380 The Great Foucault Telescope. 



THE GREAT FOUCAULT TELESCOPE. 



M. Leon Eoucattlt has laid before the French Academy an 

 account of the great telescope constructed upon his principle 

 for the Observatory at Paris. He observes that his efforts to 

 obtain large instruments with reflectors of silvered glass could 

 not be deemed completely successful until he had reached di- 

 mensions exceeding those of the largest achromatic objectives, 

 and that it was only by way of establishing a claim to the recog- 

 nition of his plans that he announced the formation of mirrors 

 of 10, 20, and 40 centimetres in diameter. Now, he is able to 

 speak of one nearly 80 centimetres in diameter, having a focal 

 length of 4| metres,* which has been completed in the esta- 

 blishment of M. Secretan. This mirror, mounted in a Newtonian 

 telescope, has been at work for three months at the Observatory, 

 performing to the entire satisfaction of the director, M. Cha- 

 cornac. 



The thick glass disc was cast in a curved form (hombe), at 

 the factory of St. Gobain, in a mould prepared by M. Sautter, 

 the director of the works for the lenticular lighthouse apparatus, 

 and although possessing sufficient homogeneity for its intended 

 purpose, showed before it was silvered that a flaw had occurred 

 during the process of cooling. On its arrival at M. Sautter's 

 workshops, it was reduced in dimensions by bringing it nearer 

 to the required shape, and by cutting a groove to fix the me- 

 chanism necessary for its manipulation. It then passed into 

 the hands of M. Secretan' s skilful operatives, who ground it with 

 a counter piece of glass, 50 centimetres in diameter, assisted by 

 emery and water. This process, which was frequently tested by 

 the spherometer, occupied a week, at the end of which time a 

 fine grained and exactly spherical surface was obtained. Hav- 

 ing been thus prepared, it was polished by hand, the polisher 

 employed being 22 centimetres in diameter, and covered with 

 rouge. This polishing was completed by one able workman in 

 another week, and the mirror was changed from the spherical to 

 the paraboloid form. From this moment its success appeared cer- 

 tain, and it was removed, with the necessary tools, to the Obser- 

 vatory, to be optically tested, and to receive the finishing touches. 



The frame and stand were made by M. Eichens, the director 

 ofM. Secretan's works. The telescope is suspended from its 

 centre of gravity by two trunnions, resting on two solid vertical 

 columns. It possesses vertical and azimu.th.al movements, so 

 that it only requires to have its inclination adjusted to the lati- 

 tude of the place in which it may finally rest, to constitute a 

 veritable equatorial. In consequence of the complaints made 



* The metre is 39'3779 inches; the centimetre 0-3937 of an inch. 



