The reader is aware that the point of the | tions of the moon's shadow-track to which moon's conical shadow sometimes extends it has been judged advisable to send obbeyond and sometimes falls short of the servers. The track crosses the southern earth. In the former case an eclipse is extremity of the Indian peninsula, and total, in the latter it is annular. But in along this part of its course there will the eclipse of November 30, 1872, the apex probably be several observing parties, of the shadow falls short of the earth's the arrangements being superintended by surface at the beginning of the eclipse; it Mr. Pogson, the Government Astronomer encounters the earth as the shadow-track at Madras, and by Colonel Tenant and passed onward towards the bulging cen- Captain Herschel, both known to fame tral part of the earth's illuminated hemi- through their observations during the sphere; and presently, towards the close great eclipse of 1868. Thence the shadowof the eclipse, falls again short of the earth's track passes to the northern part of Ceysurface. So that there are two points lon, and along this part of its course the on the earth's surface where, on Novem- English eclipse expedition will be staber 30, 1872, the eclipse will be exactly tioned. It will probably be in the rememtotal, the moon just hiding the sun and no brance of most of my readers that the more, and only for a single instant. The English Government granted (several totality will nowhere last more than about months back) the sum of £2,000, as well three-quarters of a minute; and as the as transport and the means of camping, place where this will happen lies very far for an expedition to Ceylon. It was south in the Pacific Ocean, it is not likely hoped that Professor Stokes would have that any observer will witness this eclipse. been able to take charge of this expediIt is, however, the most considerable solar tion; but these hopes were disappointed. eclipse of the year 1872. The last eclipse Mr. Lockyer, however, has been able to of the series occurs on November 19, 1873, give his services, and doubtless the expeand, like the last of the former series, it is dition will be a highly effective one. The altogether unimportant. The moon, as shadow-track passes from Ceylon to Java, supposed to be seen from the sun, will just where a French party under M. Janssen graze the most southerly part of the will be stationed. Lastly, the shadowearth's disc. "The circumstances of the track passes to the northern part of the eclipse are such," says the Nautical Alma- Australian continent, and a strong observnac, "that a map has not been considered ing party has proceeded from Sydney and requisite." There will be no total solar Melbourne to the stations along this part eclipse at all in 1873. of the shadow's course. Not until April 16, 1874, will any total eclipse worth observing take place, after the eclipse of the present month. Nor are the circumstances of the eclipse of 1874 such as to encourage favourable hopes that much will be learned during its progress. On April 6, 1875, there will be, I believe, a much more important eclipse visible (as I judge from a rough calculation) in America; but I shall probably be excused from entering into an exact calculation of its circumstances, more especially as the Nautical Almanac for 1875 will, I believe, be published before this essay appears. It will be inferred that a considerable degree of interest is attached by astronomers to the eclipse of the present month, followed as it will be by two years and four months during which there will be no solar eclipses worthy of special observation. Although the eclipse of the 12th inst. is not nearly so favourable for observational purposes as the great Indian eclipse of 1868, yet there is a considerable variety as respects the choice of stations. In fact there are no less than four distinct sec The totality will last longest in North Australia, where its duration will be more than four minutes, or nearly two minutes longer than the duration of the eclipse of last year at the best stations. In Java the totality will last more than three minutes. In Ceylon the duration of totality will barely exceed by a few seconds the duration of totality last December. A somewhat curious mistake was made on this point in a scientific journal. Mr. Hind, in his first and comparatively rough estimate of the course traversed by the moon's shadow, had placed Trincomalee on the border of the track, so that the duration of totality at Trincomalee would have been very short. But after his final and more elaborate calculation, he set Trincomalee close to the centre of the shadow-track, with a duration of total obscuration amounting to two and a half minutes. Strangely enough the increase of the estimated duration was alone noticed by the writer of an article in Nature and it was reasoned that since the duration is so considerable at Trincomalee on the border of the track, it must be very much greater at places on the centre of the track. I need scarcely point out that