The eclipse of December, 22nd, 1870.
By Professor Roscoe, F.R.S.
It is satisfactory to know that in spite of the worst Possible weather, as well as of misfortunes and losses of Various kinds which fell to the lot of the English and American savants who went to Africa, Spain, and Sicily to 9bserve the eclipse, the results are of real value and importance, and that the grant of £2000 of public money as, therefore, not failed to purchase substantial benefits to science and the nation.
In the first place, then, this eclipse was only total on coming into the western hemisphere near Cape Farewell, leaving it at Taganrog, covering only a narrow strip of the earth’s surface. The duration of the totality (the only time when the wished for observations could be made) in no place exceeded two minutes, and in many situations, even on the central line, was several seconds less; so that it was before all things necessary to choose a good situation, and for each observer to have a definite and tolerably simple set of instructions to follow. Such instructions were carefully drawn out by Professor Stokes and Mr. Lockyer as Soon as the despatch of the expeditions were settled and placed in the hands of every member of the party. In order to avoid disappointment from bad weather, detachments of observers, complete in every necessary branch, and fitted with instruments for the examination of all the Various phenonema of the eclipse, which the experience and foresight of the leading astronomers and spectroscopists of the country could suggest, were dispatched to three chief stations. The conduct of the Oran division on the coast of Africa was undertaken by Dr. Huggins, facile princeps in spectroscopic astronomy; the Cadiz division under the Rev. S. J. Perry; the Gibraltar party under Captain arsons; and the Sicilian party led by Mr. Lockyer. The American astronomers, under the leadership of Professor Peirce, of Harvard, were likewise divided between Spain and Sicily. Each of these various parties contained men charged with four sets of observations: first, spectroscopic; secondly, Polariscopic; thirdly, photographic; fourthly, sketches with naked eye or with telescope. he main obect of all the observations was concentrated on the examination of the solar corona – that halo of white silver whose light strange and varying forms, as drawn by different persons, even during the same eclipse, has so long proved an enigma to astronomers. In spite of the numerous attempts which had previously been made to certain the true character of the corona, this secret of nature had yet to be unravelled, but it was to be fairly expected that the united and systematic endeavours of English, American, and Italian observers of the eclipse of 1870 would do something to settle the question.
To describe the various hypotheses concerning the corona which have from time to time been set up by astronomers after viewing one eclipse, generally only to be knocked down by other savants on observing the next eclipse, is here out of the question. Suffice it to say that it has been supposed first, that the corona was altogether a solar phenomenon; secondly, that it was none of it solar, but all due to diffraction or irradiation; thirdly, that part of the phenomenon is really caused by outlying zones of incandescent solar atmosphere, whilst a portion is to be ascribed to the action of our air; perhaps another portion to irradiation Produced only in the eye of the observers. To give an idea of the singular differences existing between the drawings of the corona we give the following figures. No. 1, copied from an illustration given in a pamphlet on the eclipse of December last by Professor Angello Agnello; No. 2, from the eclipse of 1868, as seen by Dr. Mayer, at Burlington, U.S.
Let us next see in what way each of the four modes of attack was expected to force the capitulation of this citadel. In the first place, as regards the spectroscope, contradictory results had been obtained in the former eclipses. Colonel Tennant, in India, observed that the corona, in the eclipse of 1868, gave a continuous spectrum ; whilst in 1869 Professors Young, Harkness, and Pickering, in America, saw one or two bright lines in the corona spectrum. If, now, the Indian accounts were correct, the light from the corona is not due to reflected solar light, for then the dark solar lines would have been seen, neither is it caused by incandescent gas, like that of the red prominences, for then bright lines would be seen, but it just be, in all probability, emitted from incandescent solid bodies, such as the clouds of meteors which we know circle in enormous numbers round the sun. If, on the or hand, the American observations are correct, the gaseous and self-luminous nature of the coronal matter is ascertained beyond shadow of doubt.
