Polarization by Reflection, 553 



lo the incident heat being about 55° ; but when all the six 

 plates were combined into one bundle, and the mica plate I 

 used along with it, not far from a half of the transmitted heat 

 was polarized*. 



The following is a very convenient mode of mounting the 

 mica plates for polarizing. A cylindrical wooden tube is cut 

 across at an obliquity of 34° to the axis. The plate of split 

 mica is interposed and the parts reunited. The plane of po- 

 larization or analysation may thus be made to shift through 

 any angle by turning the tube containing the mica round its 

 axis, and a small support is provided to preserve it in any 

 position; whilst a graduation may easily be applied to the ex- 

 terior of the tube, so as to mark the angular revolution. The 

 convenience of this construction will afterwards appear. 



§ 4. On the Laws of Polarization of Heat by Rejlection. 



The general fact of the polarization of heat by reflection 

 was ascertained by me in December 1834-, and I stated the 

 result in my former paper, art. 45. Under any circumstances 

 the experiment is a troublesome one, but I have succeeded in 

 arranging it in perhaps as satisfactory a way as it admits of 

 being done. The great difficulty arises from the minuteness 

 of the quantity of heat reflected, and consequently the large 

 quantity absorbed by the plates, which complicates and ob- 

 scures the effect. This is more particularly true with dark 

 heat, which, at the same time, furnishes the most important 

 case to be examined. The effect of the absorbed heat is to 

 produce a powerful secondary radiation. 



My first inquiry on resuming the subject was to ascertain 

 the relative order of several different substances as to their 

 power of reflecting heat. This was not proposed to be done 

 with a view to a general inquiry into that important subject, 

 which I reserved to another occasion, but simply to ascertain 

 what reflecting surfaces might be best employed in polarizing 

 by reflection. Several series of experiments gave the follow- 

 ing arrangement of substances according to their power of re- 

 flecting heat, at an incidence of 45°, beginning with the most 

 perfect reflector. 



Polished speculum metal. 



Mica, split by the hand into thin plales. 



Mica, split by heat (see p. 86). 



Thick plate of mica. 



* Such plates being equally permeable to every kind of heat, as M. Mel- 

 loni's admirable experiment shows, would probably enable us to polarize 

 cold, or to show the negative effects due to a reduction of temperature. 

 This experiment I have not tried. 



