430 



REFLECTION BY MIRRORS. 



the divisions istinctly marked with numbers (see p. 75, line 9) ; 

 apertures l cm wide being made in the proper places, and the strip 

 is finally fixed permanently with glue and tacks. 



The small piece of looking-glass, 2 cm wide and 4 cm high, should 

 be cut by a glazier. It is fixed with a cement, made by melting- 

 together in a metal spoon equal parts of resin and yellow bees' wax, 

 and stirring the mixture. When gently warmed, this mixture be- 

 comes so soft that it may be moulded with the fingers. It is applied 

 only to the edges of the mirror, not to the silvered back, which 

 would be injured by it.. While fixing the mirror the eye should be 

 applied to the aperture at 0, so as to give the correct position to 

 the mirror. It is still better to have a groove cut in the wooden 

 rim into which the mirror closely fits. 



Let S S in fig. 244 represent the surface of a mirror 

 upon which rays of light fall from the point a, and 



FIG. 244 (an.proj). 



let the lines a b, ai, ad, ae, af represent some of 

 the rays. If the ray a b is supposed to be perpendicular 

 to the surface, it will coincide with the perpendicular 

 at the point of incidence of the ray a b, and the lines 

 c #, d A, e i, fk will be the perpendiculars at the points 

 of incidence of the four other rays. The ray a b is re- 

 flected back in its own direction, while the reflection of 



