VISION. 407 



from its original course to a particular side, we 

 have only to turn the surface of the medium in 

 such a manner as that the perpendicular line to 

 that surface, contained within the denser medium, 

 shall lie still farther on the same side. Thus, in 

 Fig. 408, if we wish to turn the ray r s, from s o 

 to s T, we must place the dense medium so that the 

 perpendicular s p, which is within it, shall be still 

 farther from s o, than s t is ; that is, shall lie on 

 the other side of s t. The same rule applies to the 

 contrary refraction of tlie ray s t from t v to t u, 

 when it passes out of a dense, into a rare medium ; 

 for the perpendicular t i must still be placed on 

 the same side of t v as t u is situated. 



Let us now apply these principles to the case 

 before us ; that is, to the determination of the form 

 to be given to a dense medium, in order to collect 

 a pencil of rays, proceeding from a distant object. 



409 



accurately to a focus. We shall suppose the object 

 in question to be very remote, so that the rays com- 

 posing the pencil may be considered as being 

 parallel to each other ; for at great distances their 

 actual deviation from strict parallelism is wholly 

 insensible ; and let a, b, c, d, e, (Fig. 409), repre- 

 sent 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 (u) 



