OPTICAL IMAGES. 



431 



the other rays takes place in such a manner as to 

 make the following angles equal : a c g = g c /, adh 

 = lid m, a e i = i e n, and afk = kfo. Thus the rays 

 will have the same divergence after reflection as they 

 had before it, and will proceed as though they came 

 from a luminous point a', situated upon the prolonga- 

 tion of the perpendicular a b, as far behind the surface 

 of the mirror as the point a is in front of it. Upon 

 an eye placed before the mirror the reflected rays will 

 thus make the same impression as if they actually pro- 

 ceeded from a!, the eye will in fact see the point a' . 

 This point a' is called an optical image of the point a 



Every point in a body which is placed before a mirror 

 gives rise in the same way to an optical image of itself, 



FIG. 245 (\ real size). 



and the images of all the separate points produce to- 

 gether the image of the whole body, equal to it in magni- 

 tude and corresponding to it in form. Nevertheless, a 

 change in position is observable in the images of bodies 



