Dec. 27, 1877] 



NA TURE 



165 



THE NEW PARIS TRANSIT CIRCLE^ 



OF the numerous instruments with which Leverrier 

 enriched the Paris Observatory during the twenty 

 years of his direction, the last which he was able to see 

 completely installed was the new transit circle. This 

 instrument was not, like all the others, constructed at the 

 expense of the State ; an inscription on the marble pillars 

 that support it informs the visitor that it was presented 

 to the Observatory by the generous munificence of M. 

 Raphael Bischoffsheim. This is not the only gift of M. 

 Bischoffsheim to astronomy ; the Observatory of Lyons 

 is also indebted to him for its fundamental instrument. 



The project of erecting a new meridian circle at the 

 observatory goes back to the time of the debate raised 

 before the Academy of Sciences on the subject of the 

 transfer of the observatory to a site outside Paris. 

 Those who would not admit the legitimacy of the 

 complaints made by the adversaries of the present situa- 

 tion of the observatory, were obliged to admit that the 

 great meridian room, constructed in 1830 by Arago, 

 did not offer any of the guarantees necessary to observa- 

 tions of great precision. The thickness of the walls and 

 of the double roof of that room, the small breadth of the 

 openings, the nearness of the observatory buildings, the 

 difference of level between the two faces north and south, 

 must necessarily affect the equilibrium of the neighbouring 

 layers of the atmosphere and hinder them from taking 

 that horizontality which admits of the correction of the 

 observations from the influence of refraction. 



Since the astronomer cannot get rid of this troublesome 

 influence, his first business ought then to be to reduce it to 

 conditions in which it may be possible to calculate the 

 effect. Thus what strikes the visitor admitted to the new 

 meridian circle of the Observatory is the small building 

 in which it is placed. In the middle of a green lawn rises 

 a hut made entirely of sheet-iron, the roof formed of two 

 plates which, by sliding upon rollers, may be separated 

 from each other, and leave all the upper part of the build- 

 ing open. The walls are formed of two envelopes of thin 

 iron, between which the air freely circulates, thus main- 

 taining the whole structure at the temperature of the air 

 itself. Large windows may also be opened, and the 

 observer and the instrument be thus placed in the same 

 conditions as if the observations were made in the open 

 air. All these conditions are to avoid as far as possible 

 the disturbances arising from atmospheric refraction, the 

 greatest source of inaccuracy in astronomical observation. 

 The only obstacle which may yet be a hindrance to per- 

 fection in the conditions of observation is the presence of 

 those beautiful trees which make the terrace of the obser- 

 vatory a magnificent garden, but which store up the warm 

 air during the day and slowly distribute it during the 

 night. No doubt some day the astronomers will be 

 obhged to sacrifice to the precision of their observations 

 the enjoyment of this beautiful foliage. 



The meridian circle is composed, as its name indicates, 

 of two instruments : the meridian telescope, intended, by 

 its association with an astronomical clock, to fix the 

 moment of the passage of a star across the meridian of 

 the place of observation, and the mural circle, which 

 gives the measure of the angular distance of this same 

 star from the pole or the zenith. When, forty years ago, 

 Gambey constructed the two meridian instruments of the 

 Paris Observatory, so justly celebrated and on the model 

 of which those of most other observatories have been 

 designed, he had to reconcile, by prodigies of skill, the 

 hghtness resulting from the means of construction then in 

 use, with the rigidity of the parts necessary for precision 

 of observation. It is the alliance of these two almost 

 contradictory qualities which renders so interesting the 

 instruments of that celebrated artist and especially his 

 machine for dividing the circles, which the Baron Sdguier 



From an article In La Nature by M. C. Wolf. 



has restored in the galleries of the Conservatoire. But 

 there resulted from this at first the necessity of separating 

 the measure of the two co ordinates of the stars — the 

 instant of the meridian passage and the polar distance. 

 There also resulted the necessity which Gambey was under 

 to fix on his mural circle of two metres in diameter, a tele- 

 scope altogether insufficient in optical power. 



A simple glance at the great meridian circle of the 

 observatory, the western equatorial, the great telescope, 

 the new instrument of M. Bischoffsheim, all from the 

 workshop of the great mechanician, M. Eichens, shows 

 the revolution which has been effected in the processes of 

 construction. In place of instruments formed of pieces 

 of sheet brass connected by simple screws or even soldered 

 together, we have the bodies of the telescope of cast-iron 

 bolted on axes of cast-iron and steel, strong and elegant 

 in appearance ; circles of bronze cast in a single piece and 

 protected against all deformation by numerous cross-bars. 

 It is the art of the engineer applied to the construction of 

 astronomical instruments, with the power given by the 

 choice of metals and the thickness of pieces, and the 

 precision which the employment of engineering tools 

 secures. 



This revolution was begun in England about 1847 by 

 the Astronomer- Royal, Sir George Airy. In 1863, M. 

 Leverrier successfully installed a meridian circle greater 

 still than that of Greenwich, and intended, like it, for the 

 observation of the small planets. But these gigantic 

 instruments, veritable siege-guns of long range, since they 

 reach the farthest depths of the heavens, want, simply on 

 account of their weight, one essential quality— they are 

 not reversible. Whatever be the rigidity of the pieces, 

 the instrument is subject, in each successive position, to 

 flexions necessarily unequal, which the astronomer must 

 investigate and measure in order to correct his observa- 

 tions. But this investigation and this measurement can 

 only be made by turning round the instrument. It will 

 be understood, in fact, that the apparatus, directed suc- 

 cessively to the same point of the sky, first with one of 

 its faces up, then the same face below, gives, if it is really 

 perfectly rigid but elastic, two results differing equally 

 from the truth, one minus and the other plus, so that the 

 mean of the two observations gives the exact position of 

 the star. It is this which may be expected from the new 

 meridian circle of M. Bischoffsheim. Fig. i represents 

 the telescope upon its car, which serves to raise it above 

 its pillars and 'to turn it right round by a movement of 

 rotation around a vertical axis. 



Since 1852 M. Brunner has constructed small portable 

 instruments answering to these conditions. Improved by 

 his sons, by M. Rigaud, and by M. Eichens, these rneri- 

 dian circles are now only used in geodesic expeditions. 

 In 1868 M. Eichens constructed for the observatory of 

 Lima a reversible meridian circle, the telescope of which 

 was 2-30 m. in length, and the object-glass 20 cm. in free 

 opening. It is this model, successively improved, which 

 has become, in the hands of the able constructor, the 

 meridian circle of Marseilles (1876), and the circle given 

 by M. Bischoffsheim (1877). The object-glass of the first 

 was madeby L^on Foucault,the two others are by M. Ad. 

 Martin. The new observatory of Lyons, in the estabhsh- 

 ment of which M. Andre' took an active part energeti- 

 cally sustained by the Administration, will soon possess 

 a similar meridian circle, a little smaller (telescope of 

 2m., object-glass of 14 cm. aperture, by M. Praczmowzki), 

 the expense of which is borne by M. Bischoffsheim. 



The illustrations which we give then show the per- 

 fected model meridian circle employed m observatories 

 for the determination of the celestial co-ordinates of the 

 stars. To be able to understand the use of the various 

 parts' of the instrument, it will suffice to describe a com- 

 plete observation of a star. 



Some minutes before the passage of the star across the 

 meridian, the astronomer gives to the telescope such 



