io6 Dr. Brewster on new properties of heal s 
Proposition XL. 
Radiant heat is not susceptible of refraction , and is incapable of 
permeating glass like the luminous rays. 
The propagation of radiant heat along glass can be ren- 
dered visible to the eye by the methods described in the first 
section of this paper. It advances from the heated edge of 
the plate, crystallizing the glass during its passage, and pro- 
ducing changes in those parts of the plate where it does not 
exist in a sensible state. 
If the radiant heat is received upon a convex lens, the very 
same effect is produced. Instead of being bent, like light, at 
the convex surfaces, it advances, whatever be the angle of 
incidence, in lines perpendicular to that surface, crystallizing 
the glass in its progress ; and, as soon as it has reached the 
second surface, it is again discharged, as if from a new source 
of heat. This experiment I conceive to be an ocular demon- 
stration of the first part of the Proposition. 
Dr. Herschel, in his celebrated inquiry into the properties 
of invisible heat, has deduced the very opposite result from 
several experiments ; but, independently of the minuteness of 
the effects which he observed, it is manifest, that the thermo- 
meter placed in the focus of his lens, received its heat by ra- 
diation from the lens itself ; and it is also demonstrable, that 
a convex lens, radiating heat at an uniform temperature, will 
produce *a greater effect upon a thermometer placed in its 
axis, than upon another having a different position. From the 
form of the lens, the edges are always the coldest, giving 
out their heat to the metallic ring in which they are placed^ 
