where the structure was as of intertwined jets giving an appearance resembling a hank of thread in disorder.
Certainly, for an eclipse occurring two years from the time of minimum, and five years from the time of maximum disturbance, that of July, 1842,[1] has not supplied evidence favouring the theory with which we started. Whether any other theory of association between the corona and the sun-spots will better accord with the evidence hitherto collected remains to be seen.
Turn we now to the eclipse of 1851, occurring nearly midway between the epochs of maximum solar disturbance (1848) and minimum solar disturbance (1856). I take the account given by Airy, our Government astronomer, as he was one of the observers of the eclipse of 1842.
'The corona was far broader,' he says, 'than that which I saw in 1842. Roughly speaking, the breadth was little less than the moon's diameter, but its outline was very irregular. I did not notice any beams projecting from it which deserved notice as much more conspicuous than the others; but the whole was beamy, radiated in structure, and terminated—though very indefinitely—in a way which reminded me of the ornament frequently placed round a mariner's compass. Its colour was white, or resembling that of Venus. I saw no flickering or unsteadiness of light. It was not separated from the moon by any interval, nor had it any annular structure. It looked like a radiated luminous cloud behind the moon.'
The corona thus described belongs to that which our theory associates with the period of maximum rather than of minimum solar disturbance. Definite peculiarities of structure seem to have been more numerous and better marked than in 1842. It accords with our theory that 1851 was a year of greater solar disturbance than was observed in 1842, as the following numbers show:—
| Days of observation | Days without spots | New groups observed | |
| 1842 | 307 | 64 | 68 |
| 1851 | 308 | 0 | 141 |
| 1860 | 332 | 0 | 211 |
I have included the year 1860, as we now proceed to consider the corona then seen by Airy. The year 1860 did not differ very markedly, it will be observed, from 1851, as regards the number of new groups of spots observed by Schwabe, especially when account is taken of the number of days in which the sun was observed in these two years. But 1860 was a year of maximum solar disturbance, whereas 1851 was not.[2]
Airy remarks of the corona in 1860:—'It gave a considerable body, but I did not remark either by eye-view or by telescope-view anything annular in its structure; it appeared to me to resemble, with some irregularities (as I stated in 1851), the ornament round a compass-card.'
Bruhns of Leipsic noted that the corona shone with an intense white light, so lustrous as to dim the protuberances. He noticed that a ray shot out to a distance of about one degree indicating a distance of at least 1,500,000 miles from the sun's surface. This was unquestionably a coronal appendage as neither the direction nor the length of the ray varied for ten seconds, during which Bruhns directed his attention to it. Its light was considerably feebler than that of the corona, which was of a glowing white, and seemed to coruscate or twinkle. Bruhns assigned to the inner corona a height varying from about 40,000 to about 80,000 miles. But this was unquestionably far short of the true height. In fact, Secchi's photographs show the corona extending to a distance of at least 175,000 miles from the surface of the sun. Therefore probably what Bruhns calls the base of the corona was in reality only the prominence region, and the inner corona was that which he describes as varying in breadth or height from nearly one-half to a quarter of a degree—that is from about 800,000 to about 400,000 miles. De la Rue gives a somewhat similar general description of the corona seen in 1860. He remarks that it was extremely bright near the moon's body, and of a silvery whiteness. The picture of the corona by Feilitsch (given at p. 343 of my book on the Sun) accords with these descriptions.
On the whole, the eclipse of 1860 affords evidence according well with the theory we have been considering, except as regards the brightness and the colour of the corona, which correspond more closely with what was observed in July, 1878, with the lustre and colour of the corona in 1870 and 1871. In this respect, it is singular that the eclipse of 1867, which occurred (see preceding note) when the sun spots were fewer in number, presented a decided contrast to that of 1860—the contrast being, however, precisely the reverse of that which our theory would require, if the colour and brightness of the corona be considered essential features of any law of association.
Herr Grosch, describing the corona of 1867, says, 'There appeared around the moon a reddish glimmering light similar to that of the aurora, and almost simultaneously with this (I mean very shortly after it) the corona.' It is clear, however, from what follows, that the reddish light was what is now commonly called the inner corona, which last July, when the sun was in almost exactly the same condition as regards the spots, was pearly white and intensely bright. 'This reddish glimmer,' he proceeds, 'which surrounded the moon with a border of the breadth of at most five minutes' (about 140,000 miles) 'was not sharply bounded in any part, but was extremely diffused and less distinct in the neighbourhood of the poles.' Of the outer corona he remarks that 'its apparent height amounted to about 280,000 miles opposite the solar poles, but opposite the polar equator to about 670,000 miles. Its light was white. This white light was not in the least radiated itself, but it had the appearance of rays penetrating through it; or rather as if rays ran over it, forming symmetrical pencils diverging outwards, and passing far beyond the boundary of the white light. These rays had a more bluish appearance, and might best be compared to those produced by a great electro-magnetic light. Their similarity to these, indeed, was so striking, that under other circumstances I should have taken them for such, shining at a great distance. The view of the corona I have described is that seen with the naked eye. … In the white light of the corona, close upon the moon's edge, there appeared several dark curves. They were symmetrically arched towards the east and west, sharply drawn, and resembling in tint lines drawn with a lead pencil upon white paper. … Beginning at a distance of one minute (about 26,000 miles), they could be traced up to a distance of about nine minutes (some 236,000 miles) from the moon's edge.'
Almost all the features observed in this case correspond closely with those noted and photographed during the eclipse of December, 1871. In other words the corona seen in 1867, when the sun was passing through the period of least solar disturbance, closely resembled the corona seen in 1871, when the sun was nearly in its stage of greatest disturbance. Even the spectroscopic evidence obtained in 1871 and July, 1878, may be so extended as to show with extreme probability what would have been seen in 1867 if spectroscopic analysis had then been applied. We cannot doubt that the reddish inner corona, extending to a height of about 140,000 miles, would have been found under spectroscopic analysis to shine in part with the light of glowing hydrogen, as the reddish corona of 1871 did. The white corona of July, 1878, on the contrary, shone only with such light as comes from glowing solid or liquid matter. Here then, again, the evidence is unfavourable to our theory; for the corona in 1867 should have closely resembled the corona of 1878, if this theory were sound.
It would be idle, I think, to seek for farther evidence either in favour of the theory we originally proposed to discuss, or against it: for the evidence of the eclipse of 1867 disposes finally of the theory in that form. I may note in passing that the eclipse of 1868 gave evidence almost equally unfavourable to the theory, while the evidence given by the eclipse of 1869 was neutral. It will be desirable, however, to consider, before concluding our inquiry, the evidence obtained in 1871 and last July, in order that we may see what, after all, that evidence may be regarded as fairly proving with regard to coronal variations.
First,