Prof. Forb6s on the Refraction and Polarization of Heat. 285 



fore capable of reflection, they may now be partially, or wholly, 

 incapable of it. Such a mode of action may in general terms 

 be called depolarization^ 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 explanation which we adopt of 

 the curious and complicated changes which doubly refracting 

 crystals exercise in the case of light, it is clear that the esta- 

 blishment 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 v/e can adopt, and it requires us, if we admit 

 depolarization, to admit the existence of double refraction and 

 of interference. 1'lie 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 re- 

 latively to the polarizing and analysing 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 contain- 

 ing the two axes) is parallel or perpendicular to the plane of 

 polarization. In intermediate positions, light reaches the eye. 

 This is true for all thicknesses of the film of mica onl}' where 

 light of different degrees of refrangibility is combined: with 

 perfectly homogeneous light, at certain thicknesses, no light 

 would in any position reach the eye, that is, it would not be 

 depolarized. , 



49. The analogous fact, in heat, would of course be indi- 

 cated by interposing a film of mica between a polarizing and 

 analysing plate, having their planes of incidence inclined at 

 right angles to one another, and observing whether any differ- 

 ence of heating effect appeared when the Principal Section of 

 the plate was parallel to the plane of Primitive Polarization, 

 or inclined 45° to it. 



50. The very first experiments which I tried, seeined deci- 



