HALO, a luminous circle, the result of light refracted from minute bodies, which is seen under various conditions, the most common being colored circles around the sun and moon. They are subject to variation in form, double and triple circles being seen, concentric or superposed in a pattern, and sometimes accom panied by arcs and spokelike radii. The corona, sun-dog, mock-moon, parhelion, etc., are of this character. The typical solar or lunar halo is reddish in the inner edge, and violet on the outer circumference, but when there is a double halo this coloring is reversed in the larger one, exhibiting diffraction. Sun and moon halos are mainly attributed to minute ice-crystals in the earth's atmosphere, but may be due to other interference with light. A thorough study of the diffraction of light is essential to a full understanding of the phenomena. Halos are seen at fixed distances from the sun, 22 and 46 degrees, respectively, while corona are at variable distances. See LIGHT; CORONA; PAR
HELION ; SUN.
The halo of art, also called nimbus, is a disc or approximately circular display of light around the head in a painting, usually of the Christ, the Virgin Mary or a saint. Occasion ally it takes the form of light rays extending from the head. Again at times it encircles an entire figure, usually in an ellipse, and is then commonly called aureole, a more appropriate name, as it is undoubtedly an effort to ex press the aura of the subject of the picture, such aura being assumed to be luminous in one of saintly character. (See AUR.EOLA). The halo was early adopted by painters of sacred sub jects, and adorns the paintings of a vast number of figures in cathedral windows. Consult Cher rill, The Theory of Halos and Parhelia) ; United States Weather Bureau Bull. 12 (1895) ; Monthly Weather Review (Washington, D. C.).