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terest-in fact, at present of greater interest—since it can only be seen during a total eclipse, while all the above-mentioned phenomena are now visible at any period in which the sun is shining. This is what has been named the corona, that brilliant spectacle which has always been the great feature of eclipses, extending like a crown of glory round the moon when that opaque body has completely covered the glowing disc of the sun.

It is not visible while a single ray of sunlight remains; but, as the moon glides on, finally extinguishing the sun's last beam, it leaps out with startling suddenness, and a broad expanse of white light, very bright near the moon's edge, but gradually dimming outward, extending in rays whose outer limit cannot be defined from the surrounding space. It is described by some as of a faint red hue near the sun, then of a pale yellow, then white. This phenomenon was known to the ancients, but it, like all the other solar aspects, has excited close attention only in modern times.

In the eclipse of 1842, the corona was very brilliant, and seemed agitated or flickering. Baily had imagined that it would not be more luminous than the faint, crepuscular light which sometimes occurs in a summer evening. He was, therefore, somewhat astonished at the splendid scene that suddenly burst upon his view. Struve says the light was so bright that the naked eye could scarce endure it. Later observations and photographic results give to the coronal light a quadrilateral aspect, instead of the circular shape it was supposed to have. This figure is produced by an extension of the light over the four arcs between the equator and the poles.

Herr Grosch speaks of the corona in the eclipse of August, 1867, as of a white color, growing faint outwardly, not radiated itself, but appearing as if rays penetrated, or rather ran over it, in symmetrical pencils. These rays had a more bluish appearance than the light of the corona itself, and might best be compared to those produced by a great electro-magnetic light. In the coronal light appeared several dark curves.*

Student, March, 1869.

In the eclipse of 1868, the corona was greatly extended in certain directions, and crossed by a curved ray of intensely white light. It was the publication of this fact that probably gave rise to the curious delusion at that period that a ray of magnetic light was advancing from the sun towards the earth, full of fiery intentions towards this mundane sphere. In this latter eclipse, polariscopic and spectroscopic analysis was applied to the corona.* The first was without satisfactory result; but the spectroscope of Lieutenant Tennant gave a faint, continuous spectrum.

In the American eclipse of August, 1869, Prof. Pickering saw a faint, continuous spectrum crossed by three lines, while Prof. Young saw a gaseous spectrum of three bright lines. This difference is explained as caused by a difference in width of slit of the spectroscope. Prof. Young noticed a close agreement between the coronal lines and those observed in the spectroscopic examination of the aurora. He considers the corona to be simply an electric discharge, varying with great rapidity, as in the case of the aurora-in fact, that the corona is a permanent solar aurora.

In the American eclipse, the corona had not the brilliance observed on some other occasions. It is described by Prof. Eastman as resembling the pale light in the train of a meteor, the portion around the sun being a mass of nebulous light, while the outer portions were radiated. Gen. Myer, whose observations were made from the summit of White Top Mountain, near Abingdon, Va., 5,530 feet high, thus greatly avoiding atmospheric interference, describes the spectacle presented in glowing terins.

To the unaided eye the vision was magnificent beyond description. At the centre stood the intensely black disc of the full moon, surrounded by the aureola of a soft bright light, through which shot out, as from the moon's circumference, straight, massive, silvery rays, to a distance of two or three diameters of the lunar disc; the whole spectacle showing as upon a back-ground of diffused rose-colored light. The

Le Soleil, p. 165.

silvery rays were longest and most prominent at four points of the circumference, giving the corona a quadrilateral aspect. There was no motion of the rays.*

The efforts to photograph during this eclipse were more successful than any previously made, Mr. Whipple obtaining several good negatives in which the quadrilateral shape of the corona plainly appeared. Mr. Brothers, in his photographic experiments during the last total eclipse, found the light of the corona to be considerably more active than had been supposed. He found eight seconds' exposure sufficient to produce on the plate an effect of light extending at least one and a half millions of miles beyond the moon's limb. In fact, no outer limit to the corona has been established. These later photographs have shown that great gaps or rifts break the continuity of the corona, some of these dark openings extending nearly to the moon's limb.

