tions above the solar surface equaling or exceeding those attained by hydrogen itself (Fig. 1, a). Their suitability for the purpose of prominence photography is due to several causes, among which may be mentioned their great brilliancy, their presence at the center of broad dark bands which greatly diminish the brightness of the sky spectrum, and the comparatively high sensitiveness of photographic plates for light of this color.
While fairly efficient from an optical point of view, the spectroheliograph of the preceding year had possessed many mechanical defects. In a new instrument, devised for use with the twelve-inch Kenwood telescope, these were overcome, and means of securing the necessary conditions of the experiment were provided. The first trials
of the instrument, made in January, 1892, were entirely successful, and the chromosphere and prominences surrounding the sun's disk were easily and rapidly recorded (Fig. 2). The details of their structure were shown with the sharpness and precision characteristic of the best eclipse photographs. And the opportunity for making such records, previously limited to the brief duration, never exceeding seven minutes, of a total solar eclipse, was at once indefinitely extended. Thus it became possible to study photographically the slowly varying forms of the quiescent, cloud-like prominences, and, to particular advantage, the rapid changes of such a violent eruption as is illustrated in Fig. 3.
But even before this primary purpose of the work had been accomplished, the possibility of making another and much more important application of the instrument' had presented itself. A photographic