ON SBNSORIAL VISION. 415 



easier than to make either of them disappear as if 

 blotted out from the sky, by looking full and fixedly at 

 it, while the other remains conspicuously visible. In 

 this way I find stars of the second magnitude consider- 

 ably enfeebled, though they cannot be made wholly to 

 disappear. Those of the first are but little affected. I 

 have found many persons incredulous on their first hear- 

 ing of this fact, who yet have satisfied themselves by 

 trial of its reality. I at one time believed that this 

 comparative insensibility of the centre of the retina 

 arose from the greater wear and tear consequent on 

 directing the attention continually to it, and habitually 

 directing it to any more conspicuous object, but I find 

 that the same thing happens to very young persons to 

 quite as great an extent in whom, of course, this cause 

 of deterioration cannot have gone so far as in adults. 

 There is reason to believe moreover, that this compara- 

 tive insensibility of the middle part of the retina to faint 

 impressions extends over a pretty considerable area, for I 

 find that in a room but feebly lighted, and with the back 

 to the light, it is possible by long looking fixedly in the 

 direction of an object of considerable angular diameter, 

 gradually to lose sight of it,, and at length entirely cease 

 to see it and then, by an effort of the will, accompanied 

 with some kind of organic act in the eye itself, which I 

 know by sensation, but am unable to describe in words, 

 but which is not the action of adjusting the focus, it is 

 at once realized to sight, without any alteration of the 

 direction of the optic axis, or any motion given to the 

 head or person. It is an experiment which will not 



