428 Royal Society of Edinburgh. 
same laws. But if the second slice be placed unconformably upon 
the first, or turned round a quarter of a circle, the ray, which at first 
was ordinarily refracted, is now extraordinarily refracted; and the 
ray, which at first was extraordinarily, is now ordinarily refracted. 
Now, it has been found that some crystals, such as tourmaline, pos- 
sess the property, first, of dividing these rays, and then of suppress- 
ing or absorbing one of them; the result of which is, that when two 
tourmalines, cut as we have supposed, are placed conformably, the 
ray which was not suppressed by the first slice, still makes its way 
through the second; but, when placed unconformably, the ray trans- 
mitted by the first plate is wholly suppressed by the second. In the 
latter case, therefore, not a ray of light can penetrate the two plates. 
This is polarization produced by double refraction. 
Now, all these modes of polarization have been recognised by 
Mr. Forbes in the case of neat, and even in the case of heat wholly 
unaccompanied by light. The Vice-President announced that he 
had witnessed this in the most satisfactory manner in the case of 
heat polarized by reflection and transmission, for which purposes, 
instead of glass, (which permits scarcely any non-luminous heat to 
penetrate it,) Mr. Forbes employs plates of mica, divided by a pecu- 
liar process into extremely thin laminz. 
But the analogies which he has established between light and heat 
do not stop here. It has been foundin the case of light, that, when 
the two reflecting plates before spoken of, or the two crystals, are 
placed in wnconformable positions, so that little or no light reaches 
the eye, we may, by interposing between the plates or the crystals 
a thin lamina of a doubly refracting 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 ex- 
tent, its character of polarization, or it is said to be depolarized. 
Dr. Hope stated, that he had seen this to be most completely ef- 
fected 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 position, 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 re- 
sult is found in the fact, which excited so much interest when com- 
municated 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 refraction, we have here an- 
other undoubted proof of the double refraction of heat. 
