1 38 Prof. Forbes on the Refraction and Polarization of Heat, 



faith of his veracity. Thus I have obtained repeated differ- 

 ences, not exceeding half or even a quarter of a degree of the 

 multiplier (observed without a telescope), which, by the promp- 

 titude with which the needle was repelled or attracted at the 

 instant that the change of circumstances to be considered was 

 effected, left as little doubt in my mind as if the numerical 

 result had been many times greater. 



9. Having satisfied myself^ in a variety of ways, of the ex- 

 treme delicacy and promptitude of action of this instrument, I 

 thought of applying it to detect the heat of the moon's rays in 

 a more unexceptionable manner than, I am persuaded, it has 

 ever been attempted. This curious question had not escaped 

 MM. Nobili and Melloni when they first constructed the in- 

 strument, and they mention in their first account of the ther- 

 mo-multiplier their attempts at its solution*. But, like pre- 

 vious experimenters, they employed a metallic mirror to con- 

 centrate the rays of the moon, which, acting in the usual man- 

 ner of dispersing the heat of the thermometer, produced sogreat 

 a cooling effect, as completely to neutralize any positive results. 



10. It occurred to me, however, from the consideration of 

 M. Melloni'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, ra- 

 diate through glass. And this on several grounds ; as, 1. be- 

 cause 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 pro- 

 portion to the brilliancy and refrangibility of that light; and, 

 3. because the lunar rays having passed through the whole 

 thickness of the atmosphere must, according to the experi- 

 ments of De la Roche, fully confirmed by Melloni, have parted 

 with the greater part of that species of heat most easily stop- 

 ped, and hence arrive at the earth in a state comparatively 

 capable of passing through glass and similar substances. If 

 this opinion be correct (nor can I entertain any doubt upon 

 it), if we substitute a lens for a mirror to concentrate the lunar 

 rays, we shall profit by all, or nearly all, of their heating ef- 

 fect, whilst such a lens, instead of promoting the radiation 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 

 entirely prevented. 



11. I employed, therefore, a polyzonal lens made by Soleil 

 of Paris, in my custody, to concentrate the moon's light. The 

 diameter of the lens is 30 inches ; its focal distance about -il 



• Anna/es de Chimie et de Physique, December 1831. 



