and Polarization of Heat. 1 55 



59,. 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, analyzed at right angles to the plane of polariza- 

 tion. To shew 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. 



Mean 



to 



Principal Section. 



18- | 



18°.25 





18°| 



18} | 

 1?| { 



17.9 





17| 



18^ } 



18.0 





18 









.8 } 



18.25 





18+ 



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

 were observed with the naked eye, and may therefore be con- 

 sidered as coinciding in the two columns. 



53. In repeating these experiments with a single film of mica, 

 which was alternately placed with its axis parallel or inclined 45° 

 to the plane of primitive polarization, similar results were obtain- 

 ed. With incandescent platinum, the result is of the most striking 

 character ; under favourable circumstances, the needle moves 

 through from 2° to 3° degrees, (a quantity, it will be recollected, 

 of which a twentieth or a thirtieth part is capable of measurement 

 by the improved method of observation), or even more, com- 

 mencing the moment that the change in the position of the mica 

 film is effected (which I generally perform with long forceps, so 

 as to avoid the near approach of the hand to the pile). A few 

 of the first experiments gave for the ratio of the effect on the 

 pile in the two positions, with a single plate, 



u 2 



