M. MELLONI ON THE REFLECTION OF RADIANT HEAT. 387 
By diminishing the angle of incidence which the calorific rays form 
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 (/aiton) 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 $34 or about 
half the effect given by a rock-salt lens having its diameter equal to 
that of the opening of these cones. 
