mid Polarization of Heat. 153 



stance, render it capable of reflection at a surface inclined to the 

 rays of light at the polarizing angle, which they were incapable 

 of doing before the crystal was interposed, or if before capable of 

 reflection, they may now be partially, or wholly, incapable of it. 

 Such a mode of action may in general terms be called depolariza- 

 tion, an expressive term, though not quite correct, or as has more 

 lately been proposed, in conformity with the more accurate views 

 now entertained on the subject, Di-polarization, indicating that 

 the action of the interposed crystal is to separate the incident 

 polarized ray into two parts by its doubly refracting energy ; 

 which parts are polarized in rectangular planes, and by their 

 union produce the modified effect. But whatever be the explana- 

 tion which we adopt of the curious and complicated changes 

 which doubly refracting crystals exercise in the case of light, it is 

 clear that the establishment of a correlative fact in regard to 

 heat unaccompanied by light, must force us to admit an identity 

 of the laws which combine, by a singularly refined mechanism, 

 to produce an identical result. The theory of undulations is 

 in fact by far the simplest that we can adopt, and it requires us, 

 if we admit depolarization, to admit the existence of double re- 

 fraction and of interference. The demonstration, then, of such 

 a property of heat, is one of such importance, as to require the 

 fullest proof. 



48. The power of mica to depolarize heat, I discovered on 

 the 16th of December last. If in the case of polarizing light, 

 whether by reflection or refraction, the planes of incidence rela- 

 tively to the polarizing and analyzing plates be at right angles to 

 one another, the light is wholly (or at least in great part) stopped. 

 The plates remaining in this position, it is well known, that if a film 

 of mica be interposed between them, so as to be perpendicular to 

 the incident light, that light will no longer be stopped excepting 

 in two positions, namely, when the Principal Section of the mica 

 plate (or the plane containing the two axes) is parallel or perpen- 

 dicular to the plane of polarization. In intermediate positions, 

 light reaches the eye. This is true for all thicknesses of the film 



VOL. XIII. part i. u 



