Prof. Forbes on the Refraction and Polarization of Heat, 211 



the plane of incidence corresponded with one of the neutral 

 sections of the mica-plate, (the section used was th^it perpendi- 

 adar to the principal plane,) so that the transmitted pencil 

 would be polarized exactly similarly to that refracted through 

 glass or any singly refracting medium. 



34-. 1 prepared two pairs of bundles of plates of mica of this 

 description, the first (which I called A and B) having a thick- 

 ness 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 contained 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 w^ere made on the 22nd of No- 

 vember 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 

 thermometer 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 that the one fits the extremity of the thermal 

 pile, and the other slips into the first, and by turning it round 

 we get observations with plates, whose planes of incidence for 

 rays passing along the axis of the tube, are inclined 0°, 90°, 

 180°, or 270° to one another, the direction of the ray is gene- 

 rally in a single straight line, and the observations are made 

 in the same manner, and with equal facility as in ordinary ex- 

 periments on transmission. I have little doubt that in this 

 way the polarization of heat might be proved without the aid 

 of the thermo-multiplier. The plates were fixed at the po- 

 larizing angleybr light. After what has been said, art. (16), 

 on the refrangibility of heat, it is clear, that the alteration of 

 the polarizing angle, in order to accommodate it to heat, 

 could hardly amount (by Sir David Brewster's law) to a sen- 

 sible quantity. 



* I did not see M. Mellon i's second paper till the 10th of December, 

 after I had obtained the chief fundamental results contained in this paper. 

 It does not appear, 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 Chmky Iv. 374. 



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