J198 RECENT PROGRESS OF THE THEORY OF VISION. 



distance between the sensitive and the vascular layers of the 

 retina. It is done as follows : 



If the focus of the light thrown on to the white of the eye 

 (the sclerotic) is moved slightly backwards and forwards, the 

 shadow of the blood-vessels and its image in the field of vision 

 will, of course, move also. The extent of these movements can 

 be easily measured, and from these data Heinrich Miiller, of 

 Wiirzburg whose too early loss to science we still deplore de- 

 termined the distance between the two foci, and found it exactly 

 to equal the thickness 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 sensitive to 

 weak light than the other parts of the retina. It has been long 

 known that many stars of inferior magnitude for example, the 

 Coma Berenicce and the Pleiades are seen more brightly if 

 looked at somewhat obliquely than when their rays fall fall 

 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 baeii noticed above, which 

 interferes with its free communication with the life-giving 

 blood. 



All these imperfections would be exceedingly troublesome 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. FOF even when we da 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 irre- 

 gularity 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. 



