HALO

, or Corona, a coloured circle appearing round the body of the sun, moon, or any of the larger stars.

Naturalists conceive the Halo to arise from a refrac-| tion of the rays of light in passing through the fine rare vesiculæ of a thin vapour towards the top of the atmosphere.

Des Cartes observes, that the Halo never appears when it rains; whence he concludes that this phenomenon is occasioned by the refraction of light in the round particles of ice, which are then floating in the atmosphere; and to the different protuberance of these particles he ascribes the variation in the diameter of the Halo. Gassendi supposes, that a Halo is occasioned in the same manner as the rainbow; the rays of light being, in both eases, twice refracted and once reflected within each drop of rain or vapour, and that the difference between them is wholly owing to their different situation with respect to the observer, Dechales also endeavours to shew that the generation of the Halo is similar to that of the rainbow; and that the reason why the colours of the Halo are more dilute than those of the rainbow, is owing chiefly to their being formed, not in large drops of rain, but in very small vapour. But the most considerable and generally received theory, relating to the generation of Halos, is that of Mr. Huygens. This celebrated author supposes Halos, or circles round the sun, to be formed by small round grains of hail, composed of two different parts, the one of which is transparent, inclosing the other, which is opaque; which is the general structure actually observed in hail. He farther supposes that the grains or globules, that form these Halos, consisted at first of sost snow, and that they have been rounded by a continual agitation in the air, and thawed on their outside by the heat of the sun, &c. And he illustrates his ideas of their formation by geometrical figures.

Mr. Weidler endeavours to refute Huygens's manner of accounting for Halos, by a vast number of small vapours, each with a snowy nucleus, coated round with a transparent covering. He says, that when the sun paints its image in the atmosphere, and by the force of its rays puts the vapours in motion, and drives them toward the surface, till they are collected in such a quantity, and at such a distance from the sun on each side, that its rays are twice refracted, and twice reflected, when they reach the eye they exhibit the appearance of a Halo, adorned with the colours of the rainbow; which may happen in globular pellucid vapours without snowy nuclei, as appears by the experiment of hollow glass spheres filled with water: therefore, whenever those spherical vapours are situated as before mentioned, the refractions and reflections will happen every where alike, and the figure of a circular crown, with the usual order of colours, will be the consequence. Philos. Trans. number 458.

Newton's theory of Halos may be seen in his Optics, p. 155. And this curious theory was confirmed by actual observation in June 1692, when the author saw by reflection, in a vessel of stagnated water, three Halos, crowns, or rings of colours, about the sun, like three little rainbows concentric to his body. These crowns inclosed one another immediately, so that their colours proceeded in this continual order from the sun outward: blue, white, red; purple, blue, green, pale yellow, and red; pale blue, pale red. The like crowns sometimes appear about the moon. The more equal the globules of water or ice are to one another, the more crowns of colours will appear, and the colours will be the more lively. Optics, p. 288.

There are several ways of exhibiting phenomena similar to these. The slame of a candle, placed in the midst of a steam in cold weather, or placed at the distance of some feet from a glass window that has been breathed upon, while the spectator is also at the distance of some feet from another part of the window, or placed behind a glass receiver, when air is admitted into the vacuum within it to a certain density, in each of these circumstances will appear to be encompassed by a coloured Halo. Also, a quantity of water being thrown up against the sun, as it breaks and disperses into drops, forms a kind of Halo or iris, exhibiting the colours of the natural rainbow. Musschenbroek observed, that when the glass windows of his room were covered with a thin plate of ice on the inside, the moon seen through it was surrounded with a large and variously coloured Halo; which, upon opening the window, he found arose entirely from that thin plate of ice, because none was seen except through this plate. Musschenbroek concludes his account of coronas with observing, that some density of vapour, or some thickness of the plates of ice, divides the light in its transmission either through the small globules or their interstices, into its separate colours; but what that density is, or what the size of the particles which compose the vapour, he does not pretend to determine. Introd. ad Phil. Nat. p. 1037.

It has often been observed that a Halo about the sun or moon, does not appear circular and concentric to the luminary, but oval and excentric, with its longest diameter perpendicular to the horizon, and extended from the moon farther downward than upward. Dr. Smith ascribes this phenomenon to the apparent concave of the fky being less than a hemisphere. When the angle which the diameter of a Halo subtends at the eye is 45° or 46°, and the bottom of the Halo is near the horizon, and consequently its apparent figure is most oval, the apparent vertical diameter is divided by the moon in the proportion of about 2 to 3 or 4, and is to the horizontal diameter drawn through the moon, as 4 to 3, nearly.—See farther on the subject of this article, Priestley's Hist. of Discoveries relating to Vision, p. 596—613; and Smith's Optics, art. 167, 513, 526, 527, &c.