286 ProF. Forbes on the Refraction and Polarization of Heat. 



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. Tliese 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 (tempera- 

 ture about 700") the polarizing mica plates (E and F) being 

 crossed, the ratios of heat transmitted, when the principal sec- 

 tion coincided with the plane of polarization (when the depo- 

 larizing effect was nothing), and when it was incliiicd 4<5<» 

 (when the depolarizing effect ought to be a maximum), were 

 the following: 



100:120 100:110 100:122 100:125 



With different polarizmg and analysing 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 Ifica7idescent Platinum is 

 highly polarizable; it is also powerfully depolarized, as the 

 following proportions obtained with polarizing mica plates, 

 and the same interposed films as before, indicate, as the prin- 

 cipal section was inclined O*' or 45 : 



100:126 100: 138 100: 138 



52. There were two distinct interposed plates employed for 

 these experiments; their thickness was such as to transmit the 

 red of the second order of the Newtonian Scale, when viewed 

 by polarized light, analysed at right angles to the plane of po- 

 larization. To show that no appreciable difference existed in 

 their power of stopping common or unpolarized heat, and to 

 point out the accuracy of such determinations, I may quote 

 the following experiment on the transmission of unpolarized 

 non-luminous heat through the two plates. 



Plate with sides inclined 0' and 90° Plate with sides inclined 45° 

 to Principal Section. to Principal Section. 



, Q , o . Mean. 



^^i \ 18«-25 18r 



18i { 



'^ I 17-9 



18x1 



18 ) 18-25 18 + 



The reduction is performed as in art. 20. These quantities 



