among anatomists, denotes that edge of the glans penis where the preputium begins.

**Corona**, or Halo, in optics, a luminous circle, surrounding the sun, the moon, the planets, or fixed stars. Sometimes these circles are white, and sometimes coloured, like the rainbow. Sometimes one only is visible, and sometimes several concentric coronas make their appearance at the same time. Those which have been seen about Sirius and Jupiter were never more than three, four, or five degrees in diameter; those which surround the moon are, also, sometimes no more than three or five degrees; but these, as well as those which surround the sun, are of very different magnitudes, viz., of $12^\circ 0'$, $22^\circ 35'$, $30^\circ 0'$, $38^\circ 0'$, $41^\circ 2'$, $45^\circ 0'$, $46^\circ 24'$, $47^\circ 0'$, and $90^\circ$, or even larger than this. Their diameters also sometimes vary during the time of observation, and the breadths both of the coloured and white circles are very different, viz., of 2, 4, or 7 degrees.

The colours of these coronas are more dilute than those of the rainbow; and they are in a different order, according to their size. In those which Newton observed in 1692, they were in the following order, reckoning from the inside. In the innermost were blue, white, and red; in the middle were purple, blue, green, yellow, and pale red; in the outermost, pale blue and pale red. 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. Wiedler 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 1 cm of a pale red outwardly, then followed yellow, and then green, terminated by a white.

These coronas are very frequent. In Holland, M. Muschenbroeck says, 50 may be seen in the daytime, 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. M. Leopinus says, that from the 23rd of April 1758, to the 20th of September, he himself had observed no less than 26, and that he has sometimes seen twice as many in the same space of time.

Coronas may be produced by placing a lighted candle in the midst of steam in cold weather. Also, 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 will be surrounded with a coloured 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 vapour with which it is loaded will make a coloured halo round the flame. This was observed by Otto Gneick. In December 1756, M. Muschenbroeck observed, that when the glass windows of his room were covered with a thin plate of ice on the inside, the moon appearing through it was surrounded with a large and variously coloured halo; and, opening the window, he found that it arose entirely from that thin plate of ice, for none was seen except through it.

Similar, in some respects, to the halo, was the remarkable markable appearance which M. Bouguer describes, as observed by himself and his companions on the top of Mount Pichinea, in the Cordilleras. When the sun was just rising behind them, so as to appear white, each of 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 surprized 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 colour, 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 compute, the diameter of the first of these circles was about 5½ degrees, that of the second 11, that of the third 17, and so on; but the diameter of the white circle was about 76 degrees. 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 also to this curious appearance was one that was observed by Dr MacFait in Scotland. This gentleman 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, as it were, buried under a vast deluge, and nothing but the tops of distant hills appeared here and there above the flood; so that a man would think of diving down into it with a kind of horror. In those upper regions the air, he says, is at that time very pure and agreeable to breathe in. At another time he observed a double range of colours round his shadow in these circumstances. The colours of the outermost range were broad and very distinct, and everywhere about two feet distant from the shadow. Then there was a darkish interval, and after that another narrower range of colours, closely surrounding the shadow, which was very much contracted. This person seems to think that these ranges of colours are caused by the inflection of the rays of light, the same that occasioned the ring of light which surrounds the shadows of all bodies, observed by M. Maraldi, and this author*. But the prodigious variety with which these appearances are exhibited seems to show that many of them do not result from the general laws of reflexion, refraction, or inflection, belonging to transparent substances of a large mass; but upon the alternate reflexion and transmission of the different kinds of rays, peculiar to substances reduced to the form of thin plates, or consisting of separate and very minute parts. But where the dimensions of the coronas are pretty constant, as in the usual and larger halo, which is about half the diameter of the rainbow, they may, perhaps, be explained on the general principles of refraction only.

