470 Professor Forbes's Researches on Heat. 



by inclining the plane of reflection 45°, from 37 : 100 to 60 : 100 ; 

 and when heat from incandescent platinum was employed, from 

 28 : 100 to 64 : 100. 



44. It occurred to me, that, somewhat above the superior 

 angle of total reflection indicated for light, the effect of apparent 

 depolarization would be more perfect, and a ready way of doing 

 this presented itself by the use of two prisms of rock-salt, having 

 angles of 60°, with which I provided myself. The superior angle 

 of total reflection for rock-salt (whose index of refraction is 1.56) 

 is 57°28' nearly, for light, and since it increases rapidly as the 

 refrangibility diminishes, it was reasonable to expect it to be still 

 higher, or not far from 60° for dark heat (of low refrangibility). 

 The two prisms, arranged as in Fig. 10. (which is a ground-plan), 

 fulfilled the required conditions, the dotted lines indicating the 

 path of the rays of heat through the prisms, and the result cor- 

 responded to my expectation. When the plates I and K were 

 used to polarize and analyze, and the planes of total reflection 

 and polarization were parallel, the ratio in the rectangular posi- 

 tions of the analyzing plate was 40 : 100 ; whilst, when the plane 

 of first polarization was inclined 45°, the ratio was raised as high 

 (in one series of experiments) as 94.5 : 100. With the same ap- 

 paratus, and with heat from incandescent platinum, the ratio was 

 raised from 29 : 100 to 84 : 100. Thus the astonishing proper- 

 ties of rock- salt enable us most completely to extend the analo- 

 gies of light even in their most complicated cases to the pheno- 

 mena of heat. 



45. We are naturally led from the consideration of circular 

 polarization produced by two known methods in the case of light, 

 viz. by transmission through a thin doubly refracting plate, and 

 by total reflection in a refracting medium, to consider the third 

 mode in which it has been effected, that is, by metallic reflec- 

 tion. In this case, also, the analogy holds as to the general fact 

 which I have succeeded in completely establishing under several 

 circumstances. Whilst the copious reflection of heat which takes 



