456 THE SENSORIAL, FUNCTIONS. 



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 refraction will be effected by a slight 

 degree of obliquity in the medium at the points 

 (h and k) where those rays meet it. In propor- 

 tion 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 re- 

 quired, in order to bring them to the same focus. 

 The convergence of these rays, after they have 

 passed this first surface, which would have 

 brought them to the point r, may be farther in- 

 creased 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. 

 Thi« last case, also, is illustrated in the figure, 



