494 ACCOMMODATION. [BOOK in. 



the image of that needle falls upon the retina in such a way, 

 that each pencil of rays from each luminous point of the needle 

 unites in a point on the retina, while each pencil from each 

 luminous point of the far needle unites at a point in front of the 

 retina, and then diverging again falls on the retina, in a series 

 of diffusion circles. If the near needle be gradually brought nearer 

 and nearer to the eye, it will be found that greater and greater 

 effort is required to see it distinctly, and at last a point is reached 

 at which no effort can make the image of the needle appear any- 

 thing but blurred. The distance of this point from the eye marks 

 the limit of accommodation for near objects. Similarly, if the 

 person be short-sighted, the far needle may be moved away from 

 the eye, until a point is reached a,t which it ceases to be seen 

 distinctly, and appears blurred. In the one case, the eye, with all 

 its power, is unable to bring the image of the needle sufficiently 

 forward to fall on the retina : the focus lies permanently behind 

 the retina. In the other, the eye cannot bring the image suf- 

 ficiently backward to fall on the retina: the focus lies permanently 

 in front of the retina. In both cases the pencils of rays from the 

 needles strike the retina in diffusion circles. 



The same phenomena may be shewn with greater nicety by 

 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 accommodated 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 being accommodated for a distance greater than that of the 

 needle, blocking the left-hand hole causes a disappearance of the 

 right-hand or opposite image, and blocking the right-hand hole 

 causes the left-hand image to disappear. When the eye is ac- 

 commodated for a distance nearer than that of the needle, blocking 

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

 following diagram will explain how these results are brought about. 



Let a (Fig. 67) be a luminous point in the needle, and ae, af 

 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 accommodated 



