i86 The Angle of Reflexion [Book III. 



caufe which we cannot at prefent explain, and the 

 quantity which is thus loft or abforbed differs accord- 

 ing to the nature and circumftances of the reflecting 

 iurface. 



The great law of reflexion, and which ferves to ex- 

 plain all its phenomena, is this, that the angle of re- 

 Jlexion is always equal to the angle cf incidence. It was 

 already intimated, that by the angle of incidence is 

 meant the angle made by a ray of light with a per- 

 pendicular to the reflecting furface at the point where 

 the ray falls ; and by the angle of reflexion, the angle 

 which the ray makes with the fame perpendicular on 

 the other fide *. 



The angle of reflexion being thus in all cafes equal 

 to the angle of incidence, it is evident that the power 

 which caufes this reflexion is always the fame. No 

 furface however has hitherto been found, which has 

 not fome inequalities in it to be difcovered by the 

 microfcope, and yet thefe inequalities do not? affect the 

 law thus difcovered of re fleeted rays. It is therefore 

 uriiverfally concluded, that the power which produces 

 this effect in the direction of the rays acts at fome dif- 

 tance from the reficcting furface. Innumerable con- 

 jectures have been propofed to explain this pheno- 

 menon, but it muit be confc-ffed that even the fag.i- 

 cky of a Newron was unable to develope and fully 

 explain this reflecting power. Tie however attributes 

 it to the general principle of repulfion. 



A ray of light falling perpendicularly on a plane 

 furface is reflected back exactly in the fame direction 



* See note on the beginning of Chap. I. and Plate V. fig. I. 

 The angle of refl-jxicm \viH alfo be found equal to the angle of in- 

 cidence on plane furfaces, if meaiured from the ivfu-cting furface, 

 waich is indeed very coramcn in books of philofcpay acd tre;itiit"- 

 on <j\ 



in 



