470 Professor Forses’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 
