THE PERCEPTIOX OF SIGHT. 295 



on failing to fix the attention. Indeed there is scarcely 

 any phenomenon so well fitted for the study of the causes 

 which are capable of determining the attention. It is not 

 enough to form the conscious intention of seeing first 

 with one eye and then with the other ; we must form as 

 clear a notion as possible of what we expect to see. Then 

 it will actually appear. If, on the other hand, we leave 

 the mind at liberty without a fixed intention to ob- 

 serve a definite object, that alternation between the two 

 pictures ensues which is called retinal rivalry. In this 

 case, we find that, as a rule, bright and strongly marked 

 objects in one field of vision prevail over those which 

 are darker and less distinct in the other, either com- 

 pletely or at least for a time. 



We may vary this experiment by using a pair of 

 spectacles with different coloured glasses. We shall then 

 find, on looking at the same objects with both eyes 

 at once, that there ensues a similar rivalry between the 

 two colours. Everything appears spotted over first with 

 one and then with the other. After a time, however, the 

 vividness of both colours becomes weakened, partly by 

 the elements of the retina which are affected by each of 

 them being tired, and partly by the complementary 

 after-images which result. The alternation then ceases, 

 and there ensues a kind of mixture of the two original 

 colours. 



It is much more difficult to fix the attention upon a 

 colour than upon such an object as an engraving. For the 

 attention upon which, as we have seen, the whole phe- 

 nomenon of ' rivalry ' depends, fixes itself with constancy 

 only upon such a picture as continually offers something 

 new for the eye to follow. But we may assist this by 

 reflecting on the side of the glasses next the eye letters 

 or other lines upon which the attention can fix. These 

 reflected images themselves are not coloured, but as soon 



