THE ECLIPSE EXPEDITIONS.
WE have excellent news respecting the approaching Eclipse. Government has 'granted all that was asked for, and a
strong English expedition will sail before long for Ceylon. It is, in- deed, somewhat remarkable that an eclipse which had been regarded as but of slight importance promises to be distinguished among all others for the vast range whence astronomers will gather to observe its phenomena. At this moment preparations are being made in Europe, in India, and in Australia for the swift passage of the moon's shadow from the southern part of the Indian peninsula across Ceylon and Java to northern Australia. Not only so, but messages have been flashed under the Atlantic to the leading American spectroscopists inviting them to share in the work. So that for the first time in the history of astronomy men are gathering from all parts of the earth to witness the phenomena of an eclipse.
But even more promising than the mere number of observers, or than the enthusiasm which leads them to undertake these long voyages, is the quality of the observing forces. France sends M. Janssen, distinguished as the first who ever studied the solar pro- minences in full daylight. England sends Mr. Lookyer, who independently, though later, achieved the same feat. In India, Colonel Tennant and Captain Herschel, two among the inde- pendent discoverers of the real nature of the prominences, are to take part in the work. It is hoped that America will send Pro- fessor Young, one of the first successful students of the corona, and the first to prove that beneath the coloured prominences and sierra there exists a singularly complex solar atmosphere. Dr. Milner, the eminent German spectroscopist, has also been invited to join the English expedition. And it may be added that Dr. Huggins, the Herschel of the spectroscope, though unable to join the ex- pedition, and indeed likely to do even more useful work at home with the splendid telescope placed in his hands by the Royal Society, has given the observers the aid of his advice and ex- perience.
The photographic department is also well provided for. It has been decidel that the plan devised by Mr. Brothers, and success- fully applied during the eclipse of last December, shall be adopted at all the stations. Until last December, the ordinary telescope had been employed in all attempts to photograph the corona. In the Spanish eclipse of 1860, Mr. De La Rue used a fine reflecting telescope with a metallic mirror. In the great Indian eclipse of 1868, Colonel Tennant used oue of Mr. Browning's excellent sil- vered-glass reflectors. The American photographers in 1860 all used telescopes, either refloating or refracting. All the photo- graphs so taken were failures so far as the corona is concerned. The red prominences were well shown, but the corona could scarcely be recognized. It is only fair to mention that the special aim of the photographers had been, until last Decem- ber, to obtain good pictures of the prominences, so that when we speak of their attempts as failures we use the term only with reference to the object which their successors have had in view. After a careful examination of the work done during former eclipses, Mr. Brothers came to the conclusion that no telescope could give good photographic views of the corona. A telescope had been provided for his use by the Astronomical Society ; but he was not content to rest his hopes on the work which he might do with that instrument. It seemed to him that the ordi- nary photographic camera was the proper instrument for his pur- pose. Having obtained from Mr. Dallmeyer a camera excellently adapted to the work, he took this with him to Syracuse. But in celestial photography something more is wanted than the ordinary photographic appliances. The celestial bodies are not like sitters who remain still while their likeness is being taken, but are in con- tinual motion. The sun, for instance, shifts by a space equal to his own apparent breadth in two minutes ; and even though a few seconds may suffice to take a good photograph of the eclipsed sun, yet most certainly the photograph would be but a poor one if some provision were not made for the sun's seeming motion. Telescopes are made to follow celestial objects by means of clock-work ; but photographic cameras are not provided with such clockwork. To obviate this difficulty, Mr. Brothers attached his camera to the telescope, so that both were driven by the same clockwork. We have already described the success of his operations ; and it is only necessary to remind our readers that he had
very unfavourable weather. Only during the last eleven seconds of totality was the sky clear. For eight of those precious seconds the camera was fairly at work ; and in that time the corona was depicted " as it had never been seen on glass before." It cannot be doubted that with favourable weather the photogra- phers, who are to apply the same method at different stations during the coming eclipse, will be able to obtain most valuable pictures of the corona. Mr. Brothers has been the Columbus who has shown how the egg can be made to stand upright ; and we may hope that before the eclipse of next December is past many such eggs will be set fairly on end.
As respects observations with the polariscope (the principle of which we by no means purpose to explain here), little seems to be promised or hoped for. So many were successful with this instru- ment last December, and such a grievous confusion resulted from their success, that it is almost to be desired that the expeditions should leave their polariseopes behind them. Professor Airy, at a meeting of the Royal Astronomical Society last January, correctly, if in somewhat caustic sort, summed up the results of the observa- tions which were described at that meeting :—" Your observations are most satisfactory," he said,—then, after a pause,—" that is, they are satinfactorKin this sense, that they leave matters pre- cisely as they stoodjbefore."
We have so often `pointed out the true aim of the observations lately made and presently to be made on the corona, that there would be no occasion toapeak further on the subject, but for some misapprehensions which have been occasioned by remarks let fall during the recent meeting of the British Association. At that
meeting, M. Janssen[ancliAlr.Lockyer. were present, and the latter took occasion to exhibit and describe certain illuminated pictures of the corona. Commenting on these, Professor Tait, the head of the physical-science section, remarked that he had no hesitation in saying that most of the phenomena which had been described were merely atmospheric. If he had said that all the phenomena then described were atmospheric, he would have been fully justified ; but it is by no moans to be imagined that the long-vexed question whether the corona is a solar or a terrestrial phenomenon was in any way concerned. The fact is that the pictures and descriptions of which ho spoke related solely to those phenomena of eclipses which have always been referred to our own atmosphere. Nor has interest at any time [attached to the question whether some phenomena of total eclipses may not be due to the illumina- tion of the air above: and around the observer. The problem which has so long occupied astronomers has related simply to the question whether there exists or not a solar appendage extending far beyond the coloured prominences. To show that some appear- ance seen during eclipses rare certainly not due to such an append- age is an excellent thing) in itself, but it does not amount to a proof that no such appendage exists. Astronomers owe thanks to Mr. Lockyer for carefully collecting together the records of all the various eclipse-phenomena:which do not belong to the corona proper. It is most desirable:that the existence of such phenomena should be established, or rather, that observers should not be suffered to forget that] such phenomena exist. But so far as the question is concerned which has been so long under discussion, all such evidence is purely negative. It is like the evidence of those twenty;witnesses who were called in (as related in a well-known story) ;to say that they had not seen a certain act. And as their evidence was outweighed by that of the one trust- worthy witness who hod seen the act, so all those strange and beautiful phenomena during totality which are not solar must be re- garded as of less importance (so far as the question at issue is con- cerned) than those which have been shown to belong to a real solar appendage. During the last eclipse, as on all former occasions of the kind, a number:of variable beams were seen, especially at the beginning and end of totality, and at different stations these variable beams presented different aspects. They were therefore certainly not solar, but atmospheric phenomena. Fortunately, how- ever, certain sharp-sighted observers recognized rays which did not change ; when accounts were compared it was found that such rays were seen in the same positions from widely-distant stations ; and finally, photographs taken at stations so far apart as Xerez and Syracuse showed these rays, still in the same position. When the evidence of these rays is accepted, and the inevitable inference deduced that there is an amazing solar appendage having the extension thus indicated, what sort of answer is it to urge that other rays were not fixed? And what reason can be given for the amazing proposal recently made that draughtsmen during the approaching eclipse should confine their attention to the variable phenomena of the corona ? It is as though the advocate for the offender in the old story should have urged that those who (not
being present) had not witnessed the offence should alone be listened to, and that no attention should be paid to the only real witnesses. Men of science must be content to take such evidence as is avail- able and effective, without reference to their favourite hypotheses.
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