Next Thursday there will be a total eclipse of the sun, visible in Spain and Southern Europe. In England the eclipse will not be total, nevertheless about eight-tenths of the disc of the sun will be hidden by the moon. As regards the central line of the eclipse, it will first touch the shores of Europe in Portugal, near Odemira, and cross a large tract of Portuguese territory. The objection astronomers have to go to this district to observe the eclipse arises from the fact that little is known about the country, it contains few towns of any note, and there is no regular steam packet service to its shores; the duration of the total phase at this portion of the path of the shadow will be two minutes and eight or nine seconds. The shadow will quit Portugal at Tavira, and at this place the duration of the total phase will be about two minutes eight seconds.
The shadow will next cross a large bay, which indents the shores of Portugal and Spain, after which it will reach Cadiz, which is the first convenient point of observation, although the centre of the shadow will be about twenty miles to the north of the town. The total phase will begin here at ten minutes to eleven o’clock, Greenwich time. Several observers will be stationed at Cadiz to make polariscopic observations, and Lord Lindsay left the Thames in the steamship “London ” more than a fortnight ago, with a very large quantity of apparatus intended to be used in photographing the total phase. His apparatus consists of a 12¼-inch silvered-mirror reflecting telescope, with what may be described as a breechloading apparatus at the side of the tube. The dark slides carrying the photographic plates are made to run along a grooved receptacle attache-l to the side of the tube, and they are pulled out at the other end after each plate has been exposed. As the motions of the slides are all in the same direction as the axis of the tube of the telescope. there is loss tendency to set up vibrations in the apparatus than is the case when the said motions are at right angles to the axis of the tube. Mr. W. J. Lewis, of Oriel College, Oxford, will be among the observers at Cadiz, and he is going to try to ascertain whether the corona is polarised all round the sun. Mr. C. G. Talmage, F.R.A.S., assistant at Mr. Barclay’s Observatory, Leyton, Essex, is going to observe the planet Saturn, which will be very close to the sun during the eclipse, and he is going to try to ascertain by measurement whether the light from Saturn is refracted by its passage through the coronal envelope of the sun, if an envelope of the sun it be. Mr. R. Abbay, fellow’ of Wadham College, Oxford, and Captain Toynbee F.R.A.S., of the Meteorological Department of the Board of Trade, will be among the ten English observers stationed at Cadiz. Professors Pickering and Young, of one of the American expeditions, will also be at Cadiz, the former to do polariscopic work, and the other to observe with the spectroscope.
The next convenient place of observation is Gibraltar, which, however, is thirty miles south of the centre of the dark part of the shadow thrown by the moon; the duration of the total phase at Gibraltar will be two minutes ten seconds. At Gibraltar, Mr. James Buckingham, F.R.A.S. assisted by Mr. F. Beasley, will photograph the total phase with a great refracting telescope, bating an acting aperture of 9in. He has also taken out with him a portable wooden house to be used as a dark room in which to perform the photographic operations; the lower end of the telescope will pass through a large light-tight “sleeve” in the side of the wooden house, so that all the photographic operations, including the exposure of the plates, will be carried on inside the dark room. A very long camera, with a lens of long focus, will be fixed to the outside of the tube of the telescope, so as to be moved by the sawe clockwork as that which drives the telescope; with this lens and camera, an attempt will be made to photograph the corona for a very great distance round the sun. The sappers or other assistants outside the photographic house will have to expose the plates in the camera. during the total phase. In the middle of the totality, Mr. Buckingham will expose one of his plates at the chemical focus of the object-glass of the telescope for a long time, in order, if possible, to secure a picture of the corona, the light from which is much feebler than the light from the solar prominences. Mr. Ladd, the philosophical instrument maker, will be at Gibraltar making polariscopic observations. Professor Thorpe, of Anderson‘s University, Glasgow, will be at Gibraltar making experiments to determine the amount of actinism during all parts of the eclipse. Captain J. P. Maclear, R.N., of the Royal Naval College, Portsmouth, will be stationed at Gibraltar to measure the apparent distance of Saturn from the sun during the total phase, in the endeavour to ascertain whether the solar atmosphere refracts the rays of light reflected by Saturn.
Following the shadow thrown by the moon, the next eligible place of observation is Oran, in Africa; this town is nearly under the central line of the eclipse, and is also favourably situated in the very important particular of presenting a very good chance of having fine weather during the total phase. The duration of the total phase at Oran will be two minutes ten seconds. Among the observers who are on their way to Oran are Mr. William Huggins, F.R.S., Mr. William Crookes, F.R.S., Professor Tyndall, and Mr. Carpenter. The work to be done at Oran is chiefly polariscopic and spectroscopic in its nature. The observers will leave H.M.S. “Urgent” at Gibraltar, and go on to Oran by the ordinary steamboat; at Oran they will have little time to fix their instruments, do their work, and come back again, in order to catch the “Urgent” before she starts from Gibraltar on the return journey. Oran is a town which was long in the possession of the Spaniards, who built several churches therein, and added to its fortifications. It is pleasantly situated at the foot of a mountain, and the surrounding scenery abounds in rocky precipices, orange plantations, and small streams. The inhabitants of the place are Turks, Moors, Christians, and Jews.
