Second Series. —Polarization by Reflection. 361 
lity is less completely polarized by a bundle of plates placed at a 
given angle, than the more refrangible rays. 
25. Mica is pre-eminently adapted for the purpose of polariz- 
ang by its considerable diathermancy, and by the extreme thin- 
ness of its laminz. I have, however, succeeded in polarizing 
heat by transmission through a bundle of rock-salt plates with 
parallel surfaces.* Two bundles consisting of three plates, or six 
surfaces each, polarized about one-seventh of the heat which 
passed in the parallel position, the angle of inclination to 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.+ 
26. A very convenient mode of mounting the mica plates for 
polarizing is shewn in Fig. 5. 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 polarization 
or analyzation may thus be made to shift through any angle by 
turning the tube containing the mica round its axis, and a small 
support A is provided to preserve it in any position; whilst a 
graduation may easily be applied to the exterior 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 Reflection. 
27. The general fact of the polarization of heat by reflection 
was ascertained by me in December 1834, and I stated the re- 
* For a supply of this valuable substance I have been greatly indebted both to 
Sir Purtre Grey Ecrrton, Baronet, of Oulton Park, Cheshire, and to Dr Trattt. 
+ Such plates being equally permeable to every kind of heat, as M. MEttont’s 
admirable experiment shews, would probably enable us to polarize cold, or to shew 
the negative effects due to a reduction of temperature. This experiment I have not 
tried. 
3Nn2 
