256 



tity of heat reflected by tlie metals is so much greater than Mr 

 Potter's estimate for light, as to lead me to suspect that his photo- 

 metric ratios are all too small, which would nearly account for their 

 deviation from Fresnel's law. 



" The most complete verification of Fresnel's law would be found 

 in observations made on heat polarized in opposite planes. These 

 I have attempted, and, so far as they go, they seem to confirm the 

 analogies of heat and light. But the intensity is so much reduced 

 in the process of polarizing, that I fear' we must wait for yet more 

 delicate instruments to measure it. In the mean time I may state 

 the method which I propose to employ. 



" When heat is polarized by transmission through inclined mica- 

 plates, the polarization is incomplete ; but if the plates be inclined 

 at the polarizing angle, the transmitted heat is undoubtedly com- 

 posed of a portion p polarized (in a plane which we will call +), 

 and a portion 1 — p unpolarized. This latter part philosophers 



1 p 



are content to consider as compounded of a part — — ^ polarized 



-\-, and an equal part polarized — Now, let the intensity of re- 

 flected heat polarized in the plane of reflection (or — ), and that 

 perpendicular to it (or -}-), be represented by the following Table, 

 which contains the quantities to he found. 



Also let the quantities of heat actually observed to be reflected af- 

 ter incomplete polarization by passing through a mica bundle, be 

 the following : 



