148 Professor Forbes 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 



* I did not see M. Melloni'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. Melloni 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 Berard's conclusions, objections which 

 he does not consider to have been overcome. — Ann. de Chimie, lv. 374. 



