NATURE 



201 



THURSDAY, JANUARY 14, 1875 



THE APPROACHING ECLIPSE OF THE SUN 



''"P'HE energetic action of the Council of the Royal 

 -L Society, and the wise liberality of the present 

 Government in matters connected with scientific investi- 

 gation, have saved us from what would have been little 

 short of a national disgrace. 



If all goes well, the approaching eclipse of the Sun 

 — during which, as stated by INIr. Hind, better oppor- 

 tunities for the observation of totality will be afforded 

 than are likely to be again offered before the close of the 

 present century — will be observed by English parties in 

 Siam, and either in I5urmah or in some island in the Bay 

 of Bengal. 



The work to be done, as determined by the Council 

 of the Royal Society, and the investigations which have 

 led up to it and render it of so great an importance, 

 have been stated in Monday's Times in an article 

 which enters so fully into the problem, that we take the 

 following extracts from it : — 



" In i860, Mr. De la Rue, a member of the Astronomer 

 Royal's Expedition, and Father Secchi, a delegate of the 

 Italian Government, were enabled, by the photographs 

 of the eclipsed Sun, which were then taken for the first 

 time, to place beyond all doubt that the strange red pro- 

 minences seen round the dark body of the Moon at the 

 moment of total eclipse really belonged to the Sun's 

 atmosphere. This was a fine achievement, for it settled 

 a point which had been in debate for a century and a 

 half. Important though it was, however, it was fairly 

 dwarfed by the results of the expeditions sent by the 

 Indian, French, German, and Austrian Governments to 

 observe the eclipse of 186S in India — dwarfed because 

 in the meantime an instrument had been placed in the 

 hands of the astronomer of a perfectly new kind of 

 power. It was no longer a question of place and shape, 

 but of material. Janssen, Tennant, Pogson, Weiss, and 

 many others observed the eclipse with the spectroscope, 

 and its story was that the prominences which in 1S60 had 

 been proved to belong to the Sun really consisted of a 

 glowing gas, or a mixture of such gases. But the spec- 

 troscope was not of use only during eclipse:. Before 

 1868 Kirchhoff by its means had stated the approximate 

 composition of the Sun's atmosphere, taken as a whole ; 

 and immediately after the eclipse of that year it was 

 found that by its aid the brightest part of the Sun's 

 atmosphere, to which alone up to this time attention had 

 been directed, could be seen without an eclipse at all. 

 Indeed, we were soon told that outside the bright round 

 disc that we see there was an evelope of glowing hydro- 

 gen gas, to which envelope the name of chromosphere 

 was given, and into which are frequently injected from 

 below magnesium and sodium, and, more rarely, iron and 

 the other heavy metals. Here, then, we were enabled 

 roughly to sort out into strata the various substances 

 already detected by Kirchhoff ; that is, it was established 

 that the gases and vapours were not all mixed up toge- 

 ther, but that the lightest, such as hydrogen, magnesium, 

 and sodium, were generally at top, and that, as the others 

 were shot up from time to time, and some of them more 

 frequently than others, some of them were, as a rule, 

 located lower down in the solar atmosphere than the 

 others. 



"The eclipse of 1869 the Americans had all to them- 

 selves, and splendid use they made of it. It has been 

 well said that the line of totality which swept across the 

 Vol. XI. — No. 272 



United States was one continuous observatory. In this 

 eclipse the halo of light outside the prominence-envelope 

 was the subject of special inquiry, and now this was pho- 

 tographed, as the prominences themselves had been in 

 1S60. At the same time that this was done it was esta- 

 blished that there was some other substance lying even 

 outside the hydrogen. 



" The eclipse of the next year, 1870, was best seen in 

 different parts of the Mediterranean. The English 

 Government, applied to by the Royal and Astronomical 

 Societies, at once supplied the requisite iunds and ships, 

 and sent three parties ; the United States Government 

 sent an equal number ; and the French one party, the 

 Spanish and Italian astronomers observing locally. 

 Further facts were obtained of great value ; but the 

 weather was not good, and the true nature of the corona 

 was not considered to be finally established. Another 

 appeal was therefore made to the Government in 1871 by 

 the Presidents of the British Association, the Royal 

 Society, and the Astronomical Society combined, to 

 observe the eclipse of that year in India. The Govern- 

 ment responded with a remarkable promptitude, granting 

 everything that was required. The Indian Government 

 not only had strong parties of their own, but largely aided 

 the observers sent out from England ; and the French 

 Government were again represented by the illustrious 

 Janssen, who had made his exit from beleaguered Paris 

 m a balloon to observe the phenomenon. The Dutch 

 Ciovernment had an expedition in Java. The combina- 

 tion of the results of the parties, most of whom had 

 splendid weather, led to the following most important 

 conclusions : — 



■' First, the corona was now at last photographed, under 

 nearly the same instrumental conditions, from three diffe- 

 rent places, and the exact similarity of the pictures proved 

 beyond all doubt that part of the corona was a solar 

 appendage. The size of the Sun was enormously increased 

 by this result. Secondly, evidence was obtained render- 

 ing it extremely probable that the light of the outer parts 

 of the true solar corona, or coronal atmosphere, as Janssen 

 proposed to call it, was stronger in the violet and ultra- 

 violet parts of the spectrum than elsewhere. Thirdly, it 

 was proved that for some distance above the hydrogen 

 envelope, as seen without an eclipse, less bright hydrogen 

 existed. The so-called chromosphere, therefore, was a 

 layer of brighter hydrogen and other vapours. Other 

 results were obtained, but the above are those on which 

 we wish to lay the greatest stress, for reasons we now pro- 

 ceed to state. 



" Since the eclipse of 1S71 the every-day observations of 

 the Sun and of his lower atmosphere (the chromosphere), 

 which can be rendered visible by the spectroscope, have 

 gone on with great vigour, especially in Italy. A special 

 study of the chromosphere has been made at the expense 

 ot the United States Government, at an elevation of some 

 9,000 feet, on the Rocky Mountains ; and extensive 

 laboratory researches have been undertaken with the 

 view of enabling us to understand better the various 

 phenomena observed. We shall now only refer to the 

 two latter branches of the work. Prof. Young, on the 

 Rocky Mountains, in the clear air at so great a height, 

 saw that the chromosphere was much more complicated 

 than it appears to those who observe in the plains. 

 Among other things, he found that the vapour of the 

 metal calcium, the principal characteristic lines of which 

 require perfect atmospheric conditions to enable us to see 

 them, was very often present along with magnesium, but 

 his observation left it doubtful which vapour extended 

 highest generally. The laboratory experiments proved 

 that, in the case of any one metal present in the Sun, the 

 metal behaves exactly the same in the Sun's atmosphere 

 as it does when driven into vapour l^y the passage of the 

 electric current between the carbon poles of an electric 

 lamp. At the greatest distance from the poles the spec- 



