Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/858

788 implies an excessive intensity of heat. We know also, in another mannei* presently to be described, that the vapours of the solar atmosphere, although less intensely hot than the glowing mass of the sun, are yet so hot as to be brightly luminous. The evidence respecting the constitution of the sun is based on the following coincidences between dark lines of the solar spectrum and bright lines in the spectra of various elements:— Hydro &quot;en. Lines coincident. 4 Manganese Lines coincident. 57 9 Chromium 18 11 Cobalt 19 75 Nickel 33 Magnesium 4 + (3 ?) Zinc 2 2 (?) Copper. . . . 7 Iron... 450 Titanium 200 It must not be supposed either that no other terrestrial elements exist in the sun, or that no other coincidences are recognisable. The above list simply indicates the results obtained by Angstrom, and presented in the latest avail able announcement on the subject. With each improve ment in spectroscopic appliances fresh coincidences are determined. Moreover, it is certain that many elements existing in the sun may for ever escape notice, simply be cause their vapours in the solar atmosphere may be either insufficient in quantity, or too low down to produce any recognisable effect. Mitscherlich and others have lately shown that where one or two lines of a many-lined spectrum are recognised and others wanting, the evidence for the existence of the corresponding element in the sun may yet be regarded as sufficient, because under certain conditions only one or two lines of a spectrum show themselves. The evidence given by the spectroscope respecting the solar spots confirms the theory that these are due to the existence of masses of relatively cool vapours at a lower level, and therefore relatively more compressed than the vapours elsewhere existing in the solar atmosphere. For the spectrum of the umbra of a spot differs chiefly from the spectrum of the solar photosphere in the greater strength and breadth of some of the dark lines.

We pass on to the phenomena witnessed during total solar eclipses. These are the red prominences and sierra, the inner corona, and the outer or radiated corona. Some add to these &quot; Baily s beads &quot; and like phenomena, of no real interest or importance whatever.

The red prominences were first seen during the solar eclipse of July 8, 1842. During the eclipse of July 28, 1851, it was shown incontestably that they belong to the sun, since the moon was seen to traverse them, hiding those on the east and revealing those on the west. Some still maintained that the prominences are lunar or terrestrial phenomena, on the ground that observers might have been deceived. But this doubt was finally set at rest by the photographic records which De la Rue and Secchi obtained of the total solar eclipse of June 18, 1860. It was not, however, until the eclipse of August 1868 sometimes known as the great Indian eclipse that the real nature of these wonderful objects was ascertained. Spectroscopic analysis was applied successfully to the in vestigation of the prominences by Colonel Tennant, Captain Herschel, and MM. Janssen and Rayet. The spectrum was found to consist of bright lines, indicating that the coloured prominences are masses of glowing vapour. All the observers agreed that among the bright lines those of hydrogen were present : and one yellow line was mistakenly ascribed to sodium. But Janssen on the day following the eclipse applied a new method of research, the principle of which had been definitely indicated several months before by Mr Huggins. (See Report of the Council of the Astronomical Society, in the Monthly Notices for February 1868.) This method depends on the fact that prismatic dispersion reduces the brilliancy of a continuous spectrum, but only throws apart the lines of a bright-line spectrum. Accordingly, by using a spectroscope of sufficient dispersive power, and directing it (with suitable telescopic adjuncts) towards a part of the solar limb on which a prominence exists, the light of our atmosphere, which under ordinary conditions obliterates the prominences from view, may be sufficiently dispersed to leave the bright lines of the prominence spectrum dis cernible (one at a time). By noting the length of a bright line so seen, the extent of the prominence in the particular section viewed could be determined ; and by combining such determinations, the shape of the prominence could be ascertained. Hence it was that Janssen, speaking of his observations on the day following the great eclipse, said, &quot; I have enjoyed to-day a continuous eclipse.&quot; It happened, curiously enough, that Mr Lockyer, in England, had succeeded in applying the method, not indeed so early as Janssen, but before the news of Janssen s success had reached Europe. Before long, however, Mr Huggins, the first to enunciate the principle of the new method, showed how it could be improved upon in such sort that the whole of a prominence could be seen at once. Since then many observers have studied the prominences without the aid of an eclipse, the most successful being Zollner, Secchi, Lockyer, Res- pighi, and (above all) Professor Young, of Dartmouth College, New Hampshire. The following remarks respecting the prominences have been derived from a paper by Respighi:— &quot; In the circumpolar solar regions great prominences are not formed, but only small and short-lived jets. In the spot zone the great prominences are seen, the equatorial, like the polar zones, being regions of relatively small ac tivity. Where faculas are present prominences are usually seen, but they are not identical with faculre. Over spots the jets are seen, but they are not high. There is a great difference in the duration of prominences. Some develop and disappear in a few minutes ; others remain visible for several days. They originate, generally, in rectilinear jets, either vertical or oblique, very bright and well defined. These rise to a great height, often to a height of at least 80,000 miles, and occasionally to more than twice that ; then, bending back, fall again upon the sun like the jets of our fountains. Then they spread into figures resembling gigantic trees, more or less rich in branches. In general, the highest parts are the regions of the most remarkable transformations.&quot;

Before passing to the more systematic researches made by Secchi, a few words must be said about the sierra. It had been observed, even before the prominences were noticed, that there exists a border of a red colour around the solar disk, which can be recognised on the eastern side just after totality has commenced, and on the western side just before totality ceases. Late observations abund antly confirmed the discovery, and after the eclipse of 1860 it had been definitely admitted by all who had suf ficiently examined the evidence, that a continuous red envelope surrounds the sun to a depth of three or four thousand miles. This envelope was commonly called the sierra ; but recently the name chromosphere (which purists correct into chromatosphere) has been given to it. The new method of observation, ?.s might be expected, shows the sierra spectrum as successfully as that of the prominences, which must be regarded as extensions of the sierra. Secchi makes the following remarks on the sierra and prominences:—