A view of the exterior is shown in Fig. 90, with the mirror and objective uncovered. The rocking arm at the objective end, operated by a small winch beside the ocular, swings clear both mirror and objective caps in a few seconds, and the telescope is then ready for use. Its focal length is 16 ft. 10 inches and it gives a sweep in declination of approximately 80°. It gives excellent definition and has proved a most useful instrument. A second polar telescope was set up at the Harvard Observatory station in Mandeville, Jamaica, in the autumn of 1900. This was intended primarily for lunar photography and was provided with a 12 inch objective of 135 ft. 4 inches focal length and an 18 inch heliostat with electric clock drive. [Illustration: FIG. 90.—Gerrish Polar Telescope, Harvard Observatory.] Inasmuch as all instruments of this class necessarily rotate the image as the mirror turns, the tail-piece of this telescope is also mounted for rotation by a similar drive so that the image is stationary on the plate both in position and orientation. As Mandeville is in N. lat. 18° 01′ the telescope is conveniently near the horizontal. The observatory of Yale University has a large instrument of this class, of 50 feet focal length, with a 15-inch photographic objective and a 10-inch visual guiding objective working together from the same heliostat. Despite its simplicity and convenience the polar telescope has an obvious defect in its very modest sweep in declination, only to be increased by the use of an exceptionally large mirror. It is not therefore remarkable that the first serious attempt at a fixed eyepiece instrument for general use turned to a different construction even at the cost of an additional reflection. This was the _equatorial coudé_ devised by M. Loewy of the Paris Observatory in 1882. (Fig. 91.) In the diagram A is the main tube which forms the polar axis, and B the eye end under shelter, with all accessories at the observer’s hand. But the tube is broken by the box casing C containing a mirror rigidly supported at 45° to the axis of the main tube and of the side tube D, which is counterbalanced and is in effect a hollow declination axis carrying the objective E at its outer end. [Illustration: FIG. 91.—Diagram of Equatorial Coudé.] In lieu of the telescope tube usually carried on this declination axis we have the 45° mirror, F, turning in a sleeve concentric with the objective, which, having a lateral aperture, virtually gives the objectives a full sweep in declination, save as the upper pier cuts it off. The whole instrument is clock driven in R. A., and has the usual circles and slow motions all handily manipulated from the eye end.