178 



NA rURE 



[December 21, 1899 



THE GREAT PARIS TELESCOPE. 



TOURING two or three visits to Paris and Nice 

 -•-^ some years ago, I discussed with many French 

 astronomers, whom I was privileged to count among my 

 personal friends, the question of the large telescopes of 

 the future. Among the conclusions come to, the first was 

 that the glass industry was not in a position to grapple 

 with astronomical requirements, and hence when 

 reflectors of 8 or lo feet diameter were talked of it was 

 understood that they must be made of porcelain with a 

 glass surface. Other conclusions were that the coude 

 mounting designed by M. Loewy, and carried out so far 

 as the optical parts were concerned by the brothers 

 Henry, should be replaced with object-glasses of or about 

 25 inches by the use of a siderostat. 



I subsequently (1884) gave two lectures at the Society 

 of Arts on these and other questions,^ in which I pointed 

 out what I considered the best way of using an 8-foot 

 reflector, and with regard to refractors I said : "With an 

 object-glass of 30 inches diameter for physical observ- 

 ations I should certainly prefer the siderostat, thus 

 reducing the cost of an instrument of this size to about 

 one-third of the present price." 



During the last few years we have heard a great deal 

 of an enormous telescope to be constructed on the 



K loTSo 



Fig. I.— The siderostat. 



occasion of the Paris Exhibition of 1900 ; a reflector of 

 10 feet aperture, such as was discussed in 1875, was indeed 

 spoken of at one time, and renewed one of the old dis- 

 cussions, but it would seem that now as then the glass 

 mdustry is not able to furnish a disc of this size, for after 

 all It has been determined to construct a refractor, and 

 mount It as I suggested nearly twenty years ago in front 

 of a siderostat. 



I have recently received from " Le Conseil d' Adminis- 

 tration de la Societe I'Optique" details of the scheme 

 which It IS proposed to carry out ; while information 

 regarding the telescope and siderostat has been given in 

 the Annuaire du Bureau des Longitudes, and more 

 recently in the Scientific American. 



The Council in its memorandum is at some pains to 

 excuse the exaggerations which have been so generally 

 made regarding the power of the new telescope. They 

 state that they hope for such a magnifying power that 

 the surface of the moon will be seen as if our satellite 

 were only 67 kilometres away from us. Under these 

 conditions it was calculated that an object of one metre 



1 Cantor Lectures : "Some New Optical Instruments." 



NC. T573, VOL. 61] 



square might be seen. Hence the short phrase " La lune 

 k un metre," and the consequent nonsense. One of the 

 objects which finally determined the siderostat arrange- 

 ment was the desirability of having a very long focus, 

 and a focal length of 100 metres (328 feet) has been 

 decided upon. 



M. Deloncle seems to be the chief of the band of 

 astronomical amateurs who have enabled MM. Gautier 

 and Mantois to employ their well-known skill. M. 

 Despret, the Director of the Jeumont Glass Works, has 

 produced the siderostat mirror which has a diameter of 

 2 metres, a thickness of 30 centimetres, and a weight of 

 3600 kilos. This certainly could not have been pro- 

 duced with the appliances in use twenty years ago. 



The siderostat avoids all the expense of a dome — even 

 if one of 340 feet diameter could be constructed — and 

 saves considerable expenses for installation ; it secures 

 greater stability, and saves the astronomer unnecessary 

 fatigue and serious loss of time. 



The apparatus constituting the instrument termed a 

 siderostat comprises a pedestal of cast iron, the north part 

 of which supports the polar axis, and the scuth part the 

 mirror with its frame. The cast iron pedestal, 8 metres 

 long by 8 metres high, is furnished with six screws, 

 which fit in sockets fixed to the stone base 170 metres 

 high. 



The north part of the pedestal supports the polar axis 

 with its divided and driving circles. This axis is driven 

 by a clock-work movement by means of a tangent 

 screw. 



At the lower end of the polar axis a fork is fixed, to 

 which are adiasted the pivots of the declination circle. 

 The toothed declination wheel is set in motion at the 

 foot of the instrument by a handle placed beside that 

 one which produces movement in right ascension ; both 

 of these are near the two telescopes which serve for the 

 reading of the two circles. 



The mirror, with its cell, has a total weight of 6700 kg. 



This cell of cast steel is furnished with two pivots ; to 

 the back is fixed the directing rod. The interior of 

 the cell is covered entirely with felt, in such a way 

 that the mirror has no point of contact with the metal. 

 Being supported by as great a surface as possible, all 

 deformations are avoided. 



The mirror and its cell are kept in equilibrium by a 

 system of levers and counterpoises ; the pivots rest on 

 rollers adjusted at the top of the frame, which permits a 

 circular movement by a vertical shaft and a system of 

 independent rollers between two rails. 



The base of this frame floats in a cavity two metres 

 in diameter on the south side of the pedestal, containing 

 sufficient mercury to float ^q of the total weight of the 

 movable part, which weighs 1 5,000 kg. 



The clock-work movement is set in action by a weight 

 of 100 kilos. The total weight of the siderostat is 

 45,000 kg. 



To cast the mirror a special furnace was made at the 

 Jeumont Works, capable of holding over twenty tons of 

 glass. This enormous plane mirror was, naturally, the 

 most difficult part of the apparatus to make. 



The mould, 2*05 metres in diameter and 0*30 metres in 

 height, was placed on a wagon near the furnace, in 

 order to receive the melted glass coming from the 

 crucible. When the mould was full, the wagon was 

 immediately taken to an annealing oven of the right 

 temperature and then walled up ; the cooling lasted a 

 month. The operation of anneaHng the glass is very 

 difficult to carry out ; numerous experiments had to be 

 made ; out of twelve discs only two have been 

 successful. 



The transportation of such a huge disc of glass to 

 Paris was a difificult matter, and a special tram carried 

 it there, and it was conveyed to the optical establishment 

 at night, in order to have a clear roadway. 



