f 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.

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 observations on halos and parhelia: "The light which comes through drops of rain by two refractions, without any reflection, ought to appear the strongest at the distance of about twenty 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 flatted, as it often is, the transmitted light may be so strong, at a little less distance than that of twenty-six 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 further thoughts of Sir Isaac Newton 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 quicksilvered 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 reflection 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 further 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 71°, that of the second 101°, that of the third 12° 33', 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 the year 1692. He saw by reflection, 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 of the first, or innermost crown, were blue next the sun, red without, and white in the middle, between the blue and red. Those of the second crown were purple and blue within, and pale red without, and green in the middle; and those of the third were pale blue within, and pale red without. These crowns inclosed one another immediately, so that their colours proceeded in this continual order from the sun outward; blue, white, red; purple, blue, green, pale yellow, and red; pale blue, pale red. The diameter of the second crown, measured from the middle of the yellow and red on one side of the sun, to the middle of the same colour on the other side, was 91 degrees or thereabouts. The diameters of the first and third he had not time to measure, but that of the first seemed to be about five or six degrees, and that of the second about twelve. The like crowns appear sometimes about the moon; for in the beginning of the year 1664, on February 19th, at night, he saw two such crowns about her. The diameter of the first or innermost was about three degrees, and that of the second about five degrees and a half. Next about the moon was a circle of white; and next about that the inner crown, which was of a bluish green within, next the white, and of a yellowish and red without; and next about these colours were blue and green on the inside of the outer crown, and red on the outside of it.

At the same time there appeared a halo at the distance of about 22° 35' from the centre of the moon. It was elliptical, and its long diameter was perpendicular to the horizon, verging below farthest from the moon. He was told that the moon had sometimes three or more concentric crowns of colours encompassing one another next about her body. The more equal the globules of water or ice are to one another, the more crowns of colours will appear, and the colours will be the more lively. The halo, at the distance of 22° 41' degrees from the moon, is of another sort. By its being oval and more remote from the moon below than above, he concludes that it was made by refraction in some kind of hail or snow floating in the air in a horizontal posture, the refracting angle being about 50 or 60 degrees. Dr Smith, however, makes it sufficiently evident, that the reason why this halo appeared oval and more remote from the moon towards the horizon, is a deception of sight, and the same with that which makes the moon appear larger in the horizon.

Corona, among botanists, the name given by some to the circumference or margin of a radiated compound flower. It corresponds to the radius of Linnaeus, 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, the stars of which in Ptolemy's catalogue are thirteen, and in the British catalogue twelve.