Each type of telescope has its astronomical advantages and disadvantages. The refractor is better suited for visual observations such as the measurement of double stars and the study of planetary detail and is less affected by temperature changes than the reflector. On the other hand the reflector, on account of its perfect achromatism, is the instrument par excellence for photographic observations, and, as more than three-fourths of modern astronomical work is photographic, it appears to be superseding the refractor. This advantage is increased by the fact that the refractor has apparently reached the useful limit in size and that it costs at least three times as much as a reflector of the same aperture. Although each type of telescope has its characteristic type of mounting for astronomical purposes, the principles are the same for each and can probably be most easily followed by describing the essential parts of the mounting of the 72-inch telescope. The Telescope Tube The tube performs the important function of carrying in relatively invariable position and adjustment the optical parts of the telescope. The tube of the 72-inch telescope is 31 feet long, 7 feet 4 inches outer diameter and weighs 15 tons. Its form and construction are well shown in Figs. 2 and 3. It consists of the main or central section A, Fig. 2 the lower section B which carries the main mirror and the skeleton section C which carries the secondary mirrors. The central section is a cylindrical steel casting heavily ribbed on the inside about 6 feet high and weighs 7 tons. The lower section is securely bolted to it through the flanges shown and with the mirror and its supporting mechanism weighs about 6 tons. The upper skeleton section is built up of structural steel, 3 inch I beams, firmly braced and rivetted together in the manner shown in the figures. A special feature of this skeleton tube, making it more rigid than any previous design, consists of the diagonal tension rods in each rectangular compartment screwed up each to a tension of about 2,000 pounds, so that the whole tube is under tension in every position. This stiffness is essential for the proper performance of the optical parts, as the principal and secondary mirrors at bottom and top of tube respectively should occupy the same relative positions in whatever direction the tube is pointed. The Declination Axis