M, MELLONI ON THE POLARIZATION OF HEAT. 151 



of each pile may afterwards be knoAvn by means of the degrees 

 marked by the two extremities of this Hne. Thus, when the 

 drums both mark 0°, or 360°, these planes are parallel, and 

 always preserve the same direction, if any number of degrees 

 whatever upon each division be passed before the index. But 

 if one drum be left at 0°, and all the degi'ees of its circumference 

 be successively marked at the other, the plane of refraction of 

 the second pile inclines more and more upon that of the first, 

 becomes perpendicular at 90% still advances towards the primi- 

 tive direction, and reaches it at 180°. The same successive 

 changes of inclination are afterwards produced fm-ther ; that is 

 to say, the planes of refraction are gradually separated, and take, 

 at 270°, a perpendicular direction, in order to approach and re- 

 sume the initial position of 0°, or 360°. 



To avoid the diverse incidence of the rays upon the piles, I 

 placed the source of heat in the focus of a lens of rock salt, suf- 

 ficiently distant from the tube, and in the prolongation of its 

 axis. A horizontal pencil of concentrated heat is thus obtained, 

 which traverses the piles of mica in a direction parallel to the 

 axis, and is propagated beyond, still preserving its cylindrical 

 form, and a considerable portion of its primitive energy, which 

 allows of the removal of the thermoscopic instrument intended 

 to investigate the properties of calorific radiation in the diflferent 

 positions of the piles, to such a distance that the effect of their 

 proper heating becomes perfectly insensible. 



The employment of the salt lens has, therefore, two great 

 advantages : 1 . The giving of intense and sensibly parallel rays ; 

 2. The possibility of completely securing the thcrmoscope from 

 the influence of the heat absorbed by the mica lamina. 



As to the heating of the apparatus which supports the piles, 

 that may be easily avoided by covering every part of it with a 

 double or ti'Iple metallic screen, having an opening of an equal 

 or smaller diameter than the smallest dimension of the laminfe. 



We will now recapitulate, fixing our ideas by a particular 

 example. Let the source of heat be a Locatelli lamp : the lumi- 

 nous and calorific rays emanating from it are received at a proper 

 focal distance upon a lens of rock salt ; they issue from it sen- 

 sibly parallel and horizontal, travel over a free space of forty or 

 fifty centimeters, reach the metallic screen which covers the 

 polarizing apparatus, enter by its central opening, fall only upon 



