212 ELEMENTARY PHYSIOLOGY 



photographer's camera. If we look at an object at a certain 

 distance, we see that object distinctly because its image is distinct 

 on the retina ; but other objects in the same direction which are 

 either nearer or more remote are more or less indistinct. If we 

 now direct the attention to another object nearer than the first, 

 the image of this object becomes sharp on the retina. And, since 

 the retina has not moved, the refractive power of the eye must 

 have increased in order to 'focus' this nearer object. This 

 change has been brought about by the ciliary muscles acting in- 

 directly on the crystalline lens (see p. 208). When we turn from 

 near to distant objects, the opposite change takes place. The 

 ciliary muscle relaxes, the suspensory ligament is pulled towards 

 the edge of the choroid, and the lens becomes less convex. Thus 



Fig. 201. The Photograph ar's Camera. 



A, a brass tube containing the lens and the diaphragm ; B, the 

 screen on which the image falls. 



the adjustment of the eye depends on the ciliary muscle and its 

 consequent action on the crystalline lens. 



In some persons the cornea is more convex than usual, and 

 in others the refractive power of the crystalline lens is above 

 the average. In these cases the retina is too far back for the 

 image to be sharply denned, and consequently the vision is in- 

 distinct. Such persons are said to be short-sighted; and they 

 may remedy their defective vision by wearing concave glasses, 

 which cause rays of light to diverge. 



As old age advances the cornea frequently becomes less con- 

 vex, and the refractive power being as a consequence decreased, 

 the retina is not far enough back to receive the sharply-defined 

 image of near objects. In such cases the persons are long-sighted, 



