[Illustration: Fig. 231.] Fig. 231 shows the same corona as seen by Foenander. [Illustration: Fig. 232.] Fig. 232 shows the same corona as photographed by Davis. [Illustration: Fig. 233.] Fig. 233 shows the corona of 1878 made up from several views as combined by Professor Young. 204. _The Spectrum of the Corona._--The chief line in the spectrum of the corona is the one usually designated as 1474, and now known as the _coronal_ line. It is seen as a dark line on the disk of the sun; and a spectroscope of great dispersive power shows this dark line to be closely double, the lower component being one of the iron lines, and the upper the coronal line. This dark line is shown at _x_, Fig. 234. [Illustration: Fig. 234.] Besides this bright line, the hydrogen lines appear faintly in the spectrum of the corona. The 1474 line has been sometimes traced with the spectroscope to an elevation of nearly twenty minutes above the moon's limb, and the hydrogen lines nearly as far; and the lines were just as strong _in the middle of a dark rift_ as anywhere else. The substance which produces the 1474 line is unknown as yet. It seems to be something with a vapor-density far below that of hydrogen, which is the lightest substance of which we have any knowledge. It can hardly be an "allotropic" form of any terrestrial element, as some scientists have suggested; for in the most violent disturbances in prominences and near sun-spots, when the lines of hydrogen, magnesium, and other metals, are contorted and shattered by the rush of the contending elements, this line alone remains fine, sharp, and straight, a little brightened, but not otherwise affected. For the present it remains, like a few other lines in the spectrum, an unexplained mystery. Besides bright lines, the corona shows also a faint continuous spectrum, in which have been observed a few of the more prominent _dark_ lines of the solar spectrum. This shows, that, while the corona may be in the main composed of glowing gas (as indicated by the bright lines of its spectrum), it also contains considerable matter in such a state as to reflect the sunlight, probably in the form of dust or fog. V. ECLIPSES. [Illustration: Fig. 235.]