Page:The American Cyclopædia (1879) Volume XV.djvu/653

 TELESCOPE 623 time for a "protection" or other equivalent for a patent. Metius made a similar present id a similar application later in the same month, but said that he had manufactured ich instruments two years before. It has been frequently said that Zacharias Jansen so invented the telescope more than a year iter ; but the evidence adduced only proves, according to Olbers, that he made telescopes rhich may have been imitated from those of jippersheirn ; and this is the more likely as )th were spectacle makers in the same city, md it is hardly possible that the public trans- action with the latter could have escaped the knowledge of Jansen. The attempt was made by the states general, it is said, to retain to themselves the knowledge of this invention, the importance of which in war was at once perceived by Prince Maurice ; but it is also believed that the French ambassador soon ob- tained from them an order for two telescopes for his own government. It is certain that the report of the invention soon spread abroad, and the instruments found their way to Lon- don, Paris, and Venice. But by no one was the idea more eagerly welcomed, or its great importance more quickly recognized, than by Galileo, then visiting Venice. He was evi- dently willing, at a later day, to be thought the second inventor, guided only by an uncer- tain rumor ; but it is said that he actually saw one of the Dutch telescopes. Returning to Padua with some lenses, he immediately began to improve upon what he had seen, if not to experiment independently under guidance of the mere report, and he soon found a better and more certain result than had been chanced upon by the original inventor. He made a lead- en tube, and fitted at one extremity a double convex lens for object glass, and at the other a double concave for eye piece. This, his first telescope, magnified only three times ; he then made another of more than double this power, and soon after, with a magnifying power of 30, he began to study the heavens, where his first discoveries excited more wonder than that of the "optic glass " itself. The popular cu- riosity was so great, as he himself tells us, that he was compelled night after night to stand by his glass to show its wonderful perform- ances. The phases of Venus, questioned hith- erto, were revealed to sight ; the satellites of Jupiter and the oblong shape of Saturn were distinctly seen; the lunar mountains were mea- sured ; spots were found upon the sun's disk ; and the milky way was resolved into stars. The Galilean telescope produces an erect im- age. The object glass A B would form an inverted image at 5 a, fig. 3, but the concave eye lens N refracts the rays, which being pro- duced backward forms an upright image at a' V. In 1609, the same year in which Galileo's tele- scopes were made, others found their way into England, and were soon sought after with an avidity that was stimulated by the report of Harriot's discoveries. This young astronomer 780 VOL. xv. 40 made drawings of the moon, discovered the satellites of Jupiter, and observed the spots upon the sun. The new " cylinders," as they were called, were soon in general use, and were exposed for sale in Paris in the early part of the same year. These first telescopes are sup- Fio. 8. Galileo's Telescope. posed to have been all made with a concave eye lens. Kepler in 1611 suggested the use of a convex eye lens ; but the first actual ap- plication of one was made by the capuchin Schyrle de Rheita, who describes it in his work Oculus Enoch et Elice (1645). This eye lens gives a much larger field of view, but shows objects inverted. On the other hand, the Gal- ilean telescope had the advantage of greater distinctness and brightness than was found in the "astronomical" form. The true cause of this advantage is now known to lie in the par- tial compensation by the negative eye piece of the aberrations caused by the object glass, the result being in this case the difference, while in the astronomical telescope it is the sum, of the aberrations of the two lenses. Rheita invented also the binocular or double telescope, a construction which frequently re- curs afterward, but always as a thing of cu- riosity rather than of practical utility until in modern days, as the double opera glass or lorgnette, it has become serviceable in recon- noissances, terrestrial and celestial. The very first attempts to gain magnifying power and light by enlarging the object glasses of tele- scopes, revealed a most unexpected and for- midable obstacle. It was found that all ob- jects appeared strongly tinged with prismatic colors. This obstacle remained unexplained until the time of Newton, and unconquered more than half a century longer. But if at the time insurmountable, it did not prove una- voidable, for it was ascertained that by making the focal distance of the object glass very great in proportion to the diameter, the colored fringes could be rendered practically imper- ceptible. Enormously long telescopes were therefore constructed, and it was with them that the brilliant discoveries of that time were made. Huygens used telescopes of his own manufacture, and one of his object glasses, 123 ft. in focal length, is still to be seen in the library of the royal society of London, Eng- lish makers also produced telescopes of nearly equal dimensions, and Auzout in Paris spoke of surpassing all others, but it does not appear whether he succeeded. The elder Campani, at Rome, made lenses of from 70 to 136 ft. focus, and with these Cassini discovered two of the satellites of Saturn. Cassini also used