14 ACCOMMODATION. [BOOK in. 



versely, 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 

 from each luminous 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 

 luminous 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 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, 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 forward to fall on the retina : the focus lies 

 permanently behind the retina. In the other, the eye cannot 

 bring the image sufficiently 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. 



709. 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, with the holes 

 horizontal before one eye, the other being closed, 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 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 distance of the needle, 

 the only effect produced by blocking up one hole of the card is 



