VISION. 455 



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 st. The same 

 rule applies to the contrary refraction of the ray 

 s T from TV 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 



409 



object, accurately to a focus. We shall suppose 

 the object in question to be very remote, so that 

 the rays composing the pencil may be consi- 

 dered as being parallel to each other; for at 

 great distances their actual deviation from 

 strict parallelism is wholly insensible; and let 



