December 21, 1899] 



NATURE 



179 



The discs for the object-glasses, visual [and photo- 

 graphic, of I '25 metres in diameter, were cast by M. 

 Mantois ; the flint weighs 360 kilos., and the crown 220, 

 and the figuring, polishing and mounting of these 

 enormous discs have been confided to M. Gautier. 



The following interesting account of the casting of the 

 glass for the lenses is given in the Scientific American : — 



"Great attention was paid to the castmg of the glass. 

 Specimens of the glass were constantly taken out during 



Objectif visuel 



Objectif photo^raphique 



Fig. 2. — The object-glasses. 



the heating and examined with a lens under different 

 conditions of illumination in order to judge of the degree 

 of purity which they have reached. After several speci- 

 mens have been found to be free from bubbles the tem- 

 perature is reduced, the glass thickens, the crucible is 

 opened, and a certain portion of the surface is skimmed 

 off to get rid of impurities. The glass is then stirred, 

 and the cooling is allowed to proceed rapidly for five or 

 six hours until the surface of the glass emits a well- 

 defined sound when it is struck with an iron bar. After 

 this step it is necessary to proceed 

 with annealing. The furnace is walled 

 up and a cooling is allowed to pro- 

 ceed, which requires from four to six 

 weeks. When the crucible is opened - 

 the glass is found to have been 

 broken into pieces of varying sizes. 

 In order to obtain a 792 pcmnd flint 

 glass lens it is necessary to find a 

 block which weighs nearly 1300 

 pounds, and such a block having 

 been found among those in the furnace 

 it is removed and placed upon a car. 

 Slabs of glass are sawed from two 

 parallel sides in order to obtain 

 polished surfaces that facilitate a 

 perfect examination of it. The striae 

 in the surface are removed, and if 

 after this the block exhibits any defects 

 situated at such a depth that they 

 cannot be removed, it is submitted to 

 a moulding which changes its form 

 and brings the chief defects near the 

 surface. The block is placed in a 

 mould of refractory clay and put into 

 a furnace and heated to 800' to 900° Centigrade. By 

 this means it becomes slowly heated and softened until 

 it assumes the form of the mould, but it must not be- 

 come fused, or the whole operation must be gone 

 over again. If the outcome of the process is suc- 

 cessful, the glass is slowly annealed, and is then taken 

 fr(J|n the mould and examined anew. If any defects deep 



NO, 1573, VOL. 61] 



in the glass are seen, a second operation is begun with a 

 mould of another form. Kjnally, when the glass is very 

 pure and perfect, another and final mouldmg produces 

 the plano-convex lens. After this comes another heating 

 and cooling which takes two or three weeks." 



Telescope. — The telescope tube, which is of sheet steel 

 2 mm. thick, weighs 21,000 kg. ; it is 1-50 metres in 

 diameter, and is composed of twenty-four pieces united 

 by bolts. It rests on eight supports of cast iron resting 

 on stone pillars. In view of ex- 

 pansion, the supports move on rails 

 fixed to the pillars. 



I confess this iron telescope tube 

 astonishes me. 



The two object-glasses are both 

 mounted on the same carriage, 

 which moves on rails in such a 

 manner as to place them easily, the 

 one or the other, at the end of the 

 tube ; the weight of each of these 

 object-glasses, without their cell, is 

 about 600 kg., and with the cell 

 900 kg. Each lens is adjusted in 

 a separate cell ; that of the crown is 

 carried on rollers, in order to be 

 able to remove it from the flint and 

 render the cleaning of each disc 

 easy. 



The tube carrying the eye-piece 

 is supported by four wheels on rails. 

 It is attached to the telescope tube by 

 an adjusting screw i"5o metres long, which is used for 

 focussing. In the interior of this tube there is another 

 1*20 metres in diameter, which can be rotated by clock- 

 work. This carries the adapter for the eye-piece end, 

 which is made to slide in two rectangular directions by 

 means of screws. The eye-piece end can carry either an 

 eye-piece, a micrometer, a photographic plate, or a 

 projecting lens. 



By means of the arrangements realised in the eye- 

 piece the observer is rendered independent of the 



Fig. 3.— The eye-piece and travelling adapter. (Side view.) 



apparent movement of the heavens, and is enabled to- 

 follow the object in right ascension and declination. 



It is stated that M. Gautier has been entirely suc- 

 cessful, not only with the plane mirror, but with the two 

 object-glasses. • „ r 1 



The grinding apparatus consists essentially of a large 

 cast iron plate, C, covered with an inch of flannel, upon 



