Prof. Wartmann on the Polarization of Atmospheric Heat. Ill 



in length, make only a single oscillation in twenty-four seconds. 

 When the calorific radiations are weak, I found the compensator 

 of M. Melloni to be of service, more especially as the object was 

 not to obtain absolute measures, but the ratios of the deviations. 

 Operating in the manner just described, it is found there are 

 two positions of the analyser 180 degrees apart, at which the 

 deviations are equal and maximum ; and two other positions at 

 90 degrees from the former and at 180 degrees from each other, at 

 which the deviations are equal and minimum. The positions of 

 the analyser, which for a given point of the heavens procure the 

 maximum and minimum transmissions, correspond to those of 

 greatest intensity of the direct bands and inverse bands of the po- 

 lariscope of Savart*. They are thus determined without diffi- 

 culty. 



The atmospheric heat can be depolarized by. means of a plate 

 of mica placed near the extremity of the exterior tube, and per- 

 pendicular to the incident rays. The analyser being in the 

 position of the minimum of transmission, the deviation of the 

 index experiences no sensible diminution when the principal sec- 

 tion of the interposed mica coincides with the plane of polarization, 

 while the deviation is augmented when the rotation of the mica 

 in its own plane brings its principal section to an angle of 45° 

 with the primitive plane of polarization. 



The phenomena of the polarization of atmospheric heat are 

 much less apparent in winter than in summer. The difference 

 is doubtless due to the want of sufficient sensibility in the appa- 

 ratus, to the greater difficulty of experimenting at low tempera- 

 tures, and to the small proportion of polarized rays which on the 

 most favourable days accompany the natural heat. The serenity 

 of the air exercises a very marked influence on this proportion, 

 which becomes probably null when the heavens are obscured. 

 Finally, it is easy to satisfy oneself, particularly if the atmo- 

 sphere be calm and without clouds, that the polarization aug- 

 ments from the environs of the sun up to a certain limit, from 

 which forward it decreases. I have found it inappreciable in the 

 regions occupied by the neutral points. 



It may be concluded from these researches, that the heat and 

 the light of the atmosphere proceeding from the sun are similarly 

 polarized in the same circumstances. 



* I name direct bands those which are in the plane of polarization of the 

 light which hai traversed a plate of tourmaline ; they present in the middle 



a I. lack band between two white ones. The invert* bands, perpendi- 

 cular to the primitive plane of polarization, present a central white band 

 between two black ones. 