The path of the eclipse afterwards passes to the south of Algeria, then bends northwards once more, recrosses the Mediterranean and passes through Sicily. A large party of observers have left London by the overland route to observe the eclipse in Sicily, and among the astronomers now on their way there are Mr. J. Norman Lockyer, F.R.S.; Mr. C. E. Bowen, M.A., of Harrow School; Mr. J. Brett, 6, Pump-court, Temple, E.C.; Mr. A. Brothers, F.lt.A.S., 14, St. Ann’s-square, Manchester; Mr. W. K. Clifford, Fellow of Trinity College, Cambridge; Mr. G. H. Darwin, 14, Arlington-street, Piccadilly: Mr. George Griffiths, M.A., secretary to the British Association; Mr. W. A. Harris, B.A., New University Club, St. James-street; Mr. A. C. Ranyard, M.A., New University Club, St. James-street; Professor H. E. Roscoe, F .R.S., Owen’s College, Manchester; Mr. G. M. Seabroke, 24. New Ormond-street, Queen’s-sqnare, W.C. ; Mr. O. Vignoles, F.R.S., President of the Institution of Civil Engineers; and Mr. Vignoles, C.E., 21, Duke-street, Westminster. In all, ten observers are on their way to Cadiz, twelve to Gibraltar, ten to Oran, and seventeen to Sicily.
The Sicilian party will do their work near Syracuse, a town a few miles to the south of the central line of the dark shadow ; at Syracuse the total phase will begin at twenty-seven minutes to twelve Greenwich time, and the duration will be one minute fifty-two seconds.
Of all the work to be done during next Thursday’s eclipse perhaps the most important is that which will be performed by Mr. Norman Lockyer, who takes out some new apparatus specially constructed to examine the light from the corona with a spectroscope. This “coronal spectroscope” has a single prism of great size, furnished with a slit the whole length of the prism—about 15in. Then there is an eyepiece with a very large field, which takes in the whole of the light passing through the slit and the prism. This “Kellner” eyepiece gives a very wide field of view, so that with this apparatus Mr. Lockyer will be able to collect much light from the corona, and to see at a glance whatever may be taking place at any part of the spectrum. Attached to the slit he has a special contrivance for bringing into the field of view the spectrum of hydrogen and two or three metals at the same time as the spectrum of the coronal light, so that he will thus stand a good chance of noting the position of coronal lines relatively to known lines of the spectrum, supposing the said coronal lines do not coincide absolutely therewith. This coronal spectroscope will be used in conjunction with a 9in. reflecting telescope, with a mirror about 6ft. in focal length. By this plan plenty of light will be collected from the corona, and Mr. Lockyer will take care in collecting the light to keep away from the chromosphere and red protuberances in order that the results may not be vitiated. He can easily do this, because, practically speaking, the corona is of unlimited dimensions.
The polariscopic observations are intended to determine whether the corona shines by reflected light or whether it is self-luminous. Suppose the light direct from the sun be polariscopically examined, and then the light of the sun reflected from a sheet of white paper, the polariscope will show the difference between the two. Very possibly one part of the corona may shine by reflected light and another part be self-luminous; to determine this point the attention of different observers will be given to different parts of the corona. The chief object of the whole ‘of the coronal observations is to determine whether the corona itself belongs to the sun or is an optical effect produced by the atmosphere of the earth. There is at present much division of opinion on these points, and even after the observations are made there will probably be room for plenty of further arguments.
After leaving Sicily the central line of the moon’s shadow will cross another stretch of ocean and enter Turkey in Europe as well as northern Greece. On the sea coast of Turkey the duration of the total phase will be 1min. 42sec., and it will begin at ll 43 o’clock, Greenwich time. Turkey is not only too far off to be a convenient place of observation but there are no large towns in it scattered along the path of the eclipse, and but little is known of the country traversed by the shadow. The path of the moon’s shadow comes to an end in the neighbourhood of Russia and the Black Sea.
Such are all the particulars we have at present to give about the eclipse of next Thursday, and our readers, by taking a map and tracing the path of the eclipse through the towns mentioned in this article, will be able, by the aid of the telegrams printed in the daily papers next Friday morning, to post themselves well up in all the particulars relating to the eclipse, as well as the work done. The coronal spectroscope and much of the apparatus described in this article, was made by Mr. John Browning, F.R.A.S., optician, of the Minories, and he has very generously lent a great deal of valuable spectroscopic apparatus to different observers for the purpose of examining the solar phenomena presented by the eclipse.
The mechanics Magazine, 1870