CHAPTER V.

THE PROMINENCES AND THE CHROMOSPHERE.

THE coloured prominences which have recently attracted so large a share of the attention of solar physicists were first fully recognised during the total solar eclipse of 1842. It is probable, however, that they were seen more than a century before that date, though their real nature was not suspected. During the total solar eclipse of May 2, 1733, Vassenius, at Gottenburg, observed several red clouds floating, as he supposed, in the atmosphere. One of them seemed larger than the rest, and appeared to be composed of three masses placed one above the other, and completely detached from the Moon's limb. "These spots seemed,' he writes, composed in each instance of three smaller parts or cloudy patches of unequal length, having a certain degree of obliquity to the Moon's periphery. Having directed the attention of my companion, who had the eyes of a lynx, to the phenomenon, I drew a sketch of its aspect. But while he, not being accustomed to the use of the telescope, was unable to find the Moon, I again, with great delight,

perceived the same spot, or if you choose, rather the invariable cloud occupying its former situation in the atmosphere near the Moon's periphery.' We need not be surprised that Vassenius assigns the spots without scruple to the Moon's atmosphere, since it was thought by many in his time that the Moon has an atmosphere of appreciable extent. Yet it was unfortunate for science that the prominences (for we can scarcely doubt that the appearances seen by Vassenius were really prominences) should have been thus explained away as relatively unimportant phenomena, since otherwise observers during succeeding eclipses would probably have searched for similar objects, and we might thus have possessed a long series of observations tending to indicate the laws according to which these objects make their appearance.

For more than a century eclipse passed after eclipse, and no observer recognised these flames of coloured light, which have seemed to the observers of recent eclipses so striking and obvious. Ferrer, indeed, in 1806, and Van Swinden, in 1820, noticed faint traces of some peculiar coloured appendages; but their observations were not satisfactory, nor was any attention drawn to the subject.

During the great eclipse of 1842, however, a number of first-rate observers were distributed along the line of total obscuration. Airy, Arago, and the younger Struve; Littrow, Baily, Santini, Valz, and Biela,-a host, in fine, of the most skilful astronomers in Europewatched the eclipse with careful scrutiny. All of them

recognised with surprise the presence of rose-coloured prominences round the disc of the eclipsed Sun.

The Astronomer-Royal saw three prominences at the summit of the disc; Arago, Struve, and Schidlowski saw two near the lowest point of the dise; Schumacher, of Vienna, saw three-two below, and one above.

It will be instructive to consider the account given by the first observers of these interesting objects.

The Astronomer-Royal compared them to the inclined teeth in a circular saw, and estimated their height at about one minute of arc.

Schumacher compared the protuberances to icebergs, and the pictures which illustrate his paper represent them as much more closely resembling icebergs than any protuberances seen in recent times. We may not unfairly conclude that Schumacher's drawings are somewhat idealised.

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Baily compared the prominences to Alpine peaks coloured by a setting Sun. He noticed that one was bifurcated almost to its base. M. Mauvais employs a similiar comparison. He had seen a reddish point soon after the Sun was totally obscured. When fifty-six seconds had passed after the commencement of totality,’ he writes, this reddish point transformed itself into two protuberances, resembling two adjacent mountains, and well defined. Their colour was not uniform, streaks of a deeper red marking their flanks. I cannot describe them better than by comparing them to distant Alpine peaks, illuminated by the rays of the

setting Sun. One minute and ten seconds from the time of total obscuration a third mountain became visible to the left of the other two. In colour it resembled the others. Beside it were some smaller peaks, all of them well defined.'

Mauvais noticed that the other two protuberances grew higher while the third was making its appearance. Near the end of the eclipse they were no less than two minutes of arc in height.

Biela, Schumacher, and others recognised a border of rose-coloured light surrounding a part of the Moon's limb at a lower level than that attained by the prominences. It is worthy of note that this phenomenon had been noticed earlier than the prominences themselves; for during the total eclipse of 1706, Captain Stannyan remarked that a blood-coloured streak of light appeared, before the Sun's limb emerged from behind the Moon. In 1715, also, Halley noticed that two or three seconds before the emersion, the Moon's limb appeared to be tinged with a dusky but strong red light, forming a long and narrow streak; and during the same eclipse, Louville saw what he describes as an arc of deep red colour along the edge of the Moon's disc. The latter astronomer was careful to assure himself that the appearance was no illusion, and to this end he brought the red arc into the middle of the telescopic field of view, when he found that the red colour remained unchanged. Don Ulloa, in 1778, and Ferrer, in 1806, had noticed a similar pheno

menon.

When the various accounts of the eclipse of 1842 came before the astronomical world, several theories were propounded in explanation of the red prominences. The theory that they are mountains in the Sun was for a while in favour; but Arago pointed out that some of them were too considerably inclined to the perpendicular to be so regarded. Others supposed

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them to be clouds in the solar atmosphere; while others again suspected them to be enormous flames. ordinarily happens in such cases, there were not wanting those who denied that the coloured prominences had any real existence whatever. M. Faye, for example, asserted his belief that they are purely optical illusions mirages, perhaps, produced near the Moon's surface.'

The eclipse of 1851 removed these doubts for the most part, though it is to be noted in passing that despite the evidence obtained then, and yet again in 1860, there were some who continued, even until the great Indian eclipse of 1868, to deny that the coloured prominences and the rose-tinted arcs seen at a lower level could really be regarded as solar appendages.

During the total eclipse of 1851 many observers of great skill made drawings of the very remarkable prominences which were visible on that occasion. These pictures exhibited a sufficiently satisfactory agreement to convince the observers that they had all witnessed the same phenomena; though the discrepancies between the pictures afford instructive evidence of the difficulty of delineating with exactness the details presented