370 Prof. Forbes on the Refraction and Polarization of Heat, 



I'acy of the following results, it is worth quoting them as con- 

 firming the general fact, that obscure heat has longer undu- 

 lations than luminous heat. The numbers derived from plate 

 No. 2. (see 65,) are most to be depended upon, and the agree- 

 ment of the different series made with dark heat is highly sa- 

 tisfactory. The numbers correspond to those of the last column 

 in the example of (71). 



Mica Plate, No. 1. Retardation for Light, Ratio of Total Polarization and 



or o — e = '00004 inch. Depolarization or F* : E'^. 



Number of Comparisons. 



Argandlamp, 4 100:80 



Incandescentplatinum, ... 4 100:78 



Brass about 700°, 4 100:69 



Mica Plate, No. 2. Retardation for Light, 

 or o — e = -00002 inch. 



Argandlamp, 3 100:66 



Incandescentplatinum,... 6 100:47 



Brass about 700°, 7 100 : 52 



Ditto, 4 100:51 



Mercury about 500°, ... 5 100 : 52 



In discussing these observations, it would be necessary to at- 

 tend to the remark of (73), respecting the want of homoge- 

 neity 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 

 crossed) red of the first order, almost exactly circularly polar- 

 izes obscure heat, for it depolarizes half the heat. The cha- 

 racteristic 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 analysing plate 

 was at 0° or 90°. With incandescent platinum the effect is 

 exceedingly striking ; for, if the mica film be at 0°, the po- 

 larizing 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*. 



* These conclusions were stated nearly in their present form (excepting 

 the 6th), to the Royal Society [of Edinburgh] at their meeting of the 5th 

 January. The whole of the experiments detailed in this paper (excepting 

 only the repetition of M. Melloni's experiment on the refraction of heat 

 (16), were made between the 22nd November and the 16th January, but all 

 the general consequences had been clearly made out before the close of 

 1834. 



