TELESCOPE, an optical instrument employed to view dis tant objects (Gr. Tip?e, far, oworei), to see).
Telescopes were made in considerable numbers and found their way over Europe soon after their invention. Galileo states that, happening to be in Venice about May 1609, he heard that a Bel gian had invented a perspective instrument for making objects appear nearer and larger. The day after his return to Padua he made his first telescope by fitting a convex lens in one end of a leaden tube and a concave lens in the other end. A few days afterwards having succeeded in making a better telescope, he took it to Venice, where he communicated the details to the public, and presented the instrument itself to the doge Leonardo Donato. The senate, in return, settled him for life in his lectureship at Padua and doubled his salary, which was previously 500 florins. Galileo devoted all his time to improving the telescope. He con quered the difficulties of grinding and polishing the lenses, and soon succeeded in producing telescopes of greatly increased power. His first telescope magnified three diameters; he soon made in struments magnifying eight diameters, and finally one magnifying thirty-three diameters.
With this last instrument Galileo discovered in 1610 the satel lites of Jupiter, and soon afterwards the spots on the sun, the phases of Venus, and the hills and valleys on the moon. He dem onstrated the rotation of the satellites of Jupiter round the planet, and gave rough predictions of their configurations, proved the rotation of the sun on its axis, established the general truth of the Copernican system as compared with that of Ptolemy, and fairly routed the fanciful dogmas of the philosophers. These brilliant
achievements, together with the immense improvement of the instrument under the hands of Galileo, overshadowed in a great degree the credit due to the original discoverer, and led to the universal adoption of the name of Galilean telescope for the form of instrument invented by Lippershey.
In the Galilean telescope the object-glass is a convex lens and the eye-piece concave. Kepler was the first to explain the theory and some of the practical advantages of a convex eye-piece in his Catoptrics (16i'). The first person who actually constructed a telescope of this form was the Jesuit, Christoph Scheiner, who gives a description of it in his Rosa Ursina (163o). William Gas coigne pointed out one great advantage of the form of telescope suggested by Kepler; viz., the visibility of the image of a distant object simultaneously with that of a small material object placed in the common focus of the two lenses. This led to his invention of the micrometer and his application of telescopic sights to astronomical instruments of precision. It was not till about the middle of the 17th century, however, that Kepler's telescope came into general use, and then, not so much because of the advantages pointed out by Gascoigne, but because its field of view was much larger than that of the Galilean telescope. The first powerful tele scopes of this construction were made by Huygens, after much labour, assisted by his brother. With one of these, of 12 ft. focal length, he discovered the brightest of Saturn's satellites (Titan) in 1655, and in 1659 he published his Systema Saturnium, in which was given for the first time a true explanation of Saturn's ring, founded on observations made with the same instrument.