1895.] Dr Rivers, On Binocular Colour-mixture. 275 



In the literature of the subject I have only been able to find 

 mention of experiments in this direction in papers by Fechner^ 

 and Chauveau ^. Fechner looked at a white patch on a black 

 ground, holding a coloured glass before each eye, blue before 

 one, and yellow before the other. The patch appeared white, 

 and this was ascribed to binocular neutralization of the two 

 complementary colours. He then removed the glasses, and the 

 patch remained white, which was held to prove that the after- 

 images also neutralized each other by binocular mixture ; on 

 doubling the patch by convergence or divergence of the visual 

 axes, each patch appeared coloured ; on uniting again, the patch 

 appeared white. I have repeated the experiment and it is a 

 very striking one, but I do not think it affords satisfactory 

 evidence of binocular mixture in the after-image. Under the 

 conditions of the experiment with glasses complementary in 

 colour to each other, a colourless after-image is necessary to 

 prove the existence of mixture, and in order to obtain it a white 

 patch is looked at with its whiteness heightened by contrast with 

 a black background. The influence of a white surface in masking 

 the perception of colour is well known, and in using such a white 

 patch Fechner was taking the best means of obscuring any colour 

 which might otherwise appear. In other words ; in order to prove 

 the existence of a subjective white, Fechner looked at an objective 

 white. 



This is not merely a theoretical objection. I repeated Fechner's 

 experiment using glasses not complementary to each other, green 

 and yellow, red and blue. The combined after-images, which 

 should now have occurred, were violet or green, but using 

 Fechner's conditions the patch remained white, and I could see 

 no trace of colour in it. I then repeated the experiments using 

 grey patches of different degrees of darkness instead of white, 

 and by their means I obtained appearances which I think showed 

 the existence of binocular mixture in the after-image, but I 

 obtained no results as definite as with the method I have de- 

 scribed above. Chauveau's experiments were almost identical with 

 those of Fechner and are open to the same objections. The ex- 

 periments of Fechner and Chauveau are mentioned by Titchener^, 

 who has raised another objection to them, viz. that the appear- 

 ances described may partly at any rate depend on contrast. 



Ebbinghaus* has described "binocular after-image," but this 

 expression is used by him to describe quite a different pheno- 

 menon, viz. the after-image which appears in one visual field 

 when the stimulus has been presented to the other eye. 



^ Abhand. d. k'onig. sacks. GesellscJiaft, Bd. lxx. S. 469. 1861. 

 ^ Coinptes rendus de VAcad. des Sciences, T. cxiii. p. B94. 1891. 

 3 Philos. Studien, Bd. viii. S. 243. 1893. 

 ^ Pfiiiger's Archiv, Bd. xLvi. S. 498. 1890. 



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