Second Series — Polarization by Reflection. 363 



also very superior to a single surface of mica. Glass, salt, and 

 alum seemed to reflect but a third or a fourth part, or even less, 

 of that furnished by the laminated mica. 



29. I did not, as I have said, stop to prosecute these experi- 

 ments ; they clearly shewed that of the substances which I tried, 

 mica split into thin films afforded the most copious reflection 

 (next to the metals), and this was the very substance which from 

 the first I had employed. They likewise satisfactorily shewed 

 the cause of the failure in former attempts to polarize by reflec- 

 tion, seeing that, for dark heat, glass is almost the worst reflector 

 that could have been used, and as it likewise absorbs almost all 

 the heat, transmitting very little, the effect of secondary radia- 

 tion is increased. Hence the difficulty experienced by Pro- 

 fessor Powell and others in getting any results at all* before 

 the thermo-multiplier was devised, and the failure of the at- 

 tempts of Signor Nobili of Florence even with its aid.f The 

 last-named eminent philosopher failed also in obtaining traces of 

 polarization by metallic reflection, which was not to be wonder- 

 ed at, as on another occasion we shall be able to make to appear. 



30. The form of apparatus which I have more recently em- 

 ployed for experiments on polarization by reflection was suggest- 

 ed to me by the present astronomer royal, Mr Airy, after the 

 publication of my first paper. It is represented in section in Fig. 

 6, and a perspective view is given in Fig. 7. AB and CD are two 

 reflecting surfaces of mica fixed to blocks G and H ; the former 

 of which is attached to a board TU, carrying the lamp or source 

 of heat S, and revolving in a horizontal plane round T as a 

 centre ; — the latter (H) is permanently fixed relatively to the 

 pile P, provided with its conical reflector. The surfaces AB, 

 CD are parallel, and make angles of about 56° with the horizon ; 



* Edinburgh Journal of Science, N. S., vols. iii. and v. 

 f Bibliotheque Universelle, Sept 1884. 



