110 difliculty in measuring. And since the discovery in the case of 

 lieat, of retraction, single and double, of polarization, of total re- 

 flection, and the change it produces on polarized light, as well as 

 the change of the plane of polarization by simple reflection, there 

 seems the greatest reason to suppose that the laws of reflection for 

 heat and light, if not identical for both, would be connected by 

 some simple analogy. 



" Accordingly, during the month of December 1837, I made some 

 preliminai'y observations, which encouraged me to proceed. It ap- 

 pears to me, from those observations, that the quantity of heat re- 

 flected from dift^^erent transparent bodies is independent, or nearly 

 so, of the nature of the source of heat, and the diathermancy of the 

 reflecting body, and that at 55 of incidence the intensity of re- 

 flected heat is nearly that which Fresnel's theory gives. The sub- 

 stances, however, were not all prepared so as wholly to exclude 

 the action of second surfaces. 



" I have this winter resumed the subject. I have had an appara- 

 tus constructed for securing sufficient accuracy in determining the 

 angle of incidence, and I have used reflecting surfaces, both trans- 

 parent and metallic ; the former are wedges of plate-glass, by means 

 of which reflection from the first surface only maybe observed, and 

 the latter are plane specula of steel- and silver. The prosecution, 

 however, of these apparently simple experiments has been attended 

 with unforeseen difficulties ; and although the relative proportions 

 of heat at diflFerent angles of incidence are now pretty well deter- 

 mined for glass in several cases, I am not prepared to say whether 

 the absolute amount is exactly the same as Fresnel's formula would 

 give, assigning to heat its proper refractive index. It is satisfac- 

 tory, however, to know, that the approximation to it is much 

 greater than direct photometrical measures have yet given, with 

 the single exception of two experiments of JVI. Arago already re- 

 ferred to ; and that I have reason to believe that the experimental 

 law which Mr Potter has given from direct observation in the case 

 of light, represents my results much less accurately than the theory 

 of Fresnel. 



" With respect to reflection at the metals, I believe I may 

 assert that I have verified the remark of Mr Potter, that me- 

 tallic reflection is less intense at the higher angles of incidence. 

 I have attempted to ascertain whether it reaches a minimum. 

 Hid (hen increases up to 90° of incidence, as Mr Maccnllagh 

 iipposcs, hut I have not obtained decisive results. The (juan- 



