290 Prof. Forbes on the Refraction and Polarization of Heat, 



complicated character, which nothing but an acquaintance 

 with the corresponding facts with regard to Hght could have 

 taught us how to look tor, and which, by coinciding with these, 

 indicate a common mechanism. Hence, too, were our senses 

 or our instruments capable of perceiving them, we should ne- 

 cessarily discover, by the passage of heat along the axes of 

 doubly refracting crystals, all the elegant forms of rings and 

 brushes, defined by heating, instead of luminous rays. 



63. But this analogy may be carried still further. So de- 

 finite are the experimental results in depolarization, that I 

 thought of comparing the intensities of the effects with those 

 produced in light; and for this purpose, our method of esti- 

 mating heat is far more satisfactory than those for estimating 

 the intensity of illumination. The fundamental law, which I 

 felt most anxious to verify, was the complementary nature of 

 the transmitted heat, when \\\e jjlane of analysation is parallel, 

 and when it is perpendicular, to the plane of polarization, 



64-. It is well known in the case of light, that when no cry- 

 stal is interposed between the polarizing and analysing plates, 

 or when the crystal has its principal section parallel or per- 

 pendicular to the plane of primitive polarization, the whole of 

 the light is stopped* when the plates are perpendicular or 

 crossed; the whole is transmitted when they are parallel. If 

 the principal section of the crystal be now inclined 45° to the 

 plane of polarization, the depolarizing effect is a maximum, a 

 portion of light now being transn'iitted to the eye, the plates 

 remaining crossed, which was not transmitted before, and, in 

 like manner, a portion of the light which was formerly trans- 

 mitted when the plates were parallel being now stopped. Now 

 these two quantities are equal to one another, and therefore 

 the sum of the intensities of illumination in the two cases 

 (plates parallel and plates crossed) is a constant quantity. Now 

 these two pencils correspond to the ordinary and extraordi- 

 nary image in an analysing prism of calcareous spar. Let us 

 call these intensities O^ and E^. Let the whole quantity of 

 polarized light, or the value of O^, when the principal plane 

 of the crystal coincides with that of polarization, be F^, and, 

 under the same circumstances, E^= zero. Then since the 

 two effects are complementary, whatever be the position of the 

 principal plane, O^ + E^ = const. = F^ 

 ,ind E2 = F — O^; 



or the whole intensity gained by the extraordinary pencil 



* That is, not reflected when the light is analysed by reflection, or not 

 transmitted when it is analysed by refraction. In these experiments the 

 latter method was always used. 



