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 proportion as the rays (such as those at a and 

 e) are moi3 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 further 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 sur- 

 face must be similar to that already described ; 

 if rarer, they must be in an opposite direction. 

 This last case is illustrated in the figure, where 



