Third Series. — Refrangihility of Heat. 191 



index of mean refraction obtained for heat from this source 

 than that of dark heat of higher temperature. 



The following method might perhaps be used with success 

 for obtaining more exact data respecting the refrangihility, 

 and especially the dispersion, of heat, than that just described 

 pretends to give. It must insure a beam of parallel rays of 

 heat of sufficient intensity and uniform in every part of its 

 section. A small point of heat placed behind a lens (or two 

 or three lenses to diminish aberration) is the most obvious 

 plan. But the intensity would be inadequate. I would, 

 therefore, propose a platinum wire, heated by one of Mr. 

 Daniell's constant voltaic batteries, placed behind a refracting 

 semicylinder of rock-salt*. The central rays should be alone 

 employed, and the prism for total reflection should be high 

 and narrow as well as the aperture of the pile. It is possible 

 that in this case the transition from partial to total reflection 

 would be so rapid as to make the error arising from the 

 varying intensity of partial reflection inconsiderable. By 

 changing the force of the battery, heat of all temperatures 

 might be employed in succession. The numerical analysis of 

 the heat spectrum would then take place as described in 

 p. 189. 



Conclusion. — My object in these, as in former researches, 

 has not been to group experiments of mere curiosity indiscri- 

 minately selected, but to present a basis for a proper theory 

 of heat. Without some such end in view I should have 

 thought the time and labour spent on these experiments in 

 some degree misapplied. Mere numerical results, though 

 ultimately of the highest consequence to science, should never 

 form the exclusive object of the philosopher. I trust to have 

 shown that though many of the conclusions in this paper are 

 based upon quantitative results, these have not been the ulti- 

 mate aim of the inquiry. 



The mutual bearing of the three sections of this paper, and 

 of all upon what (from analogy to physical optics) we may call 

 physical thermoiics, is now evident. (] .) In the first section we 

 have minutely discussed a point apparently perhaps of minor 

 importance, namely, the unequally polarizable nature of the 

 rays of heat. The importance of the doctrine lies in this : 

 that the common theory of undulation recognises no such 

 variation, nor perhaps does it exist in the case of light (I 

 know, however, of no decisive experiments on this point), with 

 the exception of the small effect due to the difference of re- 

 frangihility. Now, having proved in the third section that 

 this difference of mean refrangihility is from most sources 



* Such a one I have had executed. 



