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in the beams of the sun sinning from C, there will not only be twocircles of coloured light on the side next the sun, and which con-stitute the two rainbows; but there will also be another on thepart opposite to the sun, the rays belonging to which, meeting atE, afterwards diverge, and form the coloured circle G, as will bevisible, il the light that is transmitted through the globe be receivedon a piece of white paper. The colours also will appear to an eyeplaced in anv part of the surface of the cone EEG . Measuringthe angle EEJi, he found it to be 23 degrees. They were onlythe exueme rays of this cone that were coloured like those of therainbow. This experiment he thought sufficiently illustrated thegeneration of the halo; so that whenever the texture ol the cloudsis such, as not entirely to intercept the rays of the sun or moon,and yet have some degree of density, there will always lie a haloround them, the colours ot the rainbow appearing in those dropswhich are 23° distant from the sun or moon. If the sun be at A,and the spectator in B, tig. 13, the halo will be the circle DEE,DUE being 4ti° or twice 23. The reason why the colours of thehalo are more dilute than those of the rainbow, he says, is owingprincipally to their being formed not in large drops of rain, but invery small vapour; for if the drops of water wcie large, the cloudwould he so thick, that the rays of the sun could not he rcirularlvtransmitted through them; and, on the other hand, he observed,that when the rainbow is formed by \ory thin vapours, the coloursl'.ardlv appear. As for those circles of colours which are some-times "seen round candles, it was his opinion that they are owing tonulhingbut moisture on the eye of the i.b-erver; for that he couldnever produce this appearance by means of vapour only, if hewiped his eyes carelullv ; and lie had observed that such circles arevisible to some persons and not to others, and to the same personsat one time and not at another.

Halos , Huygens’s Theory cr. The most considerable andgenerally received theory, respecting halos, is that of Mr. Huy-gens. Sir Isaac "Newton mentions it with respect, and Dr. Smith,in his complete System of Optics, does not hint at any o'her. M.Huygens takes it for grunted that at those tunes (here are in theatmosphete certain bodies of the figure of round balls, opaque inthe centre, but covered with a transparent shell. By the help ofthis he endeavours to account for halos. Those bodies which M.Huygens requires, in order to explain these phenomena, are not,how ever, a mere assumption; for some such, though of a largersize than his purpose requires, have been actually lound, consist-ing ol snow within and ice without. They are particularly men-tioned by Descartes . These things premised, he addresses himselfto the solution, as follows. LetABCD, lig. 14, represent sucha globule with the opaque nucleus EF in the middle of it; and letus suppose the rays coming from G, H, to fall on the side AD. Itis manifest thev will be refracted inwards from the surface AD ;from whence it follows that a great number ot them must strikeupon the kernel EF. Eet GA, and HD, be the rays which, afterrefraction, touch the sides of the kernel EF, and let them be re-fracted again at B and C, emerging in the lines BK, and CK,crossing each other in the point K, whose distance from the glo-bule is somewhat less than its semidiameter. \V herefore, it BKand CK he produced towards M and E, fig. 13, it follows that nolight coming from the sun through the globule, can proceed to theeve any where placed within the angle EKM, or rather in thecone winch that represents; supposing that the obliquity ot theincident rays il D and GA is such as shall make the arc BC theRivalest possible : for then all the rays exterior to HD and GA,w ill !>,* refracted nearer to Q, and after emergence cioss each otherhi a p,lint k, nearer the globule than the former; and therefore can-not come at the eye placed within the said cone LKM. Suppose■low the eve placed at N, fig. 15, and let N R, NQ, he drawnparallel to" LK, and MK; then it is plain none ot the globules, the*ame as ABCD, within the cone KNQ, can come to the eve at N.Flius *c globules at O and P have their redacted rays uk, kc, anddk, ., including the eye in the cone of obscures. But otherglobules w hich lie without the cone QN'B, as S and 1, do not in-volve the eye N by their shadow-cones tkd, aw\Jkm; and there-fore some ot those ravs which are more retracted than Ao, or Itf,Will fall upon the eye, and produce a luminous cucmar ring ort'orona, including a dark area within, and whose light outwardlydecreases as it is more remote lrom the ccnlie. 1 o verify thishypothesis, M. Jluvgeus advises us to expose to the sun a thin ivol. n,— n*. 100. I

