446 Analyses of Books. [Dec. 



mass of nebulous light. This the author conceived to be an im- 

 perfect image, and drew as a consequence that the agate is fn a 

 state of approach to that particular kind of crystallization which 

 affords double images. His subsequent experiments on depolariza- 

 tion appear to have demonstrated the truth of this opinion. 2. On 

 the Structure of the Agate as connected with its Optical Properties. 

 The agate is composed of layers (usually concentric) partly transpa- 

 rent and partly opake. The opake layers appear milk-white by 

 reflected light, but brown by transmitted light. The transparent 

 layers are formed of waving lines, in some specimens very fine, in 

 others coarser. When the incident ray is parallel to these layers it 

 is transmitted in greater abundance than when oblique ; and when 

 it is transmitted in one oblique direction more of it passes than 

 when in another. These differences obviously depend upon some- 

 thing peculiar to the structure of the agate. 3. On the peculiar 

 Colours exhibited by the Agate. The author observed the existence 

 of a coloured image on each side of the colourless image, and 

 polarized in the same manner. This coloured image is equally 

 distinct in every position of the agate; it is alike produced by 

 polarized and unpolarized light, and suffers no change when 

 examined bv a plate of agate or by a doubly refracting crystal. 

 4. On the Depolarization of IAght. Almost all transparent bodies 

 possess in two positions the property of depolarizing light, while in 

 other two positions they do not affect it. The first of these positions 

 the author calls the depolarizing axes ; the second, the neutral 

 axes. 5. On the Eliplicul- Coloured Rings produced by depolurizing 

 Crystals. For this curious subject we must refer to the paper itself, 

 as it cannot be intelligibly explained without figures. 



12. On the Polarization of Light by oblique Transmission through 

 till Bodies, whether Crystallized or Uncn/stallizcd. By David 

 Brewster, LL.D. F. R.S. Edin. and F.S.A. Edin.— When Dr. 

 Brewster wrote this paper he was not aware that the important dis- 

 covery which it communicates had been already made by Mains. 

 The little intercourse between Great Britain and France, together 

 with the death of Mains, having prevented his recent discoveries 

 from being known in this country, except to one or two individuals 

 who had made optics their favourite pursuit : so far therefore as the 

 merit of discovery goes, Dr. Brewster stands upon the same footing 

 as Malus; though the priority of the latter no doubt deprives him 

 of a great part of that eclat to which he would have otherwise been 

 entitled. When light is transmitted obliquely through bundles of 

 plates of transparent bodies, as glass, it is polarized, according to 

 a certain law which depends upon the ancle of incidence of the 

 light and the number of plates through which it passes. Let n, n', 

 represent the number of plates in any two parcels, and <p, <p\ the 

 angles at which the pencil is polarized ; we have n : r£ :: cotang. $ : 

 cotang. p', and n x tang, p = n' x tang. <p\ 



Hence it follows that the number of plates in any parcel multi- 

 plied by the tangent of the angle at which it polarizes light is a 







