Todd — Untried Form of Mounting for a Telescope. 459 



Aet. XXXIX. — On a Hitherto Untried Form of Mounting, 

 either Equatorial or Altazimuth, for a Telescope of excep- 

 tional size, either refractor or reflector, in which Telescope, 

 observing-floor and dome are combined in one / by David 

 P. Todd, Director Amherst College Observatory. 



Neakly all the great telescopes of the world have in turn 

 signalized their extraordinary power by an important astro- 

 nomical discovery. Sir William Herschel's reflector first 

 brought to light the planet Uranus ; Lord Posse's Leviathan, 

 the spiral nebulae ; the 15-inch Cambridge telescope, Saturn's 

 dusky ring; the 18-inch Chicago refractor, the companion of 

 Sirius ; the Washington 26-inch, the satellites of Mars ; the 

 30-inch Pulkowa objective, the nebulosities of the Pleiades ; 

 and the 36-inch Lick telescope revealed a new satellite of 

 Jupiter. 



With such a record is it not safe to predict farther advance 

 with larger telescopes still ? There is as yet no indication that 

 a refracting telescope of five or even six feet aperture would 

 fall short of a gratifying success to its projector. Two Ameri- 

 can opticians, of wide experience and high competency, can be 

 relied upon to grind and figure such an object glass. To be 

 most effective, it must be as thin as possible — which means 

 that its focal length must be very great. Therefore the ques- 

 tion of a suitable mounting becomes all important. 



The difficulties inherent to the conventional type of mount- 

 ing practically prohibit any great increase of size, because the 

 tube is hung at or near the middle. This is in many respects 

 the least advantageous point. The tube should be rigidly sup- 

 ported at or near both ends, as well as in the middle. If, then, 

 we invoke the fertile resources of the modern bridge engi- 

 neer, and his relatively light but rigid constructions of steel, 

 the fixedness of axis in all positions of the great telescope 

 becomes a requisite easily met. 



The object of this paper is to outline one method of apply- 

 ing modern engineering resources to the mounting of a tele- 

 scope of exceptional proportions. 



A sphere 100 feet in diameter can readily be designed and 

 built, with an interior, meridional rib-structure so rigid that 

 flexure deformation will not exceed a small fraction of an inch. 

 We coat this sphere, first with steel plates, then with oak 

 planks about three inches thick. These not only allow the 

 exact spherical figure to be trued up, but also, like the wooden 

 sheathing sometimes employed on men-of-war, permit attach- 

 ment of an outer coating of copper, — or better still, of Tobin 

 bronze. 