glass-bubble, filled with water, and having some opaque substancein the centre of it; and he says we shall find, that we shall not bebe able to see the sun through it, unless at a certain distance froma place opposite to the centre of it: blit as soon as we do perceivethe light, the image of the sun will immediately appear the bright-est, and coloured red, for the same reason as in the rainbow. Thesehalos, lie says, often appear about the moon ; but the colours areso weak as to appear only white. Such white coronas he had alsoseen about the sun, when the space within them appeared scarcedarker than that without. This he supposes to happen when thereare but few' of those globules in the atmosphere; for the moreplentiful they are, the more lively the colours of the halo appear;at the same time also the area within the corona will be the darker.The apparent diameter of the corona, which is generally about 45°,depends upon the size of the darker kernel; for the larger it iswith respect to the whole globule, the larger will be the dark conebehind it. The globules that form these halos, Huygens supposesto have consisted of soft snow, and to have been rounded by con-tinual agitation in the air, and thawed on the outsides by the heatof the sun. In order to make the diameter of the halo 45° hedemonstrates that the semidiameter of the globule must beto the semidiameter of the kernel of snow very nearly as 1000to 480, and that to make a corona of 100° it must be as 1000to 680.

Halos , Sir Isaac Newton’s Theory of. This great philo-sopher considered the larger and less variable appearances of thiskind as produced according to the common laws of refraction, butthat the less and more variable appearances depend upon the samecause with the colours of thin plates. lie concludes his explica-tion of the rainbow with the following observation on halos andparhelia. “ The light which comes through drops of rain by tworefractions, without any reflexion, ought to appear the strongest atthe distance of about 26“ from the sun, and to decay graduallyboth ways as the distance from him increases. And the same is tohe understood of light transmitted through spherical hail-stones:and if the hail he a little flatted, as it often is, lhe transmitted lightmay be so strong, at a little less distance than that of 26° as to forma halo about the sun or moon; which halo, as often as the hail-stones are duly figured, may be coloured, and then it must be redwithin by the least refrangible rays, and blue without by the mostrefrangible ones; especially if the hail-stones have opaque glo-bules of snow in their centres to intercept the light within the halo,as Iluygens has observed, and make the inside of it more distinctlydefined than it would otherwise be. For such hail-stones, thoughspherical, by terminating the light by the snow, may make a halored within, and colourless without, and darker within the red thanwithout, as halos use to be. For of those rays which pass closeby the snow, the red-making ones will be the least refracted, andto come to the eye in the straightest lines.”

Halos , Weidlek’s Theory of. Mr. Weidler, in his “ Com-mentary on Parhelia,” published at Wirtemburg in 1733, observesthat it is very improbable that such globules as Mr. Huygens’s hy-pothesis requires, with nuclei of such a precise proportion, shouldexist; and if there were such bodies, he thinks they would be toosmall to produce the effects ascribed to them. Besides, he ob :serves that appearances exactly similar to halos are not uncommon)where fluid vapours alone are concerned ; as when a candle isplaced behind the steam of boiling water in frosty weather, or int he midst of the vapour issuing copiously from a hath, or behind areceiver when the air is so much rarefied as to he incapable of sup-.porting the water it contains. The rays of the sun twice reflectedand twice refracted within small drops of water are sufficient, hesays, without any opaque kernel, to produce all the appearancesof the halos that have the red light towards the sun, as mav heproved by experiment. That the diameter of the halos is gene-rally half of that of the rainbow, he accounts- for as Gassendi didbefore him.

I1AEORAGUS, in botany, a genus of the tetragynia order,and oct.imlria class of plants." Calyx four-cleft, superior; petalsfour ; drupe dry, inclosing a four-celled nut. There are twospecies, viz. II. prostrata and H. cercodia.

HALS TEAD, a town of Essex, 47 miles N. E. of Lon-don .

HALTER, [ixviij, Gr. halter, Latin, ] a peculiar kind of dis-cus. See Disc, Discus, and I1alterist.e.

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