VISION. 



457 



M, N, o, p, Q, represent the inclinations of the 

 surfaces of a rarer medium, calculated to in- 

 crease the convergence of the rays, that is to 

 bring them to a nearer focus (f). The result 

 of the continued change of direction in the 

 refracting surface, is a regular curvilineal sur- 

 face, which, in the present case, approaches very 

 nearly to that of a sphere. Hence by giving 

 these refractive media spherical surfaces, we 

 adapt them, with tolerable exactness, to produce 

 the convergence of parallel rays to a focus, and 

 by making the denser medium convex on both 

 sides (as shown in Fig. 410), both surfaces will 



410 m 



conspire in producing the desired effects. Such 

 an instrument is termed a double convex lens; 

 and it has the property of collecting into a focus 

 rays proceeding from distant points.* 



Having obtained this instrument, we may now 



* The refraction by spherical surfaces does not, strictly speak- 

 ing, unite a pencil of parallel or divergent rays into a mathe- 

 matical point, or focus; for in reality the rays which are near 

 the central line are made to converge to a point a little more 



