Feb. 25, 



One of Helmholtz's^ objections to the reality of binocular 

 colour-mixture was that the appearance obtained might depend 

 on after-images. He describes an experiment in which he com- 

 bined a pink and a green patch ; after fixation of some duration, 

 he was able to call the combined image grey, but on closing each 

 eye, he found that his grey was monocular and not binocular : 

 that the retina exposed to pink had become fatigued to pink 

 and consequently saw the pink as grey, and so with the other 

 eye to green. If he had gone one step further and had closed 

 both eyes, he would probably have seen a colourless image, 

 which could only have been explained by binocular colour- 

 mixture. 



I bring forward this after-image phenomenon chiefly as an 

 additional fact in favour of the existence of binocular mixture, 

 but it might also be held to throw light on the vexed question 

 of the seat of after-images, and might be held to favour the 

 view that they are central phenomena. This does not however 

 seem to me to be necessary. If an after-image depends upon 

 a definite physiological process in the retina, the occurrence of 

 mixture in the after-image would be explained by the same 

 process of central fusion as will explain binocular mixture in 

 general, a process however which we do not yet understand. 



As regards the second point of my paper, various suggestions 

 have been offered to explain the failure of some observers to see 

 binocular mixture. Bezold^ and Dobrowolsky^ believed that the 

 difficulty in seeing the mixture depended on the difference of 

 focus for different colours. They found that a binocular mixture 

 of red and blue occurred more readily when a weak concave 

 glass was held before the eye exposed to blue. It is possible 

 that equal refraction may be the cause of failure in some cases, 

 and possibly in that of Helmholtz, for in the account of his ex- 

 periments, he describes one colour as seen through the other. 

 In my own case, however, and in the case of several others who 

 have made observations for me, the equality or inequality of 

 refraction has no influence, and mixture is equally well seen 

 whether the concave glass is placed before the eye exposed to 

 red or to blue. 



Dobrowolsky also suggested that difference in the power of 

 unequal accommodation of the two eyes in different individuals 

 might be of importance. It is a disputed point whether unequal 

 accommodation ever occurs. Experiments of Bonders and Hering 

 are generally held to have disproved its occurrence. A. E. Fick'' 



1 Handbuch d. phys. Optik, l'^ Aufl. S. 779. 



2 Annal. d. Physik. u. Chemie, 1874. Jubelband, S. 585. 

 8 Pfliiger^s Archiv, Bd. x. S. 56. 



^ Archiv f. Augenheilkunde, Bd. xix, S. 123, 



