37 
so closely with the value 1:3095 obtained by Bravais as to 
leave no room to doubt the adequacy of the received hypo- 
thesis. Its truth has been still further confirmed by the 
occurrence of a solar halo formed on the ground. (Listing, 
“Ueber einen in Russland von Herrn Korsakoff beobachteten 
terrestrischen Sonnen halo, nebst bemerkungen iiber das 
krystallisirte Wasser.” Poggendorff’s Annalen, B. 122, 8. 161.) 
M. Korsakoff found the radius of the middle of the red ring 
equal to 22°15’ by measurement with a sextant, and traced 
the light to small prisms of ice scattered on the ground. When 
examined under a microscope these were found for the most 
part to be hexagonal prisms terminated at one end by a plane 
normal to the axis of the prism, and at the other end by a 
hexagonal pyramid. They were from 0°38 to 0°66 millimetres 
long, and from 0°13 to 0°19 millimétres thick. Crystals of ice 
exhibiting combinations of hemihedral forms with inclined faces 
had been approximately measured and described by N. A. E. 
Nordenskiold, (Poggendorff’s Annalen, B. 114, 8. 662). They 
were combinations of the rhombohedral forms 
121, Ot 2a Sah, ple 0, ps F 
The angles were | 
pine Oise? Baer ar, Wl 20 = 53" 146 
111, 15 3=81°31’. 
Another instance in which the formation of a halo was 
distinctly traced to the refraction of the light of the sun by 
an assemblage of crystals of snow is recorded in the first 
Jahrbuch des Schweizer Alpenclub. On the 3rd of January, 
1864, Professor Ritz saw a halo in the cloud of loose snow 
stirred up by a violent wind at the entrance of the upper 
valley of Kandersteg. 
This view of the formation of halos is supported by the 
observations of Arago and Sir David Brewster, who ascertained 
that the light of halos was polarized in the direction of a 
tangent to the are at the point observed. Bravais says that 
