During a total eclipse of the sun, when the light of the disk is completely cut off by the moon, the solar atmosphere is momentarily revealed. The exceedingly faint corona, extending millions of miles into space, can be seen only at such times. But the more brilliant chromosphere, the comparatively shallow atmosphere of luminous gases which completely encircles the sun, and the prominences which rise out of it, can be observed on any clear day with the aid of a spectroscope. First applied to this purpose in 1868, the spectroscope has yielded a large store of information regarding the number, distribution, and nature of the prominences and the structure of the upper chromosphere, as seen in elevation at the sun's limb. It has also permitted the observation of certain phenomena of the solar atmosphere in projection against the disk, but on account of the brilliant background, only their general outlines can be thus detected. In order to study their details, we must have recourse to the spectroheliograph.