THE TOTAL ECLIPSE OF NEXT MONDAY.
will be interesting to learn whether any noteworthy advance 1. will be made in our knowledge respecting the Sun during the total solar Eclipse of Monday next, visible in the Western States of America. It almost seems as though the limits of what can be learned with our present methods during the total obscuration of the sun had been reached. The spectroscope has little to teach us, it should seem, respecting the coloured prominences, and not much more about the solar corona. Ordinary methods of tele- scopic observation can now tell nothing new about the promin- ences, which are studied on every fine day at many observatories in Europe and America, and even in the Southern hemisphere. Nor perhaps can we learn much from the telescopic study of the corona, though this appendage of the sun has never been seen without the aid of an eclipse, even with the most elaborate spec- troscopic contrivances for making its faint light discernible through the veil of illuminated air which, to ordinary vision, hides pro- minences and corona equally when the sun is not eclipsed. Photography is the chief reliance of astronomers on this occasion, and photography has made such rapid strides during the last few years, especially in its application to astronomy, that we may well hope to hear of a greater success in obtaining pictures of the corona on this occasion than in 1871, when the best views yet obtained by this method were secured in India.
For our own part, we expect much from the approaching eclipse, especially as respects the corona. There can be no question that much more might have been done in 1870 and 1871, but for the doubts which some ill-informed students of astronomy expressed respecting the solar character of the corona. It was, in reality, clear to every mathematician who studied the matter with due reference to optical laws, that a light appearing where the corona was seen, and fading off gradually in the distance from the eclipsed sun, could not possibly belong to the moon or to our own air, but must belong to the sun. It was also clear—though this even now is not admitted by all—that although the air which lies between the observer and the sun in total eclipse must of necessity be illuminated by the solar corona (for we receive the rays of the coronal light through that air, which must therefore also receive thew), yet but the minutest and probably a quite un- discernible portion of the outer corona could be ascribed to such illumination of our air. But unfortunately, in 1869 and 1870, the idea was vaguely promulgated (vaguely, in the sense of not being definitely reasoned out) that the corona is an atmospheric effect. If a round hole is cut in a shutter which darkens a window facing sunwards, and this hole be imperfectly stopped by a plug or bung irregularly cut, it will be found that on looking sunwards through the incomplete circular crack thus left, an appearance something like that of the corona in a solar eclipse will be per- ceived. The plug will look black, like the dark body of the eclipsing moon, and all round it will be seen a glory of light, where the solar rays illuminate the dust in the darkened room. It was Secchi, we believe, who suggested this experiment, though we trust, for the sake of his reputation as an exact man of science, that he was not one of those who, on the strength of the resemblance between this artificial and a real eclipse, adopted the idea that the solar corona resembled the glory of sunlit dust. For in reality, except that the moon hides the sun as the plug hides him in the experiment, there is not the slightest resemblance between the conditions. It can be shown, and it was shown in 1869, that the rays of the sun illuminating parts of our air during total eclipse must produce an inversion of the true corona,—viz., a dark space round the sun, gradually growing less dark, until at a distance of about fifteen sun-breadths from him, the sunlit air would be seen growing brighter and brighter as the distance from the sun increased. But this reason- ing was "caviare to the general," and unfortunately "the general" included several good observers. Attention was thus chiefly directed in 1870 and 1871 to the solution of the question whether the Corona belongs to the Sun or not, and many observa- tions were made which, while disposing effectually of this already settled question, were otherwise either useless or of very little value. As an illustration of the mischievous effects of the obsti- nately maintained doubts of those who denied the solar nature of the corona, consider the photographic operations in 1871. They were in a sense most successful, and as we have already mentioned, the photographs then obtained are the best we have. But because, despite the admirable photographs obtained by Brothers of Manchester and Winlock of America, in 1870, it WRS maintained that the photographic evidence respecting the corona did not conclusively settle the question, Mr. Davis, the photographer of Lord Lindsay's party, and Colonel Tennant, instead of devoting the whole duration of totality in 1871 to obtain at each of their respective stations one really good photograph of the corona, divided up that short time into six short spaces, to obtain six views. Thus twelve views were obtained instead of two : and we do not say that the twelve views were poor ones. But manifestly they cannot be nearly so
good as two views would have been, to each of which the full time of totality had been devoted. And it is well known that Mr. Davis, to I whose skill we owe the success with which the six views taken at Baicull were obtained, was altogether dissatisfied when he com- pared the best of these views, produced by aid of a nine-inch reflector, with the corona as he had himself seen it through a two-inch telescope. It is true the twelve views disposed finally of the atmospheric theory of the corona. But there was no real value in this result. Every one competent to form an opinion on this point knew already that the corona belongs to the sun, and to make the matter clear to less competent persons was not an achievement of any moment, though it had proved a task of some difficulty. There are folks who believe the earth to be flat, and perhaps few more difficult tasks could be set the astronomer than to convince such persons of the earth's rotundity ; but no astronomer would undertake the task, or be grateful to any one who should accomplish it,—if such a feat can be imagined.
