the others, found a focus of its own. The red ray, being most refracted, reached its focus first; next came the yellow ray, then the green and last and nearest of all to the eye-piece, the blue ray made its image. When a star is viewed with such a telescope the image seen consists of a yellowish point at the centre, surrounded by a mixture of green and blue light, with red outside. This difficulty in refracting telescopes, called chromatic aberration, checked the progress of astronomy for 150 years, until, in 1750, the English optician, Dolland, invented the achromatic objective as shown in Fig. 1. In this diagram E is a double-convex lens of crown glass and D a flint-glass lens of nearly plano-concave form. The difference between the crown and flint material in light refraction and dispersion, together with the compensating form of the two lenses, results in clear and distinct "definition," the image of the star being sharply outlined and colorless. Optical glass for such lenses is of special manufacture. The world's supply comes from three makers, one each in England, France and Germany. The newer objectives are made of several thin lenses cemented together, usually four in number and alternately crown and flint glass. Some makers are using five lenses successfully, though which much difficulty. The mirror or

W&S

FIG. 6.-Universal Prism Field Glass (sectional view). speculum of the reflecting telescope is made from a casting of ordinary glass of sufficient thickness to be handled without flexure, preferably one-sixth of the diameter of the disc, although one-eighth is a commonly accepted pro

portion in the case of very large discs. The reflecting surface is ground and polished, with great precision, to a parabolic form of the focus required, and then, by a chemical process, coated with silver, which may be easily renewed when tarnished or otherwise injured.

The making of the optical parts of telescopes is a rare art, which, however, has been cultivated with peculiar success in America. Alvan Clark and Sons of Cambridge, Mass., attained world-wide fame in this connection during the lifetime of the gifted men composing the firm. At the present time the largest reputation as a maker of large telescopic objectives belongs to John A. Brashear of Pittsburgh, Pa.

The telescope tube is usually carried by an equatorial mounting. This form of instrument has its principal or polar axis set parallel to the axis of the earth, its inclination, therefore, corresponding to the latitude of the observatory. At right angles to the polar axis is the declination axis, which, in turn, carries the telescope tube at right angles to itself. Each axis is supplied with a graduated circle, indicating, respectively, the position of the star in hours, minutes and seconds of right ascension, and in degrees, minutes and seconds of declination. It will be evident that when the tube is pointed to a star in any part of the visible heavens, a revolution of the polar axis from east to west, in sidereal time, will make the telescope follow the apparent motion of the star. A driving clock, which is usually located inside the column of the instrument, controls the revolutions of the polar axis so that the star observed remains steadily in the field of vision. The equatorial principle has been applied to photographic telescopes in such manner as to allow the continuous exposure of the photographic plate during the entire night, if desired. One of the most ingenious forms of mounting is the Equatorial Coudé. In this instrument the polar axis is enlarged so as to serve as the main tube of the telescope, the eye-piece being at the upper end where the observer can sit comfortably in his warm room and observe any star in the visible heavens as easily as he uses his microscope.

An elbow is rigidly attached to the lower end of the tube. At the intersection is an accurately polished mirror set at an angle of 45 degrees. At the outer end of the elbow is another mirror, similarly set. The objective is so placed that the light it gathers from the star is reflected by the mirrors through the tube to the eye-piece. The combined movements of the polar axis (the telescope tube) and the objective and mirror carried on the elbow enable the observer to bring into view any star in the visible heavens. The polar axis, with its elbow carrying the objective and revolving in sidereal time by means of a driving clock, follows the apparent motion of the star in the usual way. Two of these instruments are in successful use in the Paris Observatory.

It will be evident that the equatorial telescope with its various modifications as above described, while giving facilities for examining and photographing the heavenly bodies, does not enable the astronomer to determine with required accuracy the positions of the stars and planets. These fine measurements are secured only by special forms of telescopes. The Me

ridian Circle is one of the most approved instruments for this purpose. From the middle of the tube trunnions extend on either side, carrying finely graduated circles and terminating in accurately ground pivots which are exactly at right angles to the optical axis of the tube. Two piers are so set as to form a rigid and accurate support for these pivots, east and west, carrying the tube so that the movement of the telescope is in the true meridian only. In the exact focus of the telescope a fixed system of cross-hairs or wires is placed. The best materials for this purpose are taken from the cocoon of the field spider, the web being only one five-thousandths of an inch in diameter. Finely drawn platinum wires are also used. These vertical spider webs are equally spaced and so adjusted that the central wire is exactly in the optical axis of the telescope as measured east and west. A horizontal wire is adjusted exactly in the optical axis as measured north and south. Parallel to these central wires there are two movable wires, one horizontal and one vertical, each governed by a micrometer screw.

In measuring transits of stars for determining right ascension, or for time, the telescope, by means of the graduated circles, is set to the declination of the star required, and when the star appears, its transit across each of the wires is recorded on a chronograph, by the observer tapping an electric key. In determining declinations, the telescope, by means of the graduated circles, is set to the approximate declination of the star to be observed, and when the star appears at the edge of the field, the observer carefully adjusts the telescope until the star seems to be exactly bisected by the horizontal wire as it threads its way across the field. By reading the fine circle the declination of the star is obtained. Other types of telescopes for similar observations are known as transits, zenith telescopes, mural circles, etc., but the illustrations given will suffice.

