The eclipse of 22 Dec. 1870 was of particu lar interest to the American observers who had studied that of 1869. This resulted in the despatch of two large parties to Europe, one directed by the United States Naval Observa tory and the other by the officers of the Coast Survey. If there remained any doubt as to the gaseous nature of the inner corona it was to be dissipated by the fine drawing and descrip tion of the inner corona furnished by Professor Watson of Ann Arbor, stationed at Carlentini, Sicily, and observing this gaseous shell to an elevation about 5 minutes; and by Professor Young's observations in Spain of the now familiar green line of coronium, visible plainly in this area, and traceable as far as 16 minutes from the solar limb. This eclipse will always be peculiarly famous for Professor Young's discovery of the reversing layer as a thin shell lying immediately above the photosphere. The existence of this reversing layer has since 1870 been repeatedly verified visually, and more re cently by photographic records. It comprises an envelope of glowing vapors of an estimated depth of about 600 miles lying at the base of the chromosphere. Its thinness causes the bright lines to appear for but a few seconds, or as a brief flash, unless the observation be made at some station near the edge of the lunar shadow.
The eclipse of 12 Dec. 1871, whose path lay across southern India, and northern Australia, was photographed according to a consistent plan, and photographs were subsequently studied with consummate skill and the delicate results reproduced in detail with masterful accuracy. (Vol. XLI, Royal Astronomical So ciety.)) Here for the first time was available a representation of the corona in all its com plex glory of spaced polar rays, dark rifts, filaments straight, curved, and interlacing, and of stupendous synclinal structures with rich nebulous mottling. The delicate portrayal of the wet-plate process was only equaled by the scientific and artistic manner in which the re sults were integrated.
The eclipse of 16 April 1874 was visible only in southern Africa, and observed at Klip fontein by Mr. E. J. Stone, the astronomer royal of the Cape Observatory. He confirmed Young's observation of the reversal of the Fraunhofer lines near the photosphere, and traced the main, green coronal line to more than a degree from the moon's edge. The corona exhibited extended equatorial wings, covering in some parts more than three lunar diameters. The spectroscopic character of the corona, and its unchanged features, as viewed from different stations in Africa, led Stone to insist on the solar origin and cosmical nature of the outer corona.
The path of the total solar eclipse of July 1878 lay diagonally across North America from Bering's Strait to the Gulf of Mexico, and afforded a memorable opportunity to many American and foreign astronomers. The rection of the line of totality across the Rocky Mountains permitted several astronomers to observe the phenomena at extraordinary tudes. Langley on Pike's Peak, at an altitude of 14,100 feet, traced the corona in the direction of the ecliptic for a distance of at least 12 lunar diameters, and felt great confidence in saying that this, though covering a range of over 10,000,000 miles, was abut a portion of its The polar rays and filaments of the inner corona received careful study from such experienced observers as Ormond, Stone, Boss, Paul, Trouvelot and Upton. The dry-plate
photographs obtained by Professor Hall's party at La Junta, and by Professor Harkness' party at Creston, were in remarkable agreement as to i the details of the inner corona, the same n tensities of shading, to great delicacy, being•in common, and the pronounced filamentous char acter of the polar rays, as well as the marked curvature and brightness of the equatorial ex tensions being clearly shown in both sets of plates. It was the concurrent opinion of many observers that although the corona exhibited vast wings, it lacked, particularly in its inner portions, the brilliancy of the corona of 1869. As to the spectroscopic observations, the gase ous corona manifested, as it seemed, a pro nounced sympathy and intimate relation with the 11-year sun-spot period, giving faint spec troscopic lines at the minimum, 1878, and bril liant ones at the maximum, 1869, 1870, and 1871, of the period. It was also discovered that a greater development of the corona in the equatorial than in. the polar regions is one of the characteristics of coronas which have been observed during periods when there were few sun-spots.
Thus, during total eclipse of 17 May 1882 the photographs of the corona reproduced thz form which it had in 1871, near a sun-spot maximum, being most extended at points away from the solar equator and with no special structure at the poles. It was thus distinguished from the coronas of 1878 and 1867: near sun spot minima, which were greatly elongated along the sun's equator and of marked struc ture at the poles.
During the eclipse of 29 Aug. 1886 Prof. W. H. Pickering, of the Harvard College Ob servatory, made a valuable investigation on the photographic photometry of the corona, the re sult of which led Holden to employ a similar method in the two eclipses of 1889. The values obtained by these two observers are stated in the following table. From this eclipse also, a study of the largest prominence of the eclipse, which rose in a somewhat spiral form to the altitude of 150,000 miles, the Italian astrono mer Tacchini was enabled to verify the dis covery of white prominences and immediately announced to the Academy of the Lincei at Rome: *That during a total solar eclipse of the sun there may be seen most beautiful eclipse and of the following total eclipse four years later, Professor Schaeberle deduced his 'Mechanical Theory of Solar Corona' — that the corona is caused by light emitted and reflected from streams of matter ejected from the sun, the stream lines of the corona coincid ing with elliptical areas having one focus at the sun's centre, the origin of the streams being, in the main, confined to the spot-zone regions. Professor Schaeberle applies his theory to the location of half a dozen solar areas productive of the actual streamers photographed in the eclipse of 1893. Whatever the merit of the par ticular theory, it has had the advantage of ing out the importance of dynamical and spatial study of the coronal streamers.