Chap. 3.] equal to the Angle of Incidence. 187 



in which it came to the reflecting furface ; rays falling 

 obliquely obferve the general law of reflexion, and 

 their angle of reflexion is exactly equal to the angle 

 of incidence. In Plate V. (fig. i.)/Vis a ray of light 

 falling perpendicularly on the plane furface a b y and it 

 is reflected back exactly in the fame direction j e c is a 

 ray falling obliquely on the furface at c 3 and it is re- 

 jected in the direction c d, making the angle of re- 

 flexion c d P exactly equal to the angle of incidence 

 c e P, as may be feen by the infpection of the figure *. 



Parallel rays falling obliquely on a plane reflecting 

 furface are reflected parallel, converging rays are re- 

 flected with the fame degree of convergence, and di- 

 verging rays equally diverging. In other words, plane 

 furfaces or mirrors make no change in the natural dif- 

 pofition of the rays of light. 



A mirror is a body the furface of which is poliihed 

 to fuch a degree as to reflect moft copioufly the rays 

 of light. In Plate V. fig. i, 2, 3, are plane mirrors: 

 in fig. 2, the rays d b and c a, which are parallel, after 

 having reached the furface a b are reflected, the one 

 "towards b and the other towards k, and in both in- 

 fiances the angle of reflexion is evidently equal to the 

 angle of incidence. 



The rays d b and c a (fig. 3.) are convergent, an4 

 without the interpofition of the mirror would unite in 

 the point E ; but being reflected they unite in the op- 

 pofite point F, the angle of reflexion with refpect tc* 

 each being ftill equal to the angle of incidence, as may 

 be feen by drawing perpendiculars to the points a 

 and b. 



* The reader will fee that the angles d c b and ec a, made 

 u-'uh the lines of incidence and reflexion and the reflecting fur- 

 face, are alfo equal. 



The 



