156 Professor Forsxs on the Refraction 
138:100 118:100 116:100 
Another series, | 130:100 125:100 125:100 
A third, 120:100 120:100 
A fourth,* 128:100 123:100 122:100 
54. The depolarizing effect of this mica plate (which also 
cives by polarized light the red of Newron’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 O° or at 45°, I totally failed in obtaining a sensible effect with 
* Observed by Dr Traitt. 