Descartes observes, that the halo never appears when it rains: from which he concludes that this phenomenon is occasioned by the refraction of light in the round particles of ice, which are then floating in the atmosphere; and though these particles are flat when they fall to the ground, he thought they must be protuberant in the middle, before their descent; and according to this protuberancy he imagined that the diameter of the halo would vary.—In treating of meteors, Gassendi supposed that a halo is the same thing with the rainbow, the rays of light being in both cases twice refracted and once reflected within each drop of rain or vapour, and that all the difference there is between them arises from their different situation with respect to the observer. For, whereas, when the sun is behind the spectator, and consequently the rainbow before him, his eye is in the centre of the circle; when he views the halo, with his face towards the sun, his eye is in the circumference of the circle; so that according to the known principles of geometry, the angle under which the object appears in this case, must be just half of what it is in the other. Though this writer says a great deal upon the subject, and endeavours to give reasons why the colours of the halo are in a different order to those of the rainbow, he does not describe the progress of the rays of light from the sun to the eye of the spectator when a halo is formed by them, and he gives no figures to explain his ideas.

Dechales, also, endeavours to show that the generation of the halo is similar to that of the rainbow. If, says he, a sphere of glass or crystal, AB, (no. i.) full of water, be placed in the beams of the sun shining from C, there will not only be two circles of coloured light on the side next the sun, and which constitute the two rainbows; but there will also be another on the part opposite to the sun, the rays belonging to which meeting at E, afterwards diverge, and form the coloured circle G, as will be visible, if the light that is transmitted through the globe be received on a piece of white paper. The colours also will appear to an eye placed in any part of the surface of the cone FEG. Measuring the angle FEH, he found it to be 23 degrees. They were only the extreme rays of this cone that were coloured like those of the rainbow.

This experiment he thought sufficiently illustrated the generation of the halo; so that whenever the texture of the clouds is such, as not entirely to intercept the rays of the sun or moon, and yet have some degree of density, there will always be an halo round them, the colours of the rainbow appearing in those drops which are 23 degrees distant from the sun or moon. If the sun be at A, and the spectator in B, the halo will be the circle DFE, DBE being 46 degrees, or twice 23.

The reason why the colours of the halo are more dilute than those of the rainbow, he says, is owing principally to their being formed not in large drops of rain, but in very small vapours; for if the drops of water were large, the cloud would be too thick, that the rays of the sun could not be regularly transmitted through them; and, on the other hand, he had observed, that when the rainbow is formed by very thin vapours, the colours hardly appear. As for those circles of colours which are sometimes seen round candles, it was his opinion that they are owing to nothing but moisture on the eye of the observer; for that he could never produce this appearance by means of vapour only, if he wiped his eyes carefully; and he had observed that such circles are visible to some persons. and not to others, and to the same persons at one time and not another.

The most considerable of all the theories respecting halos, and that which has met with most favourable and the longest reception, is that of Mr Huygens. Sir Isaac Newton mentions it with respect, and Dr Smith, in his Complete System of Optics, does not so much as hint at any other. The occasion of M. Huygens publishing his thoughts on this subject, was the appearance of a halo at Paris, on the 12th of May 1667, of which he gave an account in a paper read at the Royal Academy in that city, which was afterwards translated, and published in the English Philosophical Transactions, and which may be seen in Lowthorp's Abridgment, Vol. II. p. 189. But this article contains nothing more than the heads of a discourse, which he afterwards composed, but never quite finished, on this subject; and which has been translated, with some additions, by Dr Smith, from whom the following account is chiefly extracted.

Our philosopher had been first engaged to think particularly upon this subject, by the appearance of five suns at Warlsway, in 1658; presently after which, he says, he hit upon the true cause of halos, and not long after of that of mock suns also.

To prepare the way for the following observations, it must be remarked, that if we can conceive any kind of bodies in the atmosphere, which, according to the known laws of optics, will, either by means of reflection or refraction, produce the appearance in question, when nothing else can be found that will do it, we must acquiesce in the hypothesis, and suppose such bodies to exist, even though we cannot give a satisfactory account of their generation. Now, two such bodies are assumed by Mr Huygens; one of them a round ball, opaque in the centre, but covered with a transparent shell; and the other is a cylinder, of a similar composition. By the help of the former he endeavours to account for halos, and by the latter for those appearances which are called mock suns. Those bodies which Mr Huygens requires, in order to explain these phenomena, are not, however, a mere assumption; for some such, though of a larger size than his purpose requires, have been actually found, consisting of snow within and ice without. They are particularly mentioned by Descartes.

