Telescope

(těles-kōp.) [From Gr. tēle, far off, and sco peo, I view.) (Opt.) An instrument employed in viewing distant objects. It a ists 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, and thus rendering objects distinct and visible which would otherwise be indistinct or invisible. T. are either refracting or reflecting; the former consist of different lenses through which the objects are seen by rays refracted by them to the eye; and the latter consist of specula from which the rays are reflected and pa ed to the eye. The lens turned towards the object is called the object-gla ; that to which the eye is applied, the eye-gla ; and if the T. consists of more than two lenses, all but the object-gla are called eye-gla es. It was not till the middle of the 16th century (1549) that the application of gla es to this instrument was made. In fact, no advances were made in the construction of T. before the time of Galileo, who, while at Venice, accidentally heard that a sort of optic was made in Holland, which brought distant objects nearer; and, considering how this thing might be, he set to work and ground two pieces of gla into a form, as well as he could, and fitted them to the two ends of an organ pipe, with which he produced an effect that delighted and astonished all beholders. After exhibiting the wonders of this invention to the Venetians on the top of the tower of St. Mark, he devoted himself wholly to the improving and perfecting the T., in which he was so succe ful, that it has been usual to give him the honor of being the inventor.-Refracting Telescopes. The Galilean T. consists of a convex converging object gla , and a concave diverging eye-gla . Its magnifying power is equal to the focal length of the object gla divided by the focal length of the eye-gla ; the length of the tube is the difference between the focal lengths of the lenses. It causes an object to be seen erect, and its principle is applied in the opera-gla , for which purpose its magnifying is rarely greater than 4,: and often as low as 2.-The Astronomical T. consists of a converging object-gla and a converging eye-gla . Its magnifying power is equal to the focal distance of the object-gla , divided by the focal distance of the eye-gla ; the length of the tube is the sum of the focal distances of the lenses. It causes the objects to appear inverted.-The Terrestrial T. differs from the astronomical telescope in producing images in their right positions. This is effected by means of two condensing-gla es,P and Q(Fig. 627), placed between the object-gla , M, and the eye-piece, R. The object being supposed to be at AB, at a greater distance than can be shown in the drawing, an inverted and much smaller image is formed atba on the other side of the object-gla . But the second lens, P, is at such a distance that its M principal focus coincides with the image ab; from which it follows that the luminous rays which pa through b, for example, after traversing the B lens,P, take a direction parallel to the secondary axis, 60. Similarly the rays pa ing by a take a direction parallel to the axis, aO. After cro ing on H, these various rays traverse a third lens, Q, whose principal focus coincides with the point H. The pencil, BbH, converges towards b ', on a secondary axis, O'b, parallel to its direction; the pencil AaH, converging in the same manner at a', an erect image of the object, AB, is produced at a'b'. This image is viewed, as in the astronomical T., through a condensing eye-piece,R, so placed that it acts as a magnifying gla , that is, its distance from the image, a'b', is le than the principal focal distance; hence, there is formed, at a"b", a virtual image ofa'b', erect, and much magnified. The lenses P and Q, which only serve to rectify the position of the image, are fixed in a bra tube, ata constant distance, which is equal to the sum of their principal focal distances. The object-gla , M, moves in a tube, and can be moved to or from the lens P, so that the image, ab, is always formed in the focus of the lens, whatever be the distance of the object. The distance of the lens R may also be varied so that the image a"b" may be formed at the distance of distinct vision. This instrument may also be used as an astronomical T. by using a different eye-piece; this must have a much greater magnifying power than in the former cases. In the terrestrial T. the magnifying power is the same as in the astronomical T., provided always that the correcting-gla es, P and Q, have the same convexity.-REFLECTING TELESCOPES. In these, the speculum or mirror performs the same office as the object-gla in refracting T., and is called the object mirror. They were invented by Gregory, and described by him in 1663, but were first used by Newton. The Newton i an T. consists of a tube, at the end of the interior of which is a speculum, which exactly fits it; the rays from the object, entering the open end of the tube, are reflected back in a convergent state to a small diagonal speculum which throws them out through an aperture at the side of the tube not far from its open end. The small diagonal speculum is intended to prevent the nece ity of the observer looking down into the tube, and thus intercepting a large quantity of light from the object. The magnifying power is equal to the focal length of the object-mirror divided by that of the eye-gla which is used. There are telescopes of very great power at Cambridge in Ma achusetts, and at Cambridge in England. Telestereoscope, (tel-e-ste're-o-skop.) [FromGr. tēle, afar off, stereos, complete, and skopco, I view.] (Opt.) A stereoscope po e ing telescopic al properties. Telfair, (těl'für,) in Georgia, a S. co.; area, 600 sq. m.; C. Jacksonville. Pop. 3,245. Tell, WILHELM, (tel,) a Swi patriot, B. in the canton Uri towards the close of the 13th cent., and said to have been drowned in 1350, in attempting to save a friend during a great flood of the river Schächen. His story, too well known to need to be here recorded, has been much discu ed by modern historians, and is now very generally considered as apocryphal; be this as it may, it has been the means of creating for the world Schiller's noble drama, and Ro ini's operatic masterpiece. [s. 882]