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



form of the instrument, a mixture-colour appearing which is not 

 to be distinguished in colour-tone from that obtained by ordinary- 

 methods of mixture. 



I also see good binocular mixture when the combination 

 of two coloured patches is brought about by appropriate con- 

 vergence or divergence of the visual axes; and it is chiefly by 

 the help of this method that I have observed mixture in the 

 after-image. If two patches, a red and a blue for instance, are 

 combined by directing the visual axes so that they would meet 

 beyond the patches, three patches are seen. In the central patch, 

 the rivalry of the visual fields shows itself as a change from one 

 colour to the other, with a transient mixture in the intervals of 

 change. In my own case, after 10 to 20 seconds, the rivalry 

 ceases or becomes almost imperceptible, and I see a purple patch 

 corresponding to the proper mixture-colour, often with slight 

 variations of colour-tone in the direction of one or other of its 

 components. If, after further fixation, I close my eyes, or look 

 at a grey surface, I see under favourable circumstances three 

 after-images ; blue-green on one side from the red ; yellow on 

 the other side from the blue ; and a green or yellowish-green in 

 the centre from the combined purple patch. In many cases the 

 three after-images cannot be seen simultaneously, but a green 

 or yellow-green image is seen which can be recognized to differ 

 in colour from the after-image of either the red or the blue. 

 In other cases, especially when the original mixture has not 

 been good, and the rivalry has not ceased, change of colour may 

 be seen to take place in the central image ; i.e. the phenomenon 

 of rivalry of the retinal fields may be observed in the after-image. 

 The experiment succeeds with any colours, including those com- 

 plementary or approximately complementary to each other ; thus, 

 with a green and red patch, I obtained as binocular mixture a 

 yellowish-grey ; the central after-image was a bluish- white which 

 differed very greatly both in tone and saturation from the blue- 

 green and pink lateral images. The experiments require some 

 practice to enable one to keep up steady convergence or di- 

 vergence of the eyes for a sufficient time to obtain good after- 

 images, but the same results may be obtained when the binocular 

 combination is brought about by other means, as by Wheatstone's 

 stereoscope. The nature of the experiment does not admit of 

 satisfactory comparison of the colour of the after-images with 

 objective colours ; but, so far as I can judge, the central after- 

 image has the colour which would arise from mixture of the 

 colours of the lateral images, and also, so far as I can judge, is 

 complementary to the mixture-colour of the two patches used, 

 and it seems to me impossible to explain this in any other way 

 than by binocular colour-mixture, 



