118 



THE MICROSCOPE. 



ence 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 the quinidines has a very different 

 mode of crystallisation ; but it occasionally presents 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 

 discriminate the two. Philosophical Magazine, 1853. 



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 when viewed by polarised light. Some of the 

 Wenham Lake ice answers very well. 



During the intense frost of February 1855, Mr. Glaisher made 

 many beautiful drawings of snow crystals, figs. 83, 84, 85, and 86. 



He writes: "This morning, February 21, with a temperature of 

 21, they are falling sparingly, but are intensely beautiful; they are 

 also minute, and highly crystalline. Up to the present time, 9h. 30m., 

 all that I have observed are made up of prisms of six facets; and many 

 are double, that is, two crystals alike in form are falling, united to an 

 axis at right angles to the plane of each. These are generally fine 

 specimens, and less minute ; the rays of the under crystal in most cases 

 filling the intermediate spaces between those of the upper, and, as 



fig. 83. /Snow Crystals. 



