OPTICAL PHENOMENA.
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BY THOMAS MILNER, M. A.
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Parhelia.
Parhelia. Mock suns, in the vicinity of the real orb, are due to the same cause as haloes, which appear in connection with them. Luminous circles, or segments, crossing one another, produce conspicuous masses of light by their united intensities, and the points of intersection appear studded with the solar image. This is a meteorological rarity in our latitude, but a very frequent spectacle in the arctic climes. In Iceland, during the severe winter of 1615, it is related that the sun, when seen, was always accompanied by two, four, five, and even nine of these illusions. Captain Parry describes a remarkably gorgeous appearance, during his winter sojourn at Melville Island, which continued from noon until six o’clock in the evening. It consisted of one complete halo, 45° in diameter, with segments of several others, displaying in parts the colors of the rainbow. Besides these, there was another perfect ring of a pale white color, which went right round the sky, parallel with the horizon, and at a distance from it equal to the sun’s altitude; and a horizontal band of white light appeared passing through the sun. Where the band and the inner halo cut each other, there were two parhelia, and another close to the horizon, directly under the sun, which formed the most brilliant part of the spectacle, being exactly like the sun, slightly obscured by a thin cloud at his rising or setting. A drawing of this parhelion is given by Captain Parry, who remarks upon having always observed such phenomena attended with a little snow falling, or rather small spicula or fine crystals of ice. The angular forms of the crystals determine the rays of light in different directions, and originate the consequent visual variety. We have various observations of parhelia seen in different parts of Europe, which in a less enlightened age excited consternation, and were regarded as portentous. Matthew Paris relates in his history:—“A wonderful sight was seen in England, A. D. 1233, April 8, in the fifth year of the reign of Henry III., and lasted from sunrise till noon. At the same time on the 8th of April, about one o’clock, in the borders of Herefordshire and Worcestershire, besides the true sun, there appeared in the sky four mock suns of a red color; also a certain large circle of the color of crystal, about two feet broad, which encompassed all England as it were. There went out semicircles from the side of it, at whose intersection the four mock suns were situated, the true sun being in the east, and the air very clear. And because this monstrous prodigy cannot be described by words, I have represented it by a scheme, which shows immediately how the heavens were circled. The appearance was painted in this manner by many people, for the wonderful novelty of it.”
Paraselenæ. Mock moons, depending upon the causes which produce the solar image, or several examples of it, as frequently adorn the arctic sky. On the 1st of December, 1819, in the evening, while Parry’s expedition was in Winter Harbor, four paraselenæ were observed, each at the distance of 21½° from the true moon. One was close to the horizon, the other perpendicular above it, and the other two in a line parallel to the horizon. Their shape was like that of a comet, the tail being from the moon, the side of each toward the real orb being of a light orange color. During the existence of these paraselenæ, a halo appeared in a concentric circle round the moon, passing through each image. On the evening of March 30, 1820, about ten o’clock, the attention of Dr. Trail, at Liverpool, was directed by a friend to an unusual appearance in the sky, which proved to be a beautiful display of paraselenæ. The moon was then 35° above the southern horizon. The atmosphere was nearly calm, but rather cloudy, and obscured by a slight haze. A wide halo, faintly exhibiting the prismatic colors, was described round the moon as a centre, and had a small portion of its circumference cut off by the horizon. The circular band was intersected by two small segments of a larger circle, which if completed would have passed through the moon, and parallel to the horizon. These segments were of a paler color than the first mentioned circle. At the points of intersection appeared two pretty well defined luminous discs, equaling the moon in size, but less brilliant. The western paraselenæ had a tail or coma, which was directed from the moon, and the eastern also, but less clearly-defined.
The Rainbow. The most glorious vision depending upon the decomposition, refraction, and reflection of light, by the vapor of the atmosphere reduced to fluid drops, is the well-known arch projected during a shower of rain upon a cloud opposite to the sun, displaying all the tints of the solar spectrum. The first marked approximation to the true theory of the rainbow occurs in a volume entitled De Radiis Visus et Lucis, written by Antonius de Dominis, Archbishop of Spalatro, published in the year 1611 at Venice. Descartes pursued the subject, and correctly explained some of the phenomena; but upon Newton’s discovery of the different degrees of refrangibility in the different colored rays which compose the sunbeam, a pencil of white or compounded light, the cause of the colored bands in the rainbow, of the order of their position, and of the breadth they occupy, was at once apparent. The bow is common to all countries, and is the sign of the covenant of promise to all people, that there shall no more be such a wide-spread deluge as that which the sacred narrative records.
“But say, what mean those colored streaks in heaven