1 8 S Reflexion from plane Surfaces. [Book III. 



The rays db and c a (fig. 4.) are on the contrary 

 divergent, and after reflexion towards b and k pr<:: : .-rve 

 exactly the fame diftance from each other, as they 

 would have had if they had proceeded without inter- 

 ruption towards F and E, the angle of reflexion being 

 with rcfpect to each ray itill exactly equal to the an- 

 gle of incidence. 



Thus it is that .plane furfaccs reflect the; rays of 

 light j but the effects are materially different when the 

 furfaces are convex or concave, though the fame law 

 flill obtains with rcfpect to thefe. From a convex 

 furface parallel rays when reflected are made to di- 

 verge j convergent rays are reflected lefs convergent, 

 or are even made to diverge in proportion to the cur- 

 vature of the furface compared with their c'^ree of 

 convergence j and divergent rays are rendered more 

 divergent. Thus, it is the nature of convex furfaces 

 to fbatter or dilperle the mys of light, and in every 

 inftance to impede their convergence. From a con- 

 cave ' furface, on the contrary, parallel rays when re- 

 fieded are made to converge ; converging rays are 

 rendered more convergent ; and diverging rays are 

 made lefs divergent, or even in certain cafes may be 

 made to converge. 



To underftand this part of the fubject, it is neceifary 

 to be aware that all curvilinear furfaces are compofed 

 cf right lines infinitely ihorr, or prints -, and the reader 

 will recollect that only thofe rays which fill perpendi- 

 cularly on a reflecting furface are reflected back in the 

 fame direction. Ail curves are arches or fegments of 

 circles ; if therefore any curvilinear or fpherical fur- 

 face .is prefentcd to a number of parallel rays, it is 

 evident that only that ray which Rrikes the fpherical 

 furface in fuch a direction that it would proceed in a 

 right line to the center of that circle, of which the re- 

 flecting 



