468 Professor Forbes's Researches on Heat. 



an equal quantity of heat reaching the pile in all positions of the 

 analyzing plate. 



39. Last summer it occurred to me that it was probable that 

 rock-salt, refracting heat almost as it does light, would cause it 

 to undergo total reflection at a proper incidence. Supposing 

 this to be the case (and I had afterwards reason to believe that 

 such had been shewn to be the fact by M. Melloni), I then 

 foresaw the possibility of trying an experiment of the most con- 

 clusive character, as to the nature of heat, — its susceptibility of 

 becoming circularly polarized by means of two total internal re- 

 flections, as in the admirable experiment of Fresnel in the case 

 of light. 



40. Various circumstances prevented me from trying this ex- 

 periment until the end of January last, when I procured a rock- 

 salt rhomb, similar to that of glass used by Fresnel, but having 

 its angles calculated by Fresnel's formula, for the refractive in- 

 dex for light of rock-salt. I took the smaller of the two singles 

 which the double solution of the quadratic equation gives, on 

 account of the smaller dimensions required for the rhomb. This 

 angle is nearly 45°. On the 1st of February I performed the 

 experiment with complete success, though with an apparatus less 

 perfect than I afterwards procured. The arrangement represent- 

 ed in Fig. 9. proved exceedingly convenient. R is the rhomb 

 of salt laid on its side, so that the plane of reflection within it is 

 horizontal ; S the source of heat, P the pile, I the polarizing mi- 

 ca bundle, K the analyzing mica bundle. The following facts 

 were observed : 



41. When the plane of reflection coincided with, or was per- 

 pendicular to, the plane of primitive polarization, the heat (whe- 

 ther wholly dark, or derived from incandescent platinum ) came 

 out unchanged, that is, on placing the analyzing plate in azimuth 

 0° and 90° relatively to the polarizing plate, the ratio of the ef- 

 fects was the same as if no reflection had taken place. 



42. When the plane of first polarization was inclined + 45° 



