ON THE SEPARATION OF LIGHT AND HEAT. J2J 



x. 



''Experiments on the Separation of Light and Heat by "Refraction. 

 In a Letter from Sir H. C. Englefield, Bart. F. R. S. 

 to Thomas Young, M. D. F. R. S. From the Journal of the 

 Roi/al Injlitution, p. 202. 



Dear Sir, 



IN purfuance of your defire, I communicate to you the ex- Letter to Dr. 

 periments which I made in confequence of Dr. Herfchel's Youn S« 

 moft interefting difcovery of the feparation of folar heat and 

 light by the prifm. They carried with them, to my mind, a 

 complete conviction of the truth and accuracy of the Doctor's 

 allertions on that fubject. Should you judge them worthy of 

 infertion in the Journals of the Royal Institution, I fhall think 

 myfelf honoured by their publication. 



I am, Dear Sir, 



Your faithful Servant, 



H. C. ENGLEFIELD. 



As I was defirous not only to afcertain, by actual obferva- Introduftory 

 tion, the very curious difcovery of Dr. Herfchel, but to obvi- t&nulu. 

 ate the objection made by Mr. Leflie * to the mode in which 

 his experiments had been made, I contrived an apparatus 

 totally different in its arrangement from that which had been 

 ufed by the Doctor ; and difpofed in fuch a manner that it was 

 impoflible that the thermometers mould be affected by any 

 foreign heat reflected from any part of it. As to the heat re- 

 flected from the floor, it could not influence the refults, fince 

 it was the fame, whatever colour was thrown on the ball of 

 the thermometer. 



As I had nothing to do with light, it was not neceflary to A prifm was 

 darken the room; and as I wiflied to accumulate as large a ? ^ ce< ). *" t ! )e 

 portion of folar heat as poflible, I placed the prifm in an open room not dark- 

 window, inftead of admitting a beam through an aperture in ened * 

 a flvutter, as is the afual practice. The prifm I ufed is a 

 very good one ; and it was lent to me for this purpofe by Mr. 

 Walker, of Conduit-Street. It is three inches long, and 

 equilateral, each fide being 1.15 inch broad. 



• In his communications to the Philofophic Journal (quarto) 

 Vol. IV. p,344, 416. 



It 



