M. MELLONI ON THE REFLECTION OF RADIANT HEAT. 38? 



By diminishing the angle of incidence which the calorific rays fonn 

 with the surface of the rock crystal, an increase of reflection is obtained, 

 especially in the small incidences. But this effect is nearly imperceptible 

 on the metallic surface, for, in passing from 80° to 20°, I have been able 

 to verify with the plate of brass (laiton) no more than a difference of 4 or 

 5 hundredth parts. The concentration of radiant heat by the action of 

 metallic mirrors of any form will therefore be always inferior to that 

 produced by rock-salt lenses of the same breadth. Thus, for example, 

 the conical mirrors of polished brass which are applied to one face of 

 the pile of the thermomultiplier will never give more than ^|^ or about 

 half the effect given by a rock-salt lens having its diameter equal to 

 that of the opening of these cones. 



