364 Professor Forbes's Researches on Heat. 



consequently the heat falling on AB at an angle of 34° with the 

 surface, is reflected in the direction EF, which, by the construc- 

 tion, is a vertical line. From the surface CD, on which, at inci- 

 dence, it also falls at an angle of 34°, it is reflected to the pile, 

 whose opening inclines downwards at an angle of 22°, so as to re- 

 ceive the rays directly. From this it is clear that the whole ap- 

 paratus connected with the first plate AB may revolve round the 

 vertical line EF as an axis, until the plane of section be perpen- 

 dicular to the plane of the paper, and that yet the heat shall be 

 correctly reflected to the pile. In this case it is clear that the 

 planes of reflection becoming perpendicular, a minimum of heat 

 will be reflected if polarization take place.* 



31. Such appears to be the case with all the kinds of heat 

 that I have tried. The disturbing influence of conduction is 

 here more difficultly avoided, and serves to diminish the appa- 

 rent effect. The quantities of heat reaching the pile from any 

 non-luminous source are always small. The results, however, 

 are well marked, and seem decidedly to indicate that under the 

 particular circumstances of the observation, dark heat is more 

 completely polarized than .the more reflexible heat from an Ar- 

 gand lamp, whilst that from incandescent platinum was more 

 polarizable than either. The following results were obtained on 

 the 12th March 1836. The source of heat was in all cases at a 

 distance of six and a half inches from the centre of the first re- 

 flecting plate, and the whole length of the dotted line PFES 

 Fig. 6, was about sixteen inches. The reflecting plates were 

 composed of ten or twelve laminae of mica, split with a pen-knife, 

 and the plane of reflection was perpendicular to the principal 

 section of the mica. 



* Square tubes of wood, seen distinctly in the perspective view, serve to enclose 

 the apparatus and facilitate its adjustment. Other means not represented in the 

 figure were also used for preventing direct heat from reaching the pile in any position. 



