120 



case of heat polarized by reflection and transmission, for which 

 purposes, instead of <rlass, (which permits scarcely any non- 

 luminous heat to penetrate it), Mr Forbes employs plates of 

 mica, divided by a peculiar process into extremely thin lamina?. 



But the analogies which he has established between light and 

 heat do not stop here. It has been found in the case of light, 

 that, when the two reflecting plates before spoken of, or the 

 two crystals, are placed in unconformable positions, so that 

 little or no light reaches the eye, we may, by interposing be- 

 tween the plates or the crystals a thin lamina of a doubly re- 

 fracting substance (such as mica) in a certain position (relatively 

 to its internal structure), cause a portion of light, which before 

 was incapable of reaching the eye, to become capable of so doing. 

 In other words, the polarized light, which at first was incapable 

 of reflection or transmission at the second plate or crystal, now 

 becomes capable of it; it has lost, to a certain extent, its charac- 

 ter of polarization, or it is said to be depolarized. 



Dr Hope stated, that he had seen this to be most completely 

 efi^ected in the case of heat, by Mr Forbes. A lamina of mica is in- 

 terposed between the bodies used to polarize heat unconformably 

 placed. When the lamina of mica has a certain ])osition, no effect 

 is produced beyond stopping a small portion of the heat, which 

 would otherwise reach the thermometer; but when this interposed 

 lamina is turned 45° in its own plane, a portion of the heat which 

 before was incapable of reaching the thermometer in consequence 

 of its polarization, is now capable of doing so, and the influx of 

 heat is instantly indicated. The most striking exemplification 

 of this result is found in the fact, which excited so much interest 

 when communicated more than a year ago to the Society, that in 

 certain cases the mere interposition of a piece of mica (in the 

 proper situation), will cause an immediate indication of increased 

 temperature, the mica depolarizing- more heat than it stops. Since 

 depolarization takes place only in consequence of double refrac- 

 tion, we have here another undoubted proof of the double re- 

 fraction of heat. 



The Vice-President terminated his general and rapid sketch, 

 in which he alluded to the brilliant discoveries of Brewster, 

 Arago, and Fresnel, respecting the polarization of light, by ob- 



