558 Prof. Forbes's Researches on Heat, Second Series. 



When the plates I and K were used, the ratio was raised 

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

 60 : 100; and when heat from incandescent platinum was em- 

 ployed, from 28 : 100 to 64?: 100. 



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, ar- 

 ranged as in the figure below (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 corre- 



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

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

 and polarization were parallel, the ratio in the rectangular 

 positions of the analysing 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 apparatus, and with heat from incandescent 

 platinum, the ratio was raised from 29 : 100 to 84 : 100. 

 Thus the astonishing properties of rock-salt enable us most 

 completely to extend the analogies of light even in their most 

 complicated cases to the phaenomena of heat. 



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 con- 

 sider the third mode in which it has been effected, that is, by 

 metallic reflection. In this case, also, the analogy holds as 

 to the general fact, which I have succeeded in completely esta- 

 blishing under several circumstances. Whilst the copious 

 reflection of heat which takes place at metallic surfaces, ren- 

 ders it easier to obtain distinct results than in some other 



