168 Miscellaneous Intelligence, 



O' 



These results were obtained with flat diamonds regularly crys- 

 tallized, but on examining Mr. Allan's collection, Dr. Brewster 

 found one of a perfectly octoedral form, having the same 

 structure, and containing also an air-bubble of considerable 

 size, which had produced, by its expansion, the polarizing struc- 

 ture already described. Edin. Phil. Journal, iii. p. 98. 



2. Optico-Metcorological Question. — ^The following subject has 

 been given by the class of mathematics in the Royal Academy 

 of Sciences of Prussia: 



*' To give a mathematical explanation of the luminous and co- 

 loured crowns, which are observed at times round the sun and 

 moon, conformable to experiments on light and the constitution 

 of the atmosphere, and in accordance with observations of the 

 phsenomena made with as much precision as possible." 



Memoires cannot be received after March, 1822. The prize 

 is fifty ducats, and will be adjudged at the public sitting, on the 

 anniversary of Leibnitz, on the 3d of July following. 



3. Double Refraction of Minerals. — The means of ascertain- 

 ing the double refraction of substances are very simple ; the 

 two following are quoted by M. Soret : — " Tw^o plates of tour- 

 maline, cut parallel to the axis of the crystal, and placed cross- 

 ways, so as to absorb all the light, form the apparatus. The sub- 

 stance to be examined is to be placed between the plates ; if it 

 is doubly refractive, the light re-appears through the tourma- 

 lines ; if not, all remains dark.'' 



The other method is to place the crystal to be examined over 

 a hole in a card, and examine the light transmitted through it 

 by an- achromatic prism of Iceland spar. If the two images 

 produced are differently coloured it indicates double refraction. 

 Journal de Physique, xc. p. 353. 



4. Restoration of the White in Painting. — M. Thenard has 

 lately applied his oxygenated water with great success to the 

 arts. It is constantly happening in paintings where the white 

 used is prepared from lead, that sulphurous vapours change the 



