Telerpeton (telerpeton)

[NL., lenses of different kinds of gla , objectives could be made nearly free from chromatic aberration, and all the refracting telescopes now constructed have achromatic object gla es of some form. The usual construction is a double convex lens of crown-gla combined with a (nearly) planoconcave lens of flint-gla , 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. But other forms are po ible and even preferable. Fig. 2 shows some of those most used. For IMG:content-0045.png:[blocks in formation] Fig. 3.- The Lick Telescope, Lick Observatory, California. the rays are reflected out at the side of the tube. Finally, in the front-view or Herschelian form the small mirror is dispensed with, the speculum being slightly tilted so as to throw the image to one side of the mouth of the tube. This saves the lo of light due to the second reflection, but involves some injury to the definition. Although the reflecting telescope is free from chromatic aberration, it seldom gives as perfect definition as an achromatic instrument, and is much more subject to atmospheric disturbance; the image also is le brilliant than that given by a refractor of the same aperture; but the speculum is much easier and le costly to construct than an achromatic object-gla of the same size, so that the largest telescopes ever made have been reflectors. At the head of the list stands the six-foot "leviathan" of Lord Ro e, erected in 1845, and still in use: it is of the Newton i an form. The five-foot silver-on-gla Cas seg rain i an reflector of Mr. Common, erected in 1889, stands next, and there are in existence a number of instruments with apertures of 3 and 4 feet. Herschel's great telescope, erected in 1789, but long since dismantled, was 48 inches in diameter and 40 feet long. The magnifying power of a telescope depends upon the ratio between the focal length of the object-gla and that of the eyepiece. (See eyepiece.) It can therefore be altered at pleasure by merely exchanging one eyepiece for another. As a rule, the highest power practically available, with the best object-gla es and under the best circumstances, is from 75 to 100 to every inch of aperture. The illuminating power is proportional, other things equal, to the area of the object-gla or the speculum; so that a telescope of 12 inches aperture ought to give four times as much light as one with a 6-inch lens. Practically, however, the larger lenses, on account of the increase in the thickne of the gla , do not reach their theoretical performance. Reflecting telescopes vary greatly in their light gathering power. A Newton i an reflector with a silver-on gla speculum freshly polished is not very greatly inferior in light to an achromatic of the same aperture; but as a rule a reflector in its ordinary working condition has only about half the light of the corresponding refractor. Small telescopes for terrestrial purposes are usually unmounted, but the tube is ordinarily made in several sections which slide into one another, reducing the length of the instrument, and making it more portable, as in the common spygla . Larger telescopes are mounted upon stands of some kind, and the practical efficiency of the instrument depends greatly on the firmne and convenient arrangement of the stand. At present telescopes for astronomical use are almost always mounted equatorially - that is, the telescope-tube is attached to an axis, which itself is carried by another axis with its bearings so arranged that it points toward the pole. This principal axis is called the polar axis, and a clockwork is usually arranged to make it turn at the rate of one revolution in a sidereal day. When the telescope is once pointed at a celestial object, the clockwork will keep it apparently stationary in the field of view for any length of time. By the help also of graduated circles attached to the two axes it is easy to "set" the telescope so as to find any object whose right ascension and declination are known. Fig. 5 represents diagrammatically the equatorial of the usual German form. IMG:content-0046.png:[graphic] It is quite certain that previous to 1600 the telescope was unknown, except pos sibly dis who failed to see its practical importance, and who confined its use to "curious practices" or to demonstrations of "natural magic." Encyc. Brit., [XXIII. 135. T. Yerkes to the University of Chicago. The Lick telescope (fig. 3) is of 36 inches diameter and 57 feet in length, the object-gla by Clark of Cambridge, Ma achusetts. That of Pulkowais30inches. The achromatic objective construct- ed of flint-and crown-gla is, however, by no means perfect, and cannot be made so while these kinds of gla are used. When the correction for the rays of mean wave-length in the spectrum is the best po ible, the extreme rays-the red and violet-refuse to coincide with the others, so that the image of a bright object is surrounded by a purple halo, which renders it somewhat indistinct. This "secondary spectrum," as it is called, is not very obtrusive in small 2. instruments, but is a serious defect in large ones, and un- fits the ordinary achromatic refractor for photography. For this purpose it is nece ary to use an object-gla spe- cially corrected for the violet rays, and therefore practi- telescope (tel'e-skōp), n . [F. télescope = Sp. Pg. It. telescopio = D. teleskoop = G. Sw. Dan. teleskop, etc., telescopium (NGr. τηλεσκό πων), σκοπεῖν , view.] 1. An optical instrument by means of which distant objects are made to appear nearer and larger, It originated in the first decade of the seventeenth cen- tury, 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. The tele- scope consists e entially of two members: one, the ob- jective , a large converging lens, or a concave mirror (tech-cally worthle for visual observations. But while it is nically speculum ), which forms an optical image of the object; the other, the eyepiece, a small lens or combination of lenses, which magnifies this image. 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. Telescopes are cla ed as refracting or reflecting, according as the objective is a lens or a speculum. The simple refracting telescope has for an objective a large convex lens, A (fig. 1), of long Fig. 1.