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Luminous circles called "halos" have often been observed surrounding the sun and moon in certain conditions of the atmosphere, usually more or less colored. They are of two kinds, one kind of small dimensions, which are generally composed of two or three rings; they appear when a small quantity of aqueous vapor is diffused through the atmosphere, or when light fleecy clouds pass over the sun or moon. These are usually called coronce. The second kind are much larger, some measuring from H to 46O in diameter. The lunar halo is a simple white luminous circle, without color, excepting a pale red, which sometimes fringes -* the inner edge. The solar halo is a beautiful ^* play of colors; although not so bright as those C of the rainbow, they are marked with suf-|fioient distinctness to be seen, The red occu- pies the inner part of the circle, the violet and indigo the outer, and they shade away until lost in the surrounding space. Sometimes a secondary circle, full of bright spots, or parhelia, is seen. Various reasons have been assigned as the cause of these phenomena, but the most probable is that of Mariotte, who supposes they are due to the refraction of light through the transparent and prismatic crystala of ice which float in the upper regions of this atmosphere.
