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water and ice may be divided into two classes, according to the relative position of the luminous ring and the source of light. In the first class we have halos, and coronae, or " glories," which encircle the luminary; the second class includes rainbows, See also:

fog-bows, mist-halos, anthelia and See also:

mountain-spectres, whose centres are at the See also:

ANTI, or CAMPA

anti-See also:

SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar point. Here it is only necessary to distinguish halos from coronae. Halos are at definite distances (22° and 46°) from the sun, and are coloured red on the inside, being due to refraction; coronae closely surround the sun at variable distances, and are coloured red on the outside, being due to diffraction. The phenomenon of a solar (or lunar) halo as seen from the See also:

earth is represented in fig. 1; fig. 2 is a diagrammatic See also:

SKETCH (directly adapted from Dutch schets, which was taken from Ital. schizzo, a rough draft, Lat. schedium, something hastily made, Gr. oxE&os, sudden, off-hand, axeb6v, near by; Ger. Skizze and Fr. esquisse are from the same source)

sketch showing the See also:

APPEARANCE (from Lat. apparere, to appear)

appearance as viewed from the See also:

ZENITH (from the Arabic)

zenith; but it is only in exceptional circumstances that all the parts are seen. Encircling the sun or moon (S), there are two circles, known as the inner halo I, and the See also:

OUTER

outer halo 0, having radii of about 22° and 46°, and exhibiting the See also:

COLOURS, MILITARY

colours of the spectrum in a confused manner, the only decided tint being the red on the inside. Passing through the luminary and parallel to the See also:

HORIZON (Gr. 6pfTwv, dividing)

horizon, there is a See also:

white luminous circle, the parhelic circle (P), on which a number of images of the luminary appear.

The most brilliant are situated at the intersections of the inner halo and the parhelic circle; these are known as parhelia (denoted by the See also:

LETTER (through Fr. lettre from Lat. littera or litera, letter of the alphabet; the origin of the Latin word is obscure; it has probably no connexion with the root of linere, to smear, i.e. with wax, for an inscription with a stilus)

letter p in the figures) (from the Gr. aapa, beside, and ijXtos, the sun) or " See also:

MOCK

mock-suns," in the See also:

case of the sun, and as paraselenae (from See also:

rap& and rrelltimrt, the moon) or " mock-moons, in the case of the moon. Less brilliant are the parhelia of the outer halo. The parhelia are most brilliant when the sun is near the horizon. As the sun rises, they pass a little beyond the halo and exhibit flaming tails. The other images on the parhelic circle are the paranthelia (q) and the See also:

ANTHELION (late Gr. avOil]uos, opposite the sun)

anthelion (a) (from the See also:

Greek avri, opposite, and ijXcos, the sun). The former are situated at from 90° to 140° from the sun; the latter is a white patch of light situated at the anti-solar point and often exceeding in See also:

SIZE

size the apparent See also:

DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter of the luminary. A See also:

VERTICAL (from Lat. vertex, highest point)

vertical circle passing through the sun may also be seen. From the parhelia of the inner halo two oblique curves (L) proceed. These are known as the " arcs of Lowitz," having been first described in 1794 by Johann Tobias Lowitz (1757-1804). Luminous arcs (T), tangential to the upper and See also:

LOWER

lower parts of each halo, also occur, and in the case of the inner halo, the arcs may be prolonged to See also:

FORM (Lat. forma)

form a quasi-elliptic halo." The physical explanation of halos originated with Rene See also:

DESCARTES, RENE (1596-1650)

Descartes, who ascribed their formation to the presence of ice-crystals in the atmosphere. This theory was adopted by Edme See also:

MARIOTTE, EDME (c. 1620-1684)

Mariotte, See also:

Sir See also:

ISAAC (Hebrew for " he laughs," on explanatory references to the name, see ABRAHAM)

Isaac See also:

Newton and See also:

Thomas See also:

Young; and, although certain of their assumptions were somewhat arbitrary, yet the See also:

general validity of the theory has been demonstrated by the researches of J. G.

See also:

GALLE, or POINT DE GALLE

Galle and A. Bravais. The memoir of the last-named, published in the See also:

Journal de l'Ecole royale poly-technique for 1849 (xviii., 1–270), ranks as a classic on the subject; it is replete with examples and illustrations, and discusses the various phenomena in See also:

minute detail. The usual form of ice-crystals in clouds is a right hexagonal See also:

PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism, which may be elongated as a See also:

NEEDLE (O. Eng. ncedl; the word appears in various forms in Teutonic languages, Ger. Nadel, Dutch naal, the root being ne-, to sew, cf. Ger. nahen, and probably Lat. nere, to spin, Gr. vi)ats, spinning)

needle or foreshortened like a thin See also:

