ERRORS OF REFRACTION AND ACCOMMODATION. 155 



which pass between the posterior processes of the body and the anterior surface 

 of the lens are slackened by these changes and the lens is allowed to bulge forward 

 by its own elasticity. 



The extent to which the curvature can be increased by accom- 

 modation is limited only by the elasticity of the lens. 

 Experiment 59. The Near Point. The least distance at which 

 an object may be held away from the eye and still be clearly 

 seen is the NEAR POINT. Find how far away your own near 

 point lies by looking fixedly at a pin or pencil-point with one 

 eye closed and gradually bringing the object closer to your face. 

 A distance is found nearer than which if the object is held it is 

 seen blurred. The blurring means that your accommodation is 

 no longer sufficient to bring all the rays from a single point to a 

 focus on the retina, the focus lies behind the retina, and the 

 rays from each point reach the sensitive cells as a pencil, and 

 not as a point of light. 



The near point in a young child's eye is generally about 10 or 

 12 cms. away; as time goes on this distance increases and as a 

 rule at 40 or 50 years of age the near point has receded past the 

 length at which it is convenient to hold a book. Convex glasses 

 are then used for reading and hand-work and by this means the 

 divergence of the rays is reduced so that objects may be comfort- 

 ably held and still throw sharp images on the retina. The change 

 in elasticity has no effect on distant vision, the shape of the lens 

 in the eye at rest is the same as before. 



When the gaze is directed to an object near at hand two other 

 changes occur, associated with the act of accommodation. The 

 visual axes, which during rest or distant vision are parallel, are 

 converged so that they meet on the object at which one looks, and 

 make the image of it fall on the fovea centralis of each eye. This is 

 done by the contraction of the internal recti, which rotate the eye- 

 balls inward. At the same time the ACCOMMODATION REFLEX of the 

 pupil occurs; the circular muscle fibres which are contained within 

 the free margin of the iris contract and reduce the size of the open- 

 ing. The iris acts as a curtain or adjustable diaphragm for the 

 eye, limiting the size of the pencil of rays which enter it. By this 

 reflex contraction of the opening in near vision all rays are shut 

 out except those which pass through or near the centre of the 



