154 Professor Forbes 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 coincided 

 with the plane of polarization (when the depolarizing effect was 

 nothing), and when it was inclined 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 



