Tte/mngiUlity of Light. 163 



same dense medium, and entering the rare me- 

 dium, or the air, which, from the form of the 

 glass, you know must present a concave surface. 

 Now rays are equally converged by entering a 

 convex surface of a dense medium, and a concave 

 surface of a rarer medium. The focus of a 

 double convex lens, then, is at only half the 

 distance of the focus of one which has only one 

 convex surface, that is, a plano-convex. The 

 focus of a double convex lens, therefore, as you 

 have already seen, fig. 52, is the length of the 

 radius, or semi-diameter of that circle, which is 

 formed by the convexity of either of its surfaces. 

 That branch of optics which respects the re- 

 frangibility of light is usually called dioptrics, 

 from the Greek dia, through, and optomai, to 

 *ee ; so that it means to see through. 





