and Polarization of Heat. 1 67 



In discussing these observations, it would be necessary to attend 

 to the remark of (73), respecting the want of homogeneity in the 

 heat. 



75. From the last series it appears that a plate of mica which 

 transmits by polarized light (when the polarizing plates are cross- 

 ed) red of the first order, almost exactly circularly polarizes ob- 

 scure heat, for it depolarizes half the heat. The characteristic 

 property of circularly polarized light was observed, viz. that little 

 or no difference of result was obtained whilst the mica film was 

 interposed (its principal section being inclined 45° to the plane 

 of polarization), whether the analyzing plate was at 0° or 90°. 

 With incandescent platinum the effect is exceedingly striking ; 

 for, if the mica film be at 0°, the polarizing effect on crossing the 

 plates is about 40 per cent, of the whole. 



76. It is almost unnecessary to add, that what we have now 

 said, inferring the undulatory theory of light to be true, might 

 be translated into the language of the Newtonian theory of 

 emission. 



77. In conclusion, I would recapitulate the chief results at 

 which I have arrived.* 



1. Heat, whether luminous or obscure, is capable of polariza- 

 tion by tourmaline. 



2. It may be polarized by refraction. 



3. It may be polarized by reflection. 



4. It may be depolarized by doubly refracting crystals. Hence, 



5. It is capable of double refraction, and the two rays are 



* These conclusions were stated nearly in their present form (excepting the 6th), 

 to the Royal Society at their meeting of the 5th January. The whole of the expe- 

 riments detailed in this paper (excepting only the repetition of M. Melloni's expe- 

 riment on the refraction of heat (16)), were made between the 22d November and 

 the 16th January, but all the general consequences had been clearly made out before 

 the close of 1834. 



7 



