M. MELLONI ON THE POLARIZATION OF HEAT. 149 



diately transmitted rays of each sovirce, which is the reason 

 that Mr. Forbes found, in both cases, the same proportion of 

 polarized heat. 



Thus the action derived from the proper heating of the piles, 

 an action varied by the nature of the source, or the interjiosition 

 of a glass lamina, is of itself sufficient for the explanation of all 

 the alterations observed by Mr. Forbes in the index of calorific 

 polarization ; and it has been already observed that the greater 

 or less obliquity with which the various rays fall upon the po- 

 larizing laminae may also produce analogous variations. 



In order to obtain exact and comparable results, it is therefore 

 necessary to avoid these two causes of error ; for which purpose 

 T have successfully employed the means which we proceed to 

 examine. But it must first be seen how the piles of mica in- 

 tended for experiments of polarization are prepared. 



There are several different methods, but the following appears 

 to be preferable : First, carefully determine, by any one of the 

 known optical methods, the directions of the axes or neutral 

 sections of luminous polarization for a natural sheet of mica, one 

 or two millimeters in thickness, and cut, according to these two 

 perpendicular directions, a rectangle eight or ten centimeters 

 long, by four or five in width. Then take another rectangle of 

 very thin card, a little larger than the piece of mica which has 

 been cut, and remove all the inner part in a direction parallel to 

 the sides, so as to form a rectangular frame, of which the open- 

 ing will be six or eight millimeters smaller each way than the 

 mica ; then separate from the rectangle of mica, by means of a 

 lancet, the thinnest lamina possible ; fix it with a little gum 

 upon the frame of card, carefully keeping its sides exactly paral- 

 lel to those of the opening ; and after having fastened upon 

 those portions of the longer sides that rest upon the frame two 

 narrow bands of gummed paper, detach a second lamina of mica, 

 and superpose it exactly upon the first ; cover the sides in the 

 same manner with gummed paper, and proceed thus with all 

 the laminae subsequently separated from the rectangle of mica. 

 When the pile is finished, place a second frame of thin card 

 equal to the first on the top of it, apply some glue between 

 the free parts of the two cards, and fasten together their exterior 

 edges by bands of glued paper, in such a manner that no move- 

 ment may take place in the sheets of mica, and that their sides 

 may remain perfectly parallel or perpendicular to the sides of 



