The advance of photography : its history and modern applications (1911)

284 THE ADVANCE OF PHOTOGRAPHY Even these very large dimensions are dwarfed by those obtained when this telescope is arranged as a Cassegraincoude for spectroscopic work. The spectrograph used is of very large size, and is mounted on stationary piers in an underground pit, which can be kept at an absolutely constant temperature, a very necessary arrangement where such large apparatus is used, if distortion of the image is to be reduced to a minimum. In this case the equivalent focal length reaches 150 feet. The large 60-inch paraboloidal mirror of this telescope has a focal length of 299 inches, is 7f inches in thickness at the edges, 6| inches in thickness at the centre, and weighs 1900 lbs. All the mirrors are polished approximately flat on their backs, and when in use are silvered on their back as well as on their front surfaces, so as to ensure a symmetrical temperature effect. So as to avoid any ill-effects due to the strain inevitable when such large masses are moved, a good portion of the weight is borne, not on the axis, but by a float in mercury. At the upper end of the polar axis is a large flange to the lowTer side of which is bolted this float, which is a very rigid hollow disc of steel boiler plate, 10 feet in diameter, 2 feet deep, weighing 8600 lbs. To the upper side of the flange of the polar axis is bolted the fork between the arms of which the great tube of the telescope swings in declination on nickel steel trunions 7 inches in diameter. The fork itself is made of cast-iron and weighs 10,400 lbs. These enormous parts are clamped firmly together by twelve nickel steel bolts, each 2 J inches in diameter and 3 feet long, so as to give great strength and rigidity at a region of the mounting where the greatest tendency to flexure occurs. The float dips into a cast-iron trough which nearly fits the float, leaving a space of J inch only all round. This