Now on the present occasion there are no doubts of this sort, or if any such doubts are entertained, astronomers are not so foolish as any longer to consider them. The corona is known to be a solar appendage, known to be of most amazing extent and singularly complex in structure. As regards its physical con- stitution, we know that it is partly gaseous, though of what gas or gases it is formed we have as yet no knowledge, for no terres- trial element has yet been identified with the substance which produces the most characteristic line of the corona's spectrum. It is known also that the corona shines partly by reflecting sun- light ; for a portion of its light, when analysed by the spectroscope, gives a faint copy of that rainbow-tinted streak crossed by dark lines, which is called the solar spectrum. Other evidence seems to show that this part of the corona is of the nature of dust ; that is, it consists of small particles of solid matter, travelling probably in flights or clouds around the attracting mass of the sun. We cannot expect that the nature of the gas present in the corona will be determined during the eclipse of Monday next, for already the position of the bright lines belonging to the corona vapour has been determined very exactly, and what wanting is to resolve this question is not further astronomical observation, but research in th e laboratory. We can scarcely hope, indeed, that any better information will be obtained respecting the physical structure of the corona than we possess already. But photography may probably be much more successfully applied on this occasion than ever before ; and we may thus have most interesting facts respecting the structure of the corona, and the distance to which it extends from the sun's globe. On this last point the views obtained already show that the coronal streamers have an exten- sion of at least eight hundred thousand miles, which would imply that the corona occupies a region in space exceeding about twenty-five times the volume of the sun,—which itself, be it re- membered, exceeds 1,250,000 times the volume of this earth on which we live. But we believe that photographs may be obtained showing the coronal streamers to twice this distance, correspond- ing to an eightfold enlargement of the region occupied by the -corona. And there are reasons for thinking that by carefully- arranged naked-eye studies of the corona a still greater exten- sion might be recognised. For after all, naked-eye vision is the most effective means of studying the outer and fainter portions of such an object as the corona. Every telescopic appliance diminishes in some degree the apparent luminosity of the object studied, except in the case of bodies like the stars, which appear only as points under the strongest magni- fying power.
But probably the most interesting result of the study of the Sun during the approaching solar eclipse will be the determination of some difference in character between the corona, as seen at a time when the sun is greatly disturbed and many spots are visible on his surface, and at a time like the present, when he is compara- tively quiescent, and no spots, or very few, are seen for weeks together. Already the spectroscopic study of the prominences has shown that those tongues of glowing gas, which at times of great disturbance leap up to heights even exceeding a hundred thousand miles, are greatly reduced at times when the sun is little disturbed. In our opinion, these studies of the sun's sur- roundings, and of their changes in the great spot-period, are even more interesting than Schwabe's great discovery itself of the existence of that strange periodic change in the sun's .condition.
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