A very curious halo, with its accompanying phenomena, was seen by Mr Barker on the 22d of January 1771, a little before two o'clock, at Fort Gloucester, on the river of Lake Superior. -The weather was extremely cold. " There was a very large circle, or halo, round the sun, within which the sky was thick and dusky, the rest of the hemisphere being clear, and a little more than half way from the horizon to the zenith was a beautifully en lightened circle, parallel to the horizon, which went quite round, till the two ends of it terminated in the circle that surrounded the sun, where, at the points of intersection, they each formed a luminous appearance, about the size of the sun, and so like him when seen through a thick hazy sky, that they might very easily have been taken for him. Directly opposite to the sun was a luminous cross, in the shape of a St Andrew's cross, cutting at the point of intersection the horizontal circle, where was formed another mock sun like the other two. The two lower limbs of the cross appeared but faintly a little way below the circle. The two higher reached a good way above the circle towards the zenith, very clear and bright. In this horizontal circle, directly half way be tween the sun and the cross,• and those at the ends of the same circle, were other two mock suns of the same kind and size, one on each side; so that in this horizon tal circle were five mock suns, at equal distances from each other, and in the same line the real sun, all at equal heights front the horizon. Besides these meteors, there was, very near the zenith, but a little more towards the circle of the real sun, a rainbow of very bright and beau tiful colours, not an entire semicircle, with the middle of the convex side turned towards the sun, which lowered as the sun descended. This phenomena continued in all its beauty and lustre till about half after two. The cross went gradually off first, then the horizontal circle began to disappear in parts, while in others it was visible ; then the three mock suns farthest from the sun, the two in the sun's circle continuing longest ; the rainbow began to de crease after these, and last of all the sun's circle ; but it was observable at three o'clock or after it. See Phil. Trans. 1787, vol. lxxvii. p. 44.
On the 18th June 1790, a complicated system of halos and parhelia was observed at St Petersburg by M. Lowitz. They are represented in Fig. 10. The arches A, B, and C were coloured, and, like all the other coloured parts, had the red towards the sun. Two anthelia appeared at D and E.
A curious halo, observed by Mr Hall in Berwickshire, on the 18th of February 1796, about 10 o'clock, is shewn in Fig. 11. The moon was about southwest, and the alti tude of her limb nearly The diameter of the great halo was about 112°; and that of the small halo, having the moon in its centre, was between 8° and 12°. The weather was remarkably mild, and there was little or no wind.
On the 20th of November 1802, at 2 o'clock, Sir Hen ry Englefield observed at Richmond in Su•ry, two un common halos and parhelia. The altitude of the sun was 14°. The circle nearest the sun (Fig. 12.) was about 24° distant from him, and about a degree broad. Its light was a pale yellow, without any of the prismatic colours. The exterior circle was 48° from the sun, and about 1 i° broad. It was tinged with the prismatic colours, the red being nearest the sun. In the left branch of the inner circle, in a line parallel to the horizon, and passing through the sun, was a very faint parhelion ; but in the upper point of the same circle was a very remarkable one. Its light was so vivid that it could scarcely be viewed, and it was rather brighter than the real sun. " It was of a whiter light than the rest of the circle in which it was, and had'a pearly appearance, as partaking a little of pris matic tints. It was large, perhaps in its brightest part near 2° broad, very ill defined every, where, but most dif fused in the part farthest from the sun. From each side of the bright light proceeded a bright ray, which had a double curvature very distinct, being first convex to wards the sun, and then concave. The lower edge of these rays (or that nearest the sun) was tolerably well de fined ; the upper edge melted away into the sky with a sort of streakiness. They grew both narrower and fainter towards thei• termination, and they reached pretty near to the other circle. The whole form of this parhelion and its rays bore so striking a similitude to the body and ex tended wings of a long winged bird, such as an eagle, hovering directly over the sun, that superstition would really have had little to add to the image. See the Journ als of the Royal Institution, vol. ii. or Nicholson's Journal, vol. vi. p. 54. 'A coloured drawing of this phenomenon, will be found in Dr Thomas Young's Natural Philosophy, vol. i. plate xxix. Fig. 431.
