34 
Note on the Halo of 22°. By W. H. Miter, M.A., 
For, Sec. R. S., Professor of Mineralogy in the 
University of Cambridge. 
It had long been conjectured that the halos of 22° and 46° were 
produced by the refraction of the light of the sun through 
prisms of ice descending slowly through the atmosphere. The 
physicists who undertook to compare this hypothesis with 
observation assumed the value of the refractive power of ice, 
and in some cases they also assumed the existence of forms 
m ice which had never been observed. M. Brayais in an 
exhaustive Memoir on Halos, inserted in the 31° Cahier du 
Journal de VEcole Polytechnique, and in l Annuaire Météoro- 
logique de lu France, Année 1851, gives an account of a very 
careful determination of the refractive power of ice made by 
himself, and shows that the radius of the halo of 22° agrees 
with sufficient accuracy with the minimum deviation of light 
through the alternate faces of the regular six-sided prism 
0 11, and that of the halo of 46° with the minimum devi- 
ation through a face of the form 11 1, and a face of the 
form 0 1, 1, faces making a right angle with one another. By 
measuring the minimum deviations through a prism of ice, 
Bravais obtained for the indices of refraction the following 
values :— 
Middle Jot ved te... oc eae ee £302, 
Middle of orange ............ 13085, 
Middle of yellow ............ 13095, 
Middle of green ............. 13115, 
Middle of blue and indigo 1:315, 
Middle of violet ..x.¢....0 Lowe 
As the observation presents many difficulties, and the value 
of the research depends entirely on the accuracy of these 
