REPORT ON RADIANT HEAT. 293 
the mica, and their interference according to the same laws as 
those of light. Hence also follow the constancy of the sum 
of the intensities of the rays in the rectangular positions, or 
their complementary character, agreeably to the formulas of 
Fresnel for light. This again involves their retardation, ac- 
cording to the well-known principles of the undulatory theory ; 
and hence from Fresnel’s formulas we are assured theoretically 
of the existence of circular and elliptic polarization in the rays 
of heat, under the appropriate conditions: we have thus also 
the means of deducing the length of a wave of heat. 
The whole of this most important series of investigations 
was completed between November 1834 and January 1835, and 
their originality and priority are thus placed beyond dispute. 
The main practical improvement (which led to all the rest of 
the discoveries) was the employment cf the piles of mica for 
polarizing the heat. In the summer of 1835, Prof. Forbes was 
at Paris; and finding both M. Biot and M. Melloni sceptical 
as to his results, he exhibited them with mica piles, which he 
himself prepared on the occasion, and which he left in M. Mel- 
loni’s hands. 
In these experiments the utmost care was taken to guard 
against all the sources of fallacy from secondary radiation, &c. ; 
but as Prof. Forbes observed, these always tended to disguise 
and not to exaggerate the results. One consideration of this 
kind arising from the mere mathematical question of the dif- 
ferent amount of heat which might be radiated from one pile to 
the other in the two rectangular positions (regarded merely as 
a mathematical problem), was proposed by myself at the Dublin 
meeting of the British Association, 1835, but was completely 
shown to be inapplicable as a practical objection by Prof.Forbes, 
in a short paper in the London and Edinburgh Journal of Sci- 
ence, November, 1835 ; and further by direct experiment de- 
scribed in the same Journal for March, 1836. 
Circular and Elliptical Polarization of Heat: Forbes. 
On the Ist of Feb. 1836, Prof. Forbes announced to the Royal 
Society of Edinburgh, that he had that day succeeded in esta- 
blishing the circular polarization of heat, even when unaccom- 
panied by light, by direct experiment. It has been already 
noticed, that theoretically this would follow from the laws of 
depolarization. But in the present instance, Prof. Forbes, fol- 
lowing up the analogies of Fresnel with regard to the internal 
reflexion of light, found the very same thing verified with heat 
by similar internal reflexion in a rhomb of rock salt, where the 
