162 M. MELLONI ON THE POLARIZATION OF HEAT. 



The precise value of the angle at which the complete polariza- 

 tion of heat is effected, is not so easily obtainable as at first sight 

 it appears to be. Indeed all the calorific rays do not traverse the mica 

 laminae in the form of the phaenomena of polarization that we have 

 just described ; this may be proved by placing the laminae perpen- 

 dicularly to the incident pencil, for with this arrangement a sensi- 

 ble effect of heat is still obtained through the system. Now, we 

 know that the polarizing action is null under the perpendicular 

 incidence ; there is, therefore, a portion of heat which passes in- 

 dependently of polarization ; and though it cannot weU be de- 

 monstrated excepting when the rays fall perpendicularly upon 

 the laminae, yet it is not the less certain that it must exist under 

 every other incidence. If this portion of heat, transmitted inde- 

 pendently of polarization, had the same value, whatever were the 

 obliquity of the rays upon the laminre, the angle under which 

 the greatest calorific effect would occur would be always that of 

 complete polarization by reflection. But this value varies with 

 the incidence, according to a progression that differs entirely 

 fi-om the law observed by that portion which passes by virtue of 

 the polarizing forces ; for we have seen that instead of first in- 

 creasing, as this does, until it reach the angle of complete polari- 

 zation, it constantly decreases from 90° of incidence to 0°. If 

 the influence of non-polarized heat, upon the transmission of 

 polarized heat, be sensible, it must produce a displacement in the 

 position of the maximum, and bring it evidently nearer the 

 perpendicular incidence. 



To obtain security from this cause of error, it may be observed, 

 that the absolute quantity of non-polarized heat Avhich traverses 

 the laminae diminishes as their number increases. The probable 

 error, in the determination of the angle of polarization, will 

 therefore follow the same decreasing progression, and become 

 null for a series composed of a sufficient number of lamiiice. 



In accordance with this principle, I procured a supplementary 

 pile of forty-four elements, Avhich, added to the other piles, 

 formed a series of a hundi-ed and twenty laminae. Here the 

 quantity of non-polarized heat could have no appreciable influ- 

 ence upon the calorific rays that traverse the system by virtue of 

 the polarizing forces ; for the transmission was sensibly null un- 

 der the perpendicular incidence. Thus, the maximum trans- 

 mission, in oblique incidences, would give exactly the angle of 

 complete polarization. 



