OF OPTICS. 



251 



the direction m n ; the ray c in the direction op, &c. 

 The ray 3, that falls directly upon the middle of the 

 glass, suffers no refraction in passing through it ; but 

 goes on in the same rectilineal direction, as if no glass 

 had been in its way. 



If the glass had been concave only on one side, and 

 the other side quite plane, the rays would have diverged, 

 after passing through it, as if they had come from a 

 radiant point at double the distance of C from the glass ; 

 that is, as if the radiant point had been at the distance 

 of a whole diameter of the glass's concavity. 



If rays come more converging to such a glass, than 

 parallel rays diverge after passing through it, they will 

 continue to converge after passing through it ; but wiU 

 not meet so soon as if no glass had been in the way ; 

 and will incline towards the same side to which they 

 would have diverged, if they had come parallel to the 

 glass. Thus the rays ./and h, going in a converging 

 state towards the edge of the glass at B, and converg- 

 ing more in their way to it than the parallel rays di- 

 verge after passing through it, they will go on con- 

 verging after they pass through it, though in a less 

 degree than they did before, and will meet,' at /: but 

 if no glass had been in their way, they would have 

 met at i. 



When the parallel rays as dfa, Cmb, elc, fall upon 



a concave mirror A B, (which is not transparent, but 

 has only the surface A b B of a clear polish) they will 



LECT. 

 VIL 



