PHYSICAL FORCES 



101 



medium be similar to the first, and the opposite surfaces 

 of the intermediate medium be parallel. 



If, however, both its surfaces be curved in a convex 

 manner, parallel rays falling through one convex surface 

 will converge, and will converge again when passing 



FIG. 19. 



out of the opposite convexity till they all meet at a focus 

 F (Fig. 19). 



Thus a doubly convex lens will bring the hot rays of 

 the sun to a focus where they may set matter in com- 

 bustion the lens acting as a " burning glass." 



FIG. 20. 



Doubly concave surfaces (Fig. 20) will, in an analogous 

 manner, increasingly scatter and disperse the heat rays. 



The refrangibility of heat differs according to the 

 medium it traverses, and this is another indication that 

 heat rays differ by some other quality than mere intensity. 



