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LONDON ENCYCLOPÆDIA.

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HALO, or Corona, in optics, is a luminous circle surrounding the sun, moon, planets, or fixed stars. Sometimes these circles are white, and sometimes colored like the rainbow. Sometimes one only is visible, and sometimes several concentric halos appear at the same time. Those which have been seen about Sirius and Jupiter were never more than 3°, 4°, or 5° in diameter; those which surround the moon are, also, sometimes no more than 3° or 5°; but these, as well as those which surround the sun, are of very different magnitudes, viz. of 12° 0', 22° 35′, 30° 0′, 38° 0′, 41° 2′, 45° 0′, 46° 24′, 47° 0′, and 90°, or even larger than this. Their diameters also sometimes vary during the time of observation, and the breadths both of the colored and white circles are very different, viz of 2°, 4°, or 7°. Their colors are more diluted than those of the rainbow; and they are in a different order, according to their size. Mr. Huygens observed red next the sun, and a pale blue outwards. Sometimes they are red on the inside and white on the outside. M. Weidler observed one that was yellow on the inside and white on the outside. In France one was observed, in 1683, the middle of which was white; after which followed a border of red, next to it was blue, then green, and the outermost circle was a bright red. In 1728 one was seen of a pale red outwardly, then followed yellow, and then green, terminated by a white. In Holland, M. Muschenbroeck says, fifty may be seen in the day-time, almost every year; but they are difficult to be observed, except the eye be so situated, that not the body of the sun, but only the neighbouring parts of the heavens, can be seen. Mr. Middleton says, that this phenomenon is very frequent in North America; for that there is generally one or two about the sun every week, and as many about the moon every month. Halos round the sun are very frequent in Russia.

them saw his own shadow projected upon it, and no other. The distance was such that all the parts of the shadow were easily distinguishable, as the arms, the leg, and the head; but what surprised them most was, that the head was adorned with a kind of glory, consisting of three or four small concentric crowns, of a very lively color, each exhibiting all the varieties of the primary rainbow, and having the circle of red on the outside. The intervals between these circles continued equal, though the diameters of them all were constantly changing. The last of them was very faint; and at a considerable distance was another great white circle, which surrounded the whole. As near as M. Bouguer could com pute, the diameter of the first of these circles was about 53°, that of the second 11°, that of the third 17°, and so on; but the diameter of the white circle was about 76°. This phenomenon never appeared but in a cloud consisting of frozen particles, and never in drops of rain like the rainbow. When the sun was not in the horizon, only part of the white circle was visible, as M. Bouguer frequently observed afterwards. Similar to this curious appearance was one seen by Dr. M'Fait in Scotland; who observed a rainbow round his shadow in the mist, when he was upon an eminence above it. In this situation the whole country round seemed buried under a vast deluge, and nothing but the tops of distant hills appeared here and there above the flood. In those upper regions, the air, he says, is at that time very pure and agreeable. At another time he observed a double range of colors round his shadow. The colors of the outermost range were broad and very distinct, and every where about two feet distant from the shadow. Then there was a darkish interval, and after that another narrower range of colors, closely surrounding the shadow, which was very much contracted. He thinks that these ranges of colors are caused by the inflection of the rays of light, the same that occasioned the ring of light which surrounds the shadow of all bodies, observed by M. Maraldi, and this author.

Halos may be produced by placing a lighted candle in the midst of steam in cold weather. If glass windows be breathed upon, and the flame of a candle be placed some feet from it, while the spectator is also at the distance of some feet from another part of a window, the flame M. will be surrounded with a colored halo. And if a candle be placed behind a glass receiver, when air is admitted into the vacuum within it, at a certain degree of density, the vapor with which it is 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 Deremarkable 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

pinus says, that from the 23d of April, 1758, to the 20th of September, he himself had observed no fewer than twenty-six, and that he has sometimes seen twice as many in the same space of time.

