Scientific amusements . lenses, he managed toconstruct a telescope, and began to study the. was in i6og. Galileos Tube became celebrated,and all the first telescopes were made with the concaveeye-lens. Rheita, a monk, made a binocular telescope, asnow used in our opera and field-glasses approximately. But the prismatic colours which showed themselves inthe early telescopes were not got rid of, nor was it till 122 OPTICAL APPARATUS. 1729 that Hall, by studying the mechanism of the eye,managed a combination of lenses free from colour. Tenyears before (in 1718) Hadley had established
Scientific amusements . lenses, he managed toconstruct a telescope, and began to study the. was in i6og. Galileos Tube became celebrated,and all the first telescopes were made with the concaveeye-lens. Rheita, a monk, made a binocular telescope, asnow used in our opera and field-glasses approximately. But the prismatic colours which showed themselves inthe early telescopes were not got rid of, nor was it till 122 OPTICAL APPARATUS. 1729 that Hall, by studying the mechanism of the eye,managed a combination of lenses free from colour. Tenyears before (in 1718) Hadley had established the ReflectorTelescope ; Herschel made his celebrated forty-foot re-flector in 1789. However, to resume. In 1747, Euler declared that itwas quite possible to construct an arrangement of lenses soas to obtain a colourless image, but he was at first chal-lenged by John Dollond. The latter, however, was afterwardsinduced to make experiments with prisms of crown andflint glass. He then tried lenses, and with a concave lens. Image on the Retina, of iflint, and a convex lens of crown, he corrected the question of proper curvature was finally settled, andthe Achromatic Telescope became an accomplished fact There are two classes of Telescopes—the reflecting andrefracting. Lord Rosses is an instance of the Grubbs immense instrument is a refractor. The Microscope has been also attributed to ZachariasJansen, and Drebbel, in 1619, possessed, the instrumentin London, but it was of little or no use. The lensinvented by Hall, as already mentioned, gave an impetusto the Microscope. In the simple Microscope the objectsare seen directly through the lens or lenses acting as compound instrument is composed of two lenses (or THE MICROSCOPE. 123 a number formed to do duty as two), an eye-lens, and anobject-lens. Between these is a stop to restrain alllight, except what is necessary to view the object large glass near the object bends the rays on to th
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