A GIGANTIC TELESCOPE.

New Zealand Herald, Volume XXXVI, Issue 11200, 21 October 1899, Page 7 (Supplement)

A GIGANTIC TELESCOPE.

FOR THE PARIS EXHIBITION. o.\'F. of the most remarkable features of the coming Universal Exhibition at Paris will be the gigantic telescope with which astronomers of 1900 and succeeding years will explore the heavens. It was at the initiative of M. Francois Deloncle, Minister Plenipotentiary in the French Diplomatic Service, that a' group of amateur astronomers decided to devise for the International Exhibition an instrument of exceptional dimensions and power, far exceeding anything before attempted. With this end in view, it was determined to give the object glass a diameter of 49.2 inchesthat is, 9.2 inches more than that of the celebrated Yerkes glass at Williams Bay, Wisconsin, and 13.2 inches larger than that of the Lick telescope, at Mount Hamilton, California. In order to utilise such an aperture to the best advantage, and especially to check as far as possible the obnoxious effect of chromatic aberration, it was decided not to subordinate, as ut,ual, the optical work to mechanical difficulties by a reduction of the focal distance, but boldly to give the tube the enormous length of nearly 200 ft. SOME FIGCBES. The fixed tube of the great Paris reflector is of steel, very slightly less than 0.1 inch thick, and weighs some 21 tons. Its diameter is 59 inches. The cylinder is formed of 24 separate parts, screwed together, and rests on eight cast-iron supports, placed on eight stone pillars. In order to facilitate expansion by heat, the supports can glide on a system of rails attached to the piers. There 'arc two object glasses, the one for visual observations, the other being reserved for photographic work. Each glass weighs 16001b. They are both mounted on a track gliding along a- railway, thus allowing of their easy transfer in front of the tube. Ihe eyepiece is also movable on a railway, and the focussing is effected by a screw 60 inches long, uniting the two tubes, lhe side rostat proper, on which the telescope is mounted, which weighs some 45 tons, consists of a huge brass foot, measuring <!&ft in length and as much in height, and resting on a marble pier. The diameter of the great mirror is 78J inches, or rather more than 6jft, and its weight, mounting included. more than six and a-half tons. it is held in equilibrium -by a system of levers ana counterpoises, rolling in a well more than six and a-half feet in diameter, tilled with mercury. It was no easy task to g and polish the surfaces of the colossal minor and of the two object glasses. Here new methods had to be devised. The plane figure of the mirror has been obtained by the molar action of two flat metallic sliders. The same process in grinding the object glasses, with this difference, however, that owing to the curved surfaces to be men to the lenses, the sliders, instead of being straight, had the curvature of the discs. The rectilineal motion of the system thus gave rise to a cylindrical scction on thglass which, nowever, in virtue of he revoluto of the lenses on their .axis, was transformed into a spherical surface.