HALOS, PARHE'LIA, CORO'N/E, etc. It would not be easy even to enumerate the various distinct phdnomena which belong to the above classes; we must, therefore, be content to consider a few only of the principal varieties; and, in fact, if the causes of these be thoroughly understood. those of the others present no further difficulties, except such as are of a purely mathematical nature.
The first class we have to consider is very common. When the sun or moon is partially obscured by a mist or cloud, the latter not being of the species called cirrus (see CLOUD), it is almost invariably surrounded by colored rings of a few degrees only in diameter, called arrow (crowns). Those surrounding the sun cannot always be seen directly; but by reflection at the surface of still water, or of a glass-plate blackened at the back, the glare of the sun-light is sufficiently diminished to permit the corona to be seen. This meteor depends on the diffraction (q.v.) of light, caused by the small spheres or vesicles of water which compose the cloud, and can easily he imitated by looking nit a bright object through a piece of glass which has been breathed upon, or dusted with lyeofiodium seed. If the diffracting particles be all of the same size, the rings are very well marked; but since they become smaller as the particles increase in size, ordinary fogs and clouds, which generally contain particles of very different dimensions, give a composite effect, which spoils the distinctness, and greatly limits the number of the rings. Thus, ,no general rule can be given for the number or colors of the coronae, but it may be observed that their diminution in diameter is a sign of the increase in size of the watery spheres which cause them, and therefore in general betokens approaching rain, which comes when the particles are no longer able, on account of their size, to float in the air without sensibly falling. As before mentioned, this appearance is very common, and, in fact, we scarcely see a fragment of a cloud near the sun which does not give traces of color, depending on the average size of the particles of which it con 'ists. and its angular distance from the sun.
A. different form of corona is sometimes seen to surround the shadow of the specta tor's head, when cast by the sun on a bank of fog—in this case it is sometimes called a glory. To this class belong the colors generally seen about the famous " Specter of the Brocken." See BaocKEN. The same appearances are very frequently seen round the
shadow of the spectator when thrown on muddy water, or water carrying numerous small particles of sand. The optical explanation, founded mainly upon reflection and interference, is complete, but not suited to our pages.
So far the phenomena depend merely on the cloud or fog consisting of small particles; nothing has yet been said about the shape of the particles. Spherical drops of water produce rainbows (q.v.), and upon the vesicular form that moisture often assumes in the air, probably depend the blue of the sky and the gorgeous tints of sunrise and sunset. But halos (Gr.) and parhelia (Gr. false or mock suns) depend upon the presence in the air of innumerable crystals of ice, generally forming a light cirrus cloud. We cannot enter upon a complete explanation of these phenomena, but we shall give a general idea of their origin. referring the student who wishes a thorough knowledge of the subject toga Memoir by Bravais (Journal de l'Ecok Polgleehnique, xviii.), who has himself repeatedly witnessed and carefully measured the various appearances in question.
The theory of halos was first roughly attempted by Huyghens; but although his explanations are in the main correct (at all events, as regards the very simplest of the appearances), they are based on the utterly inadmissible supposition, that the halo-pro ducing clouds are formed of cylinders of water, each having an opaque, frozen nucleus. It will be seen that the results of this supposition agree with those of the correct one in a few cases only. Further progress was impossible until the crystalline forum and the refractive index of ice were observed. Both of these observations are of great difficulty; but they have been carried out by Wollaston and others with considerable accuracy. After Ilnyghens, Mariotte, admitting the crystalline form of ice-particles, made some great steps in advance, and much of what Ile left unexplained was successfully supplied by Young, and after him by Kaemtz. The most complete and systematic explanation of the whole subject, however, is that of Bravais, already referred to. There, references are given to nearly all the accurately recorded observations of halos and parhelia—the great mass of which, of course, are due to arctic voyagers, especially Scoresby and Parry.