The halo A B exhibited at times bright prismatic tints, and was attended by an ellipse or oval, as seen in the figure. The other circles were white, and fainter according as they were situated farther from the sun. Fig. 39. B 488. On the 30th of March 1660, at Dantzic, Hevelius beheld, about one o'clock in the morning, the halo shown in figure 39. When first perceived the moon at M was surrounded by a complete whitish circle, A B C, 45° in diameter, while at A and C were two mockmoons displaying various colors, and shooting out at intervals very long and whitish streams of light. At two o'clock the larger circle, DE F, was seen, reaching down to the horizon, having a diameter of 90°. D F M LUNAR HALO. The tops of both circles were touched by colored arches, like inverted rainbows, the red tint being next to the moon. The arch at B was a part of a circle equal in size to DE F, while that at E was a portion of a circle of the same magnitude as A B C. 489. Such is the general structure of haloes, and the identity that exists in the magnitude and arrangement of the several parts clearly shows, that they must originate in certain fixed laws; but what those laws are has not yet been fully determined. 490. ORDINARY HALO OF 45°. The most satisfactory explanation of this halo is that given by Mariotte and Dr. Young, who suppose it to be caused by the refrac tion of the sun's rays, as they pass through crystals of frozen vapor, floating in the upper regions of the atmosphere. What does the general structure of haloes indicate? 491. For the sake of illustration we will suppose that A, figure 40., is a crystal of ice or snow, having its refracting angle equal to 60°, which is the usual angle of such crystals, and that SE and S P are parallel sunbeams, and E the eye of the observer. The ray, S P, passes through the crystal as through a prism, and is decomposed into its original colors, the greatest amount of prismatic light reaching the eye when the angle of deviation, SER, is about twentytwo degrees and a half. Fig. 40. REFRACTION THROUGH ICE-CRYSTALS. 492. Now, it is well known, that in cold weather the air near the earth is often filled with fine needle-shaped crystals of ice, and that in the higher regions of the atmosphere, above the limit of perpetual congelation, crystalized vapor exists in summer as well as in winter. (Art. 237.) 493. If we then suppose a stratum of ice-crystals floating in the air so thin that the sun is distinctly seen through it, though veiled as by a slight mist; an observer will behold this luminary encircled by rings of colored light, proceeding from those crystals whose angular distance from the sun is about twenty-two degrees and a half. The diameter of this circle or halo, will of course be nearly 45°, and the red tint will be next to the sun since it suffers less refraction than the blue. 494. It might be objected, that the crystals of snow, when floating in the air, would not naturally assume such positions as to refract the light properly to the eye; but it can be proved by rigorous calculations, that if the vast number of crystals which compose the stra tum, take every possible position, one-half of the sun How is the objection answered, that the crystals of ice would not naturally assume such a position as to refract the light to the eye? light will pass through them; and that one-third of the transmitted rays will reach the eye within a range of one and a half degrees, viz., when the angle of deviation SER varies from 21° 50' to 23° 22'. Such is the theory in regard to the origin of the ordinary halo, and the probability of its truth is strengthened by the fact, that fine crystals of ice are known to produce curves and circles of prismatic light. 495. On the 23d of March, 1845, Prof. Snell, of Amherst College, beheld a most beautiful phenomenon. As he stood facing the sun, which had just arisen, he observed upon the dead grass before him a curved, horizontal band of light, three or four feet broad, glowing with all the colors of the rainbow. The top of the curve was twelve or fifteen feet distant, while the two branches extended several rods to the right and left. The long spires of dead grass were fringed with frostcrystals, and the cause of this brilliant arch was justly attributed to the refraction of the sun's rays as they traversed these minute prisms. 496. If a distant light, as a street-lamp, is viewed through a pane of glass upon which the vapor of a room has crystalized, two or more fine haloes will be distinctly seen surrounding the luminous object. The same appearances are presented to the eye if we substitute a plate of glass upon which a few drops of a saturated solution of alum have rapidly crystalized. 497. EXTRAORDINARY HALO OF 90°. The halo of ninety degrees is also supposed to be owing to the refraction of light through crystals of ice or snow; the crystals being six-sided prisms. (Art. 283.) 498. CIRCLES PASSING THROUGH THE SUN. These are often highly colored, and when the sun is near the horizon, a portion of a vertical circle sometimes presents the appearance of an upright, luminous column. What facts are stated to show that minute ice-crystals can produce haloes? How is the halo of ninety degrees caused? What is said respecting the circles passing through the sun? Many years ago, on a very cold morning, there were seen at West Point, above the sun, vertical columns of light of exceeding splendor, tinted with all the prismatic colors, and surpassing in brilliancy the hues of the rainbow. A similar phenomenon was observed at the same place, by Prof. Twining, on the 5th of Jan. 1835; but the prismatic tints were wanting. On the 2d of January, 1586, an extraordinary display of this kind was seen by Roth, at Cassel. Before the sun rose, an upright column of light illumined the sky, at the point where the sun was about to appear. Its breadth was equal to that of the sun, and it glowed like a vivid flame. An image of the sun next appeared so brilliant as to be taken for the orb itself. This was immediately followed by the true sun, which was directly succeeded by a second image. The luminous column with its three suns was visible for the space of an hour; the three suns were exactly similar, only the true one was the brightest. 499. A similar phenomenon was observed on the 21st of February, 1847, by Lieut. Abert and his party, during their exploration of New Mexico, and is thus described by Abert in the official report of the expedition: "The snow had heaped up around the rest of the tents so that the inmates were obliged to desert them, and take refuge in the wagons. About mine, the wind had swept in such a way as to keep open a path around it, although the snow was on a level with the ridge-pole of the tent. We now broke up some boards that were in the wagons, and kindled a little fire. Soon the sun rose; but, instead of one sun, we had three; all seemed of equal brilliancy; but, as they continued to rise, the middle one only retained its circular form, while the others shot into huge columns of fire, which blended with the air near their summits. The breadth of the columns was that of the sun's apparent diameter, and their height about twelve times the same diameter; they were between twenty and thirty degrees distant from the sun. Before the sun had risen more than ten degrees the phenomenon entirely disappeared." 500. The origin of these circles, as well as of those which belong to the third class, is attributed to the action of snow-crystals upon the rays of light, and philosophers have discovered much ingenuity in framing hypotheses to account for these phenomena. 501. PARHELIA AND PARASELENE. The images of the sun observed in haloes, are called parhelia, from the Greek words para, near, and helios, the sun; while those of the moon are termed paraselenæ, from para, near, and seléna, the moon: they have also received the names of mock-suns, and mock-moons. These images are found at the intersection of the different circles, and are formed by the accumulation of light at these points. That such an increase of light occurs is obvious: for if two equally bright circles cut each other, the place where they cross will be twice as bright as the circles themselves. The parhelia and paraselenæ are tinged with the colors of the ordinary halo, and have frequently appended to them a waving stream of light. What is said as to their origin? What are parhelia and paraselenæ ? |