Therefore, in his experiments, although different colours stimulated the two eyes, they did so at points whose correspon- dence had been so established by habit that there could result only a single impression, which consequently was composed of the effects of the two stimulations.

This kind of explanation fails to meet the facts, for if binocular colour mixture be determined solely by similarity of the contours given in the uniocular fields, then there should be no rivalry between fields alike in form but different in colour : and all observers are agreejl that rivalry does arise under these conditions provided that the colours are sufficiently different.

While Helmholtz never found that the result of binocular mixture was like that of uniocular mixture, yet he did not deny that others might succeed in doing so, and showed how such mixture might be explained by his theory of colour vision and his assumption that the "contents of each visual field come to consciousness separately without being fuseci with the other by means of any physiological mechanism, the fusion of the two fields being a purely psychical act" (23, p.

771).

According to his theory of colour vision any complex colour is due to the projection of three different colour sensations in the same part of the visual field.