Professor Young made in the American eclipse a discovery of great interest, all doubt of which has since been removed by a similar observation made during the last eclipse. This was the discovery at the base of the chromosphere, and, of course, in immediate contact with the photosphere, of a thin layer in whose spectrum the dark lines of the solar spectrum were all reversed. He noticed them gradually fading out as the solar crescent grew narrower, and at the instant that the moon covered the whole photosphere the field was at once filled with brilliant lines, which suddenly flashed into brightness, fading out again in less than two seconds, and apparently occupying the exact positions of the dark lines.‡

The significance of the fact given is this. The solar spectrum is produced by the light of an intensely hot photosphere, which shines through a cooler atmosphere occupied largely by the vapors of various elements, which on earth exist only as solids. These stop certain of the rays, causing black lines in the spectrum. He had apparently then seen the light of the photosphere unfiltered by these atmospheric vapors, and found

*Naval Obs. Report, p. 150.
† Nature, Feb. 23, 1871.
Franklin Institute Journal, Feb., 1871.

it to consist of the same elementary substances in an intensely heated condition.

Numerous theories have been advanced in explanation of the corona. Those attributing it to a lunar atmosphere, to diffraction of the solar rays in passing the moon, and to action of light on the earth's atmosphere, all seem to be disproved by the results of photography, which seem to show conclusively that it is an appendage of the sun. If so it is a phenomenon of the most imposing character, compared with whose volume that of our earth, and even that of the sun itself, sink into insignificance. The inner radiance which incloses the sun on every side indicates a luminous region of vast extent, while the problems suggested for our consideration in the aspect of this region, and by the physical state of the matter distributed through it, are of the most interesting description. Proctor hypothesises on its origin as follows. He ascribes it to countless millions of meteoric bodies which rotate round the sun, probably so thickly aggregated as to form a cosmical cloud. It is known that such meteorites exist in incalculable numbers, the earth encountering every year, in its movement round the sun, more than a hundred systems of meteors. Each of these systems appears to consist of a ring of matter, rotating round the sun, and composed of innumerable fragments of solid matter. The earth besides meets myriads of meteors not yet associated with any system. Every year we encounter 2,700,000,000 visible to the naked eye, while, if we include telescopic meteors, the number is calculated at 146,100,000,000.

Yet the earth traverses but a narrow line of immeasurable space. Analogy teaches that they occur in equal quantities in that portion of space unoccupied by the earth's orbit, there being millions of millions of such systems for every one known to us. Their lately proved association with comets adds greatly to this probability, and shows that the periheliæ of their excentric orbits closely approach the sun.

The conclusion is that these innumerable meteors, whose orbits are closely grouped round the solar orb, and to whose continual fall into this orb Mayer attributes its heat emis

sions,* must form a cloud of moving matter, incandescent or vaporized by the intense heat, their dense aggregation decreasing but gradually to a long distance from the sun. Between these moving and possibly colliding particles intense electric action might take place; and there is reason to believe that in an eclipse of the sun they would form round him an aureola of splendor.t

Such a hypothesis seems to necessitate a faint luminousness still more distant from the sun, which should be visible after his setting. Our author finds this in the zodiacal light, the annual variation in this light being in consonance with the meteoric variations. One fact certainly seems to closely connect the corona, the zodiacal light, and the aurora, and this is the close connection, if not identity, of the spectral lines of these three distinct phenomena.

Whatever may hereafter be shown to be the origin of the aurora will possibly aid in the explanation of these solar mysteries. It is known now to occur far above the formerly supposed limits of the atmosphere, and may possibly be an electric excitation in cosmical matter occupying exterior space.

Our knowledge of the sun has undoubtedly greatly increased of late years, through the numerous investigations above mentioned; yet, if we seek to deduce from them a general idea of the solar constitution, we find ourselves greatly in the dark. We are simply increasing the complexity of conditions, and rendering the problem more intricate as we approach a knowledge of its true elements.

There is no parallel in the conditions of matter on the sun and on the earth. Metals which to us exist only as solids or liquids are present in the sun as glowing vapors. Gases which no powers at our command can liquify may exist under the enormous pressure of the sun's interior as liquids, or even solids. On one hand an extreme temperature volatilizes our most rigid elements, perhaps dissociated substances which we consider elements. On the other hand, an immense pressure,

*Heat as a Mode of Motion, p. 438.

The Sun, pp. 364-8.

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