Chap, hi.] SIGHT. 849 



together in the point c, the retina occupying the position of the 

 plane nn ; the point in the needle appears as one point, and the 

 needle will appear as one needle. When the eye is accommo- 

 dated for a distance beyond a, the retina may be considered 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 point in the needle 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 (§ 529) 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. Simi- 

 larly when the eye is accommodated for a distance nearer than 

 the needle, the retina may be supposed to be removed to 11, 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^>' 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. 



§ 532. 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 



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. 



