DIOPTRIC MECHANISMS. 



747 



what is called Schemer's Experiment. If two smooth holes be pricked in a 

 card, at a distance from each other less than the diameter of the pupil, 

 and the card be held up before one eye, with the holes 

 horizontal, and a needle placed vertically be looked at 

 through the holes, the following facts may be observed : 

 When attention is directed to the needle itself, the image 

 of the needle appears single. Whenever the gaze is 

 directed to a more distant object, so that the eye is no 

 longer accommodated for the needle, the image appears 

 double and at the same time blurred. It also appears 

 double and blurred when the eye is accommodated for 

 a distance nearer than that of the needle. When only 

 one needle is seen, and the eye therefore is properly ac- 

 commodated for the distance of the needle, no effect is 

 produced by blocking up one hole of the card, except 

 that the whole field of vision seems dimmer. When, 

 however, the image is double on account of the eye be- 

 ing accommodated for a distance greater than that of 

 the needle, blocking the left-hand hole causes a disap- 

 pearance of the right-hand or opposite image, and 

 blocking the right-hand hole causes the left-hand image 

 to disappear. When the eye is accommodated for a 

 distance nearer than that of the needle, blocking either 

 hole causes the image on the same side to vanish. The 

 diagram will explain how these results are brought 

 about. 



Let a (Fig. 165) be a luminous point in the needle, 

 and ae, a /the extreme right-hand and left-hand rays 

 of the pencil of rays proceeding from it, and passing 

 respectively through the right-hand e, and left-hand/, 

 holes in the card. (The figure is supposed to be a 

 horizontal section of the eye.) When the eye is accom- 

 modated for a, the rays e and /meet together in the 

 point c, the retina occupying the position of the plane 

 n n ; the luminous point appears as one point, and 

 the needle will appear as one needle. When the 

 eye is accommodated for a distance beyond a, the retina may be consid- 

 ered to lie l no longer at n n, but nearer the lens, at m m for example ; the 

 rays a e 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 at 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 

 shall see in speaking of the judgments pertaining to vision, 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 a e falling on it there, gives rise to the 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 / I, and 

 the right-hand a e and left-hand a f rays, after uniting at c, will diverge 



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. 



Diagram of Schemer's 

 Experiment. 



