TELESCOPE. 109 TELFORD. the direct rays from the heavens fall. The side- rostat, which was invented by Foueault, is a movable mirror with a plane surface mounted on such a mechanism that the light falling on the mirror from a given portion of the heavens will always be rellected to the object glass of the telescope. In other words, instead of the telescope being moved about its polar axis, as would happen with an equatorial, the mirror is given sueli a motion that it neutralizes the move- ment of the earth. This idea is said to have been suggested by Robert Hooke at a time when the long-focus lenses of Huygens were being used and the difficulties attending their mounting were under consideration. On the occasion of the transit of Venus in 1882 Lord Lindsay em- ployed a heliostat in connection with his 40-foot telescope, while for taking photographs the Amer- ican observers used the same method most suc- cessfully. When it w-as proposed to erect at the Paris Exposition a telescope larger than any previously attempted, it was realized that an equatorial of greater size than those of the great observatories in the United States was prac- tically impossible, and accordingly the fixed form was adopted. Both visual and photographic ob- jectives are provided, and with the latter the largest photographs of the moon yet produced have been taken. Each lens weighs half a ton and they were constructed by JI. Gautier, who also made those for the University of Chicago telescope. Although in this form of telescope there is no elaborate mounting with mechanical devices, yet as much care must be expended on the side- rostat as on the object glasses. The mirror form- ing the essential part of this instrument is 78% inches in diameter, is 11 inches in thickness, and weighs 3600 kilograms (nearly 8000 pounds). It is believed by astronomers that for direct visual observations the refractor is the better instrument, while for the study of stellar, plane- tary, and nebular spectra and the photography of these bodies the reflector is the more useful. Also the latter instrument can be used to advan- tage in photometric and radiometric investiga- tions of the moon and the planets. The reflector is free from chromatic aberration, and there is not the absorption of light which occurs when lenses of large apertui-ej; are employed. Larger angular and linear apertures can be used and the mount- ing is far more simple than for a refractor of corresponding size. The development of the fixed refracting telescope is at present being awaited by astronomers, as it will obviate many difficul- ties of construction, mounting, and expense. List op Large Telescopes in Existence in 1900 Refractors 15 inches and upward Inches Paris fExhIbition) 50 Terkes 40 Lick 36 Pulkova (Rnssla) 30 ,'ice 29.9 Paris 28.9 Greenwich 28 McCorralck Observatory. Virginia 26 Washington, United States 26 Newall's, Camhridpe. England '. 25 Cape of Oood Hope 24 Harvard 24 Princeton, N. J 23 Mount Etna 21.8 Strassburg 19-1 Milan 19.1 Dearborn. Chicago 18.5 Warner Observatory, Rochester, United States 16 Washburn Observatory, Madison, Wis 15.5 Edinburgh ^"-^ Brussels }^'^ Madrid 1? Rio de Janeiro }2 Paris 15 Sir William Huggins ^^ reflectors 2 FEET 6 INCHES AND UPWARD ft. ins. Lord Rosee 6 Dr. Common 5 Melbourne * Paris 4 Meudon 3 3 south Kensington 3 Crossley (Lick Observatory) 3 Greenwich 2 6 South Kensington 2 6 Bibliography. Drude, Theory of Optics (London, 1902) ; Chauvenet, Spherical and Prac- tical Astronomy, vol. ii. (Philadelphia, 1863) ; for history and theory, consult Lockyer, Stargaz- ing, Past and Present (London, 1878). For more recent developments consult various arti- cles in the Astrophysical Journal, especially by the following: Wadsworth, 1897 and 1902; Reese, 1902; Ritchey, 1901. TEL'ESIL'LA OF AR'GOS (Lat., from Gk. TeWo-iXXa) ( C..510 B.C. ) . A Greek poet. She was born at Argos and is said to have served in the Argive army against Sparta. Even if these deeds are mythical, her name became a symbol for pa- triotic enthusiasm and her statue was placed in the Temple of Aphrodite. Her lyrics are lost ex- cept for two lines found in Bergk's PoetcE Lyrici Orceci (vol. iii., 4th edition). TELETS LAKE. A lake of Siberia. See Altin. TEL'FORD, Thomas (1757-1834). An emi- nent Scottish engineer, born in Eskdale, Dumfries- shire. At the age of fourteen he adopted the trade of a stone-mason. In 1780 he removed to Edin- burgh, and in 1783 he repaired to London, where he was appointed in 1784 to superintend the erec- tion of the resident commissioner's house at Portsmouth dockyard. In 1787 he was appointed surveyor of public works for Shropshire; and his two bridges over the Severn at Montford and Buildwas and other works gained for hirn the planning and superintendence of the projected EUesmere Canal, to connect the navigation of the Severn, Dee, and Mersey — a work which occupied tenyears (1795-180.5). "in 1801 he received a com- mission from the Government to report on the state of Scotland, and on the desirable public works for that country. As a consequence, the plan of a canal from Inverness to Fort William was revived, and its planning and construction intrusted to Telford. (See Caledonian Canal.) In the same capacity he constructed more than 1000 miles of road in the Highlands, Lanark- shire, and Dumfries.shire and about 1200 bridges, besides churches, manses, harbors, etc. In 1808, and again in 1813, he was invited to Sweden, to report on the projected scheme for connecting Lake Vener with the Baltic, and superintended the construction of the Gfita Canal, by which this was eflFected. His next great work was the construction of the road from London to Holy- head, including the erection of numerous bridges —among others, of the Menai suspension bridge. For the last few years of his life he re- tired from the active duties of his profession, emploved himself in collecting and arranging materials for a complete history of his various works, and had the greater portion of the MSS.
