Looked at from another slant it is a polar heliostat, of which the telescope forms the driving axis in R. A. The whole mount was a substantial casting on wheels which ran on a pair of rails. For use the instrument was rolled to a specially arranged window and through it until over its regular bearings on a pier just outside. A few turns of the wheel D lowered it upon these, and the back of the frame then closed the opening in the wall leaving the instrument in the open, and the eye end inside the room. The example first built was of only 4 inches aperture but proved its case admirably as a most useful and convenient instrument. This mount with various others of the fixed eyepiece class may be regarded as derived from the horizontal photoheliographs used at the 1874 transit of Venus and subsequently at many total solar eclipses. Given an equatorially mounted heliostat like Fig. 81 and it is evident that the beam from it may be turned into a horizontal telescope placed in the meridian, (or for that matter in any convenient direction) and held there by rotation of the mirror in R. A., but also in declination, save in the case where the beam travels along the extension of the polar axis. [Illustration: FIG. 88.—Grubb’s Original Polar Telescope.] For the brief exposure periods originally needed and the slow variation of the sun in declination this heliostatic telescope was easily kept in adjustment. The original instruments were of 5 inches aperture and 40 ft. focal length, and the 7 inch heliostat mirror was provided with ordinary equatorial clockwork. Set up with the telescope pointing along the polar axis no continuous variation in declination is needed and the clock drive holds the field steadily, as in any other equatorial. Figure 89 shows diagrammatically the 12 inch polar telescope used for more than twenty years past at the Harvard Observatory. The mount was designed by Mr. W. P. Gerrish of the Harvard staff and contains many ingenious features. Unlike Fig. 88 this is a fixed mount, with the eye-end comfortably housed in a room on the second floor of the main observatory building, and the lower bearing on a substantial pier to the southward. [Illustration: FIG. 89.—Diagram of Gerrish Polar Telescope.] In the figure, _A_ is the eye end, _B_ the main tube with the objective at its lower end and prolonged by a fork supported by the bearing _C_ and _D_ is the declination mirror sending the beam upward. The whole is rotated in R. A. by an electric clock drive, and all the necessary adjustments are made from the eye end.