a Art. 530.

Fig. 481..

Art. 528.

between the eye and the fun, and will decay more and more in the globules that lye farther and farther from that line or from the apparent place of the fun. In this conclufion I agree with Hugenius in his differtation concerning Corona's and Parhelia, printed in the next chapter; though I differ from him in the reafon he gives for it, namely because the drops that lye nearest to the fun make the largest images of him; (maximam folis imaginem exhibent); whereas thefe images being lefs than the globules have no fenfible apparent magnitude; and therefore appear brighter or duller only becaufe the rays fall thicker or thinner upon the eye.

526. Nevertheless to think different from so great a master of reafon as Sir Ifaac Newton, is but little better than a degree of uncertainty; or at leaft a fort of pain which the mind would willingly be freed from. I have therefore been very particular in demonftrating this conclufion because I b Opt. p. 155. cannot reconcile it with one of his ". Where he fays that the light which comes through drops of rain by two refractions without any reflection ought to appear ftrongest at the distance of about 26 degrees from the fun; [that is when the angle LMN is about 26 degrees;] and to decay gradually both ways as the distance from him increases and decreases. And that the fame is to be understood of light tranfmitted through spherical hail-ftones. And if the hail be a little flatted, as it often is, the light tranfmitted may grow fo ftrong at a lefs diftance than that of 26 degrees as to form a halo about the fun or moon. I have already obferved that Hugenius was of a contrary opinion; and befides he tells us in another place of his differtation that though he had examined all the reflections and refractions of the fun's rays in globules of water, yet he could find no caufe at all for the appearance of a ring of light about him of the ufual bignefs of a halo. And thence he began to fufpect that they are formed by hail-ftones that have opake globules of fnow in their centers. In this opinion Sir Ifaac Newton feems to agree d Opt. p. 155. with him. For he goes on in thefe words, which halo, as often as the hail-ftones are duly figured, may be coloured; and then it must be red within by the leaft refrangible rays, and blue without by the moft refrangible ones; efpecially if the hail-ftones have opake globules of fnow in their centers to intercept the light within the halo (as Hugenius has observed) and make the infide thereof more diftinctly defined than it would otherwife be. For fuch hail-ftones, though spherical, by terminating the light by the fnow, may make a halo red within and colourless without, and darker within the red than without as halo's ufe to be. For of those rays which pafs clofe by the fnow, the rubiform will be leaft refracted, and fo come to the eye in the directeft lines. The light which paffes through a drop of water after two refractions and three or more reflections, is fcarce ftrong enough to caufe a fenfible bow [as appears by the different ftrength of the first and second rain-bows] but in thofe cylinders of ice by which Hugenius explains the parhelia it may perhaps be fenfible. This

is all I can find of Sir Ifaac Newton's concerning this fubject, excepting another short paffage where he seems to fuggeft a reason why halo's appear oval, an inftance of which he had mentioned juft before; and then adds thefe words. By its being oval, and remoter from the moon below than above, I conclude that it was made by refraction in some sort of hail or fnow floating in an horizontal pofture, the refracting angle being about 58 or 60 degrees. In this alfo I must confefs I cannot reft satisfied; a Opt. p. 291. because all halo's, though formed by fpherical hail with fpherical globules of fnow in their centers, muft appear oval in the pofition he mentions upon account of the flattish figure of the fky, as it is explained in the 167th article.

CHAPTER XI.

Concerning Corona's and Parbelia, commonly called Halo's and Mock-Suns.

IN

N this chapter I have tranflated Hugenius's whole differtation upon Design. this curious subject; and as this excellent Author intended to demonftrate the more difficult things at the end of all, which the learned Editors of his Pofthumous Works have fupplyed in few words, I have done the fame more at large in an Appendix to this chapter; which contains also the observations referred to in this differtation.

I. Corona's what

527. Though the caufes of corona's and parhelia, which fhall here be affigned are much alike, yet I will firft treat of corona's, because they are and how large. easier to be explained; and being understood, will facilitate the explanation of parhelia. Now corona's are circular rings of light, which by day appear round about the fun, and sometimes by night about the moon; fometimes white, and fometimes, when brighteft, adorned with various colours like the rain-bow. Their apparent diameter is generally about 44 or 45 degrees, but fometimes a larger fort have appeared, about 90 degrees in diameter or more. It is faid that a great many fuch rings have ap peared together, with the fun in their centers. I my felf have often obferved thofe rings of 45 degrees; the first of which, being the most lively, gave me occafion to take notice of the moft remarkable appearances in it, in order to attempt their explication. I obferved this ring was made up of various colours, but fainter than thofe in the rain-bow; that its inward edge was red and its outward a pale blue, much inclining to white; that the space within the ring (called the area) was darker than that on the outside of it, which appeared every way pretty clear, excepting that it was covered with fome fmall pellucid and whitish clouds. I found the diameter of the ring about 45 degrees, by a grofs manner of meafuring it, which was by holding my cane at arms length and obferving what part'

of

fed.

II.

of it covered the diameter of the area; and thence I computed the angle as near as I could. This obfervation was made 30. Mar. 1652. I remember after this obfervation I looked into what Des Cartes had wrote concerning the causes of corona's. He would have them to be generated by the rays of the fun refracted through flat stars of pellucid ice: which I did not like, because it follows from thence, as he himself confeffes, that the fpace within the crown fhould appear brighter than that without, contrary to what I had obferved.

528. I therefore thought it proper to fearch after fome other caufe of Whence cau- this appearance, though Des Cartes afferts, that no other can be found in the heavens to produce any thing like it. I examined all the refractions and reflections of the fun's rays through globules of water, but found nothing that could cause a circle of that bignefs. I therefore framed to my self other figures confifting of congealed drops, but in vain; so hard it is to form an idea of a thing never seen. But being drawn into the fame thoughts by the appearance of 5 funs at Warfaw in the year 1658; and being diligently attentive to the figure of the corona defcribed above, at laft I hit upon their true caufe; and a little after upon that of the parhelia. For I make no fcruple to call thofe caufes true, whofe effects agree fo exactly with obfervations, as to make it feem unneceffary to fearch for others. Confidering therefore that certain particles flying about among the clouds afforded materials for thefe meteors; as appeared fufficiently from hence, that the corona kept in the fame place though the clouds were dispersed; confidering alfo that the dulnefs of the fky within the corona was an argument that the particles within that circle did not tranfmit the fun's light fo copiously as when they flew out of it; I foon apprehended that the corona might be formed, if each of these particles were round globules confifting of a coat or fhell of transparent ice or water, that contained an opake kernel within it. For I eafily perceived, if a great number of fuch particles were difperfed between us and the fun, that those which lay within a certain distance from the fun, could tranfmit no rays to the eye, whereas others more remote would let them pafs; as will appear presently. Now it is not only probable but matter of fact, that such fort of hail-ftones are difperfed among the clouds; because as Des Cartes attefts, they fometimes fall down upon the ground. For speaking in his Meteors about the formation of such particles, he says, hence it comes to pass, that as the outward fhell of fuch particles ufually confift of a continued pellucid ice, there is often found fome now at their centers, as appears by breaking them. Nor will it appear improbable that particles of fnow fhould ftick in the middle of globules of water, confidering that these globules are fustained by exhalations or vapours raised upwards; by which means the water is hindered from descending and from deferting the fnow in its center. Neither is there any reafon, why the kernels which are of the fame

gravity