224 EECENT PROGRESS OF THE THEORY OF VISION. 



which actually separates the layer of rods and cones from 

 the vascular layer of the retina. 



The condition of the point of clearest vision (the yellow 

 spot) is disadvantageous in another way. It is less sensi- 

 tive to weak light than the other parts of the retina. It 

 has been long known that many stars of inferior magni- 

 tude for example, the Coma Berenice? and the Pleiades 

 are seen more brightly if looked at somewhat obliquely 

 than when their rays fall full upon the eye. This can be 

 proved to depend partly on the yellow colour of the 

 macula, which weakens blue more than other rays. It may 

 also be partly the result of the absence of vessels at this 

 yellow spot which has been noticed above, which interferes 

 with its free communication with the life-giving blood. 



All these imperfections would be exceedingly trouble 

 some in an artificial camera obscura and in the photographic 

 picture it produced. But they are not so in the eye so 

 little, indeed, that it was very difficult to discover some 

 of them. The reason of their not interfering with our 

 perception of external objects is not simply that we have 

 two eyes, and so one makes up for the defects of the other. 

 For even when we do not use both, and in the case of 

 persons blind of one eye, the impression we receive from 

 the field of vision is free from the defects which the 

 irregularity of the retina would otherwise occasion. The 

 chief reason is that we are continually moving the eye, 

 and also that the imperfections almost always affect those 

 parts of the field to which we are not at the moment 

 directing our attention. 



But, after all it remains a wonderful paradox, that 

 we are so slow to observe these and other peculiarities 

 of vision (such as the after-images of bright objects), so 

 long as they are not strong enough to prevent our seeing 



