156 Professor Forbes on the Refraction 



138:100 118:100 116:100 



Another series, 130:100 125:100 128:100 



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 shew, 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 constantly tends to diminish the ratio of the true differ- 

 ence of effect, as observed in (23). 



55. It occurred to me, that since thin plates of mica present 

 comparatively little resistance to the passage of heat, that 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 po- 

 larizing light circularly (nearly), was employed for this experi- 

 ment. But not only was I unable to detect any increase of effect 

 when it was placed between the polarizing and analyzing 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 and with that from incan- 

 descent platinum. When I proceeded to estimate its depola- 

 rizing power by the usual method of placing the Principal Section 

 at 0° or at 45°, I totally failed in obtaining a sensible effect with 



* Observed by Dr Traill. 



