Proceedings of the British Association. 325 



glass from St, Andrews, a change of a very different kind is ef- 

 fected. In some cases the sihceous and the metallic elements of 

 the glass are separated in a very singular manner, the particles of 

 silex having released themselves from the state of constraint pro- 

 duced by fusion and subsequent cooling, and arranged themselves 

 circularly round the centre of decomposition ; while the metallic 

 particles, which are opake, have done the same thing in circles 

 alternating with the circles of the siliceous particles. This resto- 

 ration of the silex to its crystalline state, is proved by its giving 

 the colors of polarized light, and possessing an axis of double re- 

 fraction. The most valuable glass articles manufactured by 

 Fraiinhofer, of Munich, seemed to be peculiarly liable to some su- 

 perficial decomposition of this kind. A prism of this glass in the 

 Observatory of Paris had become absolutely black. A prism be- 

 longing to himself had become quite blue on the surface, although 

 as yet its action on light was not affected. The largest object 

 glass of the principal telescope in the Observatory of Edinburgh 

 had begun to show decided symptoms of superficial decomposi- 

 tion, and many other instances could also be mentioned. M. La- 

 ment, professor of astronomy at Munich, who is in the constant 

 habit of using Fraiinhofer's glasses, stated that there is an easy 

 and effectual remedy for this tendency of Fraiinhofer's glass to 

 deteriorate on the surface, which was to rub it frequently with 

 the finer parts of whiting, prepared by elaborating a mass of whi- 

 ting in water, the fine powder to be dried and used on old soft 

 linen. 



On the rings of Polarized Light produced in specimens of de- 

 composed glass ; by Sir D. Brewster. — In the course of a series 

 of experiments on the connexion between the absorption of light 

 and the colors of thin plates, published in the Phil. Trans. 1837, 

 I accidentally observed under the polarizing microscope, certain 

 phenomena of polarized tints of great beauty and singularity. 

 These tints were sometimes linear and sometimes circular, and in 

 some specimens they formed beautiful circular rings traversed by 

 a black cross, resembling the phenomena of mineral crystals, or 

 those produced by rapidly cooled circular plates or cylinders of 

 glass. Having found in the decomposed glass from St. Andrews, 

 that the siliceous particles had resumed their position as regular 

 crystals, and arranged themselves circularly round the centre of 

 decomposition, I was led to suppose that this was the cause of the 



Vol. XL, No. 2.— Jan. -March, 1841. 42 



