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 ex- 

 periments. 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 de- 

 grees 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 ob- 

 servation 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 op- 

 tical evidence of the regularity of their internal texture ; and though 

 they cannot be called doubly refracting crystals, yet the author con- 

 ceives that they form two images, which are coincident, but differ- 

 ently polarized, and accordingly that these bodies should be called 

 doubly polarizing crystals. 



Other bodies, on the contrary, like the confusedly crystallized ace- 

 tate of lead, having axes in all directions, present no neutral axes, 

 but depolarize in every direction ; while others, according to the de- 

 gree of their crystalline texture, have the property of depolarizing a 

 greater or less proportion of the incident fight, 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, 



