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and then an anthelion from the same cloud when it was opposite to the sun.

HALOES.

484. Haloes are circles of prismatic colors about the sun and moon; they differ from coronas in three particulars; first, their structure is often more complicated; secondly, their diameter is greater; and thirdly, the order of colors is reversed, the red being nearest the luminary.

485. The several parts of this phenomenon may be thus classified, 1st, Circles surrounding the orb which occupies their centre. 2d, Circles passing through the orb. 3d, Arcs of circles touching those of the first class. 4th, Parhelia and paraselena or mock-suns and mockmoons, found at the points where the circles cross each other.

486. FACTS. The annexed figure represents a halo around the sun, observed by Scheiner, in 1630. In the cut, S is the sun, A B C a circle about 45° in diameter, and D E F another circle, its diameter being nearly 95° 20', the sun being in their Both the common centre. circles were colored like the primary rainbow, but the red was next to the sun, the other colors succeeding in the natural order. DSF is

a third whitish circle pass

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ing through the centre of the sun, and H E G a portion of a fourth touching D E F at E. At A, C, D, and F, were mock-suns; the same phenomena were seen at B

What are haloes?

How do they differ from coronas or crowns?

How are the several parts of the halo classified? Describe the three haloes recorded.

and E. The mock-suns, A and C, were of a purplish red next to the sun, while D and F were entirely white, the former were also more brilliant, continuing visible for three hours together, while the light of the latter was faint and fluctuating.

The mock-suns B and E were almost the first to appear and the last to fade, excepting A, and throughout the whole phenomenon, which lasted five hours, they were perpetually changing in magnitude and color. B was formed in a peculiar manner, the halo A B C was composed of several intersecting circles, and at one of these intersections the mock-sun B appeared.

487. On the 9th of Sept. 1844, a halo of a somewhat complicated structure was seen by many observers, both at New Haven and at Hartford, Ct. It continued for the space of four hours, commencing about 10 A. M. and ending at 2 P. M.

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Its appearance is shown in figure 38., where S represents the sun, A B the ordinary halo of about 45° in diameter, and C D a circle whose centre was in the zenith while its circumference passed through the sun. rectly north of the zenith, upon the circumference of C D, a parhelion appeared at the intersection of C D with the circles E F and G H, which were both equal in size to itself.

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.

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°.

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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 D E 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?
Explain the origin of the halo of forty-five degrees.

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.

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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 stratum, 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?

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