this inference was unwarranted. In fact the duration of totality can never under any circumstances be considerable for places close to the border of the shadowtrack. In southern India the eclipse will last about as long as the eclipse of last year at the best stations. It cannot be doubted that the observers this year will have a much more difficult task than those who have added so importantly to our knowledge during the eclipses of the last three years. This will appear on a brief consideration of the progress and present position of the problem with which the observers are to deal. However, as Professor Young remarks on this point (writing in 1871), "considered as a demonstration of self-luminosity one bright line is just as conclusive as many." 66 still disposed to assign to the solar element of the corona a lower relative importance than most other astronomers, concedes a thickness of from six to ten minutes"that is from a fifth to a third of the solar diameter. It was in fact demonstrated by this observation alone that the corona, for a considerable part at least of its extension, is a self-luminous object. "Nor can there be any doubt," we may add with Professor Young, "as to the location of the selfluminous matter. It cannot be in our atmosphere, for no possible reason can be assigned why the particular molecules of the air that happen to lie near the lines which join the eye of the observer with the edge of the moon should become luminous In 1868, the observers of the great Indi- rather than others in a different portion of an eclipse discovered that the solar prom- the sky. Nor can it be at the moon; inences are vast masses of glowing vapour, otherwise, of course, it would always be hydrogen being the chief constituent of visible round her disc." Accordingly." these marvellous objects. But the solar he adds, "it is now universally, I think I corona, that glory of light which appears may say, acknowledged that one important around and beyond the coloured promi- element of the corona consists of a solar nences, did not at that time receive its in- envelope of glowing gas reaching to a conterpretation. In 1869, the American ob-siderable elevation. Mr. Lockyer, who is servers directed their chief attention to this beautiful phenomenon; and they were singularly successful in their observations. One result of a very remarkable character was obtained by several observers. The light of the corona when analyzed in the spectroscope was found to be in large part monochromatic, the coronal spectrum showing one bright line. Now the reader is doubtless aware that in spectrum analysis the essential point is to determine where any bright or dark lines may lie along the range of that rainbow-tinted streak which we call the solar spectrum. In this instance the position of the bright line has been most satisfactorily deternined by a very skilful spectroscopist, Professor Young, of America. The line agrees in position with one of the lines in the spectrum of iron, a line also seen in the spectrum of the aurora borealis. But the spectrum of iron contains upwards of 400 lines, while even the simpler spectrum of the aurora contains several lines; that of the corona, on the other hand, has not been proved to contain any other bright lines except the one just mentioned. Others have been suspected, but the degree of their brightness has not been such as to prove beyond all possibility of question that they belong to the solar corona. A somewhat similar mistake occurred last year, guine expectations of the duration of totality in that whereby the Sicilian eclipse party formed too sanIsland This, as I have said, was written by Professor Young in 1871, but before a certain most important fact had come to his knowledge, which without at all affecting what he here puts forward, renders it possible to say much more as to the real extension of the corona. We have seen that a certain object, surrounding the sun on all sides to a distance of from 160,000 miles to 290,000 miles from his surface, is demonstrably a self-luminous envelope. It was to this envelope, or perhaps rather to its brighter portion as seen from the earth, that some proposed to assign the barbarous name leucosphere," to distinguish it from the bright layer of prominence-matter close by the sun, which is called the sierra, or chromatosphere. But the visible extension of the corona is greater yet, and before the eclipse of 1870 doubts still existed as to the actual extent of that solar corona, which all had now begun to recognize as a real entity. That some portion of the light seen around the sun during total eclipse is in reality only due to the illumination of our own atmosphere is altogether beyond question. It is true, indeed, as was pointed out by Professors Young and Harkness, Dr. Curtis, and my The question which arises here, then, is this -at what distance from the eclipsed sun has the light of the solar corona so diminished, and that of the atmospheric glare so increased, that the latter light predominates over the former. This ques self, that none of the coronal light for several degrees from the sun's place, can be solar light reflected by our atmosphere, as had been mistakenly supposed; but it is no less certain that our atmosphere is illuminated not merely in directions lying close up to the moon's edge, but evention is not only exceedingly nice, but, as towards the body of the moon herself, by the light of the coloured prominences and of the real solar corona. The observer himself sees these luminous objects during totality, and therefore the air all round him must be illuminated by them.