October i i, 1900] 



NATURE 



575 



their massive cells are enclosed in the large rectangular 

 glass case shown in the figure, and being mounted 

 on rails, it is a matter of but a few moments to inter- 

 change the position of the lenses with respect to the long 

 tube seen extending from end to end of the gallery. This 

 enormous (and, from an astronomical point of view, 

 probably superfluous) tube is composed of twenty-four 

 sections of sheet steel, each about 8 feet 3 inches (2"5o 

 metres) long, and 59 inches (I'So metres) in diameter, 

 which are supported on six braced columns rising from 

 the floor below. 



At the near end of the tube is shown the massive tail- 

 piece, with the various arrangements for focussing, 

 clamping and rotation of the photographic plate. The 

 focussing is done by traversing the whole of the tail- 

 piece on the two short rails, a motion of about 5 feet 

 (r5o metres) being allowed, as it is unlikely that the 

 focal lengths of the two lenses will turn out identical. 



The lens in position, corrected for the photographic 

 rays, is 49'2 inches (r25 metres) aperture, and 187 feet 

 (57 metres) focal length. The diameter of the image of 

 the sun or moon at the principal focus will therefore be 

 about 21 inches in diameter. 



Rotation of the photographic plate, about 30 inches 

 square, during exposure is necessary on account of the fact 

 that when a siderostat is used, only the central point of the 

 field of view remains stationary, all the surrounding parts 

 having a motion round this as centre. To eliminate this 

 M. Gautier has provided a subsidiary clock, placed to 

 the rear of the eye-end, which, by means of connecting 

 gear to the milled wheel seen on the circumference of 

 the end of the tube, turns the whole tail-piece of the 

 telescope at any desired rate. For visual observations 

 the plate-holder is removed, and an adapter carrying an 

 eye-piece is inserted. The whole of this eye-end section 

 is now covered in by a temporary dark-room. 



To admit the light from celestial objects to the mirror 

 of the siderostat, the roof and walls of the building for 

 some 70 or 80 feet are made in two sections, one of which 

 slides to the north, the other to the south, allowing of a 

 clear view from the zenith southwards to within some 

 few degrees of the horizon. This done (requiring some 

 six or eight attendants to work the pulley blocks), all the 

 subsequent movements and adjustments are easily made 

 by two observers. One is stationed at the base of 

 the siderostat, with the handles for working the instru- 

 ment, telescopes for reading the circles, sidereal clock 

 and telephone all within his reach. Whatever object is 

 selected from the previously prepared working list for 

 the evening, the declination handle is turned until the 

 scale reading seen through the telescope gives the correct 

 declination of the object. Then the hour-angle, or differ- 

 ence between the sidereal time indicated by the clock 

 and the right ascension of the star, is set by means of 

 another handle and the second small telescope. This 

 done, the required object will be near the centre of the 

 field of the large telescope, and the clock being set 

 running to keep it there, the astronomer takes his place 

 at the eye-end to bring it exactly to the centre of the 

 eye-piece. He also has a telephone by his side, and for 

 a space of some two or three minutes there is a con- 

 tinual rross-fire of such terms as " Declinaison," " As- 

 cension droit," " Doucement," " Au sens contraire," with 

 various endearing terms of admonition in cases of over- 

 doing the movements. Considering, however, the com- 

 paratively high power (5CX)) which is the lowest used, it is 

 astonishing how quickly an object is obtained after the 

 setting of the circles. This in itself furnishes an incon- 

 testable proof of the extremely accurate adjustment of 

 the siderostat and telescope, both as regards the angle 

 of its polar axis and its position in the meridian. The 

 object being found, all lights are extinguished and a 

 drawing or photograph made as carefully as possible. 

 On the evenmg it was my privilege to be present, our 



NO. 16 I 5, VOL. 62 J 



first object was the Ring Nebula in Lyra (Messier 57). 

 The astronomer in charge was M. Eugene Antoniadi, 

 of the Juvisy Observatory, near Paris, who has started a 

 systematic study of nebultC with the telescope. This 

 view of the nebula surpassed anything seen before, 

 although frequent observations of it have been made by 

 the writer with a telescope of 36 inches aperture. The 

 great increase of light given by the great glass made 

 it possible to use a highly magnified image, which was 

 at the same time bright enough for the eye to detect 

 detail without any strain. 



We next turned to the coloured double star /3 Cygni, 

 after that a Lyras (Vega), the companion of which was a 

 very conspicuous object. During all this time the in- 

 strument was being used in a building containing several 

 hundred people, electric lights all over heating the air, 

 and the huge searchlights and illuminations from the 

 surrounding buildings causing considerable atmospheric 

 glare — conditions under which none but first-class ap- 

 paratus would be workable. However, owing probably 

 in a great measure to the large proportion of the roof 

 which is opened, the star images were not inconveniently 

 unsteady. 



M. Deloncle appears indefatigable in doing his utmost 

 to entertain any one having special interest in his proi^g^. 

 Parties of guests are often there to listen to a short 

 address from him on its construction and installation, 

 after which they go to the eye-piece, and in turn see 

 whatever may be on view. This goes on till after mid- 

 night, and then, the last visitor away, all lights are turned 

 out, the lens case opened to permit of air circulation, and 

 about half an hour or more allowed to elapse for the 

 general temperature to be equalised throughout the various 

 parts. We turned next to an object which M. Antoniadi 

 had not previously observed (a small planetary nebula in 

 Sagitta), found it quite easily by the circles and slight 

 subsequent sweeping, and then occupied about an hour 

 and a half in careful drawing. This nebula is G.C. 4572, 

 and M. Antoniadi's drawing appears in \\\t. Bulletin de la 

 Soci^te Astronomique de France, September 1900. 



In sweeping from one object to another, thousands of 

 stars cross the field of view, and it was specially noticeable 

 that no distortion of the star images was to be detected 

 as the mirror was moved to diff"erent angles of incidence. 

 With such a high power this is in itself a severe test of 

 the planeness of the silvered surface, and it is worth 

 drawing attention to the fact that the figuring of this 

 mirror by M. Gautier has been done entirely by 

 mechanical means, controlled at every step by the most 

 delicate optical tests. 



In connection with the actual work of observing, 

 nothing is so important as accurate clock driving, in order 

 that the astronomer may not be troubled by constantly 

 having to bring the object into the centre of the field. 

 How efficient this instrument is in this respect will be 

 understood when it is stated that during the period for 

 which the clock runs at one winding, about forty-five 

 minutes, the star images do not move sufficiently in the 

 field of a power of 500 to necessitate any adjustment. 

 The angular diameter of the field of view is about 3' 

 of arc. 



By the time M. Antoniadi had finished his drawing 

 of this planetary nebula it was about 3.0 a.m., and 

 approaching daylight. The mirror cover was lowered 

 into its place, the two sliding sections of the roof pulled 

 together over the mirror, and the clock stopped. In a 

 few minutes M. Antoniadi and myself were the only occu- 

 pants of the gallery, and the institution being already in 

 such regular use that a couple of beds are provided for 

 observers staying all night, we decided that, as it was so 

 terribly hot indoors, we would take up our beds and camp 

 out in the courtyard. We did this, and sleep being some- 

 what out of the question, spent some time gazing upward 

 in the hope of seeing some forerunners of the August 



