260 KECENT PROGRESS OF THE THEORY OF VISION. 



there can of course be no doubt that the observers I first named 

 really saw what they profess, so that there must here be great 

 individual difference. Indeed with certain experiments which 

 Dove recommends as particularly well fitted to prove the correct- 

 ness of his conclusion, such as the binocular combination of 

 complementary polarisation-colours into white, I could not 

 myself seethe slightest trace of a combination-colour. 



This striking difference in a comparatively simple observa- 

 tion seems to me to be of great interest. It is a remarkable 

 confirmation of the supposition above made, in accordance with 

 the Empirical Theory of Vision, that in general only those sen- 

 sations are perceived as separated in space, which can be 

 separated one from another by voluntary movements. Even 

 when we look at a compound colour with one eye, only three 

 separate sensations are, according to Young's theory, produced 

 together ; but it is impossible to separate these by any move- 

 ment of the eye, so that they always remain locally united. 

 Yet we have seen that even in this case we may become conscious 

 of a separation under certain circumstances; namely, when it 

 is seen that part of the colour belongs to a transparent covering. 

 When two corresponding points of the retinae are illuminated 

 with different colours, it will be rare for any separation between 

 them to appear in ordinary vision ; if it does, it will usually 

 take place in the part of the field of sight outside the region of 

 exact vision. But there is always a possibility of separating 

 the compound impression thus produced into its two parts, 

 which will appear to some extent independent of each other, and 

 will move with the movements of the eye ; and it will depend 

 upon the degree of attention which the observer is accustomed 

 to give to the region of indirect vision and to double images, 

 whether he is able to separate the colours which fall on both 

 retinae at the same time. Mixed hues, whether looked at with 

 one eye or with both, excite many simple sensations of colour 

 at the same time, each having exactly the same position in the 

 field of vision. The difference in the way in which such a 

 compound-colour is regarded by different people depends upon 

 whether this compound sensation is at once accepted as a coherent 



