1897 THE MIOROSCOPE. 19 
aud even a perfectly cold room is warmer than the out 
of door temperature. 
Have the slides cold and dry and the instrument ar- 
ranged with the polariscope and focussed as nearly as 
possible. Then catch the crystals on the slide as they 
fall and examine them without delay. Snow crystals, 
but a short time after falling, break, the broken pieces 
freeze together and crystalization is destroyed. 
The light during a snow storm is not suitable for 
photo-micrographic work. As an artificial light, the 
petroleum lamp is preferred. To prevent heat action em- 
anating from the illuminating ray cone, an absorptive 
cell of alum solution should be interposed. As alum 
solution freezes solid in —5 deg. R., chloride of sodium 
is added. With Hartnark’s projection system (31 mm, 
focus distance), from 5 to 7 seconds upon an erythrosine 
plate is ample. 
Dr. Neuhaus’ pictures of ice crystals much resemble 
those of hoar trost, deposited after a cold winter’s night. 
Of snow crystals the doublets are highly interesting, two 
crystals having merged into one, and those having passed 
through a moist stratum of air, when microscopic drops 
of water will freeze into the hexagonal form, giving the 
picture an appearance very much resembling a cauliflower. 
The cause of the various forms of hexagonal crystals, 
which often change in the same snow storm, is a ques- 
tion which it is difficult to answer. Dr. Smallwood has 
traced an apparent connection between these various 
forms and the electric conditions of the atmosphere, 
whether negative or positive. Dr, Hellmann says: “It 
is better to acknowledge that we know nothing posi- 
tively in regard to this. In our knowledge of the form 
and structure of the snow we have made great advance 
since the time of Kepler, but, after nearly 400 years we 
cannot give a satisfactory answer to his question: ‘Cur 
autem sexangula?”’ 
