Prof. Forbes on the Refraction and Polarization of Heat. 287 



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 re- 

 sults were obtained. With incandescent platinum, the result 

 is of the most striking character; under favourable circum- 

 stances, the needle moves through from 2° to 3°. (a quan- 

 tity, it will be recollected, of which a twentieth or a thir- 

 tieth part is capable of measurement by the improved method 

 of observation,) or even more, commencing 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 expe- 

 riments gave for the ratio of the effect on the pile in the two 

 positions, with a single plate, 



138:100 118:100 116:100 



Another series, 1 30 : 1 00 1 25 : 1 00 123:1 00 



A third, 120:100 120:100 



A fourth*, 128 : 100 123 : 100 122 : 100 



54. The depolarizing effect of this mica plate (which also 

 gives by polarized light the red of Newton's second order,) 

 upon non-luminous heat, was also exceedingly well marked, 

 as I shall presently show, and amounted generally to between 

 0°*5 and 1°, as the statical effect; but as the source of heat 

 requires to be closer to the mica plates, more is transmitted 

 by conduction, which constantl}^ tends to diminish the ratio of 

 the true difference of effect, as observed in (23). 



55, It occurred to me, that since thin plates of mica pre- 

 sent comparatively little resistance to the passage of heat, a 

 very thin plate might perhaps depolarize more heat than it 

 stopped, and thus we should have the paradoxical effect of an 

 interposed obstacle increasing the effect, a mode of action 

 which I thought I perceived in a thicker plate. I was at first 

 surprised to find the reverse the case. 



56, A film of mica which transmitted a slightly blue white 

 of the first order (by polarized light), and which was capable 

 of polarizing light circularly (nearly), was employed for this 

 experiment. But not only was 1 unable to detect any increase 

 of effect when it was placed between the polarizing and ana- 

 lysing plates (E and F) crossed so as to give a minimum of 

 transmitted heat, but there was an evident interception when 

 it was interposed. In other words, it stopped more heat than 

 it depolarized. This was true both with non-luminous heat 



» Observed by Dr. Traill. 