The balls with the opaque kernel, which he supposed to have been the cause of them, he imagines not to exceed the size of a turnip-root; but, in order to illustrate this hypothesis, he gives a figure of one, of a larger size, in ABCDEF, (no. 3.) representing the kernel of snow in the middle of it. If the rays of light, coming from GH, fall upon the side AD, it is manifest they will be so refracted at A and D, as to bend inwards; and many of them will strike upon the kernel EF. Others, however, as GA and HD, will only touch the sides of the kernel; and being again refracted at B and C, will emerge in the lines BK, CK, crossing each other in the point K, whose nearest distance from the globule is somewhat less than its apparent diameter. If, therefore, BK and CK be produced towards M and L, (no. 4.) it is evident that no light can reach the eye placed within the angle LKM, but may fall upon it when placed out of that angle, or rather the cone represented by it.

For the same reason, every other of these globules will have a shadow behind it, in which the light of the sun will not be perceived. If the eye be at N, and that be conceived to be the vertex of a cone, the sides of which NR, NQ, are parallel to the sides of the former cone KL, KM, it is evident that none of the globules within the cone QNR can send any rays of the sun to the eye at N. But any other globule out of this cone, as X, may send those rays, which are more refracted than XZ, to the eye; so that this will appear enlightened, while those within the cone will appear obscure. It is evident from this, that a certain area, or space, quite round the sun, must appear dark; and that the space next to this area will appear luminous, and more so in those parts that are nearest to the obscure area; because, he says, it may easily be demonstrated, that those globules which are nearest to the cone QNR exhibit the largest image of the sun. It is plain, also, that a corona ought to be produced in the same manner whatever be the sun's altitude, because of the spherical figure of the globules.

To verify this hypothesis, our philosopher advises us to expose to the sun a thin glass bubble, filled with water, and having some opaque substance in the centre of it; and he says we shall find, that we shall not be able to see the sun through it, unless at a certain distance from a place opposite to the centre of it; but as soon as we do perceive the light, the image of the sun will immediately appear the brightest, and coloured red, for the same reason as in the rainbow.

These coronas, he says, often appear about the moon; but the colours are so weak as to appear only white. Such white coronas he had also seen about the sun, when the space within them appeared farce darker than that without. This he supposes to happen when there are but few of those globules in the atmosphere; for the more plentiful 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 degrees, depends upon the size of the dark kernel; for the larger it is with respect to the whole globule, the larger will be the dark cone behind it.

The globules that form these halos, Mr Huygens supposes to have consisted of soft snow, and to have been rounded by continual agitation in the air, and thawed on their outsides by the heat of the sun.

To make the diameter of the halo 45 degrees, he demonstrates that the semidiameter of the globule must be to the semidiameter of the kernel of snow very nearly as 1000 to 480; and that to make a corona of 100 degrees, it must be as 1000 to 680.

Mr Weidler, in his Commentary on parhelia, published at Würtemberg in 1733, observes that it is very improbable that such globules as Mr Huygens's hypothesis requires, with nuclei of such a precise proportion, should exist; and if there were such bodies, he thinks they would be too small to produce the effects ascribed to them. Besides, he observes that appearances exactly similar to halos are not uncommon, where fluid vapour alone are concerned; as when a candle is placed behind the steam of boiling water in frosty weather, or in the midst of the vapour rising. copiously from a bath, or behind a receiver when the air is so much rarefied as to be incapable of supporting the water it contains. The rays of the sun twice reflected and twice refracted within small drops of water are sufficient, he says, without any opaque kernel, to produce all the appearances of the halos that have the red light towards the sun, as may be proved by experiment. That the diameter of the halos is generally half of that of the rainbow, he accounts for as Gassendi did before him.

M. Mariotte accounts for the formation of the small coronas by the transmission of light through aqueous vapours, where it suffers two refractions, without any intermediate reflection. He shows that light which comes to the eye, after being refracted in this manner, will be chiefly that which falls upon the drop nearly perpendicular; because more rays fall upon any given quantity of surface in that situation, fewer of them are reflected with small degrees of obliquity, and they are not so much scattered after refraction. The red will always be outermost in these coronas, as consisting of rays which suffer the least refraction. And whereas he had seen, when the clouds were driven briskly by the wind, halos round the moon, varying frequently in their diameter, being sometimes of two, sometimes of three, and sometimes of four degrees; sometimes also being coloured, sometimes only white, and sometimes disappearing entirely; he concluded that all these variations arose from the different thicknesses of the clouds, through which sometimes more and sometimes less light was transmitted. He supposed, also, that the light which formed them might sometimes be reflected, and at other times refracted. As to those coronas which consist of two orders of colours, he imagined that they were produced by small pieces of snow, which when they begin to dissolve, form figures which are a little convex towards their extremities. Sometimes, also, the snow will be melted in different shapes; and in this case, the colours of several halos will be intermixed and confused; and such, he says, he had sometimes observed round the sun.

