154 THE MICROSCOPE. 



having a red and green fringe, whilst the four intermediate 

 sectors are of a gorgeous orange-yellow. These appear- 

 ances alter upon the revolution of the analysing-plate of 

 tourmaline ; when the blue stage is employed, the cross 

 will assume a blue or yellow tint, according to the position 

 of the aualysing-plate. These phenomena are analogous 

 to those exhibited by certain circular crystals of boracic 

 acid, and to those circular discs of salicine (prepared by 

 fusion) ; the difference being, that the salts of quinidine 

 have more intense depolarising powers than either of the 

 other substances ; besides which, the mode of preparation 

 effectually excludes these from consideration. Quinine 

 prepared in the same manner as quinidine has a very 

 different mode of crystallisation ; but it occasionally pre- 

 sents circular corneous plates, also exhibiting the black 

 cross and white sectors, but not with one-tenth part of the 

 brilliancy, which of course enables us readily to discrimi- 

 nate the two." 



Ice doubly refracts, while water singly refracts. Ice 

 takes the rhomboidic form ; and snow in its crystalline 

 form may be regarded as the skeleton crystals of this 

 system. A sheet of clear ice, of about one inch thick, and 

 slowly formed in still weather, will show the circular rings 

 and cross if viewed by polarised light. 



It is probable that the conditions of snow formation are 

 more complex than might be imagined, familiar as we are 

 with the conditions relating to the crystallisation of water 

 on the earth's surface. Dr. Smallwood, of Isle Jesus, 

 Canada East, has traced an apparent connection between 

 the form of the compound varieties of snow crystals and 

 the electrical condition of the atmosphere, whether nega- 

 tive or positive ; and is instituting experiments for hia 

 better information on the subject. 



A great variety of animal, vegetable, and other sub- 

 stances possess a doubly refracting or depolarising struc- 

 ture, as : a quill cut and laid out flat on glass ; the cornea 

 of a sheep's eye ; skin, hair, a thin section of a finger-nail ; 

 sections of bone, teeth, horn, silk, cotton, whalebone ; 

 Btems of plants containing silica or flint ; barley, wheat, &c. 

 The larger-grained starches form splendid objects; tous- 

 Its-mois, being the largest, may be taken as a type of al] 



