The next in size is the Pulkowa telescope, 30 inches in diameter, the object-glass also by Clark. 3), of 36 inches diameter and 57 feet in length, the object-glass by Clark of Cambridge, Massachusetts. The discoveries of Guinand about 1800 partially relieved the difficulty, and from about 1870 to 1890 a considerable number of instruments have been made with apertures exceeding 2 feet-the largest so far being the great Lick telescope (fig. For many years after the invention of the achromatic telescope it was impossible to obtain suitable glass for lenses of more than 5 inches in diameter. But other forms are possible and even preferable. The usual construction is a double-convex lens of crown-glass combined with a (nearly) planoconcave lens of flint-glass, the focal lengths of the two lenses being proportional to their dispersive powers, and the curves so chosen that the spherical aberration is corrected at the same time.
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About the middle of the eighteenth century it was discovered in England that, by combining lenses of different kinds of glass, objectives could be made nearly free from chromatic aberration, and all the refracting telescopes now constructed have achromatic object-glasses of some form.
By making the telescope very long in proportion to its diameter, the injurious effect of this chromatic aberration can be greatly reduced, and about 1660 Huygens and Cassini used instruments more than 100 feet long in their observations upon Saturn. The simple refracting telescope in any of its forms is a very imperfect instrument, owing to the fact that rays of different color are not alike refrangible, the focus being nearer the lens for the blue rays than for the red. But the field of view is very restricted, and this form of instrument now survives only in the operaglass. In the Galilean telescope the eye-lens is concave instead of convex, and intercepts the rays from the objective before they reach the focus, so that the object is seen erect. With this form of instrument the object is seen inverted. The “real” inverted image of the object formed at m by the object-glass is viewed by the magnifying lens B, the magnifying power being equal to the ratio between the focal lengths of the lenses A and B. 1), of long focus, while the eyepiece, B, is also a convex lens, but of short focus, the two being placed at a distance slightly less than the sum of their focal lengths. The simple refracting telescope has for an objective a large convex lens, A (fig. Telescopes are classed as refracting or reflecting, according as the objective is a lens or a speculum. The optical parts are usually set in a tube, and this is so arranged that the distance between the objective and the eyepiece can be adjusted to give the most distinct vision. The telescope consists essentially of two members: one, the objective, a large converging lens, or a concave mirror (technically speculum), which forms an optical image of the object the other, the eyepiece, a small lens or combination of lenses, which magnifies this image. It originated in the first decade of the seventeenth century, apparently earliest in Holland but Galileo in 1609 independently invented the form which bears his name, published it to the world, and was the first to apply the instrument to astronomical observation. To slide or pass one within another, after the manner of the sections of a small telescope or spyglass to come into collision, as railway cars, in such a manner that one runs into another to become compressed in the manner of a telescope, due to a collision or other force.Īn optical instrument by means of which distant objects are made to appear nearer and larger. To shorten or abridge significantly as, to telescope a whole semester's lectures into one week. To cause to come into collision, so as to telescope. Its essential parts are the object glass, or concave mirror, which collects the beam of light, and forms an image of the object, and the eyeglass, which is a microscope, by which the image is magnified. ☞ A telescope assists the eye chiefly in two ways first, by enlarging the visual angle under which a distant object is seen, and thus magnifying that object and, secondly, by collecting, and conveying to the eye, a larger beam of light than would enter the naked organ, thus rendering objects distinct and visible which would otherwise be indistinct and or invisible.
An optical instrument used in viewing distant objects, as the heavenly bodies.
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