154 Professor Forses on the Refraction 
of mica only where light of different degrees of refrangibility is 
combined : with perfectly homogeneous light, at certain thicknes- 
ses, 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 indicated 
by interposing a film of mica between a polarizing and analyzing 
plate, having their planes of incidence inclined at right angles to 
one another, and observing whether any difference of heating ef- 
fect 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, seemed deci- 
sive on this point. I employed the piles of mica for polarizing 
by transmission, and interposed successively two plates of mica 
so arranged that the Principal Section was in the one parallel 
(or perpendicular), and in the other inclined 45° to the plane 
of Primitive Polarization. These were cut from the same piece, 
and precisely of the same thickness; but I afterwards employed 
one and the same plate, inclined alternately in two positions. 
By the first experiments with dark heat (temperature about 
700°) the polarizing mica plates (E and F) being crossed, the 
ratios of heat transmitted, when the principal section cozncided 
with the plane of polarization (when the depolarizing effect was 
nothing), and when it was ¢zclined 45° (when the depolarizing 
effect ought to be a maximum), were the following : 
100:120 100:110 100:122 100:125 
With different polarizing and analyzing plates, viz. C and D, the 
following ratios were obtained also for dark heat : 
100:118 . 100:120 100:120 100:113 
51. We have seen that the heat of Incandescent Platinum is 
highly polarizable ; it is also powerfully depolarized, as the fol- 
lowing proportions obtained with polarizing mica plates, and the 
same interposed films as before, indicate, as the principal section 
was inclined 0° or 45 : 
100:126 100:138 100:138 
