496 ACCOMMODATION. [BOOK m. 



rise to an image of the spot a at P, and similarly the left-hand 

 spot q corresponds to the right-hand Q. Blocking the left-hand 

 hole, therefore, causes a disappearance of the right-hand image, 

 and vice versa. Similarly when the eye is accommodated for a 

 distance nearer than the needle, the retina may be supposed to 

 be removed to II, and the right-hand ae and left-hand of rays, after 

 uniting at c, will diverge 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 Q 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 farther and farther 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 

 horizontally, 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. 



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

 tropic eye, the near limit of accommodation is about 10 or 12 cm. 

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

 distance. The 'range of distinct vision' therefore for the emme- 

 tropic eye is very great. In the myopic, or short-sighted eye, 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, but in the vitreous 

 humour. The range of distinct vision is therefore in the myopic 

 eye very limited. In the liypermetropic, or long-sighted eye, the 

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

 passive 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 presbyopic 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 condition 

 of the accommodation mechanism, whereas in the latter the power 

 of accommodation may be good and yet, from the internal arrange- 

 ments of the eye, be unable to bring the image of a near object on 

 to the retina. When a normal eye becomes presbyopic, the far 



