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HAʼLO, n. s. A red circle round the sun or them saw his own shadow projected upon it, and
no other. The distance was such that all the If the hail be a little fatted, the light transmitted parts of the shadow were easily distinguishable, may grow so strong, at a little less distance than that as the arms, the leg, and the head; but what of twenty-six degrees, as to form a halo about the sun surprised them most was, that the head was or moon; which halo, as often as the hail-stones are adorned with a kind of glory, consisting of three duly figured, may be coloured.
Newton. or four small concentric crowns, of a very lively I saw by reflection, in a vessel of stagnated water, color, each exhibiting all the varieties of the prithree halos, crowns or rings of colours about the sun, mary rainbow, and having the circle of red on like three little rainbows, concentrick to his body. the outside. The intervals between these circles
continued equal, though the diameters of them Halo, or Corona, in optics, is a luminous all were constantly changing. The last of them circle surrounding the sun, moon, planets, or was very faint; and at a considerable distance fixed stars. Sometimes these circles are white, was another great white circle, which surrounded and sometimes colored like the rainbow. Some- the whole. As near as M. Bouguer could com times one only is visible, and sometimes several pute, the diameter of the first of these circles concentric halos appear at the same time. Those was about 53°, that of the second 11°, that of the which have been seen about Sirius and Jupiter third 17°, and so on; but the diameter of the were never more than 3o, 4o, or 5° in diameter; white circle was about 76o. This phenomenon those which surround the moon are, also, some- never appeared but in a cloud consisting of times no more than 30 or 5°; but these, as well frozen particles, and never in drops of rain like as those which surround the sun, are of very dif- the rainbow. When the sun was not in the hoferent magnitudes, viz. of 12° 0', 22° 35', 30° 0', rizon, only part of the white circle was visible, 38° 0', 41° 2°, 45° 0', 46° 24', 47° 0', and 90°, as M. Bouguer frequently observed afterwards. or even larger than this. Their diameters also Similar to this curious appearance was one seen sometimes vary during the time of observation, by Dr. M'Fait in Scotland; who observed a and the breadths both of the colored and white rainbow round his shadow in the mist, when he circles are very different, viz of 2°, 4°, or 7°. was upon an eminence above it. In this situaTheir colors are more diluted than those of the tion the whole country round seemed buried unrainbow; and they are in a different order, ac- der a vast deluge, and nothing but the tops of cording to their size. Mr. Huygens observed distant hills appeared here and there above the Ted next the sun, and a pale blue outwards. flood. In those upper regions, the air, he says, Sometimes they are red on the inside and white is at that time very pure and agreeable. At on the outside. M. Weidler observed one that another time he observed a double range of cowas yellow on the inside and white on the out- lors round his shadow. The colors of the outerside. In France one was observed, in 1683, the most range were broad and very distinct, and middle of which was white ; after which followed every where about two feet distant from the a border of red, next to it was blue, then green, shadow. Then there was a darkish interval, and the outermost circle was a bright red. In and after that another narrower range of colors, 1728 one was seen of a pale red outwardly, closely surrounding the shadow, which was very then followed yellow, and then green, terminated much contracted. He thinks that these ranges by a white. In Holland, M. Muschenbroeck of colors are caused by the inflection of the savs, fifty may be seen in the day-time, al- rays of light, the same that occasioned the ring most every year; but they are difficult to be oh- of light which surrounds the shadow of all boserved, except the eye be so situated, that not dies, observed by M. Maraldi, and this author. the body of the sun, but only the neighbouring Halos may be produced by placing a lighted parts of the heavens, can be seen. Mr. Middle candle in the midst of steam in cold weather. ton says, that this phenomenon is very frequent If glass windows be breathed upon, and the in North America; for that there is generally flame of a candle be placed some feet from it, one or two about the sun every week, and as while the spectator is also at the distance of some many about the moon every month. Halos feet from another part of a window, the flame round the sun are very frequent in Russia. M. will be surrounded with a colored halo. And if Æpinus says, that from the 23d of April, 1758, to a candle be placed behind a glass receiver, when the 20th of September, he himself had observed air is admitted into the vacuum within it, at a cerno fewer than twenty-six, and that he has sometimes tain degree of density, the vapor with which it is seen twice as many in the same space of time. loaded will make a colored halo round the flame.
Similar, in some respects, to the halo, was the This was observed by Otto Guericke. In Detemarkable appearance which M. Bouguer de- cember 1756 M. Muschenbroeck observed, that, scribes, as observed on the top of Mount Pichinca, when the glass windows of his room were coverin the Cordilleras. When the sun was just ris- ed with a thin plate of ice on the inside, the ing behind them, so as to appear white, each of moon appearing through it was surrounded with Vol. XI.- Part I.