red. Within the ring is a bluish-white surface, the aureole. If spectrum colors other than the red are observable, they follow each other in order from violet to red, reversing the order of halo colors. This sequence of color sometimes is repeated several times in the case of the corona but not with the halo.
Halo.—The most common form of halo is the ring around the sun or moon. It has a radius of about 22 degrees of a great circle. At times, however, the halo is a complex arrangement of concentric tangential and independent arcs of circles. The sim-
from a drawing made by himself.
Lunar halo observed by Gen. A. W. Greely at Fort Conger.
ple halo is practically a rainbow, red inside the ring, with colors on the outer side ranging in spectrum order. Unless the halo is strong, however, the impression to a casual observer is that of a white ring. Occasionally another fainter and incomplete ring of 46 degrees radius may be observed. Still more rarely a white ring parallel to the horizon and passing through the sun is observable. At or near the intersection of this circle with the halo, mock suns, parhelia, or mock noons, paraselenæ, appear as very bright spots, with red predominating. Mock suns and mock moons are seen at times in other positions. The
