Part 3

The light of the corona was so bright, O. Struve states, that the naked eye could scarcely endure it; 'many could not believe, indeed, that the eclipse was total, so strongly did the corona's light resemble direct sunlight.' Thus while as to extent the corona in 1842 presented the appearance to be expected at the time of maximum solar disturbance, if our theory is sound, its brightness was that corresponding to a time of minimum disturbance. Its structure corresponded with the former condition. The light of the corona was not uniform, nor merely marked by radiations, but in several places interlacing lines of light could be seen. Arago, at Perpignan, observed with the unaided eye a region of the corona 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, 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 Days without New groups observation spots 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.

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, however, as I have considered two eclipses which occurred when the sun spots were decreasing in number--namely, those of 1842 and 1851, midway (roughly speaking) between the crest and hollow of the sun-spot wave on its descending slope, it may be well to consider an eclipse which was similarly situated with respect to the ascending slope of a sun-spot wave. I take, then, the eclipse of 1858, as seen in Brazil by Liais. The picture drawn by this observer is one of the most remarkable views of the corona ever obtained. It is given at p. 339 of my book on the Sun. Formerly it was the custom to deride this drawing, but since the eclipse of 1871, when the corona was photographed, it has been admitted that Liais's drawing may be accepted as thoroughly trustworthy. It shows a wonderfully complex corona, like that of 1871, extending some 700,000 miles from the sun, and corresponding in all respects with such a corona as our theory (if established) would have associated with the stage of maximum solar disturbance. As in this respect the eclipse of 1858, when sun-spots were increasing, resembled those of 1842 and 1851, when sun-spots were diminishing in number, we find no trace of any law of association depending on the rate of increase or diminution of solar disturbance.

If we limited our attention to the eclipses of 1871 and of July, 1878, we should unquestionably be led to adopt the belief that the corona during a year of many spots differs markedly from the corona when the sun shows few spots, or none. So far as the aspect of the corona is concerned, I take the description given by the same observer in both cases, as the comparison is thus freed as far as possible from the effect of personal differences.

Mr. Lockyer recognised in 1871 a corona resembling a star-like decoration, with its rays arranged almost symmetrically--three above and three below two dark spaces or rifts at the extremity of a horizontal diameter. The rays were built up of innumerable bright lines of different length, with more or less dark spaces between them. Near the sun this structure was lost in the brightness of the central ring, or inner corona. In the telescope he saw thousands of interlacing filaments, varying in intensity. The rays so definite to the eye were not seen in the telescope. The complex structure of interlacing filaments could be traced only to a height of some five or six minutes (from 135,000 to 165,000 miles) from the sun, there dying out suddenly. The spectroscope showed that the inner corona, to this height at least (but Respighi's spectroscopic observations prove the same for a much greater distance from the sun), was formed in part of glowing gas--hydrogen--and the vapour of some as yet undetermined substance, shining with light of a green tint, corresponding to 1474 of Kirchhoff's scale. But also a part of the coronal light came from matter which reflected sunlight; for its spectrum was the rainbow-tinted streak crossed by dark lines, which we obtain from any object illuminated by the sun's rays. It should be added that the photographs of the corona in 1871 show the three great rays above and three below, forming the appearance as of a star-like decoration, described by Mr. Lockyer; insomuch as it is rather strange to find Mr. Lockyer remarking that 'the difference between the photographic and the visible corona came out strongly, ... and the non solar origin of the radial structure was conclusively established.' The resemblance is, indeed, not indicated in the rough copy of the photographs which illustrates Mr. Lockyer's paper; but it is clearly seen in the photographs themselves, and in the fine engraving which has been formed from them for the illustration of the volume which the Astronomical Society proposes to issue (some time in the present century, perhaps).

Now, in July, 1878, the corona presented an entirely different appearance. Mr. Lockyer, in a telegram sent to the Daily News, describes it as small, of pearly lustre, and having indications of definite structure in two places only. Several long rays were seen; but the inner corona was estimated as extending to a height of about 70,000 miles from the sun's surface. The most remarkable change, however, was that which had taken place in the character of the corona's spectrum--or, in other words, in the physical structure of the corona. The bright lines or bright images of the inner corona (according as it was examined through a slit or without one) were not seen in July, 1878, showing that no part, or at least no appreciable part, of its light came from glowing gaseous matter. But also the dark lines seen by Janssen in 1871 were wanting on this occasion, showing that the corona did not shine appreciably by reflecting sunlight. The spectrum was, in fine, a continuous rainbow-tinted streak, such as that given by glowing solid or liquid matter.

The inference clearly is: 1. That in July, 1878, the gaseous matter which had been present in the corona in 1871 was either entirely absent or greatly reduced in quantity; 2. The particles of solid or liquid (but probably solid) matter which, by reflecting sunlight, produced a considerable portion of the corona's light in 1871, were glowing with heat in July, 1878, and shone in the main with this inherent light; and 3. The entire corona was greatly reduced in size in July, 1878, as compared with that which formed the 'star-like decoration' around the black body of the moon in December, 1871.

We cannot, however, accept the theory that such a corona as was seen in 1871 invariably surrounds the sun in years of great disturbance, while the corona of last month is the typical corona for years of small solar disturbance. The generalisation is flatly contradicted by the evidence which I have presented in the preceding pages. It may be that such a corona as was seen in 1871 is common in years of great disturbance, just as spots are then more common, though not always present; while such a corona as was seen in July, 1878, is more common in years of small disturbance, just as days when the sun is wholly without spots are then more common, though from time to time several spots, and sometimes very large spots, are seen in such years. On the whole, I think the evidence I have collected favours rather strongly the inference that an association of this sort really exists between the corona and the sun-spots. It would, however, be unsafe at present to generalise even to this extent; while certainly the wide generalisation telegraphed to Europe from America as the great result of the eclipse observations in July, 1878, must unhesitatingly be rejected.

