Reflexibility of Light. 169 



nature of those which are convex or concave is a 

 more difficult study, though the same law pre- 

 vails with respect to them as with respect to the 

 others. To understand the principles on which 

 they act, it will be expedient to call to your 

 recollection what was said in the former lecture 

 on spherical surfaces. All curves or arches may 

 be considered as composed of a number of small 

 flat planes, lying obliquely to one another. Pa- 

 rallel rays, therefore, striking an object opposed, 

 to them in this position, will strike it more or less 

 obliquely. Thus, in fig. 57, the rays a, 6, c, d, 

 which would fall perpendicularly on a horizontal 

 surface, strike obliquely upon those which are 

 opposed to them ; and, instead of being reflected 

 parallel, are reflected divergent. For the same 

 reason, convergent rays would be reflected less 

 convergent by such a mixed surface as this, and 

 divergent rays would be rendered still more 

 divergent. Fig. 58, you see, is the reverse of the 

 preceding, and it serves very well to represent the 

 effects of a concave mirror. By this you must 

 perceive that the parallel rays a, b, c, d, which 

 would have been reflected parallel by a plane 

 mirror, are made to converge, because, instead of 

 striking this mirror in a direct line, they strike it 

 obliquely ; and you may easily conceive, that by 

 the same rule, convergent rays will be reflected 

 still more convergent, and divergent rays will be 

 made to converge less. 



As by a mirror of the convex kind convergent 



VOL. I. I 



