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 which lie at a distance from the sun's surface as great as the solar radius or diameter.

The spectroscopic testimony forms of course an exclusive source of information as to the nature of the elementary bodies present in the corona. It must be admitted that our knowledge on this subject is rather of a negative character. The spectroscope has hitherto mainly afforded us indications of elements which seem to be undeterminable by our knowledge of terrestrial chemistry. Professor Schuster, after a careful discussion of the evidence afforded by other eclipses, has come to the conclusion that it is not at present possible to identify the lines specially characteristic of the coronal spectrum with those of any known terrestrial substances. Indeed, the corona presents a curious green line that seems to denote some invariable constituent in the sun's outer atmosphere; but the element to which this green line owes its origin is wholly unknown. It has been conjectured that it is due to some body present in the sun which is unknown to terrestrial chemists. The elucidation of this question is from every point of view one of the most interesting problems in solar physics.