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NATURE 



[December 21, 1899 



which the glass disc, A^ was carefully laid. The Scientific 

 American thus describes it : — 



" This plate revolves slowly around a vertical axis by 

 gearing, G, the whole being stepped in a cone. Above 



Fig. 4. — Details of the eye-piece. (Front view.) 



there is a stationary circular bronze rubber, B, 47^ 

 inches in diameter, which is given a reciprocating motion 

 by a slider, I, thus passing across the face of the mirror 

 travelling in a circle beneath it. The perfect revolution 

 of the plate and the accurate ad- 

 justing of the slides and their paral- 

 lehsm resulted in the production of 

 a perfect mirror. It required three 

 months to adjust the slides alone. 

 The grinding of the mirror was done 

 with a mixture of emery and water. 

 During this operation a workman 

 always stood at a respectful distance 

 from the apparatus so as not to 

 change the temperature of it. P"rom 

 time to time he injected a mixture 

 of emery and water by means of a 

 syringe into a channel running 

 through the grinding plate and 

 •ending at the centre. This work 

 was carried on generally from 2 till 

 5 o'clock in the afternoon, the time 

 of day when the temperature does 

 not change perceptibly. The entire 

 morning was devoted to the cleaning 

 of the machine, and to the verifi- 

 cation of the parallelism of the 

 grinding plate with the surface of 

 the mirror, an operation which was 

 performed with four scales which were accurate to xo'oo 

 of a millimetre." 



"As the grinding proceeded, finer and finer emery was 

 used, and the closer the grinding plate was brought to 



NO. 1573, VOL. 61] 



the surface of the glass. With the finest emery the 

 distance between the plate and the glass was 0008 inch. 

 The grinding lasted eight months and was followed by 

 the operation of polishing, which required two months 

 The lower surface of the polishing plate was covered 

 with a sheet of albumenised paper like that used in 

 photography, but unsensitised. The workmen spread 

 upon this paper a small quantity of the finest Venetian 

 tripoli and as much as possible was removed with a 

 soft brush. The distance between the rubber and the 

 surface of the glass was o"ooi2 of an inch." 



" This method of treatment, notwithstanding its deli- 

 cacy, produces enough heat to render the mirror slightly 

 convex and cause it to draw away more strongly in 

 the centre, so that, upon cooling, it was hollowed at this 

 point. In order to surmount this difficulty the slides 

 were given a curve of which the pitch was 0-4 of an inch. 

 The heat was diminished by operating the machine for 

 a minute and then stopping for a quarter of an hour. 

 When the hand is applied to the mirror, there occurs an 

 extension of 00012 of an inch, which is sufficient to 

 distort completely for four or five minutes the image of 

 the flame of a lamp placed at one side of the plate and 

 observed from the other with a small telescope arranged 

 for the purpose. The next operation to be performed is 

 the silvering, and, of course, it will have to be silvered 

 anew from time to time. The mirror protrudes 5*4 inches 

 from its tube or cell, which will be made to swing so as 

 to bring the surface to be silvered underneath. The 

 reservoir containing the bath will be lifted by means of 

 a winch until the mirror enters it at a proper depth. 

 When the operation is finished, the reservoir will be 

 lowered and the silvered surface turned upward and the 

 mirror readjusted in its cell." 



I am indebted to M. Gautier for the use of the illus- 

 trations, which have already appeared m the Annuaire 

 Scientifique du Bureau des Longitudes. 



It is the intention of the Syndicate to erect in connec- 

 tion with this telescope a Palais de I'Optique near the 

 EifTel Tower, containing a hall capable of holding some 

 4000 persons, and in fine weather images of the various 

 celestial bodies are to be thrown on a screen 20 metres 

 in the side by means of secondary magnifiers. Thus an 

 image of the moon 16 metres in diameter, and of Mars 

 370 metres in diameter, are promised to the abonnes. 



Fig. 5. — The polishing machines. 



No doubt the great telescope will be largely capable of 

 advancing science, and this will certainly be taken 

 advantage of by the highly skilled \ rench astronomers. 

 Its erection, therefore, will be a great gain. 



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