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

ASTRONOMICAL PHOTOGRAPHY 285 space is filled with mercury, 650 lbs. of that element being required for this purpose. The immersed part of the float gives a displacement of about 50 cubic feet of mercury, thus carrying about 21 J tons of the moving parts of the telescope in the fluid, and relieving 95 per cent, of the weight on the large bearings of the polar axis. The whole mounting is so designed that the centre of weight of the moving parts is vertically above the centre of flotation. The telescope is, owing to this system of flotation and the care which has been given in finishing the driving clock, the clock connections, etc., able to follow the motions of the stars with exquisite smoothness and accuracy, although the moving parts weigh nearly 23 tons. The tube of the telescope is a skeleton one, octagonal in shape, and is constructed so as to ensure very great rigidity. The entire building and dome are planned so as to close air tight if necessary. In the early morning, after a night's work, the dome of the building is closed and not opened again until sunset, so enclosing 120,000 cubic feet of cool night air. It is believed that this provision, together with the complete sun protection of the dome and building, will reduce the rise in temperature within the structure during the day to a very few degrees. This protection from daily temperature changes should be sufficient for the telescope mounting, and for the smaller mirrors. To further protect the large mirror during the day a small refrigerating plant supplies air at a constant temperature, such as is expected at night ; this circulates through a jacket enclosing the entire lower end of the telescope tube. The necessity for all these precautions arises from the fact that it is important to preserve the best optical figure on the large mirror. An error in the figuring of the mirror is about three