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THE SUN'S CORONA.
In a paper which appeared in Fraser's Magazine' for February, 1870, I called attention to certain results which seemed fairly deducible from the observations made by American astronomers and physicists during the eclipse of August 7, 1869. The news of those observations reached me while I was engaged on that paper (entitled “Strange Discoveries respecting the Aurora'), and seemed to add a new importance to the discoveries which I had already recorded. The aurora had been analysed with the spectroscope, and the results were full of interest. The zodiacal light had been similarly analysed, with results indicating an association between this phenomenon and the terrestrial aurora ; and this circumstance seemed even more interesting than the facts revealed respecting the aurora itself. But scarcely had these results been recorded when there came the news that the solar corona had also been analysed with the spectroscope during the eclipse of 1869, and that its spectrum presented the same bright lines which appear in that of the terrestrial aurora! Three phenomena severally interesting, as well as severally perplexing, were thus brought into seeming association; and though the nature of any one of them was by no means definitely
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revealed, yet considerations of the most significant nature were suggested-considerations at once enhancing the interest of these several phenomena and promising to afford one day a means by which all three might be interpreted.
Let us examine what is the present state of our knowledge respecting the sun's corona, noting specially what new light, if any, has been thrown upon the problem by the recent eclipse expeditions, but also not forgetting that vast mass of evidence which former observers have accumulated for our use. It may be noted, indeed, that if we are in a position to theorise at all respecting the corona's nature, we shall certainly not theorise safely unless we consider all the evidence we have. To take this or that fact, however striking, and on it to found a theory respecting a phenomenon so remarkable, and presenting so many complex relations, would be unwise indeed. We must endeavour to bear in mind all that has been learned, to apportion to each observed fact its due weight, and where observed facts seem opposed to each other to analyse them with special care, since nearly always the most definite and striking evidence is afforded by those observations which seem most perplexing
Let us first examine what is known about the sun and his surroundings, in order that we may the more satisfactorily weigh the evidence respecting phenomena as yet unexplained. Such a course is also rendered advisable by the fact that there will be frequent occasion to refer to the prominences and other like features in speaking of the corona and the problems it presents to us for solution.
The rainbow-tinted streak which forms the basis (so to speak) of the solar spectrum tells us that the sun's light comes in the first place from matter which is incandescent, and is either solid or liquid; or, if gaseous, exists at a very great pressure. The innumerable dark lines which cross the rainbow-tinted streak show that outside this matter there are the vapours of many well-known terrestrial elements, existing at a lower temperature than the matter which gives the continuous background of the spectrum. Of the exact position of these absorbing vapours we know (or, perhaps, I should say we knew, before the recent eclipse) comparatively little; but they must necessarily lie above the regions whence the really white light proceeds.
Outside these absorbing vapours is that region into which the coloured prominences are projected. But far lower than the summits of the prominences there lies the region to which the Astronomer-Royal gave (in 1842) the expressive name the sierra. It appears in solar eclipses as an arc of red light around the sun. Its border is well defined and serrated. In colour it resembles closely the prominences; and the researches of spectroscopists have shown that it consists in the main of the same gases. It is this layer which Mr. Lockyer, unaware of its prior recognition, called the chromosphere, a name which is, for many reasons, far less satisfactory than the one devised by Mr. Airy.
Then, lastly, outside the prominences and the sierra there had been recognised the corona, a glory of light surrounding the sun during total eclipses. Precisely as the coloured matter is divisible into lofty prominences and the low sierra, so this corona had been seen to consist of two distinct portions, viz., projecting radiationis extending sometimes to a distance from the sun far exceeding his apparent diameter, and a lower, brighter, and more uniform portion extending to a distance of little more than a fifth of the sun's apparent diameter.. Since the recognition of this peculiarity has been described by those little familiar with the history of solar eclipses as the most important result of the recent eclipse expeditions, it may be as well to remark in this place that the fact has been known for at least 164 years. For in 1706 MM. Plantade and Capiés recognised the existence of a ring of very white light around the moon, within the limits of which ring "the light was everywhere equally vivid ; but beyond the exterior contour the light was less intense, and was seen to fade off gradually into the surrounding darkness, forming an annulus round the moon of about eight degrees in diameter. I quote from • Grant's Physical Astronomy, to which excellent treatise I would refer the curious reader for many other accounts respecting the ring-formed portion of the corona.
It is this seemingly compound object—the solar corona—that astronomers have been so anxiously seeking to interpret during the last two or three years. The recent acquisition of new powers of research, as well as the new knowledge lately obtained respecting the constitution of the solar system, at once suggested hopes that this problem might be at length mastered, and encouraged the expectation that the results would throw a most important light on the economy of those regions of space which immediately surround the solar orb.
It may be said that the first attempt to apply the new means of research to the phenomena presented by the corona was made during the eclipse of 1860, when Mr. De la Rue and Fr. Secchi photographed the eclipsed sun. The success of these physicists was, however, but limited as respects the corona. They succeeded in obtaining excellent photographs of the coloured prominences; but only faint indications of the corona are shown even in the best of their pictures. The photograph which showed the widest extension of the corona was one of Fr. Secchi's; and he was enabled to draw from this view the conclusion that the corona is somewhat brighter and more developed over the solar spot-zone than near either the equator or poles of the sun.
Eight years passed, and then the approach of the great Indian eclipse, one of the most remarkable which have ever occurred, led astronomers to hope that the powers of the spectroscope might reveal something of the true nature of the coronal glory. Indeed more was hoped from the study of the corona than from that of the prominences. This is evident from the words