study of the spectrum of various portions of the sun's surface had shown the existence at many points of great regions of calcium vapor, luminous enough to render their existence evident through the production of bright H and K lines on the solar disk (Fig. 1, b and c). Some of these calcium regions had indeed been known to exist through the visual observations of Professor Young, who had observed the bright lines in the vicinity of sun-spots. But the vast extent of the calcium regions, and the characteristic forms of the phenomena, could not be ascertained by such means. What was required was such a
representation of the solar disk as the spectroheliograph had been designed to give in the case of the prominences. From a consideration of the results obtained in the spectroscopic study of the disk, it appeared probable that an important application of the spectroheliograph might be made in this new direction.
Before describing this second application of the instrument, it may be well to call attention to the appearance of the sun when seen with a telescope, or when photographed in the ordinary manner, without a