M. MELLONI ON THE POLARIZATION OF HEAT. 



163 



I therefore mounted my hundred and twenty laminae on one 

 frame, which was provided with two pivots upon the transversa] 

 section, and an alhidade indicating the incUnation of the polar- 

 izing surfaces to the calorific rays, upon a vertical circle ten 

 inches in diameter. The transmissions obsen^ed at each half 

 degree comprised between 33° and 35°, ai'e subjoined : and to 

 them are added two series of similar observations made upon two 

 piles consisting of a smaller number of laminae, one of twenty, 

 the other of sixty elements. The quantity of incident heat varies 

 from one series to another, and consequently the transmissions 

 given under the same inclinations by the three systems of lamime, 

 cannot be compared together. The fulfilment of this condition 

 of comparison was neglected, in order to render the transmissions 

 from the series consisting of numerous laminae more sensible; 

 nor was it of utility for the end actually proposed. 



It will be perceived from a glance at this table, that the 

 greatest calorific transmission occurs at an incidence of 34° in 

 the first series, at 33° 30' in the second, and that it maintains 

 the same obUquity in the third. The influence of the non- 

 polarized heat upon the value of the angle of polarization, is 

 therefore sensible only when the pile is composed of a small 

 number of laminje. By comparing, in each of the last two 

 series, the number which represents the greatest transmission, 

 with the numbers that immediately precede and follow it, it 

 will easily be seen that the maximum cannot differ much from 

 33° 30', and that if there be a deviation of a few seconds, it is 

 rather in the direction of the 34th degree than in the opposite 

 one. 



According to the law discovered by Sir D. Brewster, the tan- 

 gent of the angle of polarization, for light, is given by the num- 

 ber which represents the index of refraction of the body employed 

 as a reflector. Now, we know that mica has an index of refrac- 



