HALO

any of a wide range of atmospheric optical phenomena that result when the Sun or Moon shines through thin clouds composed of ice crystals. These phenomena may be due to the refraction of light that passes through the crystals, or the reflection of light from crystal faces, or a combination of both effects. Refraction effects give rise to colour separation because of the slightly different bending of the different colours composing the incident light as it passes through the crystals. On the other hand, reflection phenomena are whitish in colour, because the incident light is not broken up into its component colours, each wavelength being reflected at the same angle. The most common halo is the 22 halo, a series of coloured arcs, or in some cases complete circles, of 22 angular radius with the Sun or Moon at its centre. The order of coloration is red on the inside and blue on the outside, opposite to that of the atmospheric corona. Less frequently observed phenomena, such as parhelia, sun pillars, tangent arcs, sun crosses, and others, also are attributable to the reflection or refraction of sunlight or moonlight by ice crystals. also called nimbus in art, radiant circle or disk surrounding the head of a holy person, a representation of spiritual character through the symbolism of light. In Hellenistic and Roman art the sun-god Helios and Roman emperors often appear with a crown of rays. Because of its pagan origin, the form was avoided in Early Christian art, but a simple circular nimbus was adopted by Christian emperors for their official portraits. From the middle of the 4th century, Christ was also shown with this imperial attribute, as was his symbol, the Lamb of God, from the end of the 4th century. In the 5th century it was sometimes given to angels, but it was not until the 6th century that the halo became customary for the Virgin Mary and other saints. For a period during the 5th century, living persons of eminence were depicted with a square nimbus. The halo was used regularly in representations of Christ, the angels, and the saints throughout the Middle Ages. Often Christ's halo is quartered by the lines of a cross or inscribed with three bands, interpreted to signify his position in the Trinity. From the 15th century, however, with the growth of naturalism in Renaissance art, the nimbus created problems in representation. At first it was treated by some Florentine artists as a solid object seen in perspective, a disk fixed to the back of a saint's head. The inadequacy of this solution led to its decline in Italian art in the 16th century and to its abandonment by Michelangelo and Titian. In Flemish painting of the 15th century, it began to be represented as rays of light; under the influence of the Counter-Reformation, which sought to restore a glorious conception to religious art, this form was adopted by Italian artists of the late 16th century, notably Tintoretto, as a realistically rendered light emanating from the holy person's head. This new interpretation was the standard one in the Baroque period and in most subsequent religious works. The halo is also found in Buddhist art of India, appearing from the late 3rd century AD. It is believed that the motif was brought to the East by Greek invaders. See also mandorla.