* actually stated, it is wholly unanswerable, Now here a question of extreme delicacy arises. The true solar corona undoubtedly be determined by means of such appliances grows fainter and fainter with increased as are at present available, yet it was extension from the sun. That is, if we possible last December to demonstrate the could see the corona from some point extension of the corona to a distance far raised above the earth's atmosphere, so exceeding the six or ten minutes acknowlthat no terrestrial illumination could de- edged by those who had once sought to ceive us, we should see the corona gradu- reason away the corona almost wholly. ally diminishing in lustre with distance It is clear that if any definite coronal from the sun, until at last it became too feature extending more than ten minutes faint to be discerned at all. On the con- from the place of the eclipsed sun, could trary, the illumination of our atmosphere be seen at stations far apart, then beyond during totality must necessarily increase all question that feature would be shewn with distance from the direction of the to be extra terrestrial. For instance, it eclipsed sun. This is obvious, because could not possibly be imagined that some those molecules of the air which lie directly peculiarity in the air over Syracuse could towards the moon's place are themselves reproduce a feature of this sort precisely suffering total eclipse from the sun's direct as it appeared to the observers near Xerez. light, and are illuminated by a rather less owing to a peculiarity of the air over this proportion of prominence and coronal light station. than the observer himself, whereas those molecules which lie in directions far removed from the place of the eclipsed sun are suffering either but a partial eclipse, or else, though their eclipse be total, they are yet illuminated by more lustrous portions of the corona and prominence-matter. So that so far as atmospheric glare alone is concerned, we should have, as I wrote in March, 1870, a relatively "dark region around the eclipsed sun and a gradual increase of light with distance from him." Now, soon after the eclipse occurred, it was announced that the observers in Spain had recognized a peculiar gap, shaped like a letter V, in the lower portion of the corona — - on the left hand. This gap was pictured and described to me by my friend, Mr. W. H. H. Hudson, M.A., and Fellow of St. John's College, Cambridge, before any of the other accounts had come under my notice; and it was with some interest that I awaited the January meeting of the Royal Astronomical Society, before which the records of the observers in Spain were One cannot but be surprised at the stress which to be presented. At that meeting a picwas laid by some soon after the eclipse of last De-ture was exhibited by Lieutenant Brown, cember, on the fact that even directly towards the moon's place, light was received which the spectro-in which this V-shaped gap was a very scope showed to be similar in character to that prominent feature. But in the discussion of the bright inner portion of the corona. Not enly was the fact dwelt on repeatedly as a proof which ensued after Lieutenant Brown's that the corona lies on our side of the moon, but paper had been read, Mr. Hudson reIt was commended to my own special attention as a marked that the gap had seemed somewhat proof that I had been mistaken in urging before the eclipse of 1970 that the corona is demonstrably a larger to him, on which Lieutenant olar appendage. In the very paper in which I Brown admitted that perhaps the size of urged this view before the Royal Astronomical Society, on March 11, 1870, I ponted out that our air the gap had not been quite adequately premust be illuminated towards the moon's place by sented in his drawing. the light of all the visible solar appendages - as the prominences, chromatosphere, and corona as well as by reflected earth-light. My words were sufficiently distinct. They ran as follows:-"The light from all these sources should extend over the moon's disc, since it would illuminate the air be-present. tween the observer and the moon's body." After the meeting a photograph, taken during the eclipse by Mr. Willard, of America, was shown to a few of those Why the picture was not exhibited and described at the meeting itself I do not know. Probably the description | be that it does not exist in the photograph, was reserved for American societies. But and that, therefore, the