Jan. 20, 1876] 



NATURE 



23t 



the telescope, suspended by its centre, must not bend 

 more than a millimetre in the most unfavourable positions, 

 according to the calculations by which M. Wolf determines 

 its dimensions. Experience has verified his calculations ; 

 the two mirrors remain exactly centred upon each other 

 in all positions of the telescope. 



It is necessary, however, to be able to direct the tube 

 toward any point of the sky, and it is necessar)', more- 

 over, that when the star is once in the field of the 

 instrument, that should be able to follow it, by a simple 

 movement, in its apparent course through the heavens. 

 This is accomplished by what is called the equatorial 

 mounting of the telescope. This revolves on an axis, 

 cast of iron and steel, whose direction is parallel to the 

 axis of the celestial sphere. Then it may be inclined 

 more or less on this axis, by turning round in a second 

 axis of steel, which crosses the former at a right angle, 

 and partakes in its movement of rotation. The whole of 

 the arrangement of double axes, a veritable wonder of 

 mechanism, from the precision and ease of the movement, 

 weighs with the telescope 10,000 kilogrammes. Such is 

 the mass which, like the hand of a gigantic chronometer, 

 must follow with precision the march of the stars in the 

 vault of heaven, obedient to the action of clockwork, con- 

 trolled by a Foucault regulator. 



To realise this wonder, M. Eichens had to put together 

 the most delicate apparatus of the mechanics of preci- 

 sion, and, preserving their delicacy intact, give them the 

 strength necessary to support great weights. We cannot 

 explain in detail the series of these wonders, tell how 

 friction is almost annihilated throughout, how all the 

 parts are in equilibrium, whatever be the position of the 

 telescope : how, in fine, at the same time that the instru- 

 ment follows the movement of the sky, the observer may 

 at his pleasure move it with perfect ease in all directions 

 by means of contrivances placed at his hand. 



The perfection of mechanism would be nothing if it 

 did not serve the purpose of observing the stars with an 

 optical apparatus of equal perfection. Let us at once say 

 that the first attempts which have been made with the 

 instrument have completely satisfied the astronomers. 

 Not only has the mirror acquired, under the hand of M. 

 Martin, the rigorously parabolic form which gives it the 

 property of collecting in a single point the rays of a star, 

 but the ver>' complex eye-piece, by means of which the 

 luminous point is observed, is itself without a single 

 defect. It now only remains to silver the surface of the 

 mirror, an operation at present easy, by the processes of 

 M. Ad. Martin, and which will be accomplished in a 

 large dish i'3o metres in diameter. Meantime, the sur- 

 face of the polished glass reflects sufficient light to make 

 it possible to observe the most feeble stars ; directed 

 towards the moon, the telescope concentrates in the eye 

 a light almost intolerable. It may be judged from this 

 what will be the brilliancy of celestial images when the 

 silvered mirror will throw upon the eye, not merely 

 scarcely one-half, but more than nine-tenths of the light 

 which it receives. 



The comparison which we made above between the 

 Marseilles telescope of o'8o metre aperture and the 

 most powerful instruments elsewhere, allows us to predict 

 the results which science has a right to expect from a 

 telescope whose mirror is greater by half, and whose 

 mechanism has reached the latest limits of perfection. 

 M. Wolf, to whom the use of the telescope has been en- 

 trusted, proposes to employ it in observing the planets 

 and their satellites. At the same time the new telescope 

 will be fitted with all the apparatus necessary *or pho- 

 tography and the spectroscopic observation of the stars. 

 It should be remembered, however, that the use of such 

 a gigantic instrument requires a long apprenticeship ; 

 the Melbourne instrument had two observers before it 

 came into the hands of an astronomer who knew how to 

 make good use of it. 



In a few weeks will be completed the first of the great 

 instruments promised to France by MM. Le Verrier and 

 Foucault. The construction of the telescope has been 

 undertaken first, that it may serve as a study for the 

 construction, much more delicate, of the great refractor of 

 16 metres in length, with an object-glass of 75 metre 

 aperture. The success of the reflector is a guarantee 

 that M. Le Verrier, with his eminent colleagues, will 

 accomplish satisfactorily the second part of his great and 

 patriotic enterprise. 



THE LOAN EXHIBITION OF SCIENTIFIC 

 APPARATUS AT SOUTH KENSINGTON 



ON the 3rd inst, as we have already intimated. Her 

 Imperial Highness the Crown Princess of Ger- 

 many invited to her palace forty of the representa- 

 tives of science of Berlin, to lay before them the plan 

 of the London Exhibition of Scientific Apparatus, and 

 to ask their co-operation for this purpose. Amongst 

 those honoured by invitations were the Ministers of 

 Education and of Commerce, the Postmaster-General, 

 and the following professors of the University: — MM. 

 Braun (botanist). Dove, Helmholtz, and KirchhofT (physi- 

 cists), Du Bois-Reymond (physiologist), Kiepert (geogra- 

 pher), Forster (astronomer), Peters (zoologist), Kronecker 

 (mathematician) jWebsky (mineralogist), Hofmann, Oppen- 

 heim, and Sell (chemists), Wichelhaus (technologist). 

 Orta (agriculturist) ; the following professors of the Poly- 

 technic School : — MM. Reulaux (mechanician), Lieber- 

 mann (chemist), Vogel (photographer), and Scheibler 

 (agricultural chemist) ; the director of the South Kensington 

 Museum, Mr. Cunliffe-Owen, the directors of the German 

 Industrisil Museum, MM. Gruner and Lessing, the manu- 

 facturers, Dr. Werner Siemens (member of the Academy), 

 and Dr. Martins. The illustrious hostess, as well as His 

 Imperial Highness the Crown Prince, pleaded warmly for 

 the worthy representation of Germany in the London 

 Exhibition. Although the short time left for prepara- 

 tion, and the coincidence of the exhibition with that of 

 Philadelphia, were generally felt as serious drawbacks, 

 some of the men of science present taking the lead 

 assembled the following day, when a general committee 

 was formed under the presidency of Dr. A. W. Hofmann ; 

 with the view of forming special committees for the dif- 

 ferent branches of the exhibition, and of inviting one 

 member of ever)' German University and Polytechnic 

 School to co-operate with them. An invitation to the men 

 of science and the manufacturers of scientific apparatus 

 has already been issued. 



"Science," the invitation says, "being the common 

 property of all nations, the exhibition of the appliances by 

 which it is promoted partakes of an international charac- 

 ter. The objects pursued by the English Commission in 

 organising the Exhibition have in Germany also been 

 recognised as worthy of attamment, and, in order to give 

 an impulse in our Fatherland to German participation in 

 the Exhibition, a Committee has been formed, at the 

 special instance of the Crown Prince and Princess of 

 Germany, which has been intrusted by the English Com- 

 mission with the collection and selection of objects 

 worthy of being exhibited." 



The invitation then proceeds to detail the conditions of 

 exhibition settled by the Science and Art Department, 

 and concludes as follows : — 



"The Exhibition of Scientific Apparatus in London 

 essentially differs from the former Exhibitions, as it pays 

 IcSs regard to merely commercial mterests, but kteps in 

 view the higher aim of disseminating as wideiy as possible 

 the knowledge of the different methods of science. In 

 order to render full justice to this task, the British Govern- 

 ment (Science and Art Department) will bear all the 

 I osts of packing, sending, and returning any objects that 

 1 may be confided to its care." 