VOL. XI.-PART I,

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THE

LONDON ENCYCLOPÆDIA.

[blocks in formation]

HALO, or Corona, in optics, is a luminous circle surrounding the sun, moon, planets, or fixed stars. Sometimes these circles are white, and sometimes colored like the rainbow. Sometimes one only is visible, and sometimes several concentric halos appear at the same time. Those which have been seen about Sirius and Jupiter were never more than 3°, 4°, or 5° in diameter; those which surround the moon are, also, sometimes no more than 3° or 5°; but these, as well as those which surround the sun, are of very dif ferent magnitudes, viz. of 12° 0′, 22° 35′, 30° 0′, 38° 0′, 41° 2′, 45° 0′, 46° 24′, 47° 0′, and 90°, or even larger than this. Their diameters also sometimes vary during the time of observation, and the breadths both of the colored and white circles are very different, viz of 2°, 4°, or 7°. Their colors are more diluted than those of the rainbow; and they are in a different order, according to their size. Mr. Huygens observed red next the sun, and a pale blue outwards. Sometimes they are red on the inside and white on the outside. M. Weidler observed one that was yellow on the inside and white on the outside. In France one was observed, in 1683, the middle of which was white; after which followed a border of red, next to it was blue, then green, and the outermost circle was a bright red. In 1728 one was seen of a pale red outwardly, then followed yellow, and then green, terminated by a white. In Holland, M. Muschenbroeck says, fifty may be seen in the day-time, almost every year; but they are difficult to be observed, except the eye be so situated, that not the body of the sun, but only the neighbouring parts of the heavens, can be seen. Mr. Middleton says, that this phenomenon is very frequent in North America; for that there is generally one or two about the sun every week, and as many about the moon every month. Halos round the sun are very frequent in Russia.

them saw his own shadow projected upon it, and no other. The distance was such that all the parts of the shadow were easily distinguishable, as the arms, the leg, and the head; but what surprised them most was, that the head was adorned with a kind of glory, consisting of three or four small concentric crowns, of a very lively color, each exhibiting all the varieties of the primary rainbow, and having the circle of red on the outside. The intervals between these circles continued equal, though the diameters of them all were constantly changing. The last of them was very faint; and at a considerable distance was another great white circle, which surrounded the whole. As near as M. Bouguer could com pute, the diameter of the first of these circles was about 53°, that of the second 11°, that of the third 17°, and so on; but the diameter of the white circle was about 76°. This phenomenon never appeared but in a cloud consisting of frozen particles, and never in drops of rain like the rainbow. When the sun was not in the horizon, only part of the white circle was visible, as M. Bouguer frequently observed afterwards. Similar to this curious appearance was one seen by Dr. M'Fait in Scotland; who observed a rainbow round his shadow in the mist, when he was upon an eminence above it. In this situation the whole country round seemed buried under a vast deluge, and nothing but the tops of distant hills appeared here and there above the flood. In those upper regions, the air, he says, is at that time very pure and agreeable. At another time he observed a double range of colors round his shadow. The colors of the outermost range were broad and very distinct, and every where about two feet distant from the shadow. Then there was a darkish interval, and after that another narrower range of colors, closely surrounding the shadow, which was very much contracted. He thinks that these ranges of colors are caused by the inflection of the rays of light, the same that occasioned the ring of light which surrounds the shadow of all bodies, observed by M. Maraldi, and this author.

Halos may be produced by placing a lighted candle in the midst of steam in cold weather. If glass windows be breathed upon, and the flame of a candle be placed some feet from it, while the spectator is also at the distance of some feet from another part of a window, the flame M. will be surrounded with a colored halo. And if pinus 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. This was observed by Otto Guericke. In December 1756 M. Muschenbroeck observed, that, when the glass windows of his room were cover

Similar, in some respects, to the halo, was the remarkable appearance which M. Bouguer describes, as observed on the top of Mount Pichinca, in 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.

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