and Polarization of Heat. 167 
In discussing these observations, it would be necessary to attend 
to the remark of (73), respecting the want of homogeneity in the 
heat. 
75. Fyom 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. 
. It may be polarized by refraction. 
. It may be polarized by reflection. 
. It may be depolarized by doubly refracting crystals. Hence, 
. It is capable of double refraction, and the two rays are 
2 
oO & OO 
* 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 
