The mounting of the 72-inch telescope has several new features not hitherto used and sets a new standard for convenience and accuracy of operation. The observatory is much indebted to the Warner & Swasey Co., who have made most of the large mountings in America, for the spirit in which they undertook and carried through this work. Their sole object was to produce the best possible mounting regardless of cost and no suggestion of the writer looking to improvement was refused. To Mr. Swasey, the president, are due many of the original features of the mounting and the beauty and harmony of the design, while Mr. Burrell, the works manager, is responsible for the simplification of the mechanism and the beautiful co-ordination of the details. No greater testimony to the perfection of design and construction can be given than to say that after five years use there is no feature the director would wish changed, and no single defect of construction has been revealed. It may be of interest to note the principal improvements in this mounting. 1. All parts of the sky can be readily reached. This is not possible with all types of reflecting telescopes. 2. The elimination of cylindrical bearings with cumbrous friction-relieving devices, formerly considered necessary for maintaining collimation and adjustment on declination and polar axes, and the use of ball bearings for both friction-relieving and collimating purposes has resulted in remarkable ease of movement of the telescope. 3. Freedom from periodic or other errors in driving and smoothness and freedom from “backlash” in slow motions. 4. Ease, speed and accuracy with which settings can be made due to careful design and original features in setting motors and setting circles. 5. Great stiffness of tube and improvements in method of attaching and changing secondary mirrors. 6. Beauty and harmony of design and appearance. SECTION 4.--OPTICAL PARTS AND SPECTROGRAPHS The Principal Mirror The great mirror is composed of hard plate glass cast in one piece and after annealing, ground and polished to the correct shape. As received from the St. Gobain Co., the disc was 73·5 inches diameter, over 13 inches thick with a central hole about 6 inches diameter and weighed nearly 5,000 pounds. It was first of all ground truly circular to a diameter of 73 inches and flat on both sides to a thickness of slightly over 12 inches, while the central hole was enlarged to 10 inches. When the back was polished approximately flat, the disc was seen to be a beautiful specimen of the glass makers art, homogeneous and almost entirely free from bubbles or other defects. [Illustration: Fig. 4.--PHOTOGRAPH OF RING NEBULA IN LYRA (_Enlarged 8 diameters_)]