Page:Proceedings of the Royal Society of London Vol 2.djvu/36

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When a ray of light has been so modified by reflection or refraction that in certain planes it is not divided into two parts by a prism of Iceland spar, that ray is said to be polarized; but it may again, by several means, be rendered divisible, and is then said to be depolarized. The object of the author, in this letter, is to comprise experiments on the depolarizing properties of a great variety of substances at the same time, and thence to deduce the general principles on which the various degrees or modes in which they exhibit this property depend.

Dr. Brewster has already, in a former communication, described the general phenomena of depolarization by mica, calcareous spar, topaz, and other regularly crystallized bodies, which have two neutral axes at right angles to each other, and two depolarizing axes also at right angles to each other, but making angles of 45 with each of the neutral axes.

Of the bodies now enumerated by the author, some have no polarizing or depolarizing properties, as rock salt, fluor spar, and spinelle ruby. Out of seven specimens of diamond, four did not depolarize; one depolarized about one fifth of the light, one about one half, and one nearly all the light in every position.

The next class of bodies subjected to experiment by the author, are vegetable and animal substances, from which he expects to deduce important conclusions.

Gum arabic depolarizes light in every position, unless in extremely thin chips. Cherry gum also depolarizes light, with some appearance of a neutral axis. Caoutchouc has the same property; and though it loses it when fused by heat, it recovers the same power gradually in the course of a few weeks. White wax, melted between two plates of glass, depolarizes in every position. Bees' wax has the same property, as may be exhibited by inclosing a portion of a cell of a honey-comb between layers of Canada balsam.

Manna, camphor, and balsam of Tolu were tried with the same results. Various fibrous vegetable substances have also the property of depolarizing, but have neutral axes in the direction of their fibres. Adipocire from various sources, spermaceti, and soups of all sorts, depolarize in every position. Various kinds of hair, wool, feather, and silk depolarize, but have neutral axes in the direction of their length, and at right angles to it.

Human cuticle, human nail, and various kinds of horn; the cornea of the eye of a man, cow, or fish; a piece of bladder, isinglass, or glue, depolarize in all positions. Certain other animal substances, which are distinctly fibrous in one direction, exhibit neutral axes in