148 Professor Forses on the Refraction 
wise be adapted to the instrument. I had previously noticed the 
large proportion of heat transmitted by thin plates of mica, and 
I thought of applying bundles of mica plates placed at the po- 
larizing angle, and so cut from the plate, that the plane of inci- 
dence corresponded with one of the neutral sections of the mica 
plate, (the section used was that perpendicular to the princi- 
pal plane.) so that the transmitted pencil would be polarized ex- 
actly similarly to that refracted through glass or any singly re- 
fracting medium. 
34. I prepared two pairs of bundles of plates of mica of this 
description, the first (which I called A and B) having a thickness of 
about one-fiftieth of an inch, and was split into about ten plates, 
whilst the others (C and D) were only half the thickness, and con- 
tained but half as many reflecting surfaces. I found that these 
plates, placed at the proper angle, polarized light very satisfactorily. 
On applying them to heat, I had the satisfaction of finding that 
not only was heat from an oil lamp most decisively polarized, 
but also that from a brass plate warmed by alcohol, but so as to 
be quite invisible in the dark, having probably a temperature (as 
before mentioned) of about 700° Fahr. These experiments were 
made on the 22d November last, and were afterwards amply 
confirmed *. 
35. It is to this mode of observing that I attribute chiefly 
the success of my after inquiries. The mode of reflection for 
polarizing is attended with so much inconvenience where a ther- 
mometer is concerned, and especially with the multiplier, as to 
render the employment of it tedious and incommodious ; whereas 
by having two bundles of mica plates arranged in square tubes, so 
* TI did not see M. Mexuonr’s second paper till the 10th of December, after I 
had obtained the chief fundamental results contained in this paper. It does not ap- 
pear, however, that M. Mrttoni had thought of applying his instrument to any 
question of polarization except that of tourmaline, and in a note he alludes to the 
objections, which had been urged against Berarp’s conclusions, objections which 
he does not consider to have been overcome.—Ann. de Chimie, lv. 374. 
