This circular steel building is capped by a circular plate supported by massive girders on which is placed a curved railroad iron rail turned and adjusted perfectly level and circular. The hemispherical dome turns on this rail on 24 massive wheels mounted on roller bearings and is rotated by an electric motor mechanism operated by reversing switches on each side of the south pier. The framework of this dome, which is 66 feet in external diameter and about 38 feet high, the lower 5 feet being cylindrical, consists essentially of a circular base to which the bearing wheels are attached and two double, very deep and rigid main ribs, one of which can be seen in Fig. 3, 16 feet apart in the clear and extending parallel to each other entirely across the dome. These are united 6 feet beyond the zenith by a cross girder and this opening, which can be revolved to any azimuth and can be closed by double motor operated shutters, enables the telescope to observe at any part of the sky. Auxiliary circular ribs reaching from the base ring up to the main ribs all around the dome except at the shutter opening, form the support for the double steel covering of 12 inches separation. This ventilating space is united through suitable weather guards with the space between the building walls and forms the protection against overheating by the sun in the day time. Dome Accessories The shutter opening 16 feet wide and extending from the base of the dome to 6 feet beyond the zenith is closed by a double shutter also double walled and ventilated. Canvas screens mounted on tubes and operated by motor can be run up from the base of the shutter opening, this one being shown in Fig. 3, and down from the top so as to limit the opening to the width of the tube of the telescope. These screens are necessary when the wind is blowing to prevent shaking of the tube and consequent jumping of the star image. For enabling the observer to reach conveniently the upper end of the telescope tube in any position a counterweighted elevating platform, moved up and down the shutter opening by motor and drum with lifting cables is provided. This platform is 20 feet long and 4 feet wide with a wing at each end, extending inwards about 6 feet, on which the observer can stand and can move it and himself by a hand wheel up to and partly encircling the telescope tube. The platform and movable wings, which are well shown in Fig. 3, can be brought to any desired height by an operating switch at one side and can be reached by an auxiliary stairway from a small stationary platform attached to the base ring of the dome. This stationary platform is reached from the observing floor by a permanent stairway moving with the dome and extending to a foot from the floor. Platform wings and stairways are made safe for observers by guard and hand rails 30 inches high. This moving platform, an essential accessory for direct photography and other work at the principal focus of the