Experiments on the Depolarization of Light as exhibited by various 

 mineral, animal, and vegetable Bodies, with a Reference of the Phe- 

 nomena to the general Principles of Polarization. By David Brewster, 

 LL.D. F.R.S. Edin. and F.S.A. Edin. In a Letter addressed to 

 the Right Hon. Sir Joseph Banks, Bart. K.B. P.R.S. Read De- 

 cember 15, 1814. [Phil. Trans. 1815, p. 29.] 



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 depola- 

 rized. The object of the author, in this letter, is to comprise expe- 

 riments 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 pro- 

 perty 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 pola- 

 rizing 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 de- 

 duce 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 



