as exhibited in its propagation along plates of glass. 107 
and therefore, the discharge of heat must be most copious in 
the direction of the axis.* 
The inability of radiant heat to pass through glass, may be 
considered as a consequence of its refusing to yield to the 
refractive force ; for we can scarcely conceive a particle of 
radiant matter freely permeating a solid body, without suffer- 
ing some change in its velocity and direction. The ingenious 
experiments of M. Prevost of Geneva, and the more recent 
ones of M. Delaroche, have been considered as establishing 
the permeability of glass to radiant heat. M. Prevost em- 
ployed moveable screens of glass, and renewed them con- 
tinually, in order that the result which he obtained might not 
be ascribed to the heating of the screen; but such is the 
rapidity with which heat is propagated through a thin plate of 
glass, that it is extremely difficult, if not impossible, to observe 
the state of the thermometer, before it has been affected by 
the secondary radiation from the screen. The method em- 
ployed by M. Delaroche of observing the difference of 
effect, when a blackened glass screen, and a transparent one, 
were made successively to intercept the radiant heat, is liable 
to an obvious error. 
\ 
* The circumstance of the glass cooling most rapidly at the edges, which may be 
proved by exposing it to a polarised ray, enables us to account for the anomalous and 
hitherto unexplained fact observed by the younger Euler, that the focal length of 
a lens is shortened when its temperature is increased. The observation having always 
been made when the lens was actually cooling, the density, and consequently the 
refractive power had increased towards the circumference of the lens, and therefore 
its focal length was diminished. 
Might not the spherical aberration of lenses be diminished, and even corrected, by 
giving them a variable density from their vertex ? I have three object glasses of this 
kind, two crystallized and one uncrystallized, and ground carefully upon the same 
tool ; but I have not yet been able to examine their optical properties. 
• P 2 