Having thus given a description of some of the most interesting halos and parhelia that have been accurately observed, we shall now proceed to give some account of the theories by which these phenomena have been ex plained.
Descartes supposes that halos are generated by the rays of the sun refracted through flat stars of pellucid ice ; but it follows from this supposition, that the space within the halo should appear brighter than that without, which contrary to observation. See Descartes, Illeteorolog. cap. x.
The subject of halos was next investigated by Huygens, who published a large dissertation concerning their cause, which has been translated and reprinted by Dr Smith, in his Treatise on Optics. Huygens assumes the existence of particles of hail, some of which are globular, and others cylindrical, with an opaque portion in the middle of each, bearing a certain proportion to the whole ; and he supposes these cylinders to be kept in a vertical position by a cur rent of ascending air of vapours, and sometimes to have a position inclined to the horizon in all directions when they are dispersed by the wind or otherise. The cylinders tre
supposed to have been at first globules, formed of the softest and finest particles of snow. As soon as a globule is formed by a collection of these particles, many more particles will adhere to the bottom of it, hut not to its sides, on account of the current of ascending vapours. The glo bules will thus have an oblong cylindrical figure ; and when the warmth of the sun or of the air shall have melted the outsides of these cylinders, a smaller cylinder of snow will remain in the middle of each of them, surrounded with water ; and after a certain part is melted, the cylin ders within will become round and perfect, and will re main in this state for some time. If this coat of water should be frozen, Huygens supposes that it may possibly remain sufficiently transparent and polished to transmit, refract, and reflect the rays of the sun in a regular man ner. By the aid of these assumptions, Huygens has in geniously explained, in a very minute manner, almost all the principal phenomena of halos which had been seen at the time when he wrote. It is extremely improbable, however, that such hailstones do exist, and still more im probable that they should have such properties as to pro duce constantly the diameter of Sir Isaac Newton ascribes the halo of 221 degrees by refraction from floating hail or snow, and he accounts for the small coloured corona by his doctrine of fits of easy reflection and transmission. " As light reflected by a lens," says he, " quicksilvered on the backside, makes the rings of colours above described, so it ought to make the like rings of colours in passing through a drop of water. At the first reflection of the rays within the drop, some co lours 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 pass ing through the middle of this globule, has 250 fits of easy transmission within the globule, and that 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 ahout 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 co lours. Suppose now that, in a fair day, the sun shines through a thin cloud of such globules of water or hail, and that the globules are all of the same bigness, and the sun seen through this cloud shall appear encompassed with the like concentric rings of.colours, and the diameter of the first ring of red shall be 7r, that of the second 104°, that of the third 12° 33'. And, according as the globules of water are bigger or less, the rings shall be less or big ger. This is the theory, and experience answers it. For, in June 1692, 1 saw, by reflection, in a vessel of stagnat ing 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 at 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 im mediately, 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 colcur on the other side, was 94.°, or thereabouts. The diameters of the first and third 1 had not time to measure ; but that of the first seemed to be about five or six degrees, and that of the third about 12°. The like crowns appear sometimes about the moon ; for, in the beginning of the year 1664, February 19th, at night, I saw two such crowns about her. The diameter of the first or innermost was about 3°, and that of the second about 52. 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 yellow and red without, and next about these colours were blue and green on the inside of the outward crown, and red on the outside of it. At the same time there appeared a halo, about 22° 35' dis taut from the centre of the moon. It was elliptical, and its long diameter was perpendicular to the horizon verging below far thest from the moon. I am told, that the moon has 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 222° from the moon is of another sort. 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 snow floating in the air in an horizontal posture, the refracting angle be ing about 58° or 60°." See Newton's Optics, Book ii. Part iv. Obs. 13.