24 REPORT—18-0. 
plane of reflexion 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 Prof. Forbes’s Researches in the London and 
Edinburgh Journal of Science, vol. xii., that philosopher de- 
scribes some additional results which he has obtained respect- 
ing the polarization of heat. These are briefly as follows :— 
1st. Heat polarized in any plane, and then reflected from the 
surface of a refracting medium, changes its plane of polariza- 
tion in a manner similar to what obtains in light ; that is, the 
plane is on one side of the plane of reflexion up to the maxi-~ 
mum polarizing angle, and on the other side after passing that 
limit. This mode of determining the polarizing angle offers 
some advantages over the more direct methods. 
2ndly. Metals polarize heat very feebly by reflexion. Yet 
the effect is perceptible, and increases, through a considerable 
range of incidences, but it does not seem to attain a maxi- 
mum; 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. 
3rdly. Heat polarized in a plane inclined 45° to the plane of 
reflexion at silver, has its nature changed, as in light, and pre- 
sents the conditions of elliptic polarization, though the ellipse 
is much more elongated. 
4thly. Two reflexions from silver increase the polarizing 
effect of metals, and an increased tendency to circular polariza- 
tion under the conditions of the last case. The effect increases 
with the obliquity of incidence. 
All these results have been verified in the case of obscure as 
well as luminous sources of heat. 
On the 15th Feb. 1836, the Keith prize was awarded to 
Prof. Forbes by the Royal Society of Edinburgh; the Vice- 
President, Dr. Hope, stating, in the course of a most able 
address delivered on the occasion, that several members of the 
council, as well as himself, had personally witnessed the satis- 
factory verification of the main facts announced, before the 
medal was adjudged. 
