The great eclipse of 8 July 1842 may be said to mark the beginning of serious and ac curate attention to the complex physical fea tures of total solar eclipses. Then the path of totality swept from Spain across France, Italy, Austria, Russia and central Asia and thus per mitted many expert European astronomers to view and record the marvelous phenomena. It is noteworthy that in reports of this eclipse the red solar prominences were first described with precision, three large ones having been seen on the upper edge of the moon. The corona was also described as of a brilliant white luminosity and extending at least 15 minutes from the moon's edge. Among the many phenomena ob served, the objects of special speculation were the newly discovered red flames or protuberances which Baily considered as "forming a portion of the corona,* and Airy, who observed the eclipse through clouds, attributed to "some irregularity in the density of the cloud's edge.* During the total solar eclipse of 28 July 1851 both the corona and the prominences were ob served in greater development than in 1842, and by several observers a scarlet sierra or reddish arc of light was noted near those parts of the lunar disc where the rays of the sun had dis appeared, or else were about to appear. The observers for the most part became certain that the prominences were true solar append ages, because they were apparently uncovered by the moon's edge on the western edge of the sun, and extinguished on the eastern limb. A precious photographic achievement was the single successful daguerreotype taken by Bar owski with a telescope of 2.4 inches aperture, attached to the Konigsberg heliometer, and with an exposure of 84 seconds. It distinctly pictured the corona and several prominences, and thus became the first unchallenged record of these features of an eclipse.
It was, however, at the eclipse of 18 July 1860 which traced its total path across northern Spain, the Mediterranean and northeastern Africa, that photography was first applied with marked scientific success. Mr. Warren de la Rue and Padre Secchi each secured good representa tions of the prominences; the former also faint traces and the latter good portrayal of the corona. Comparison of these photographs set at rest, once for all, any doubt still existing as to the true solar character of the promi nences. The photographic method of attacking the problems arising in total solar eclipses was from this time firmly established. Its progress ive development was determined chiefly by three elements: First, increase in the sensitive ness of the plates; second, knowledge of the exposure time required; and third, necessary advancement in practical optics. The gelatin emulsion process introduced by Maddox in 1871, and given the touch of great sensitiveness by Bennett, in 1878, secured the first. Successive failure and success, coupled with scientific method in photometric study, brought the sec ond. And, finally, refined theoretical discus sion of the principles of optics as applied to celestial photography, coupled with marked im provements in optical manufacture, supplied the third. So that now by separate and regulated
exposures, with specially designed instruments, either the outer long, delicate coronal wings may be pictured to an extreme limit, or the intensely brighter parts of the inner corona, shown with all their involved filaments and rich tracery, and with many a curious relation to the prominences and chromosphere.
The application of the spectroscope to the study of the brilliant red prominences during the total solar eclipse of 18 Aug. 1868 will al ways be memorable. Tracing its course across the Indian and Malayan peninsulas, and afford ing a total phase of more than five minutes and a half, this eclipse was spectroscopically ob served by Janssen, Major Tennant, Lieutenant Herschel, Pogson and Rayet. The light of the prominences was recorded by each of these ex perts as consisting of several bright lines dearly indicative of the gaseous nature of those remarkable objects. The hydrogen lines C and F were certainly identified, and a yellow line, thence to figure famously in astronomy as Di due to helium, was too hastily assigned to sodium. Another notable result of this eclipse was the discovery by Janssen of a method by which the bright tines of the promi nences could be observed apart from any eclipse whenever the sky was sufficiently clear. On the day following the eclipse he was able suc cessfully to apply the new method; to demon strate beyond controversy that the bright lines of hydrogen prevailed in the protuberances; and to make a third discovery always to be asso ciated with this great Indian eclipse, namely, that of pronounced and rapid changes of form occurring in the prominences. Lockyer in Eng land had for some time been at work on the same method for obserling the bright promi nence lines, and his final success was reported to the French Academy at the same meeting at which Janssen's achievements were announced. The coincidence was evidently rather deter mined by the progressive advancement of spec troscopic science than by accident. The spectrum of the corona of this eclipse was generally ob served as a faint continuous one without lines dark or bright. Rayet, indeed, noticed a cer tain bright line near E as extending above the prominences. The polariscopic observations of Lieutenant Campbell and Captain Branfil proved that the coronal light was polarized in planes passing through the sun's centre. The infer ence thus seemed doubly plain that some solar light was reflected by the corona, although the absence of the dark Fraunhofer lines required explanation.
During the total eclipse of 7 Aug. 1869 spectroscopic observations established the exist ence of a bright green line crossing the continu ous spectrum of the corona. It was at that time estimated as coincident with a bright chromospheric line of approximately the same wave-length, and the name Coronium was given to the element from which the light which was the source of the line came. The photographic records of the powerful spectro graphs applied to the eclipses of 1896 and 1898 furnished data, however, which definitely sepa rated coronium from its neighbor in the chromosphere.