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

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. 4, 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

ever, that the possibilities of the new method were much greater than had been indicated by the work so far accomplished. It seemed certain that our knowledge of the finer details of the calcium clouds would be greatly increased if provision could be made for photographing a much larger solar image with a spectroheliograph of improved design.

And

it was furthermore evident that other applications of the instrument, involving the use of different spectral lines, and the employment of principles which had not entered in the Kenwood work, might reasonably be hoped for.

The construction of the great forty-inch telescope of the Yerkes

Observatory provided the first requirement of this new work, namely, a large solar image, having a diameter of seven inches as compared with the two-inch image given by the Kenwood telescope. The construction of a spectroheliograph large enough to photograph such an image of the sun involved serious difficulties, but these were finally The Rumford spectroheliograph, designed to meet the special conditions of the new work, was constructed in the instrument

overcome.

shop of the Yerkes Observatory, and is now in daily use with the fortyinch telescope (Fig. 5).

In this instrument the solar image is caused to move across the first slit by means of an electric motor, which gives the entire telescope a slow and uniform motion in declination. The sun's light, after passing through the first slit, is rendered parallel by a large lens at the lower end of the collimator tube. The parallel rays from this lens fall upon a silvered glass mirror, from which they are reflected to the first of two prisms, by which they are dispersed into a spectrum.

After passing through the prisms, the light, which has now been deflected through an angle of 180°, falls upon a second large lens at the

lower end of the camera tube. This forms an image of the spectrum at the upper end of the tube, where the second slit is placed. Any line in