REFRACTION OF RAYS OF LIGHT. 647 



controlled as to enable objects to be seen at various dis- 

 tances. 



With, the help of the diagram below (fig. 177), repre- 

 senting a vertical section of the eye from before back- 

 wards, the mode in which, by means of the refracting 

 media of the eye, an image of an object of sight is thrown 

 on the retina, may be rendered intelligible. The rays of 

 the cones of light emitted by the points A B, and every other 

 point of an object placed before the eye, are first refracted, 

 that is, are bent towards the axis of the cone, by the 

 cornea c c, and the aqueous humour contained between it 

 and the lens. The rays of each cone are again refracted 

 Fig. 177.* 



and bent still more towards its central ray or axis by the 

 anterior surface of the lens E E ; and again as they pass 

 out through its posterior surface into the less dense 

 medium of the vitreous humour. For a lens has the power 

 of refracting and causing the convergence of the rays of 

 a cone of light, not only on their entrance from a rarer 

 medium into its anterior convex surface, but also at their 

 exit from its posterior convex surface into the rarer 

 medium. 



In this manner the rays of the cones of light issuing 

 from the points A and B are again collected to points at a 



