176 M. MELLONI ON THE POLARIZATION OF HEAT. 



arrived upon the thermoscopic body, without being there con- 

 densed by the collector, which was not in the least required, in 

 consequence of the intensity of the effects produced. The mica 

 lamina, interposed between the two piles, was of a circular form, 

 and in thickness equal to 0'"'""2489 ; it could only revolve in its 

 own plane around the centre, which consequently remained im- 

 mobile during this rotatory motion. 



The equality of the negative, and the corresponding positive 

 variations, is here established with all the requisite exactitude, 

 for their differences are less than j-^g, sometimes in one, and 

 sometimes in the other direction. Yet each number contained 

 in this table is the result of only ten observations. It is true 

 that these observations were made with the greatest care, and 

 that the differences between the maximum and minimum of each 

 series scarcely exceeded half a degree. 



Now, suppose that a horizontal pencil of obscure heat issuing 

 from black glass be thrown upon a vertical surface of glass or 

 mica, at the angle of complete polarization ; that the reflected 

 rays be afterwards transmitted pei-pendicularly through the cir- 

 cular lamina of mica ; and that the emergent heat be received 

 upon another surface of glass or mica, disposed parallel to the 

 first ; it will there undergo a second reflection, and return in a 

 direction parallel to the primitive direction, but always removing 

 further from the source. If the thermoscopic pile be placed at 

 a certain distance from the two reflectors, so that it may receive 

 the impression of the pencil of heat which has undergone the 

 two reflections and the intermediate transmission of the mica 

 disc, by turning this disc in its proper plane, a much less ener- 

 getic action is observable when the principal section is inclined 

 45° upon the horizon, than when it is horizontal or vertical. 

 The effects obtained are nearly as sensible as the differences re- 

 corded in the preceding table ; for the index of the galvanometer, 

 in passing from one position of the principal section to the other, 

 travels over arcs of from 20° to 25°. 



This experiment, which is perfectly analogous to the preceding 

 ones, is very interesting, as it enables us completely to insulate, 

 as to their mode of manifestation, the polarizing forces developed 

 in the act of reflection, from the similar forces developed during 

 simple refraction. Indeed, until now, it has been necessary to 

 have recourse to the second forces of polarization to render the 

 first sensible. The rays, in the experiment under considera- 



