150 M. MELLONI ON THE POLARIZATION OF HEAT. 



the frame and to the neutral sections, one of which should 

 always be found in the plane of refraction of the radiation, an 

 indispensable condition, as is known, to render the polarizing 

 action of these sorts of piles independent of their crystalline 

 state, and consequently, similar to the action of piles of glass or 

 any other amorphous substance. I procured thus four pairs of 

 mica piles, composed of three, five, ten, and twenty laminae. 



The next step was to give them the necessary arrangement 

 for experiments of polarization. The apparatus which appeared 

 to me most suitable is exactly similar to that described in M. 

 Biot's Traite de Physique, vol. iv. p. 255, with the exception of 

 a few shght modifications, which render it still more simple, and 

 more especially adapted to experiments of polarization by re- 

 fraction. 



It consists of a horizontal tube, to the two ends of which are 

 adjusted two drums, open at one end, which by hard friction 

 may be turned round, like the ordinary covers of cylindrical 

 boxes ; each of them is divided at the edge of friction into 360 

 degrees ; from two opposite points of their free circumference 

 proceed two arms parallel to the axis ; they are pierced at a cer- 

 tain distance with two small holes, in which are fastened the 

 pivots of a rectangular frame intended to receive the two piles 

 of mica : these pivots, placed in a contrary direction upon the 

 transverse line which passes through the centre of the frame, 

 allow of its being more or less inclined in relation to the axis of 

 the tube ; they may be fixed in a determinate position by a 

 clamp. The measure of the angle is furnished by an arc of the 

 circle fixed upon one of the two salient arms of each drum. 



Thus, when the piles are attached to the apparatus, they 

 may be directly placed, by means of their moveable supports, at 

 any inclination whatever in relation to the axis of the tube, and 

 by afterwards turning the drums any possible position around 

 this axis may be given them ; that is to say, they may be made 

 to travel over, in succession, all the imaginable angular positions 

 around the calorific pencil ; for we shall presently see that the 

 rays of heat always enter the tube in the direction of the axis. 



The circular divisions of the two drums mutually correspond by 

 means of a line, parallel to the axis, traced at the upper part of the 

 tube, and prolonged to the graduated edges in form of an index. 

 The exterior arms being placed symmetrically upon the two sides 

 of the tube, the reciprocal directions of the planes of refraction 



