ioo ELEMENTS OF SCIENCE 



the mirror opposite, will be so reflected as to meet again 

 exactly at the focus of the second mirror the spot to 

 which all the rays coming to the mirror in straight lines 

 are reflected and converge. Now, if a red hot iron ball 

 be placed at A (Fig. 18), the focus of one, the rays of 

 heat will then radiate from the adjacent mirror's sur- 

 face, thence they will be first reflected in straight lines 

 to the surface of the other mirror, whence they will be 

 again reflected in convergent lines to its focus B when 

 if either a piece of phosphorus or a thermometer has 

 been previously placed there, the thermometer will 

 rise or the phosphorus take fire. The interposition of 

 a screen between the mirrors will prevent these effects, 



FIG. 1 8. 



while if there be no screen, the red hot ball may be placed 

 even nearer the thermometer than the focus of the first 

 mirror and yet produce much less effect than when in 

 that focus. 



(6) Refraction. When radiant heat passes from one 

 medium into another as from air into glass then the 

 directions of its rays become thereby somewhat changed, 

 or, as it is called, refracted. If the second medium be 

 diathermous (transparent to heat) then, when the rays, 

 having traversed it, pass out of it again e.g., from 

 glass once more into air there is a second refraction of 

 the rays which completely undoes the effect of the first 

 as to direction if (as in the case supposed) the third 



