350 REFLECTION OF HEAT. 



554. Reflection of Heat. When radiant heat 

 falls upon an athermanous body, part of it is generally 

 absorbed and raises the temperature of the body. The 

 rest is reflected, the energy still existing in the ether waves. 

 The angle of incidence equals the angle of reflec- 

 tion ( 97). 



FIG. 262. 



(a.) In Fig. 262, the source of heat at A is a Leslie's cube filled with 

 hot water. S is an athermanous screen with an aperture for the 

 passage of rays from A to the reflector B. The line CB is per- 

 pendicular to the reflector. When D, the bulb of the deferential 

 thermometer, is placed so that the angle ABC equals the angle 

 DBG, the reflected rays will strike the bulb and raise the temper- 

 ature. 



555. Reflection by Concave Mirrors. By the 



use of spherical or parabolic mirrors, remarkable heating 

 effects may be produced. When parallel rays (like the 

 sun's rays) strike directly upon such a mirror, they are 

 reflected to a focus. Any easily combustible substance 

 held at the focus may be thus ignited. 



(a.) Two such mirrors may be placed as shown in Fig. 263. At 

 the focus of one reflector place a hot iron ball ; at the focus of the 

 other, a bit of phosphorus or gun-cotton. If the apparatus be 

 arranged with exactness, the combustible will be quickly ignited. 



