SIGHT. 



497 



verged to a focus at a point in front of the retina ; because, being 

 less divergent, when they strike the lens, the same amount of re- 

 fraction will bring them together sooner than before. On the other 

 hand, if the object be moved to a point nearer the eye, the rays, 

 becoming more divergent as they strike the lens, will be converged 

 less rapidly to a focus, and vision will again" become indistinct. 



This may easily be seen by the aid of a very simple experiment. 

 If two needles be placed upright, at different distances from the eye, 

 one for example at eight and 

 the other at eighteen inches, but 

 nearly in the same linear range, 

 and if then, closing one eye, we 

 look at them alternately, we shall 

 find that we cannot see both dis- 

 tinctly at the same time. For 

 when we look at the one near- 

 Fig. 161. 



Fig. 160. 



est the eye, so as to perceive its form distinctly, the image of the 

 more remote one becomes confused ; and when we see the more re- 

 mote object in perfection, that which is nearer loses its sharpness of 

 outline. This shows, in the first place, that the same condition of 

 the eye will not allow us to see two objects at different distances 

 with distinctness at the same time ; and secondly that, on looking 

 from one to the other, there is a change of some kind in the focus of 

 the eye, by which it is adapted to different distances. Indeed we 

 are conscious of a certain effort at the time when the point of vision 

 is transferred from one object to the other, by which, the eye is 

 adapted to the new distance ; and this alteration is not quite instan- 

 taneous, but requires a certain interval of time for its completion. 

 This accommodation of the eye to different distances is un- 

 doubtedly effected by an antero-posterior movement of the lens 

 within the eyeball. It will at once be perceived, on referring to 

 Fig. 161, that if the lens were moved a little backward toward the 

 32 



