352 EADIANT HEAT. 



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 Professor Forbes, in a short paper in the London and Edinburgh 

 Journal of Science, November, 1835; and further by direct experi- 

 ment described in the same journal for March, 1836. 



Circular and Elliptical Polarization of Heat: Forbes. 



On the 1st of February, 1836, Professor Forbes announced to the 

 Royal Society of Edinburgh, that he had that day succeeded in establish- 

 ing the circular polarization of heat, even when unaccompanied 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, Professor Forbes, following up the analogies of Fresnel with 

 regard to the internal reflection of light, found the very same thing 

 verified with heat by similar internal reflection in a rhomb of rock salt. 

 where the plane of reflection is inclined 45° to the plane of primitive 

 polarization. 



A short notice of this discovery appears in a paper by the author, 

 in the London and Edinburgh Journal of Science, March, 1836, in 

 which he also states the inference from the same considerations, that 

 the waves are of the same kind as those of light, viz: formed by 

 transverse vibrations. 



In a paper reported in the proceedings of the Royal Society of 

 Edinburgh, March 21, 1836, and printed along with the second series 

 of Professor Forbes' s Researches in the London and Edinburgh 

 Journal of Science, vol. xii, that philosopher describes some addi- 

 tional results which he has obtained respecting the polarization of 

 heat. These are briefly as follows: 



1st. Heat polarized in any plane, and then reflected from the sur- 

 face of a refracting medium, changes its plane of polarization in a 

 manner similar to what obtains in light; that is, the plane is on one 

 side of the plane of reflection up to the maximum polarizing angle, 

 and on the other side after passing that limit. This mode of deter- 

 mining the polarizing angle offers some advantages over the more 

 direct methods. 



2d. Metals polarize heat very feebly by reflection. Yet the effect 

 is perceptible, and increases, through a considerable range of inci- 

 dences, but it does not seem to attain a maximum; in this respect it 

 seems to agree with what Sir D. Brewster has remarked in light, viz: 

 that the maximum is greatest for the least refrangible rays, heat 

 being less refrangible than light. 



3d. Heat polarized in a plane inclined 45° to the plane of reflec- 

 tion at silver, has its nature changed, as in light, and presents the 

 conditions of elliptic polarization, though the ellipse is much more 

 elongated. 



4th. Two reflections from silver increase the polarizing effect of 

 metals, and an increased tendency to circular polarization under the 



