On the Nature of Sun-Spots

The researches described in the present paper were undertaken in connection with a more general investigation of the sun, considered as a typical star. The gradual development of the work dates from 1891, when three attacks, along converging lines, were begun by the writer at the Kenwood Observatory. One of these involved a study of the spectra of sun-spots, requiring the development of instruments and methods adequate to record photographically the minute peculiarities of their widened lines. The second was a photographic investigation of the solar atmosphere, rendered possible by the initiation and gradual improvement of the spectroheliograph. A series of laboratory experiments was also undertaken for the purpose of interpreting the phenomena detected in both of these series of researches.