PLATE

plate. There are three refracting angles possible, one of 120° between two adjacent prism faces, one of 6o° between two alternate prism faces, and one of 90° between a prism See also:

FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

face and the See also:

BASE

base. If innumerable See also:

numbers of such crystals fall in any manner between the observer and the sun, light falling upon these crystals will be refracted, and the refracted rays will be crowded together in the position of minimum deviation (see REFRACTION OF LIGHT). Mariotte explained the inner halo as being due to refraction through a See also:

PAIL

pail of alternate faces, since the minimum deviation of an ice-prism whose refracting See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle is 6o° is about 22°. Since the minimum deviation is least for the least refrangible rays, it follows that the red rays will be the least refracted, and the See also:

VIOLET

violet the more refracted, and therefore the halo will be colopred red on the inside. Similarly, as explained by See also:

Henry See also:

Cavendish, the halo of 46° is See also:

die to refraction by faces inclined at 9o°. The impurity of the colours (due partly to the sun's diameter, but still more to oblique refraction) is more marked in halos than in rainbows; in fact, only the red is at all pure, and as a See also:

RULE

rule, only a See also:

mere trace of See also:

green or See also:

BLUE (common in different forms to most European languages)

blue is seen, the See also:

EXTERNAL

external portion of each halo being nearly white. The two halos are the only phenomena which admit of explanation without assigning any particular See also:

DISTRIBUTION (Lat. distribuere, to deal out)

distribution to the ice-crystals. But it is obvious that certain distributions will predominate, for the crystals will tend to fall so as to offer the least resistance to their See also:

motion; a needle-shaped crystal tending to keep its See also:

AXIS (Lat. for " axle ")

axis vertical, a plate-shaped crystal to keep its axis See also:

HORIZONTAL

horizontal.

Thomas Young explained the parhelic circle (P) as due to reflection from the vertical faces of the See also:

long prisms and the bases of the See also:

SHORT, FRANCIS JOB (1857– )

short ones. If these vertical faces become very numerous, the See also:

eye will perceive a colourless horizontal circle. Reflection from an excess of horizontal prisms gives rise to a vertical circle passing through the sun. The parhelia (p) were explained by Mariotte as due to refraction through a pair of alternate faces of a vertical prism. When the sun is near the horizon the rays fall upon the See also:

PRINCIPAL

principal See also:

section of the prisms; the minimum deviation for such rays is 22°, and consequently the parhelia are not only on the inner halo, but also on the parhelic circle. As the sun rises, the rays enter the prisms more and more obliquely, and the angle of minimum deviation increases; but since the emergent See also:

ray makes the same angle with the refracting edge as the incident ray, it follows that the parhelia will remain on the parhelic circle, while receding from the inner halo. The different values of the angle of minimum deviation for rays of different refrangibilities give rise to spectral colours, the red being nearest the sun, while farther away the overlapping of the spectra forms a flaming colourless tail sometimes extending over as much as 10° to 20°. The " arcs of Lowitz " (L) are probably due to small oscillations of the vertical prisms. The " tangential arcs " (T) were explained by Young as being caused by the thin plates with their axes horizontal, refraction taking See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place through alternate faces. The axes will take up any position, and consequently give rise to a continuous See also:

SERIES (a Latin word from serere, to join)

series of parhelia which See also:

TOUCH (derived through Fr. toucher from a common Teutonic and Indo-Germanic root, cf. " tug," " tuck," O. H. Ger. zucchen, to twitch or draw)

touch externally the inner halo," both above and below, and under certain conditions (such as the requisite See also:

ALTITUDE (Lat. altitudo, from altus, high)

altitude of the sun) form two closed elliptical curves; generally, however, only the upper and lower portions are seen. Similarly, the tangential arcs to the halo of 46° are due to refraction through faces inclined at 90°, The paranthelia (q) may be due to two See also:

INTERNAL

internal or two external reflections. A pair of triangular prisms having a See also:

COMMON

common face, or a stellate crystal formed by the symmetrical interpenetration of two triangular prisms admits of two internal reflections by xrr.

28faces inclined at 120°, and so give rise to two colourless images each at an angular distance of 120° from the sun. See also:

Double internal reflection by a triangular prism would form a single coloured See also:

IMAGE (Lat. imago, perhaps from the same root as imitari, copy, imitate)

image on the parhelic circle at about 98° from the sun. These -angular distances are attained only when the sun is on the horizon, and they increase as it rises. The anthelion (a) may be explained as caused by two internal reflections of the solar rays by a hexagonal lamellar crystal, having its axis horizontal and one of the diagonals of its base vertical. The emerging rays are parallel to their See also:

ORIGINAL

original direction and form a colourless image on the parhelic circle opposite the sun.

End of Article: HALO

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