TELESCOPE

(Yunanca köken — orijinale bakınız.), I look at.) An optical instrument for viewing distant objects. For several reasons a distant object is seen le distinctly than a similar near one. The angle which an object subtends diminishes as the distance increases; the density of the light which renders it visible also diminishes with the distance, but in a much faster ratio; and a considerable portion of light is always lost in its pa age through the atmospher., It is found by experience that to be discernible at all in ordinary daylight, a detached object must subtend at the eye an angle of not le than 30", and that the least angle under winch contiguous objects can be satisfactorily distinguished is about one minute. By the aid of a telescope a magnified image of the object is obtained; and within certain limits the object is not only apparently enlarged, but rendered brighter than it appears to the una isted eye. The invention of the telescope, to which practical astronomy is indebted for its most important discoveries, has been ascribed to various persons. Sir David Brewster (Encyc. Brit., art. "Optics") says, "We have no doubt that this invaluable instrument was invented by Roger Bacon or Baptist a Porta, in the form of an experiment; though it had not, perhaps, in their hands a umed the maturity of an instrument made for sale, and applied to useful purposes, both terrestrial and celestial. If a telescope is an instrument by means of which things at a distance can be seen better than by the naked eye, then Baptist a Porta's concave lens was a real telescope; but if we give the name to a tube having a convex object-gla at one end, and n convex or concave lens at the other, placed at the distance of the sum or difference of their focal lengths, then we have no distinct evidence that such an instrument was used before the beginning of the 17th century." Des cartes ascribes the invention to James Metius, a citizen of Alkmaer in Holland;Huygcns to John Lippersey, or Zacharias Jansen; Borellus also to Jansen. Profe or Moll, who has discu ed these rival claims, after examining the official papers preserved in the archives at the Hague, comes to the conclusion that Metius (whose proper name was Jacob Adriaansy), on the 17th of October, 1608, was in po e ion of the art of making telescopes, but that from some unexplained reason he concealed his invention, and thus gave up every claim to the honour he would have derived from it; that on the 21st of October in the same year, 1G08, John or Hans Lippersey,asi>ectac!e-maker of Middle burg, was actually in po e ion of the invention; and that there is little reason to believe that either Hans or Zacharias Zanz (or Jansen, father and son) were inventors of the telescope, though one of them invented a compound miscroscope about 1590. (Journal of the Royal Institution, vol. i.) The telescope soon made its way into other countries. In April or May, 1609, the illustrious Galileo, having heard a rumour of the invention, set about considering the means whereby distant objects could be seen distinctly, and was soon in po e ion of a telescope which magnified three times. In subsequent trials he succeeded in. increasing the 1228 [s. 1242]