is, on the whole, well justified for the following reasons: We see the rainbow where rain is falling, while the sun is shining in the opposite part of the sky. Our rainstorms usually come from the west and pass away to the east. A morning rainbow can only be seen in the west, and indicates that rain is approaching us. An evening rainbow (ignoring lunar bows) is seen only in the east, and shows that the rain area is receding from us, giving place to clear skies. [Illustration: THE CIRCUMZENITHAL ARC (_From a drawing by L. Besson in La Nature._) Parts of the halos of 22° and 46°, upper tangent arc of the 22° halo, and two parhelia are also shown. The circumzenithal arc is always brightly colored.] Ice crystals in the atmosphere, such as those composing the higher clouds, produce a great variety of optical phenomena, known as _halos_. Some phenomena of this class are common, others exceedingly rare. Moreover, there are several theoretically possible forms of halo of which observations have never yet been reported, so that halo observing can be recommended to the amateur meteorologist as offering opportunities for making interesting discoveries. Halos take the form of narrow rings of definite angular size around the sun or moon (not to be confused with the coronas, of variable dimensions, described below), rings passing through the luminary, arcs in various other positions, and roundish spots of colored or white light. They may be seen separately or in combination. In rare cases, a dozen or more different forms of halo are visible at the same time, producing a most spectacular display. One of the most remarkable displays of this kind in the history of science was seen, in different degrees of development, over the eastern United States on November 1 and 2, 1913; an event which greatly stimulated interest in the study of halos in this country. Complex halos are quite common in the polar regions; where they are seen not only in the sky, but also in the air, charged with ice particles, close to the earth. Whenever a thin veil of cirrus or cirro-stratus clouds overspreads the sky there is a likelihood that halos will be visible. Those formed near the sun, however, frequently pass unnoticed, on account of the dazzling brightness of that luminary. Smoked or tinted glasses greatly facilitate their observation. [Illustration: DIAGRAMS OF THE PRINCIPAL FORMS OF HALO (_After Besson, Monthly Weather Review, July, 1914._)