1154 ACCOMMODATION. [BOOK m. 



and the needle will appear as one needle. When the eye is 

 accommodated for a distance beyond a, the retina may be con- 

 sidered to lie 1 no longer at nn, but nearer the lens, at mm for 

 example ; the rays ae will cut this plane at p, and the rays af at 

 q\ hence the luminous point will no longer appear single, but will 

 be seen as two points, or rather as two systems of diffusion circles, 

 and the single needle will appear as two blurred needles. The 

 rays passing through the right-hand hole e, will cut the retina at 

 p, i.e. on the right-hand side of the optic axis ; but, as we have 

 already ( 707) said, the image on the right-hand side of the 

 retina is referred by the mind to an object on the left-hand side 

 of the person ; hence the affection of the retina at p, produced 

 by the rays ae falling on it there, gives 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 af rays, after uniting at c, will diverge 

 again and strike the retina in diffusion circles 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 determination of the accommodation 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. 



710. In what may be regarded as the normal eye, the so- 

 called emmetropic 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 emmetropic eye is very great. In the myopic, or short-sighted 

 eye, the near limit is brought much closer (5 or 6 cm.) to the 



1 Of course, in the actual eye, as we shall see, accommodation is effected by a 

 change in the lens, and not by an alteration in the position of the retina ; but for 

 convenience sake, we may here suppose the retina to be moved. 