seeming gap was whatever the cause, it is certain that if the due to some peculiarity of the atmospheric picture had been shown earlier, some illumination at the Spanish stations. It doubts which were expressed respecting would not, in this case, be by any means the real nature of the corona would have demonstrated that the sun has no appendbeen obviated. For there, in the photo- age reaching so far as five or six hundred graph, and occupying the precise position thousand miles from the sun's surface, but described to me much earlier by Mr. it would be quite certain that the evidence Hudson, and publicly described and pic- given by the V-shaped gap could not be tured by Lieutenant Brown and others, accepted as demonstrative or even trustwas this V-shaped gap. worthy. The presence of the V-shaped gap in Mr. Brothers's photograph would supply an argument positive and final; its absence would supply a negative argument, proving nothing however, and leaving the matter much where it stood before the eclipse took place. Mr. Willard's photograph was taken at a station near Xerez, so that all that has hitherto been said relates to Spanish observations. To complete this portion of the evidence, I quote the following passage from an interesting account of the eclipse by one of the observers in Spain. It is extracted from the English Mechanic for January 27, 1871. "The corona proper, or glory, or radiated corona — as it is variously called extended a distance of almost the moon's diameter from the moon's edge, but not equally in every direction. It had a greater extension in four directions, at the extremities of two diameters at right angles to each other, so as to give it the shape, ronghly speaking, of a square with rounded corners. It was broken in parts, and notably by one decided V-shaped gap. This was observed, not only by one party, but at three stations, San Antonio, Xeres, and La Maria Louisa, which form a triangle, each of whose sides is five or six miles in length." con The first sketch I received was tained in a hasty note from Mr. Brothers, written soon after his arrival in England. I was surprised, and, to say the truth, somewhat disappointed, to find that the Vshaped gap was not shown, as in the Spanish pictures. There were several gaps, but not one in the lower left-hand portion of the corona. But in the next letter which I received, Mr. Brothers intimated that the sketch was only intended to show the general aspect of the corona- - to show its radiated structure, and that in fact he had not copied the sketch from the photograph, the negative not being as yet unpacked. Some days elapsed before a drawing made from the photograph was sent to me. In this drawing the V-shaped But in the meantime news had been re- gap was not only presented in the same ceived from Sicily which conveyed the un- place as in the Spanish views, but, as in pleasing impression that the observations them, it formed the most remarkable feathere had been all but complete failures. ture of the corona. Soon after, photoIn particular it was supposed that Mr. graphs taken directly from Mr. Brothers's Brothers, who had the management of the negative were in the hands of all who took photographic department there, had been interest in the subject, and there-picunable to obtain any useful results, since tured by the corona itself was the gap no mention had yet been made of his suc-on which so much was held to depend. I was indeed as much surprised as pleased, when I received a letter from him announcing that he had secured five photograps of the corona, in one of which the corona appeared "as it had never been seen on glass before." It will be conceived that I awaited with great interest I have dwelt somewhat at length on this even the first rough sketch of the corona V-shaped gap, because it is in reality of as there pictured. If the V-shaped gap extreme importance. On no former occaappeared in such sketch, the conclusion sion had any distinctive feature of the would be inevitable that a real solar ap-corona been unmistakably recognized at pendage exists having an extension at least equal to that indicated by the bounding edges of the gap-that is, an extension of at least 600,000 miles. If, on the other hand, that well-marked peculiarity failed to present itself, the inference would cess. All possibility of mistake as to the reality of the agreement between this gap and the gap shown in the American photograph was removed by the circumstance that two other gaps, less marked but still recognizable, appeared in both photographs. stations far apart. It happened strangely that on the first occasion upon which the corona was successfully photographed, a very remarkable and characteristic peculiarity was presented by the corona. vourable as are the circumstances of the Fa approaching eclipse, it is not by any means, which, as we have seen, Sir John Herchel certain that the photographs taken at dis- regarded as demonstrated. It is at least tant stations will be so well suited for unfortunate that in thus summing up the