t 261 j 



XXXIV. Note on the Polarization of Heat. 

 By G. Carey Foster, F.R.S* 



rj^HE following determinations of the amounts of heat trans- 

 -*- mitted by two Nicol's prisms, whose principal sections 

 make different angles with each other, were recently made in 

 the Physical Laboratory of University College, London, by 

 Mr. M. J. Jackson. Although the results amount to nothing 

 more than an additional verification of a relation that is already 

 thoroughly established, I venture to put them on record, not 

 only because such verifications are satisfactory in themselves, 

 but also because the apparatus required for conveniently re- 

 peating experiments of this kind is not always at hand. 



The source of heat was a rather powerful paraffin-oil lamp 

 (supplied by White, of Glasgow, for use with a Thomson's 

 quadrant electrometer). The rays from the lamp were con- 

 centrated by a lens of 7*5 centims. diameter and about 22 cen- 

 tims. focal length, placed so as to produce a real image of the 

 lamp-flame within the silvered reflecting cone of the thermo- 

 pile. Immediately behind the lens (on the side next the lamp) 

 a double screen of polished sheet brass was placed, whereby 

 the radiation could be cut off or allowed to pass at will. On the 

 other side of the lens came two Nicol's prisms, each about 20 

 Gentims. long, and giving a clear circular field about 6 # 7 

 centims. in diameter. The prisms are protected at the ends 

 by disks of thin glass, which were left on during the experi- 

 ments. It is to the possession of these fine prisms, made for 

 me by Mr. C. D. Ahrens, that the possibility of making the 

 experiments with so much ease was due. The thermopile, 

 which was about 95 centims, distant from the lens, was 

 protected from stray radiation by a double hood of tin-plate. 

 The galvanometer was a reflecting instrument of low resist- 

 ance, on Sir William Thomson's principle, made by my assist- 

 ant, Mr. Grant. By means of a commutator inserted between 

 the thermopile and the galvanometer, two opposite deflections 

 were obtained for each position of the prisms. In the follow- 

 ing Table, the column headed 8 X gives the means of the deflec- 

 tions to right and left when the angle, 6, between the prin- 

 cipal sections of the prisms was measured in one- direction; 

 and the column headed S 2 , the corresponding deflections when 

 this angle was measured in the opposite direction. The num- 

 bers denote divisions of the galvanometer-scale. 



* Communicated by the Physical Society of London, having been read 

 to the Society on March 3, 1877. 



