166 Experimental Philosophy. [Lecture 12. 



you in the tenth lecture, fig. 47, and it will serve 

 to elucidate all the phsenomena of reflection. 



Lest you should, however, have attended to 

 the maxims and definitions subjoined to that 

 lecture less assiduously than you ought, I shall 

 refer you to another figure. In PL XIII. fig. 

 54, ?io may be considered as a ray of light striking 

 perpendicularly on the surface of the mirror a &, 

 and it is consequently reflected back in the same 

 line. The ray d o, coming from the luminous 

 body d, strikes the mirror obliquely, and is re- 

 flected to the eye in the line o e, in such manner, 

 that the angle e o n is equal to the angle o d n ; 

 in other words, the angle of reflection is equal to 

 the angle of incidence. 



This, you will answer, is sufficiently clear ; but 

 how comes it that I do not see the object at o, 

 since it is there that the rays strike the mirror ? 

 And why is it, that, on the contrary, the object 

 appears behind the glass, and in the situation of 

 s ? This has been partly explained by a rule 

 which I formerly laid down ; namely, that we 

 see every thing in that line in which the rays 

 last approached us. Now an object is rendered 

 visible, not by single rays proceeding from every 

 point of its surface, but by pencils of rays, or 

 collections of divergent rays issuing from every 

 point, as was explained in the preceding lecture. 

 These pencils of rays are afterwards, by the 

 refractive powers of the eye, converged again to 



