The Russellville Democrat., 1879-10-16
THE PULKOwA TELESCOPE.
The New Instrument Ordered in this Coun. try by the Russian Government the
 Largest in the World.


 The New Instrument Ordered in this Coun.
 try by the Russian Government the
 Largest in the World.
 Boston Advertiser.
 The contract for the object glass of the new telescope for the Pulkowa Observa- tory,' which the Russian Government has made with Alvan Clark & Sons, of Cambridgeport, provides for lens with diameter of thirty inches, to be finish ed in eighteen months after the glass disks are received; but two years are al- lowed for procuring these, so that it may be three and a half years before the glass is completed. The price is to be $32, ,000. The metal work and mounting, which is to be done in Europe, will probably cost as much more. The new instrument is to be considerably more powerful than any now existing, the Washington tele- scope and it's sister, the Mccormick glass, being each twenty-six and quar- ter inches in diameter. The Vienna telescope, now in process of construction by Grubb, of Dublin, but, if reports are correct, likely to be delayed in it's com pletion by difficulties of construction greater than the contractors had expect ed, is to be of ftwenty-eight inches apera-, ture. The Newhall telescope of England of twenty-five inches aperature is the only other instrument at all comparable with these.
 The Washington telescope. the New York Times points out, is the only one of them which has yet accomplished any serious work, but its record in the dis covery of the satellites of Mars, its obser- vations upon the satellites of Saturn, Uranus and Neptune, and its perform ance upon nebule and double stars has been such as to make it clear that any observatory which does not wish to be hopelessly in the rear as regards certain important classes of astronomical work must be provided with optical appliances of equal power. It is an interesting question how much further the size of telescopes can be increased. The Rus sian astronomer would have been glad to obtain a considerably larger one than that finally contracted for, but the opti- cans would not venture on more than thirty inches. The limiting difficulty lies at present in the manufacture of the raw material. It is almost impossible to produce glass disks of such size sufficient ly homogeneous and free from stria to serve for telescopic use. There are only two or three firms in the world which undertake the manufacture of optical glass of large size, and thus far not single disk of thirty inches diameter has been produced by either. But if this difficulty were overcome, and glass of any size were procurable, any further in- crease of dimensions is sure to bring in serious difficulties which already are pre- ceptible even in our present instruments difficulties arising from the distortion of the lenses by their own weight. It IS not at all certain that an obiect-glass four feet in diameter could be made to perform at all. It is true that the dis tortion of lens injures it's performance far less than the same amount of distor- ton of mirror of similar size; but. while the lens must be supported at the edge, mirror can be so mounted that it's weight will produce very little dis tnrbance of the definition.
 Up to the present time, however, it is beyond question that refracting tele- scopes have proved far more efficient than refectors-the latter are ponderous, unwieldy, inconvenient, and require con- stant attention and adjustment to make them work. What is worse, the reflect ing surface is not permanent like that of lens, but requires constant care and frequent renewal. As a consequence re- flectors have done almost nothing for science except in the hands of their makers. The Hershels, Lord Rosse, Las sell and De Ia Rus have got good work out of their instruments; but the other great refiectors of the world have proved but little better than costly failures, like those at Paris and Melbourne. One lit- tie bit of history is worth recording in connection with the contract for the Pulkowa telescope. About the time when Weisse, of the Vienna observatory, was in this country, some years ago, the Pulkowa astronomers announced the discovery, with their fifteen-inch tele- scope, of a minute companion to the bright star Procyon. None of the in- struments in this country were, how ever, able to show it, although it was carefully looked for, especially at Wash ington. This circumstance persuaded 1 Weisse of the inferiority of the Ameri- | can lenses, and decided him, much to l the disappointment of the Clarks, to ! give the order for the Vienna telescope | to Grubb, who is still struggling to carry out his contract, with no very certain | prospect of success. A year or two later | the Russians found that their companion | Of Procyon was myth-a mere ghost, | produced by reflection between the | lenses of their telescope, and q poetical | justice now brings them to the Clarks to | be furnished. with new, more reliable, and powerful instrument