The second largest reflector in the world is the 72-inch instrument set up in 1918 on Little Saanich Mountain on Vancouver Island, British Columbia, in the Dominion Observatory there. The mirror of this instrument was ground by the John A. Brashear Company of Pittsburgh, from a disc of glass cast in Bel gium and shipped out of that country two days before the war broke out. The mirror is par abolic, 13 inches thick at the rim with a hole 10% inches in diameter through its centre. It weighs 4,340 pounds and has a focal length of 30 feet. Observations may be made from the side of the upper part of the tube, from the side at the lower end or through the opening at the centre of the mirror. The telescope with all its fittings weighs 55 tons and is moved by an amount of electric current barely sufficient to light a 16-candle power electric lamp. The mounting was built by Warner and Swazey. The only other telescope of this size in existence is that of Lord Rosse set up upon his estate in Ireland in 1842. Its first mirror was of specu lum metal and was replaced by a 72-inch silvered glass mirror in after years. For a long period it was the largest telescope in the world; but was abandoned for optical reasons.
There are three 60-inch reflectors in the Western Hemisphere, the newest one erected at the National Observatory of the Argentine Re public near Cordoba. The mirror was made by the John A. Bras-hear Company in 1916. In 1904 Harvard University secured the 60-inch reflector made by Dr. A. A. Common of Ealing, England. This instrument is of the Casse grainian type and has a rectangular tube. The
mounting is peculiar, the telescope being sup ported in position upon the end of a hollow cylinder which floats in a well or deep basin of water. The cylindrical float is 18 feet long and seven and two-thirds feet in diameter and arranged to float constantly at an .angle with the horizon equal to the elevation of the celes tial pole at Cambridge—about 45 degrees. The instrument weighs over 20 tons, but is so delicately balanced that it may be moved in any direction with the greatest ease by its elec trical controls. The eye-piece of the instrument is detached from the telescope and housed in the second story of an adjacent building, the light from the principal mirror being directed to it by accessory mirrors.
The 60-inch reflector of the Mount Wilson Observatory, constructed by Dr. Ritchey in 1908, was placed in commission in December of that year. The instrument complete weighs 211/2 tons, nearly all of which is ingeniously supported upon a float in a basin of mercury, of which it displaces the equivalent of 50 cubic feet, although the entire amount of mercury in the basin is but 635 pounds. The controls are electric and may be manipulated from several stations about the instrument.
Another fine telescope which should be men tioned in this connection is the 48-inch re flector built by Sir Howard Grubb and located at Melbourne, Australia.
For more detailed descriptions of these wonderful instruments and their achievements the files of the astronomical journals are rec ommended.