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The labour of collation was well advanced when he died at the age of fifty-two, April 16, 1901. Investigations of metallic arc-spectra have also been carried out with signal success by Hasselberg, Kayser and Runge, O. Lohse, and others. Another condition sine quâ non of progress in this department is the separation of true solar lines from those produced by absorption in our own atmosphere.
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