500 THE SPECIAL SENSES. 



This is because rays of light coming into the eye very obliquely, 

 in a lateral or vertical direction, are not brought to their proper focus. 

 Thus, in Fig. 162, the rays diverging from the point a, directly in 

 front of the eye, fall upon the lens in such a way that they are all 

 brought together at x, at the surface of the retina ; but those coming 

 from b fall upon the lens so obliquely that, for rays having an equal 

 divergence with those coming from a, there is more difference in their 

 angles of incidence, and of course more difference in the amount 

 of their refraction. They are consequently brought together more 

 rapidly, and on reaching the retina are dispersed over the space y, z. 



Fig. 162. 



The perfection of the eye, as a visual apparatus, is very much 

 increased by the action of the iris. This organ, as we have already 

 mentioned, is a nearly vertical muscular curtain, placed in front of 

 the lens, attached by its external margin to the junction of the 

 cornea and sclerotic, and pierced about its centre by the circular 

 opening of the pupil. It consists, according to most anatomists, of 

 two sets of muscular fibres viz., the circular and the radiating. 

 The circular fibres, which are much the most abundant, are arranged 

 in concentric lines about the inner edge of the irjs, near the pupil ; 

 the others are said to radiate in a scattered manner, from its central 

 parts to its outer margin. The action of these two sets of fibres is 

 to contract and enlarge the orifice of the pupil. The circular fibres, 

 in contracting, draw together the edges of the pupil, and so diminisli 

 its opening; and when these are relaxed, the radiating fibres come 

 into play, and by drawing apart the edges of the orifice, enlarge 



