scope ever constructed, having a forty-inch objective made by Alvan Clark & Sons. The focal length of the instrument is sixty-four feet. The mounting is similar to that of the great Lick telescope, but is heavier, more rigid, and improved. One important advantage introduced for the first time in this mounting is the system of electric motors, by means of which the various motions are effected. By simply touching buttons upon a little keyboard the astronomer may produce any one of ten different results, changing the position of the instrument or of parts of the observatory itself. In this simple way the great instrument may
Fig. 10.—Yerkes Observatory, Lake Geneva.
be moved, the clock may be started or stopped, the shutter of the dome may be opened or closed, the dome itself may be revolved, or the floor may be made to rise and fall. The dome covering the telescope is about ninety feet in diameter, with an observing slit twelve feet wide extending from the horizon to beyond the zenith. Two spectroscopic attachments are connected with the great telescope: (a) a spectroheliograph for photographing the solar chromosphere, prominences, and faculæ by monochromatic light combined with a large solar spectroscope for photographic and visual study of solar phenomena; (b) a stellar spectroscope for photographic and visual investigation of stellar spectra, and determination of motion in the line of sight. The world has a right to expect results from such an instrumental equipment, and the Department of Astronomy has in view important researches. Among these are micrometrical measurement of double stars and
