TELESCOPE

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TELESCOPE (from rr/A^, at a distance, and GKoncu), to see); an optical instrument, employed for viewing distant objects, by increasing the apparent angle under which they are seen without its assistance,whence the effect oa the mind of an increase in size, or, as commonly termed, ^magnified representation. (See Optics.) The telescope is perhaps one of the most impor ¦ taut inventions of science, as it unfolds to our view the wonders of the heavens, and enables us to obtain the data for astronomical and nautical purposes. As the use of the instrument depends upon the proportionate distance of the glasses, and this distance requires to be changed to suit the nearness or remoteness of the object, and the vision of the observer, the tube of the telescope is so contrived as to admit of being lengthened and shortened, according to circumstances. The invention of the telescope is ascribed to different persons, among whom are John Baptista Porta, Jansen of Middleburg, and Galilei. The time of its first construction is considered to have been about 1590; but, in 3608 and 1609, we find these instruments for sale at very high prices by Dutch opticians; and in the latter year, Galilei constructed one without having seen those of the Dutch, by fitting a planoconvex and a planoconcave lens in a tube of lead. The simplest construction of the telescope consists merely of two convex lenses, so combined as to increase the apparent angle under which the object is seen. The lenses are so placed that the distance between them may be equal to the sum of their focal distances. The lens nearest the eye is called the eyeglass, and that at the other extremity of the tube the objectglass. Objects seen through this telescope are inverted, and on that account it is inapplicable to land observation ; but at sea it is occasionally used at night and in hazy weather, when there is little light, and is, therefore, sometimes called the night telescope. The astronomical telescope is constructed in this manner, as the inversion of the object is immaterial in celestial observations. The common day telescope, or spyglass, is an instrument of the same sort, with the addition of two, or even three or four other glasses, for the purpose of presenting the object in an erect position, increasing the field of vision, and diminishing the aberration caused by the dissipation of the rays. But the aberration and chromatic error of telescopes were not completely obviated until the invention of the reflecting and achromatic telescopes, which, when accurately constructed, present the object to the vision free from all distortion or chromatic dispersion. The reflecting telescope was invented by father Mersenne, a Frenchman, in the middle of the seventeenth century. Concave mirrors have the property of uniting the rays of light which proceed from any object, so as to form an image of that object at a certain point before the mirror. (See Mirrors.) J£ the distance of the object is so great, that the rays proceeding from it strike upon the mirror parallel to each other (which is the case with the heavenly bodies), the distance of the image is equal to half die radius of the sphere, of which the mirror is an arc, and the point where it is formed is called the focus of the mirror (See Burning Mirror.) This property of the concave mirror has caused it to be used in the observation of the heavenly bodies; and the instrument constructed with such a mirror, is called a reflecting telescope. The simplest constructions of this kind were those m which the image, formed in the focus of the mirror, was used directly, and a convex eyeglass was employed to magnify the angle under which it was seen ; and this, in fact, still continues to be the principle on which reflecting telescopes are constructed. But as this construction is attended with some difficulties in practice, Newton, and, since him, Cassegrain, Gregory, Hadley, Short, and the Herschels, have introduced some modifications in it. Newton, by meav.s of a second reflection from a plane mirror, inclined at a certain angle, threw the image of the object into such a position in the tube of the telescope, that it could be easily examined from the side of the tube, through a planoconvex eyeglass, in whose focus it was situated. In the Gregorian telescope there is a large mirror with a small hole in its centre; opposite to this is placed a second small mirror in the axis of the larger one, and at a distance from it a little more than the sum of their focal distances. By means of this construction the image formed by this double reflection is viewed through one or more eyeglasses, fixed in the direction of the opening, and, therefore, the ohserver is stationed in a line with the object; while, in the JYeivtonian telescope, he is at right angles to it. The Cassegrainian is constructed in the same way as the Gregorian, with the exception of having a small convex instead of a concave speculum. Herschel gave the mirror such a position that its focus should fall directly under the edge of the upper aperture, so that the observer, in viewing the image, should not intercept the light: this he called a frontvieio telescope. It is plain that the size of the mirror, and, consequently, its focal distance, have an effect upon the magnitude of the image; and modern astronomers have^ therefore, empkryed some instruments of this kind of great bulk. Herschel's gigantic telescope, erected at Slough, near Windsor, wras completed August 28, 1789; and on the same day the sixth satellite of Saturn was discovered. The diameter of the polished surface of the speculum was fortyeight inches, and its focal distance forty feet. It weighed 2118 pounds, and was placed in one end of an iron tube four feet ten inches in diameter. The other end was elevated towards the object, and had attached to it an eyeglass, in the focus of the speculum, as above mentioned. The observer was mounted in a gallery, mov from so large a surface was truly surprising, and enabled objects, otherwise invisible, to become extremely interesting. (A full description of this instrument, illustrated with eighteen plates, may be found in the Transactions of the Royal Society for 1795.) The frame of this instrument having become much decayed, it has been taken down, and another, of twenty feet focus and,, eighteen inches diameter, erected in its place (1822), by the distinguished J. F. W. Herschel, son of sir William. The largest frontview telescope, at present in England, is that erected at the royal observatorv at Greenwich, by Mr. Ramage, in* 1820. The diameter of the reflector is fifteen feet, and its focus is twentyfive feet. Schroterhacl an excellent telescope of this kind ,at Lilienthal, of twentyfive feet focus, by which the Milky Way was separated into innumerable small stars. Schrader, at Kiel, had a similar instrument of twentyfive feet focus, at the close of the last century. Another improvement has been recently introduced in the reflecting telescope, by making the speculum of platina, so that it will not suffer from rust. Having noticed some of the most valuable modifications of the reflecting telescopes, we now return to the refracting one. The most important improvement in this instrument consists in the formation of the objectglasses free from the errors of chromatic and spherical aberration, whence they have been denominated achromatic (a, without; %pw/x", color) telescopes, or, more properly, aplanatic (a, without; xXavog, error) telescopes. These are now made in such perfection, that they have, in some degree, superseded the reflecting telescopes; and the optical institute at Renedictbeuern (q. v.) provides obseiTatories with such excellent dioptrical instruments, that the catoptric are little used. Dollond (q. v.) first made achromatic telescopes; Ramsden (q. v.), Reichenbach (q. v.), &c, have made the best. They are formed by employing a double objectglass, composed of two lenses of different refractive powers, which will mutually correct each other, and thus give a pencil of white light entirely colorless. Triple objectglasses are also used: one .of the largest ever constructed was erected at the observatory of Dorpat, in 1824, and was made by Fraunhofer. (q. v.) The diameter of the objectglass has a clear aperture of nine and six tenths inches, and a focal distance* of fifteen feet; and a focus of eighteen feet. Mr. Tully has also made one in England, of which the aperture of the objectglass is six eighttenth inches, and the focal length twelve feet. It is now in possession of doctor Pearson. (See Astronomical TVmisactions* vol. ii.)