THE SENSE OF SIGHT 405 



muscle and suspensory ligament. The former is 

 attached to the ligament, and the ligament is the 

 supporting structure of the lens. If the eye is' looking 

 at a near object the lens becomes more convex or 

 bulges in front. This is produced by the ciliary 

 muscle contracting and relaxing the ligament which 

 permits the lens to bulge forward and become more 

 convex due to its elasticity. The nearer an object 

 is to the eye the greater will be the divergence of the 

 rays of light, and as a result the lens becomes more 

 convex in order to converge and focus the rays upon 

 the retina, so that the image will be distinct. If 

 the eye is looking at a distant object the lens is less 

 convex. The further away an object is from the eye, 

 the less divergent will be the rays of light. Thus the 

 lens is not called upon to converge the rays of light as 

 they fall upon the retina to appear distinct, as is the 

 case in looking at a near object. At the same time the 

 pupil also becomes narrowed to prevent an indistinct- 

 ness of the image by permitting an excess of light 

 rays to pass. These would otherwise be too diffuse, 

 due to the angle at which they enter, to permit of a 

 proper focussing of the image on the retina. 



In seeing an object both eyes are involved, and two 

 images, one in each eye, are focussed upon the retina, 

 but there arises only one sensation. This is due to the 

 fact that both eyes converge toward the object seen, 

 and it is focussed as an image upon the fovese at 

 corresponding points, in each retina. When any 

 condition interferes with the proper convergence of 

 the eyes toward an object, double vision occurs, due 

 to the object falling on two different points of the 

 retina. 



The function of the iris is to regulate the quan- 

 tity of light entering the interior of the eye and adjust 

 the rays of light so that the formation and perception 

 of an image shall be distinct. 



