Page:The American Cyclopædia (1879) Volume XV.djvu/661

 TELESCOPE eter " is one of ,the most elaborate and beau- tiful monographs of astronomy, was able with this instrument to grapple successfully with that even now most difficult practical problem, the measurement of the parallax of a fixed star. . Several attempts have also been made, as by Eochon, Maskelyne, and Boscovich, to pro- duce the double images by refraction through prisms or pairs of prisms, either beyond the object glass or sliding within the tube, as well as by dividing the small mirror of reflecting telescopes, as Ramsden suggested for the Cas- segrainian form, and Brewster for the New- tonian. Divided-eye-lens micrometers have also been made, the best form of which is that given by Airy, who found the four-glass eye piece best adapted for this purpose, and divided the second lens, counting from the object glass. But in all the arrangements of divided lenses an essential imperfection arises from the exhibition of color and of some dif- fraction in the direction at right angles to that of the line of separation, and this practical inconvenience may be seriously felt in some classes of observations. On this account, it will probably yet be found most advantageous to make use of the double-refracting property of certain crystals for the separation of im- ages. This account would be imperfect with- out a sketch of the particular form of tele- scope employed by the American observing parties in photographing the recent transit of Venus. What was required was a large im- of the sun at the focus of the object glass, or the principal focus as it is called. The size of this image is directly proportional to the focal length, and a focal length of about 40 ft. was required to give the image the desired dimensions. It would clearly have been im- possible to provide telescopes of this length for distant stations, even if at "Washington, Greenwich, or Paris instruments of such di- mensions could be so driven by clockwork that the tube should remain constantly direct- ed toward the sun. It became necessary, therefore, to make use of a heliostat, or plain mirror, so worked by a driving apparatus as to deflect the sun's rays constantly in the same horizontal direction. The construction of a plane mirror sufficiently true for this purpose was a task which fully taxed the skill even of Alvan Clark and his sons. " The slightes^de- viation from exactness," as Newcomb points out, " would be fatal ; for instance, if a straight- edge laid upon the glass should touch at the edges, but be the 100,000th of an inch above it at the centre, the reflector would be use- less." The mirrors were tested by observing objects through a telescope, first directly and then by reflection from the mirror. If they were seen with equally good definition in he two cases, it would show that there were no irregularities in the surface of the mirror; while if it were concave or convex, the focus of the telescope would seem shortened or lengthened. The first test was sustained per- TELL 631 fectly, while the circles of convexity or con- cavity indicated by the changes of focus of the photographic telescope were many miles in diameter. During the total eclipse of April, 1875, the heliostat again came into play for photographic purposes, but unsuccessfully because of unwise arrangements. TELFAIR, a S. county of Georgia, bound- ed S. and W. by the Ocmulgee and N. E. by the Little Ocmulgee river ; area, 925 sq. m. ; pop. in 1870, 3,245, of whom 1,145 were col- ored. The surface is level and the soil sandy, with extensive pine forests. The Macon and Brunswick railroad intersects the N. part. The chief productions in 1870 were 62,429 bushels of Indian corn, 15,823 of oats, 20,569 of sweet potatoes, 19,829 Ibs. of wool, and 704 bales of cotton. There were 497 horses, 2,563 milch cows, 6,946 other cattle, 8,475 sheep, and 7,477 swine. Capital, Jacksonville. TELFORD, Thomas, a British engineer, born at Westerkirk, Eskdale, Dumfriesshire, Aug. 9, 1757, died in Westminster, Sept. 2, 1834. While working in Edinburgh as a stone mason he studied architecture and drawing. In 1783 he removed to London, was employed on the quadrangle of Somerset house, afterward for three years as architect in the Portsmouth dockyard, then upon the alterations of Shrews- bury castle, and in the construction of numer- ous bridges, one of which over the Severn had a flat arch of 130 ft. span. He superintended the construction of the Ellesmere canal, 103 m. in length, and requiring extensive aque- ducts, which he built of iron ; the Caledonian ship canal, whose locks surpassed any previ- ously built in size; and six other canals in England and Scotland, the Gotha canal in Swe- den, an immense tunnel at Harecastle on the Grand Trunk canal, besides 1,000 miles of new road and 1,200 bridges. The St. Katharine docks of London, the improvement of the Aberdeen and Dundee harbors, the construction of iron bridges with flat arches of 170 ft. span, and above all the Menai suspension bridge, were his work. See " Life of Thomas Telf ord, Civil Engineer, written by Himself" (4to, with a folio volume of plates, 1838). TELIOSTS, a name given by J. Mtiller to the common bony fishes from their well ossified skeleton ; the term has been taken by Hux- ley for one of his orders. They comprise the ctenoids and cycloids of Agassiz. TELL, William, a legendary hero of Switzer- land. According to tradition, he was a hunter, living at Burgelen in the canton of Uri. His wife was a daughter of Walter Fiirst, who with Stauffacher of Schwytz and Melchthal of tTnterwalden organized the conspiracy of the Grtitli in 1307, and founded Swiss indepen- dence. Tell's part in the revolt against Austria is related as follows : Gessler, Austrian bailiff in Kussnacht, placed his cap upon a pole in the market place of Altorf, and gave orders that passers by should do it reverence. Tell neg- lected or refused to do this, and was arrested