136 Professor Forsers on the Refraction 
tiplier their attempts at its solution *. But, like previous expe- 
rimenters, they employed a metallic mirror to concentrate the 
rays of the moon, which, acting in the usual manner of dispers- 
ing the heat of the thermometer, produced so great a cooling 
effect, as completely to neutralize any positive results. 
10. It occurred to me, however, from the consideration of 
M. Mettonr’s very decisive experiments as to the permeability 
of screens of different kinds to heat from various sources, that the 
moon’s heat must, in very great proportion at least, radiate 
through glass. And this on several grounds ; as, 1. because the 
sun’s heat, of which this may be considered as an integral part, 
does so with scarcely any loss ; 2. because heat, accompanied by 
light, always does so, and generally in proportion to the brilliancy 
and refrangibility of that light ; and, 3. because the lunar rays ha- 
ving passed through the whole thickness of the atmosphere must, 
according to the experiments of Dr La Roce, fully confirmed 
by Mettont, have parted with the greater part of that species of 
heat most easily stopped, and hence arrive at the earth m a 
state comparatively capable of passing through glass and si- 
milar substances. If this opinion be correct (nor can I entertain 
any doubt upon it), and if we substitute a lens for a mirror to con- 
centrate the lunar rays, we shall profit by all, or nearly all, of their 
heating effect, whilst such a lens, instead of promoting the radia- 
tion of the heat of the thermometer to the sky, will entirely stop 
it (because heat of this description does not pass sensibly through 
the thinnest glass), and thus its disturbing influence will be en- 
tirely prevented. 
11. I employed, therefore, a polyzonal lens made by Soue1L 
of Paris, in my custody, to concentrate the moon’s light. The 
diameter of the lens is 50 inches; its focal distance about 41 
inches, whence we may compute the size of the lunar image to 
be a circle 0.38 inch in diameter. Comparing this with the di- 
* Annales de Chimie et de Physique. December 1831. 
5 
