spectroheliograph. Such a photograph is reproduced in Fig. 4. Its most conspicuous features are the numerous dark spots scattered over the sun's surface; these are the well known sun-spots. Near the edge of the sun there may be seen certain bright regions, which are known as faculæ. The calcium regions above referred to are usually associated with the faculæ, but they lie above them, and they give no trace of their existence on ordinary photographs, like the one in Fig. 1, or to the eye when observing the sun through a telescope.
The results of the first experiments, which were made at the beginning of 1892, were such as to justify fully the expectations that had been entertained. It was at once found possible to record the forms, not only of the brilliant clouds of calcium vapor associated with the faculæ, and occurring in the vicinity of sun-spots, but also of a reticulated structure extending over the entire surface of the sun. The earliest applications of the method were made in the study of the great sun-spot of February, 1892, which, through the great scale of the phenomena it exhibited, and the rapid changes that resulted from its exceptional activity, afforded the very conditions required to bring out the peculiar advantages of the spectroheliograph. In the systematic use of the instrument continued at the Kenwood Observatory through the following years, a great variety of solar phenomena were recorded, and the changes which they underwent from day to day—sometimes, in the more violent eruptions, from minute to minute—were registered in permanent form for careful study. During this period, which ended with the transfer of the Kenwood instruments to the Yerkes Observatory, over 3,000 photographs of solar phenomena were secured. From a systematic study of these negatives, in the course of which the heliographic latitude and longitude of the calcium regions in many parts of the sun's disk were measured from day to day, a new determination of the rate of the solar rotation in various latitudes has been made. This shows that the calcium regions, like the sun-spots, complete a rotation in much shorter time at the solar equator than at points nearer the poles. In other words, the sun does not rotate as a solid body would do, but rather like a ball of vapor, subject to laws which are not yet understood.
In this first period of its career the spectroheliograph had therefore permitted the accomplishment of two principal objects. It had provided a simple and accurate means of photographing the solar prominences in full sunlight, which gave results hardly inferior to those obtained during the brief moments of a total eclipse. It had also given a means of recording a new class of phenomena, known previously to exist only through glimpses of the bright calcium lines in the vicinity of sunspots, but wholly invisible to observation either visually or on photographs taken by ordinary methods. It was not difficult to see, how-