M. Mariotte then proceeds to explain the larger coronas, namely those that are about 45 degrees in diameter, and for this purpose he has recourse to equiangular prisms of ice, in a certain position with respect to the sun; and he takes pains to trace the progress of the rays of light for this purpose; but this hypothesis is very improbable. In some cases he thought that these large coronas were caused by hailstones, of a pyramidal figure; because after two or three of them had been seen about the sun, there fell the same day several such pyramidal hailstones. M. Mariotte explains parhelia by the help of the same suppositions. See PARHELIA.

Sir Isaac Newton does not appear to have given any particular attention to the subject of halos, but he has hinted at his sentiments concerning them occasionally; by which we perceive that he considered the larger and less variable appearances of this kind as produced according to the common laws of refraction, but that the less and more variable appearances depend upon the same cause with the colours of thin plates.

He concludes his explication of the rainbow with the following observation on halos and parhelia. "The light which come through drops of rain by two refractions, without any reflexion, ought to appear the strongest at the distance of about 26 degrees from the sun, and to decay gradually both ways as the distance from him increases. And the same is to be understood of light transmitted through spherical hailstones; and if the hail be a little flattened, as it often is, the transmitted light may be so strong, at a little less distance than that of 26 degrees, as to form a halo about the sun or moon; which halo, as often as the hailstones are duly figured, may be coloured, and then it must be red within by the least refrangible rays, and blue without by the most refrangible ones; especially if the hailstones have opaque globules of snow in their centres to intercept the light within the halo, as Mr Huygens has observed, and make the inside of it more distinctly defined than it would otherwise be. For such hailstones, though spherical, by terminating the light by the snow, may make a halo red within, and colourless without, and darker within the red than without, as halos used to be. For of those rays which pass close by the snow, the red-making ones will be the least refracted, and so come to the eye in the straightest lines."

Some farther thoughts of Sir Isaac Newton's on the subject of halos we find subjoined to the account of his experiments on the colours of thick plates of glass, which he conceived to be similar to those which are exhibited by thin ones. "As light reflected by a lens quick-silvered on the back side makes the rings of the colours above described, so (he says) it ought to make the like rings in passing through a drop of water. At the first reflexion of the rays within the drop, some colours ought to be transmitted, as in the case of a lens, and others to be reflected back to the eye. For instance, if the diameter of a small drop or globule of water be about the 500th part of an inch, so that a red-making ray, in passing through the middle of this globule, has 250 fits of easy transmission within the globule, and all the red-making rays which are at a certain distance from this middle ray round about it have 249 fits within the globule, and all the like rays at a certain farther distance round about it have 248 fits, and all those at a certain farther distance 247 fits and so on, these concentric circles of rays, after their transmission, falling on a white paper, will make concentric rings of red upon the paper; supposing the light which passes through one single globule strong enough to be sensible, and in like manner the rays of other colours will make rings of other colours. Suppose now that in a fair day the sun should shine through a thin cloud of such globules of water or hail, and that the globules are all of the same size, the sun seen through this cloud ought to appear surrounded with the like concentric rings of colours, and the diameter of the first ring of red should be \( \frac{1}{2} \) degrees, that of the second \( \frac{1}{3} \), that of the third \( \frac{1}{4} \), and according as the globules of water are bigger or less, the ring should be less or bigger."

This curious theory our author informs us was confirmed by an observation which he made in 1692. He saw by reflexion, in a vessel of stagnating water, three halos, crowns, or rings of colours about the sun, like three little rainbows concentric to his body. The colours Corona, among botanists, the name given by some to the circumference or margin of a radiated compound flower. It corresponds to the radius of Linnæus; and is exemplified in the flat, tongue-shaped petals which occupy the margin of the daisy or sunflower.

Corona Australis, or Meridionalis, Southern Crown, a constellation of the southern hemisphere, whose stars in Ptolemy's catalogue are 13, in the British catalogue 12.