THE PERCEPTION OF SIGHT. 297 



This striking difference in a comparatively simple 

 observation 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 sensations 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, ac- 

 cording to Young's theory, produced together ; but it is 

 impossible to separate these by any movement 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 whole without 

 any attempt at analysis, or whether the observer is able 



