IT is now nearly three quarters of a century since the green line in the spectrum of the solar corona was first observed. Subsequent observations-few eclipse expeditions since then have neglected this field of study-have established that the inner corona emits a spectrum consisting of at least twenty -two broad bright lines superposed on a continuum. The brightest line is usually the green one at 5303 A., but considerable variations in the relative intensity of the lines are found from eclipse to eclipse, and recent coronagraph observations made without an eclipse confirm large day-to-day intensity variations. None of the lines have ever been produced in the laboratory, nor indeed have they ever been observed in any other celestial source, except for a brief appearance during a nova-like outburst by the variable star ES Ophiuchi in 1933. There has been no lack of suggestions regarding their origin: they have been attributed in turn to iron, to 'proto-nuorine', to singly- and doubly-ionized calcium, to argon, and to oxygen, neutral and ionized ; all with a certain amount of supporting evidence, and the last three within the past fifteen years. None of these assignments has had general support, however, because none of them explains more than a small selection of the facts.
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