The object of the spectroscopic observers was, therefore, simply to note the appearances presented by the spectrum of the corona spreading to a distance of at least the sun’s radious from the solar limb. None of the previous observers had determined exactly the position or wave length of the green line with exactitude, although Professor Young believes that it coincides with an iron line marked 1474 on Kirchhoff’s scale. The methods adopted for catching this line and marking it down with accuracy were various. Dr. Huggins arranged a registering scale, which he finds accurate and useful. Mr. Lockyer adopted the plan of directly comparing the corona spectrum with one of incandescent hydrogen; I, myself, did the same with the iron spectrum, and Professor Winlock devised a special plan for mapping the position of the lines. Unfortunately bad weather prevented most of these plans from being tried. D. Huggins saw nothing at Oran; Mr. Lockyer’s station at Catania was overclouded during totality; whilst I was in a snow storm on Etna with all my instruments 5000ft. above the sea. Professors Young and Winlock at Xeres, and Mr. Burton and the Italian astronomer Fr. Denza at Agosta, however,
luckily had a view; they saw and mapped the position of the green line, which really turns out to be 1474 on Kirchhoff’s scale. It extends all round the sun to at least 20 min. from the disc, and therefore plainly points out :-(1) That the corona is a solar phenomenon; (2) that part of the light is given off from a glowing gas. What the nature of the substance may be which emits this singular light it is as yet impossible to say; we may, however, affirm, in the first place, that it is probably not iron, because other equally bright lines of iron are not seen; and, in the second place, we learn from the observations of Angström and Young that a similar green line is visible in the spectra of the aurora and the zodiacal light. Here a new field of interest at once opens out. Is the solar corona in some way connected with these other phenomena about which we know so little? Is the corona simply a gigantic solar aurora? and if so, what can be the nature of this material which either occupies space or exists in the highest regions of the planetary atmospheres; and how is this substance heated so as to become self-luminous?
Thanks to the Janssen-Lockyer method, we need not trouble ourselves now with drawing, photographing, or observing the red prominences during a total eclipse. We can map them when we please. May we not with confidence look forwad to a time when the green coronal line may be also seen when the sun shines brightly? Indeed this line has already been seen high up in a prominence by Mr. Seabrooke. It was nevertheless of the greatest importance on the occasion of the last eclipse to map the prominences immediately before or during totality, in order that the relation between the extent and position of these red flames should be compared with the outline of the corona as sketched by trustworthy observers. And here one of the most satisfactory results of the late expedition may be mentioned. Mr. Seabrooke at Catania carefully mapped all the prominences existing on the sun’s disc thirty minutes before totality. Professor Watson, the celebrated astronomer of Ann Arbor University, drew the corona as it appeared to him at Carlentini, observing with a 2½ in. refractor. On comparing the two drawings thus independently made a most interesting series of coincidences presented themselves; wherever on the solar disc a large group of prominences were seen in Mr. Seabrooke’s map there a corresponding bulging out of the corona was chronicled on Professor Watson’s drawing, and at the positions where no prominences presented themselves there the bright portions of the corona extended to the smallest distance from the sun’s limb. Hence the close connection between the great solar cyclones or storms which become visible to us on the edge of the sun as red flames, and the outlying portions of solar atmosphere which we term the corona is no longer a matter of doubt or surmise, but a conclusion founded upon observational proof of the most convincing kind.
To sum up the results of the spectroscopic observations as regards the chemical nature of the corona, prominences, and chromosphere, we have the existence of the following bright lines and substances proceeding towards the photosphere: – (1) Corona (possible new element), green line (1474); (2) prominences, highest portions, hydrogen, F, C near G (27 96); (3) prominences, lower portions, h; (4) chromosphere (possibly a new element), near D; magnesium, b and other lines; sodium, D; barium and iron, several lines.
The next set of observations are those made with the polariscope. These have for their object to ascertain how large a portion, if any, of the light from the corona is polarised, and therefore reflected sunlight. Now as no dark lines had been seen in the corona spectrum, we might feel inclined to assume that the coronal rays contained no reflected sunlight, but Mr. Lockyer has pointed out cogent reasons why, as generally observed, the fine dark lines in the sunlight would probably not be seen in the corona spectrum. If, therefore, the polarised condition of the coronal light were ascertained it would go far to show that the faint continuous spectrum of the corona was due to reflected sunlight. Such proved to be the case; for Mr. Raynard at Villasmondo, Mr. Peirce, jun., North of Catania, and Mr. Ladd in Spain all observed strong radical polarisation in the corona.
