REFRACTION OF HEAT. 



351 



FIG. 263. 



Replace the iron ball with a Leslie's cube containing hot water ; 

 at the focus of the other reflector place one bulb of the differential 

 thermometer. The rise of temperature at this focus will be clearly 

 shown, even irfien the oilier bulb is nearer the source of heat than the 

 focus is. 



556. Refraction of Heat. When rays of heat 

 fall obliquely upon a diathermanous body, they will be 

 bent from a straight line on entering and leaving the body. 

 This bending of the ra,y is called refraction. Many 

 rays of heat may thus be concentrated at a focus, as in the 

 case of a common burning-glass. By the aid of a spectacle- 

 glass, the sun's rays may be made to ignite easily combus- 

 tible substances. The refraction of obscure rays cannot 

 be shown by a glass lens, since glass is athermanous for 

 such rays. But if a rock-salt lens be held before a source 

 of obscure heat, and the face of a thermopile placed at 



