VISION. 323 



viation from strict parallelism is wholly insensible; and let A, 

 B, c, D, E, (Fig. 409,) represent these rays. There must 

 evidently be one of these rays (c,) and only one, which by 

 continuing its rectilineal course, would arrive at the point 

 (R) intended to be the focus of the rays. This ray, then, 

 may be suffered to pass on, without being subjected to any 

 refraction; the surface of the medium should, therefore, be 

 presented to the ray (at i) perpendicularly to its course, so 

 that it may pass through at right angles to that surface. 

 Those rays (B and D) which are situated very near to this 

 direct or central ray (c,) will require but a small degree of 

 refraction in order to reach the focus (R:) this small refrac- 

 tion will be effected by a slight degree of obliquity in the 

 medium at the points (H and K) where those rays meet it. 

 In proportion as the rays (such as those at A and E) are more 

 distant from the central ray, a greater amount of refraction, 

 and consequently a greater obliquity of the surfaces (G 

 and L) will be required, in order to bring them to the same 

 focus. 



The convergence of these rays, after they have passed 

 this first surface, may be farther increased by interposing 

 new surfaces of other media at the proper angles. If the 

 new medium be still denser than the last, the inclination of 

 its surface must be similar to that already described; if rarer, 

 they must be in an opposite direction. This last case is il- 

 lustrated in the figure, where M, N, o, p, Q, represent the in- 

 clinations of the surfaces of a rarer medium, calculated to 

 increase the convergence of the rays, that is to bring them 

 to a nearer focus (r.) The result of the continued change 

 of direction in the refracting surface, is a regular curvilineal 

 surface, which, in the present case, approaches very nearly 

 to that of a sphere. Hence by giving these refractive me- 

 dia spherical surfaces, we adapt them, with tolerable exact- 

 ness, 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 conspire in pro- 

 ducing the desired effects. Such an instrument is termed 



