CHAP, in.] SIGHT. 847 



to the far needle, the near one appears blurred and indistinct, 

 and that, conversely, when the near one is distinct, the far one 

 appears blurred. By an effort of the will we can at pleasure 

 make either the far one or the near one distinct ; but not both 

 at the same time. When the eye is arranged so that the far 

 needle appears distinct, the image of that needle falls exactly on 

 the retina, and each pencil of rays reflected from each point of 

 the needle unites in a point upon the retina ; but when the far 

 needle is seen distinctly, the focus of the near needle lies behind 

 the retina, and each pencil from each point of this needle falls 

 upon the retina in a series of diffusion circles ; hence the image 

 of the near needle is blurred. Similarly, when the eye is 

 arranged so that the near needle is distinct, the image of that 

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

 rays from each point of the needle unites in a point on the retina, 

 while each pencil from each 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, and the far needle is now 

 seen indistinctly. 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 anything but blurred. The distance of this point from 

 the eye marks the near 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 at which it 

 ceases to be seen distinctly, and appears blurred ; the far limit of 

 accommodation is reached. In the one case, the eye, with all its 

 power, is unable to bring the image of the needle sufficiently for- 

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

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

 ciently backward to fall on the retina: the focus lies perma- 

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

 from the needle strike the retina in diffusion circles. 



531. The same phenomena may be shewn with greater 

 nicety by what is called Scheiner'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, with the 

 holes horizontal before one eye, the other being closed, and a 

 needle placed vertically be looked at through the holes, the fol- 

 lowing facts may be observed. When attention is directed to 

 the needle itself, the image of the needle appears single. When- 

 ever the gaze is directed to a more distant object, so that the eye 

 is no longer accommodated for the needle, the image of the 

 needle 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 



