Page:Proceedings of the Royal Society of London Vol 2.djvu/72

Rh : but the reverse will happen in each case if one glass be compressed and the other dilated.

If a compressing force be applied to the centre of a plate of glass it will exhibit the black cross and other phenomena to be seen by means of doubly-refracting crystals.

If a plate so compressed be inclined to the polarized ray, the tints of colour will ascend or descend, according to the direction in which it is inclined.

If a plate to which the power of depolarization has been given by heat be compressed, the tints of the interior fringes rise in the scale, and those of the exterior descend, when the axis of pressure is perpendicular to their direction.

The same effects which are thus produced upon glass by compression, are produced in a similar manner upon such crystallized bodies as do not possess these properties in their natural state. But those bodies which already possess the doubly-refracting structure in a high degree, as calcareous spar, rock crystal, beryl, &c., suffer no change by any degree of mechanical compression to which the author has subjected them.

Since the tints of colour communicated to polarized light depends on the degree of force applied to glass, through which it is transmitted, Dr. Brewster conceives that a convenient instrument might be constructed for measuring the intensity of forces, which he would call a Chromatic Dynamometer; and in the same manner might variations of temperature, or humidity of bodies be measured, with the assistance of a little ingenuity, by chromatic thermometers and hygrometers.

In the prosecution of these experiments, the author examined the properties communicated to jelly by variations of its density from drying, and contrived means of giving it permanent power of depolarization, by the constrained position in which it was allowed to harden. And he found that the polarizing force of distended isinglass far exceeds any which can be given to glass, either by heat or pressure, and is even greater than that of beryl, which owes its power to crystalline texture.

In conclusion, the author expresses his hope that the principles here investigated afford a solution of the most important part of the problem of double refraction, by ascertaining the mechanical condition of both classes of doubly-refracting crystals, although the division of incident light into two portions oppositely polarized yet remains to be accounted for: and he thinks we must remain satisfied with referring this to the operation of some peculiar fluid, which he conceives to be the principal agent in producing all the phenomena of crystallization and double refraction.

Dr. Brewster adds, that a recent experiment (which he does not describe) upon the polarizing qualities of a body of which the densities vary in regular minute strata, induces him to think more favourably than heretofore of the undulatory system of light.

