748 SIGHT. 



again, and strike the retina at p' and q'. The blocking of the hole e will 

 now cause the disappearance of the image q' on the left-hand side of the 

 retina, and this will be referred by the mind to the right-hand side, so that 

 will seem to vanish. 



If the needle be brought gradually nearer and nearer to the eye, a point 

 will be reached within which the image is always double. This point marks 

 with considerable exactitude the near limit of accommodation. With short- 

 sighted persons, if the needle be removed further and further away, a point 

 is reached beyond which the image is always double ; this marks the far 

 limit of accommodation. 



The experiment may also be performed with the needle placed horizon- 

 tally, in which case the holes in the card should be vertical. 



The adjustment of the eye for near or far distances may be assisted by 

 using two needles, one near and one far. In this case, one needle should be 

 vertical and the other horizontal, and the card turned round so that the holes 

 lie horizontally or vertically according to whether the vertical or horizontal 

 needle is being made to appear double. 



622. In what may be regarded as the normal eye, the so-called emme- 

 tropic eye [Fig. 166], near the limit of accommodation is about 10 or 12 cm. 



[FIG. 166. 



Emmetropic Eye. Parallel Rays focussed on the Retina.] 



and the far limit may be put for practical purposes at an infinite distance. 

 The " range of distinct vision," therefore, for the emmetropic eye is very 

 great. In the myopic, or short-sighted eye [Fig. 167] the near limit is 

 brought much closer (5 or 6 cm.) to the cornea ; and the far limit is at a 

 variable, but not very great distance, so that the rays of light proceeding 

 from an object not many feet away are brought to a focus, not on the retina, 



Myopic Eye. Rays coming from a distance focussed too soon.] 



but in the vitreous humor. The range of distinct vision is therefore, in the 

 myopic eye very limited. In the hypermetropic [Fig. 168], or long-sighted 

 eye, the rays of light coming from even an infinite distance are, in the pas- 

 sive state of the eye, brought to a focus beyond the retina. The near limit 

 of accommodation is at some distance off, and a far limit of accommodation 

 does not exist. The presbyopie eye, or the long sight of old^people, resembles 

 the hypermetropic eye in the distance of the near point of accommodation, 

 but differs from it inasmuch as the former is an essentially defective condi- 

 tion of the accommodation mechanism, whereas in the latter the power of 