comparison as those taken during the results of the costly eclipse expedition of eclipse of last year. So that it is well to December, 1870, Sir W. Thomson did not set store by the great fact which was es- mention what particular discovery then tablished by the observers of the latter made seemed to his judgment to demoneclipse. The following words, taken from strate the terrestrial origin (in the main) a letter addressed to Mr. Brothers by Sir of the .coronal phenomena. One can unJohn Herschel, serve to indicate the im- derstand why Professor Tait, after hearportance which he attached to the photo-ing a lecture on the general subject of graphic records of the V-shaped gap: solar eclipses, should have remarked that Assuredly," he wrote, "the decidedly what he had just heard convinced him that marked notch or bay in both photographs" the corona was of terrestrial origin; for (those taken at Cadiz and Syracuse) a variety of eclipse phenomena seem at agreeing so perfectly in situation (marked a first view to suggest the atmospheric so definitely by its occurrence just opposite theory as the only available explanation. the middle point between two unmistaka- Moreover there can be no question that ble red prominences) is evidence not to some of the most striking phenomena prebe refused, of its extra-atmospheric ori- sented at the beginning and towards the gin. . . . A terrestrial atmospheric origin close of totality, are actually due to the is quite out of the question." illumination of our atmosphere at those And here, in passing, I may venture to epochs by departing rays or returning rays note as somewhat surprising in the pres- of direct sunlight. After a lecture chiefly ence of such an opinion, announced pub-devoted to the consideration of precisely liely before the highest astromomical tri- such phenomena as these, and illustrated by bunal of this kingdom-the statement striking pictures of such phenomena, the made by the President of the last meeting opinion might well be formed that the of the British Association, that the obser chief part of the coronal radiance is simply vations during the eclipse of 1870 proved atmospheric. It is only on a complete surthe terrestrial atmospheric origin of at vey of the subject, and especially of the evileast the principal portion of the coronal dence relating to the corona as seen in the light. Even if we rejected the positive heart of the totality, that the immense evidence obtained during that eclipse, and weight of evidence in favour of the real even if we regarded Herschel's opinion as existence of the corona as a solar appenof no weight whatever, it would still be dage of amazing extent is clearly recogimpossible to point to a single fact dis-nized. But so far as could be judged by covered last December which tended to confirm the atmospheric theory. Facts were noticed then, as facts have been noticed before, which at a first view seem to suggest a terrestrial origin of the coronal phenomena; but undoubtedly none of those facts were novel. Every circumstance that was new to astronomers was in favour of the extra-terrestrial origin, the report, Sir W. Thomson's expression of opinion related solely to the new results the discoveries, in fact effected last December; and it is perplexing in the extreme to hear these results described as demonstrating the atmospheric origin of the chief portion of the corona. At The only new fact which seems in the least to countenance this remarkable statement, is the circumstance that the light • The omitted words relate to the absence of any received from the direction in which the signs which could show the corona to be a phenom-moon's dark disc lay, was found, when eton produced within the space separating the analyzed by the spectroscope, to resemble earth from the moon. On this point, further, I may remark that I had occasion to submit to Sir John the light received from the corona. Herschel certain considerations relating to a theory first sight this seems to show that the that the radiations of the corona are produced by the passage of the solar rays past the moon's corona itself is an atmospheric phenomeedge, through dispersed meteoric matter between non. For certainly the light received from the earth and the moon. I submitted, amongst other the direction of the moon's dark disc canmatters, this question to the great astronomer Whether the light due to the illumination of this not come directly from a solar appendage. dispersed matter would not be altogether inferior And as great stress was laid on this cirin amount to the light received from the illumination of similar matter lying beyond the moon, up cumstance by some, unfamiliar with what to and beyond the sun's place? His reply was, as I was to be expected when this light came had fully expected, that undoubtedly this consid- to be examined, it seems just possible that eration (which he had not before noticed) rendered the lunar theory of the corona altogether untena- Sir W. Thomson may have been guided by their strongly-expressed opinion. ble. |