Page:Proceedings of the Royal Society of London Vol 2.djvu/37

Rh that direction, and at right angles to it, though they depolarize in other directions.

Acetate of lead, confusedly crystallized between two plates of glass, depolarizes in all positions. Plates of ice have in general the same effect, though some exhibit neutral axes.

Oil of mace being a soft solid, opake from confused crystallization, depolarizes also in all positions; but it also exhibits, through a very thin margin, a peculiarity not observed in any of the preceding experiments. When the flame of a candle is viewed through it, the flame appears surrounded by a halo; but if the light be polarized before it is transmitted through the oil of mace, then the flame has four wings or luminous radiations, at right angles to each other; and accordingly if two pencils of light be received at the same time by transmission through Iceland spar, then there are two such images, with their four wings transversely situated, so that the rays of one image correspond in position with the blank spaces of the other.

A slice of tortoiseshell, which also depolarizes in every position, exhibits also, by the polarized light of a candle, faint luminous rays, similar to those seen by oil of mace.

The author next classes these bodies according to the various degrees in which they more or less perfectly depolarize, and more or less perfectly possess neutral axes, in which depolarization does not take place.

The simplest case of depolarization is that effected by a thin plate of Iceland spar, or other regularly crystallized body, the principal section of which is not in the plane of polarization, or at right angles to it, and consequently occasions the polarized ray to be subdivided into two others transversely polarized, according to the original observation of Huygens. Hence if other bodies, as hair, wool, silk, &c. have neutral axes or planes, in which a transmitted ray retains its polarization, while it is depolarized in other positions, this affords optical evidence of the regularity of their internal texture; and though they cannot be called doubly refracting crystals, yet the author conceives that they form two images, which are coincident, but differently polarized, and accordingly that these bodies should be called doubly polarizing crystals.

Other bodies, on the contrary, like the confusedly crystallized acetate of lead, having axes in all directions, present no neutral axes, but depolarize in every direction; while others, according to the degree of their crystalline texture, have the property of depolarizing a greater or less proportion of the incident light, or, according to the degree of regularity of that texture, may exhibit some appearance of neutral axes.

With regard to oil of mace, Dr. Brewster observes, that since the continuous halo which surrounds the flame of a candle seen through it, is divided, by refraction through a prism of Iceland spar, into two sets of luminous radiations surrounding the two flames seen through it, having the luminous rays of the one corresponding in position to the vacant spaces of the other, he infers that the halo itself, in fact,