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 overcome. The Rumford spectroheliograph, designed to meet the special conditions of the new work, was constructed in the instrument
shop of the Yerkes Observatory, and is now in daily use with the forty inch 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.