It remains to be considered how science may hope to obtain more trustworthy evidence than we yet have respecting the corona and its changes of form, extent, lustre, and physical constitution. In the case of the prominences, we have the means of making systematic observations on every fine, clear day. It has been, indeed, through observations thus effected by the spectroscopic method that an association has been recognised between the number, size, and brilliancy of the prominences on the one hand, and the number, size, and activity of the sun-spots on the other. But in the case of the corona, we are as yet unable to make any observations except at the time of total solar eclipse. It seems almost impossible to hope that any means can be devised for seeing the corona at any other time. Of course, without the aid of the spectroscope the corona, as ordinarily seen during total eclipses, must be entirely invisible when the sun is shining in full splendour. No one acquainted with even the merest elements of optics could hope to see the corona with an ordinary telescope at such a time. The spectroscope, again, would not help in the slightest degree to show such a corona as was shining in July, 1878. For the power of the spectroscope to show objects which under ordinary conditions are invisible, depends on the separation of rays of certain tints from the rays of all the colours of the rainbow, which make up solar light; and as the corona in July, 1878, shone with all the colours of the rainbow, and not with certain special tints, the power of the spectroscope would be thrown away on a corona of that kind. All that we can ever hope to do is to discern the gaseous corona when, as in 1871, it is well developed, by spectroscopic appliances more effective for that purpose than any which have hitherto been adopted; for all which have as yet been adopted have failed.

Now, the difficulty of the problem will be recognised when we remember that the strongest tints of the corona's light--the green tint classified as 1474 Kirchhoff--has been specially but ineffectually searched for in the sun's neighbourhood with the most powerful spectroscopic appliances yet employed in the study of the coloured prominences. In other words, when the light of our own air over the region occupied by the corona has been diluted as far as possible by spectroscopic contrivances, the strongest of the special coronal tints has yet failed to show through the diluted spectrum of the sky. Again, we have even stronger evidence of the difficulty of the task in the spectroscopic observations made by Respighi during the eclipse of 1871. The instrument, or I should rather, perhaps, say the arrangement, which during mid totality showed the green image of the corona to a height of about 280,000 miles, did not show any green ring at all at the beginning of totality. In other words, so faint is the light of the gaseous corona, even at its brightest part, close to the sun, that the faint residual atmospheric light which illuminates the sky over the eclipsed sun at the beginning of totality sufficed to obliterate this part of the coronal light.

Whether with any combination specially directed to meet the difficulties of this observation, the gaseous corona can be rendered discernible, remains to be seen. I must confess my own hopes that the problem will ever be successfully dealt with are very slight, though not absolutely evanescent. It seems to me barely possible that the problem might be successfully attacked in the following way. Using a telescope of small size, for the larger the telescope the fainter is the image (because of greater loss of light by absorption), let the image of the sun be received in a small, perfectly darkened camera attached to the eye-end of the telescope. Now if the image of the sun were received on a smooth white surface we know that the prominences and the corona would not be visible. And again, if the part of such a surface on which the image of the sun itself fell were exactly removed, we know (the experiment has been tried by Airy) that the prominences would not be seen on the ring of white surface left after such excision. Still less, then, would the much fainter image of the corona be seen. But if this ring of white surface, illuminated in reality by the sky, by the ring of prominences and sierra, and by the corona, were examined through a battery of prisms (used without a slit) adjusted to any one of the known prominence tints, the ring of prominences and sierra would be seen in that special tint. If the battery of prisms were sufficiently effective, and the tint were one of the hydrogen tints--preferably, perhaps, the red--we might possibly be able to trace the faint image of the corona in that tint. But we should have a better chance with the green tint corresponding to the spectral line 1474 Kirchhoff. If the ring of white surface were replaced by a ring of green surface, the tint being as nearly that of 1474 Kirchhoff as possible, the chance of seeing the coronal ring in that tint would be somewhat increased; and, still further, perhaps, if the field of view were examined through green glass of the same tint. It seems just possible that if prisms of triple height were used, through which the rays were carried three times, by an obvious modification of the usual arrangement for altering the level of the rays, thus giving a power of eighteen flint glass prisms of sixty degrees each, evidence, though slight perhaps, might be obtained of the presence of the substance which produces the green line. Thus variations in the condition of the corona might be recognised, and any law affecting such variations might be detected. I must confess, however, that a consideration of the optical relations involved in the problem leads me to regard the attempt to recognise any traces of the corona when the sun is not eclipsed as almost hopeless.

It is clear that until some method for thus observing the corona has been devised, future eclipse observations will acquire a new interest from the light which they may throw on the coronal variations, and their possible association in some way, not as yet detected, with the sun-spot period. Even when a method has been devised for observing the gaseous corona, the corona whose light comes either directly or by reflection from solid or liquid matter will still remain undiscernible save only during total eclipses of the sun. Many years must doubtless pass, then, before the relation of the corona to the prominences and the sun-spots shall be fully recognised. But there can be no question that the solution of this problem will be well worth waiting for, even though it should not lead up (as it most probably will) to the solution of the mystery of the periodic changes which affect the surface of the sun.

FOOTNOTES:

Days of Days without New groups observation spots observed 1837 168 0 333 1842 307 64 68 1844 321 111 52

Only it should be noticed that nearly all the spots seen in the year 1844 belonged to the next period, the time of actual minimum occurring early in 1844.]

Days of Days without New groups observation spots observed 1848 278 0 930 1851 308 0 141 1856 321 193 34 1860 332 0 211 1867 312 195 25

A comparison of the three tables given in these notes and the text will afford some idea of the irregularities existing in the various waves of sun-spots.]