on the terrestrial Rays that occasion Heat. 445 



apparatus, from so coarse an experiment, that the focus of heat, 

 in this case, was certainly farther removed from the lens than 

 the focus of light, and probably not less than £ of an inch ; the 

 heat, at half an inch beyond the focus of light, being still equal 

 to that in the focus. 



Article v. — Transmission of heat-making Rays. 



We enter now on the subject of the transmission of heat 

 through diaphanous bodies. Our experiments have hitherto 

 been conducted by the prism, the lens, and the mirror; these 

 may indeed be looked upon as our principal tools, and, as such, 

 will stand foremost in all our operations ; but the scantiness of 

 this stock cannot allow us to bring our work to perfection. 

 Nor is it merely the want of tools, but rather the natural 

 imperfection of those we have, that hinders our rapid progress. 

 The prism which we use for separating the combined rays of 

 the sun, refracts, reflects, transmits, and scatters them at the 

 same time ; and the laws by which it acts, in every one of these 

 operations, ought to be investigated. Even the cause of the 

 most obvious of its effects, the separation of the colours of light, 

 is not well understood ; for, in two prisms of different glass, 

 when the angles are such as to give the same mean refraction, 

 the dispersive power is known to differ. Their transmissions 

 have been still less ascertained ; and I need not add, that the 

 internal and external reflexions, and the scattering of rays on 

 every one of the surfaces, are all of such a nature as must 

 throw some obscurity on every result of experiments made 

 with prisms. A lens partakes of all the inconveniencies of the 

 prism ; to which its own defects of spherical aberrations must 



mdccc 3 M 



