The building for housing a large reflecting telescope requires to be of special design for the best results. It should not rise above the shade temperature during the day and should rapidly assume and follow the external temperature at night. Such materials as brick or stone are obviously not suitable and all recent telescope buildings are entirely of metallic construction in order to assume quickly the night temperature, and of double-walled, ventilated type to prevent overheating from the sun’s rays. The building for the 72-inch telescope is entirely of steel construction, circular in form, 66 feet in external diameter and with vertical walls 32 feet high. A view from the south is given in the Frontispiece and from the north in Fig. 1, showing the city of Victoria and the straits of Juan de Fuca in the background. An external and internal covering of galvanized iron separated by about 16 inches allows free circulation of air from a peripheral opening at the base up through a similar double walled dome and out of louvres at the top. The ground floor of Terrazo is laid directly on the rock base and the observing floor 22 feet above this is formed of steel girders and checkered steel plate. In the centre of the ground floor rises the massive pier to support the telescope, of reinforced concrete and symmetrical tapering form. The pier is actually double, united below the observing floor by a massive reinforced arch and extending above the floor as two piers (see Fig. 2) one for each end of the polar axis. Temporary partitions on the ground floor provide a dark room, sleeping room, and temporary office quarters. [Illustration: Fig. 1.--OBSERVATORY BUILDING FROM THE NORTH] The Dome