They are usually seen surrounding the sun or the moon, but in the tropics small halos were observed by Humboldt round the planet Venus. The formation of encircling halos of the classes A and B is generally attributed to refraction and reflexion of the rays of light by the presence of minute snow crystals in the upper strata of the air, and they occur usually when cirrus or cirro-stratus clouds exist. This theory was originated by Descartes, and was adopted by Marriotte (in 1686), and subsequently followed by Dr Young and Sir Isaac Newton. It is, however, probable that some halos and certainly all coronae (class C) are formed by refraction of light from globules of water suspended in the atmosphere. Simple halos are either about 22⅛ degrees or 47 degrees in diameter. The ice or snow crystals, being hexagonal in form, sometimes produce a double refraction, and then the large and small halos are seen at the same time, and, reflexion taking place from the surfaces of the crystals, additional halos or parts of halos are formed as in figs. 1 and 2. At the points of intersection of the halos, images of the sun or moon sometimes appear; when seen with the former, they are called parhelia or mock-sums, and when with the latter, paraselenes or mockmoons. They are sometimes accompanied with flaming tails, and are usually of prismatic colours, as also are the halos and coronae. The latter are seen when fleecy clouds or mist intervene between the spectator and the sun or moon, to which they are in immediate proximity, thereby being distinguished from the halos, which are formed at a distance from those luminaries. The predominating colour in coronae is red or orange.

The second principal division of halos includes those which are seen opposite to the sun and moon. The first class of these comprises rainbows (a), which are occasioned by refraction of the light from the sun or moon, produced by falling rain, causing a bow with the prismatic colours in concentric bands and arranged as they appear in the solar spectrum. When the rain is abundant an inner bow is formed, the colours in which are in inverted order. Rainbows when seen from mountain tops and the topmasts of ships are completely circular. Fog-bows and mist halos (β) are somewhat similar.

The former are frequently white in colour, and are seen in the English Channel, and also off the banks of Newfoundland, being called by sailors “fog-eaters,” from their indicating a dispersion of the fog or mist. They appear to be formed in a horizontal or very oblique plane. Mist halos are seen in the vapour of fountains or waterfalls, and resemble rainbows. Mountain spectres (γ) are caused by reflexion, and often appear accompanied by chromatic halos (see fig. 3). They generally occur on mountains when the sky between the observer and the sun is clear and there is a mist or cloud on the side of the mountain opposite to the sun, on which the summit of the mountain and the buildings and persons standing near are reflected. The most notable of mountain spectres is that of the Brocken, in the Harz mountains. Howitt says of this :—

“If the fog is very dry, you see not only yourself but your neighbour; if very damp, only yourself, surrounded by a rainbow-coloured glory, which becomes more lustrous and beautiful the damper and thicker the fog is, and the nearer it approaches.”

Many of the ancient descriptions of miraculous appearances in the heavens may be attributed to the phenomena before mentioned. Among the chief of these is the appearance of a cross which the emperor Constantine alleged he saw in the heavens about noon when marching against Maxentius (about 313 a.d.), and which was the cause of his conversion to Christianity (Eusebius, Vita* Constant., i. 27). Some writers have doubted the occurrence; but it is quite possible Constantine saw part of a system of solar halos, and two of these at right angles would form a cross (see fig. 4). The Brighton Herald of April 10, 1852, records a similar phenomenon on April 1st of that year at Brighton :—

‘ ‘ Sun setting, evening very fine with slight haze westward, when a ray of brilliant light shot upwards about 20 degrees above the horizon, directly perpendicular from the sun, and soon after its appearance it was crossed by a horizontal band of light of a paler colour, but about the some breadth. It formed a perfect cross, and lasted half an hour.”

Halos and rainbows are considered valuable prognostications of future weather. Fragments of rainbows seen on detached clouds are called wind-dogs, and are said to be a sign of wind. When the diameter of a corona contracts it shows the water particles are uniting into larger ones which may fall in rain, whereas if the corona extends, the particles are growing smaller, indicating increasing dryness. The open side of a halo is in some places considered to foretell the quarter from which bad weather may be expected.

See Μ. A. Bravais, “Mémoire sur les Halos et les Phénomènes optiques qui les accompagnent” (Journal de VÉcole Royale Polytechnique, 31 cahier, tome 18, Paris, 1847); Buchan, Introductory Tert- Book of Meteorology, 1871; Proc. Met. Soc., 1875, vol. ii., newser., pp, 438 to 444.