Next we come to the results of photographic work, perhaps the most interesting chapter of the whole. It is clear that any impression produced upon a photographic plate by such a phenomenon as the coronal light is of the greatest possible value as a true record of its extent and distribution. Only once before has a satisfactory photograph of the corona been obtained, by Mr. Whipple at Shelbyville, Kentucky, in August, 1869. Hence great preparations were made both by American, English, and Italian observers to photograph the phenomenon in December last. The American photographers in Sicily saw nothing, and obtained no picture; the well-known photographer Dr. Vogel, of Berlin, was with me on Etna in a snowstorm; at Oran no photograph was obtained, but at Syracuse, thanks to the energy and skill of Messrs. Brothers and Fryer, at any rate one perfect photograph of the corona was secured, and the results to which this single photograph may lead will in themselves repay for the expense and trouble of the expedition.
The first important fact which this negative reveals is the actual existence of dark, nearly radial bands shooting out from various portions of the sun’s disc through the luminous portions of the corona; the second fact which it records is that a distinct chemical action is observed on the plate, extending in certain directions from the sun’s disc to a distance which we should previously have conceived to be incredible, viz., more than two solar diameters! These two facts are quite new, and as entirely unexpected as they are unexplained.
Lastly come the sketchings and drawings, a class of observations with which it is most difficult to deal, because they are less precise and definite, and, therefore, more liable to error. According to the instructions, the sketchers — situated at different as well as the same station—were to draw the corona in the same way, using similar telescopes. So that if two men—one in Spain and the other in Sicily—were to draw the streamers or dark radial bands in actually the same positions on the sun’s disc, we should be convinced that these appearances were truly solar, and not produced either in our own atmosphere or in the eye of the observer. Whereas, if two sketchers — drawing independently at the same place – drew pictures which were identical, but different from those drawn at a distance, we should infer that the rays, &c., were not solar, but due to our air; and if even the pictures of the neighbouring sketchers differed, we should fairly conclude that the difference in the drawing was due to some effect produced in the eye of each observer. It is difficult, not to say impossible, to estimate truly the differences in the drawings without careful scrutiny. Still, we have good evidence to show that the sketches made in December last, even from the same locality, exhibit differences at least as marked as those drawn on former occasions Dr. Macdonald, on board the Lord Warden, off Catania saw eight rays ranged with perfect symmetry. Captain Brandreth, on the same ship, saw only two elliptical hoops crossing each other at right angles. Captain Cochrane, of the Caledonia, saw, besides the ring, a complicated stellate figure with rays of nearly equal length; whilst Mr. Dexter, at sea, between Catania and Syracuse, saw only one ray of great length. From the differences between these sketches we should therefore conclude that a part of the phenomena of the corona have a subjective origin.
Under these circumstances it would seem as if these sketches of the several appearances of this singular phenomenon were not of much value for recording the physical outlines of the outermost portions of the corona. The permanent impression on sensitive plate can and must always be substituted for the transient one on the retina. Still there are certain parts of the corona exhibiting structure to the eye which probably never . can be reproduced by photography. Thus, a laminated structure similar to that well known to exist in the heads of comets was plainly seen in the bright portions of the corona by Professor Watson, and this may prove to be a most essential feature of the coronal light.
Until all the various observations of the “AngloAmerican Eclipse Expedition of 1870” are collected together, and the numerous drawings compared, it is impossible to form an opinion as to the exact conclusions to which they may lead; enough has, however, been said to show that this expedition, undertaken and carried out under difficulties (not to say dangers) such as few have encountered and surmounted, has succeeded in the great object of its mission, and that man’s knowledge concerning the great luminary which supports our life and being is more complete than it was before the hundred seconds of darkness fell on the face of the earth at 2 p.m. on the 22nd of December, 1870.
The Engineer, 1871