telescope

[Gr. tēle = far off; skopeo to see, to observe.] Optical Instruments: An instrument for magnify ing distant objects so as to make them look nearer the eye than they actually are. Its e ential parts are: : an object-gla or a concave mirror to render the rays of light convergent, and form an image of the object, and an eyepiece to magnify it after the manner of a microscope. About A. D. 1000, Gerbert of Auvergne viewed the stars through a tube in which, however, there were no lenses. Roger Bacon seems to have known that lenses in com bin at i on had a magnifying power. Dr. Dee, in 1570, speaks of "perspective gla es," apparently used in war to survey the enemy's forces. Jansen and Lippersheim, Lippers hey, or Laprey, spectacle makers at Middelburg, and Jacob Adriansz or Metius, seem to have first become aware of the power of instruments constructed like the modern telescope, and, on Oct. 2, 1608, Lippers hey offered to the States-General three instruments "with which one can see to a distance." Galileo, hearing of this, divined how the result was effected, and constructed the Galilean telescope which had a double concave eyepiece, and made many astronom ical discoveries with it, including the satellites of Jupiter. His telescope is still well known in the advantage of making telescopes with two convex familiar opera-gla . Kepler first pointed out the lenses, and Scheiner carried the suggestion into practice in 1650. De Rheita made a telescope with three lenses, and another of the binocular type. Huyghens made a telescope of 123 feet focal length, only the object-gla of which was in a short tube, telephone.