- The Simple Refracting Telescope. focus, while the eyepiece, B, is also a convex lens, but of short focus, the two being placed at a distance slightly le than the sum of their focal lengths. The "real" in verted image of the object formed at m by the object-gla 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. With this form of instrument the object is seen inverted. 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 ob impo ible to secure a perfect color-correction with any lens composed of ordinary crown and flint-gla , there is no reason why kinds of gla may not be invented which will render it po ible; and since 1880 experiments, under the auspices of the German government, by Profe or Abbé at Jena, appear to have resulted in at least partial succe . Lenses as large as 12 inches in diameter have been made of the new gla . If large disks of this gla can be obtained sufficiently homogeneous, and not corrosible under exposure to the air, the art of telescope-making will immediately make enormous progre . The reflecting telescope was invented between 1660 and 1670, independently by Gregory and Newton, by the latter as the result of his discovery of the decomposition of light by refraction, which led him to conclude (erroneously) that the faults of the refracting telescope were nece arily incurable. There are four different forms of the instrument, differing only in the method by which the rays reflected by the concave speculum which forms the objective are brought to the Fig. 4.- The Greg or i an Reflecting Telescope. [ cap .] Same as Telescopium . - Axis of a telescope. See axis1. Binocular telescope, an instrument composed of two similar small telescopes fastened together side by side and parallel, so that both eyes can be used 며 Fig. 5.-The Equatorial. at once in looking through it. The opera-gla is its most common form. Brachy-telescope, or brachyte, a form of silver-on-gla reflector in which the small mirror, convex in form, is placed out of the axis of the large speculum, which is slightly inclined, the distortion thus produced in the image being partly compensated by the corresponding inclination of the small mirror. This construction avoids the perforation of the speculum, and leaves its whole area unobstructed; it also considerably diminishes the length of the instrument. - Broken telescope, a telescope which has a reflecting prism or mirror inserted about half-way between the object-gla and its focus, the tube being thus bent at right angles: much used in transit-instruments and theodolites. - Cane telescope, a telescope or spygla fitted in a walking-stick. - Cas seg rain i an telescope, a form of reflector in which the small mirror is convex. See def. 1.- Catadioptric, catoptric telescope, a reflecting telescope. - Dialytic telescope. See dialytic. - Equatorial telescope. See equatorial, n., and def. 1. Galilean telescope, the form of refracting telescope invented by Galileo, and still used as the opera-gla : it is telescope characterized by having a concave lens as the eye-gla , and shows objects erect.- Greg or i an telescope. See Greg or i an and def. 1. - Herschelian telescope, a form of reflecting telescope in which no small mirror is used, but the large speculum is slightly inclined, so as to make the image acce ible at the side of the mouth of the telescope tube.-- Keplerian telescope, a form of refracting telescope which is characterized by the use of a convex lens of em short focus for the eyepiece: sometimes referred to simply as the astronomical telescope, because, exhibiting objects inverted, it cannot be advantageously used for any but astronomical observations. - Magnifying power of a telescope. See magnify. - Newton i an telescope, the usual form of reflecting telescope, which ploys a small plane mirror set at an angle of 45°, throwing the image through the side of the tube.Night telescope, a spy-gla of wide Newton i an Telescope. aperture and low power, useful in twilight or moonlight. -Photographic telescope, a telescope fitted for photography. It may be a refractor with an object-gla spefocus, or a reflector, which requires only mechanical adaptations.-Prism-telescope. See teinoscope. Sciatheric a which has a concave speculum gla telescope. See sciatheric. - Silver-on-gla telescope, on the front surface. Most of the reflectors now made are of this kind.-Terrestrial telescope, a telescope having two additional lenses in the eyepiece, by means of which the inverted image is brought to an erect position, in contradistinction to an astronomical refracting telescope. cially constructed to bring the actinic rays an accurate View-telescope, the small telescope which usually forms part of a spectroscope. -Watch-telescope, a small telescope attached to a theodolite or other geodetic instrument, and intended to enable the observer to a ure himself of the stability of the parts of the instrument which ought to remain immovable while the observations are being made.-Water-telescope. ( a ) A simple tube, five or six inches in diameter, with a plane gla inserted watertight at the end. It is used by Norwegian fishermen and others to enable them to see objects under water. (b) A telescope with its tube completely filled with water. Such an instrument was used by Airy at Green wich, about 1870, as part of a zenith-sector, in order to settle by observation certain questions relating to the aberration of light.Zenith-telescope, an instrument designed for the purpose of determining the latitude of a place by measuring the difference between the zenith-distances of two stars culminating north and south of the zenith at nearly equal altitudes: introduced by Capt. Talcott of the United States Engineers about 1840. The principle involved had been discovered as early as 1740 by Horrebow, but the method was much for want of suitable star-catalogues, telescope (tel'e-skop), v.; pret. and pp. tele scoped, ppr. telescoping. [ and had been quite lost sight of. trans. To drive into one another like the movable joints or slides of a spy-gla : as, in the collision the forward cars were telescoped; to shut up or protrude like a jointed telescope. II. in trans. To move in the same manner as the slides of a pocket-telescope; especially, to run or be driven together so that the one partially enters the other: as, two of the carriages telescoped.