tances—all constituted so many riddles for astronomers till 1868. In that year one of the great eclipses of the sun took place. We might say that, at the very moment when the heavens had just suffered their most precious secrets to be revealed, the star of day had deigned to invite us to the study of his admirable structure.
The eclipse was observed, and the result surpassed even the general expectation. The nature of the protuberances was immediately recognized, and a method was discovered that permitted the study of these phenomena every day, without having to wait for the rare occasions of eclipses. This method led in a short time to the discovery of the chromospheric atmosphere, and this completed and explained the phenomena of the protuberances. The first results of the spectroscopic investigations may be stated thus:
The sun of Herschel and Arago, formed of a central nucleus and a luminous envelope, the photosphere, has an additional stratum formed chiefly of incandescent hydrogen. This stratum, in immediate contact with the photosphere, is very thin, being only from eight to ten seconds thick; it is the seat of small eruptions of metallic vapors rising from the photosphere, in which sodium, magnesium, and calcium predominate. Frequently, however, principally at the time when the sun-spots become abundant, there rise from the solar globe formidable eruptions of hydrogen, which pass through this same envelope and rise to a height of sixty thousand or ninety thousand miles. These eruptions are the protuberances of the total eclipses, the nature of which is thus revealed and the forms explained.
The corona and the phenomena exterior to it were the objects of study in the next eclipses. In 1874 French observations showed that the corona constituted a new solar atmosphere, a very rare one and enormously extended, in which hydrogen still dominated, while it presented spectral conditions as yet unexplained. This atmosphere seemed to borrow a part of the appearances of the protuberance-eruptions which penetrate it and are extinguished in it. It also seemed probable, and that opinion was expressed by the author of these observations, that the figure of the corona would vary with the condition of external activity of the sun. At the times of the maximum of spots, when the protuberance-eruptions were in the highest activity, the coronal atmosphere would be intersected by numerous and rich jets which would increase its extent and density, and change its aspect. This opinion was confirmed by one of the observers of the last eclipse in Egypt.
I shall conclude this brief review of the methods of physical astronomy with a word upon an art which has recently brought a really wonderful aid to all our scientific studies—I mean photography. Considered in its old and primary object, the aim of photography is to fix the images of the camera-obscura. Its aim, however, and its means have been singularly extended. We have to consider here only