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Scientific Intelligence, 



spar, one set of laminiB may be composed of lime, the other of 

 carbonic acid. The only double refracting crystal incompatible 

 with this supposition is sulphur, which, however, may hereafter 

 be ascertained to be a compound. 



Another very singular discovery of Dr. Brewster is, that when 

 polarized light is transmitted through certain transparent bodies, 

 it is unpolarized by these bodies in certain positions, and 

 unaltered by them in others. The transparent bodies which 

 possess this property are 7'ock crystal, topaz, chrysolite, lorax, 

 sidpkate of lead, felspar, selenite, citric acid, sulphate of potash, 

 carlonate of lead, leucite, tourmalhie, pistazite, mica, Iceland 

 spar, agate without veins, some pieces of plate glass* Gum 

 arable, horn, glue, and tortoiseshell, depolarize light in every 

 position. 



Dr. Brewster has observed that mica and topaz exhibit some 

 singular phenomena with 

 light. Let the rectangle 

 A BCD represent a piate 

 of mica. When a prism 

 of calcareous spar is placed 

 in a vertical, or a ho- 

 rizontal line, upon this 

 - plate, polarized light viewed 

 through dKui both suffers no 

 change. The horizontal and 

 vertical lines E F, G H upon the plate of mica may be called 

 the neutral axes of the mica. When the Iceland spar is placed 

 in the diagonals AC, B D of the plate, the polarized light ig 

 depolarized, and hence these diagonals may be called depolar- 

 izing axes. If we examine a polarized image by the prism of 

 Iceland spar, placed upon the vertical neutral axis of the mica, 

 the polarity of the light will of course continue, and only one 

 image will be seen ; but if we incline the plate of mica forwards^ 

 so as to make the polarized light fall upon it at an angle of about 

 45^, the image that was formerly invisible starts into existence, 

 and therefore the light from which it was formed has beeu 

 depolarized. If the same experiment is made upon the horizon-- 

 ial neutral axis, no such effect is produced; and hence it follows 

 that the vertical neutral axis is accompanied by an oblique 

 depolarizing axis. By making the same trials with the depola- 

 rizing axes, it will be found that each is accompanied by aa 

 oblique neutral axis ; and therefore each plate of mica possesses 

 two oblique neutral axes, and one oblique depolarizing axis. The 

 oblique depolarizing axis is represented by the line o n, and the 

 two oblique neutral axes by. the lines om. and op. The angles 

 G 0 Q 0 m, Go p, being about 45^, and the planes of 

 these angles being perpendicular to the plate of mica. Topaz 



