408 THE SENSORIAL FUNCTIONS. 



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 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, which would have brought 

 them to the point r, 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, 

 also, is illustrated in the figure, where m, n, o, p, q, 

 show the inclinations of the surfaces of a rarer 

 medium, calculated to increase 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, 



i 



