54 Analyses of Books. [Jan. 



XI. On the Action of Crystallized Surfaces tipon Light. By 

 Dr. Brewster. 



Mains was of opinion that the forces which produce extraor- 

 dinary refraction begin to act only after light has penetrated the 

 surface of a doubly refracting body. The author had been 

 induced, from experiments made before the perusal of Malus's 

 book, to conclude that the polarizing forces extend beyond the 

 surface of the crystal. This led him to repeat the experiments 

 on which Malus's opinion was founded. The result of the inves- 

 tigation, which overturns the doctrine of Malus, is contained in 

 this paper. If we take a six-sided prism of nitre, and observe a 

 luminous object through two of its inclined surfaces that have a 

 good polish, we perceive two distinct and perfectly formed 

 images. If we now I'oughen these surfaces and cement upon 

 each of them a plate of glass by means of balsam of capaivi, the 

 character of the two images will be greatly changed. The image 

 that has suffered the greatest refraction will be as distinct as 

 before ; but the other image will be either of a faint-reddish 

 colour, or wholly invisible, according to the degree of roughness 

 induced upon the refracting surfaces. If we substitute pure 

 ahohol or the white of an egg for the balsam, the least refracted 

 imaoe will become distinct, and the most refracted imag-e will 

 be either a mass of nebulous light, or almost invisible. The 

 reason of this is, that balsam of capaivi has nearly the same index 

 of refraction as the ordinary image, but not of the extraordinary. 

 It therefore removes the roughness of the surfaces as far as the 

 ordinary image is concerned, but leaves the roughness for the 

 extraordinary image. The index of refraction of alcohol and 

 white of egg is nearly the same as that of the extraordinary 

 image. It therefore removes the roughness as far as the extra- 

 ordinary image is concerned; but leaves it with regard to the 

 ordinary image. Similar experiments were made with calcareous 

 spar and arragonite. The author draws from them the following 

 conclusions : 



1. The force of double refraction and polarization extends not 

 only without the interior limit of the ordinary refractive force, 

 but also without the surface of the crystal. 



2. The force of double refraction and polarization emanates 

 from the surface of bodies, though its intensity depends upon 

 the inclination of the surface to the axis of the crystal. 



3. The ordinary or the extraordinary image may be extin- 

 guished at pleasure in any doubly refracting crystal ; and the 

 crystal is thus converted into a singly refracting crystal, like 

 certain specimens of agate. 



4. In soft crystals that do not admit of a perfect polish, the 

 distinctness of any of the two images may be made a maximum, 

 by giving the ciystal the best polish of which it is susceptible, 

 and then cementing plates of glass upon its surface by a trans- 

 parent cement of the same refractive power as that of the pencil. 