Even with all the caution used in the construction of these delicate instruments, errors are sure to develop, due to refraction, flexure of the tube, variation resulting from changes in temperature and other contributing causes, for all of which allowance must be made in the final reduction of the observations. About the middle of the last century Professor Airy, then Astronomer Royal at Greenwich, designed and constructed a vertical telescope, which he believed would eliminate the errors so manifest in his other instruments. He named it the "Reflex Zenith Tube." The principle is shown in Fig. 7. Every part of the instrument is stationary and no part need be touched when in use by the astronomer. The light from the star as it passes the zenith is concentrated by the objective upon a surface of mercury in the base of the column, by which it is reflected back through a hole in the objective; the cone of rays then meets a diagonal prism, is reflected at right angles and enters the eye-piece to be observed as in other instruments. Contrary to the expectations of the Astronomer Royal, errors in the observed position of the stars were still manifest and the most careful investigations failed to trace them to their source. The instrument was, therefore, discarded. Fifty years later, Professor Chandler, of Cambridge, Mass., discovered that the pole of the earth "wobbles" slightly, causing a variation in latitude. The

Tube" were at once traced to the variation in latitude. The old instrument which had been condemned is thus proven to be correct both in theory and practice. It, therefore, represents the latest development in astronomical telescopes, and a large Reflex Zenith Tube is now in the service of the University of Pennsylvania.

In recent years the mounting of great equatorials has passed from the domain of the instrument-maker to that of the engineer, who finds abundant scope for ingenuity and technical expertness in combining very massive and so, rigid, construction with very delicate mechanism. At the present time the largest telescopes in the world are owned and made in America.

The table on following page gives a list of the larger refracting telescopes in the equipment of the more important American observatories.

While the refracting telescope still holds its advantages for photographic astronomical work the tendency of late years has reverted to the reflecting telescope for visual work. Astronomers prefer it because of the much clearer images obtained, due in large part to the entire absence of the secondary spectrum. The images formed by the best refractors have a turbid quality as compared with the clean crispness of the reflector. As in the case of the refracting instruments, America boasts the largest reflectors. The largest of all is the

Hooker reflecting telescope of the Mount Wilson Observatory, Pasadena, Cal., with a clear aperture of 1003% inches (2.549 meters) and a primary focal length of 507.5 inches (12.891 meters); it can be used directly on the axis either (a) as a focal plane instrument, or (b) in the Newtonian form; at its secondary foci it may be used (c) as a Cassegrain instrument with a focal length of 1,606 inches (40,792 meters), or (d) as a Coudé with focal length of 3,011 inches (76.480 meters). Celestial objects from 65° north to 53° south declination can be observed at the principal focus. The telescope is mounted after the English fashion, the skeleton tube containing the mirror at its lower end swinging between the sides of the open polar axis yoke which has a bearing at either end resting on cast-iron pedestals built up from the main concrete pier below. The mounting was designed by Mr. Pease and other members of the observatory staff, assisted by Prof. Peter Schwamb of the Massachusetts Institute of Technology. The pier is 33 feet high while the intersection of the axes is 50 feet above ground. Both the declination and right ascension bearings are composite; in part, of spherical type, serving to define the axes, while the remainder carries the load. The declination load is carried by means of counterweight systems while the polar axis is supported by means of steel drums built integral with the axis and floating in cast-iron tanks filled with mercury. The tube complete weighs 35 tons and the total moving parts weigh 100 tons. They are constructed of cast and structural steel throughout and in as large units as could be machined as a whole. The larger portions of the mounting, some weighing 10 tons, were made in Quincy, Mass., and their transportation up Mount Wilson to an elevation of 5,700 feet was a difficult feature in the erection of this instrument. The mirror is 101.2 inches diameter, 124 inches thick and weighs about 9,000 pounds. It was cast in Saint Gobain, France, and figured by G. W. Ritchey in the observatory optical shops. It is mounted on a counterweight support system in the rear and on four edge arcs at the sides. To control its temperature it is surrounded with a lining of cork board built integral with its cell, and a system of piping is installed within this through which liquid at any desired temperature may be circulated from tanks in the pier below. Fans serve to equalize the temperature around mirror and coils. When it becomes necessary to resilver the mirror it is removed from the tube and lowered in its cell by an electric

elevator into the pier, where all apparatus and material is at hand and where the temperature may be controlled. The telescope has motordriven fast and slow motions, while the diurnal motion is supplied by the typical form of driving clock having a conical pendulum isochronously governing a falling weight. This operates through a worm and a worm wheel (17 feet in diameter) cut and ground with high precision. The instrument is controlled, settings are made and the dome is turned from a station on the pier at which the readings of the circles are made; the "remote" system of electrical control is used throughout, and most of the operations are duplicated by auxiliary controls at the three observing stations or foci. To reach the principal focus an observing platform is provided which travels along the shutter opening. The dome is of structural steel throughout, 100 feet in diameter, 100 feet high and weighs 730 tons. The rotating portion weighing 500 tons is carried on 28 trucks and is traction driven by two motors at opposite sides. The trucks and rails are carefully machined to conical surfaces to eliminate vibration at the telescope when the dome is moved. The shutter is in halves which open sideways and permit free working of the telescope from the zenith to the horizon. The walls and dome are of double construction and are ventilated at the top to prevent excess heating during the summer. A 10-ton crane is provided to assist in the erection and in the transfer of the various auxiliary sections of the tube.

The second largest reflector in the world is the 72-inch instrument set up in 1918 on Little Saanich Mountain on Vancouver Island, British Columbia, in the Dominion Observatory there. The mirror of this instrument was ground by the John A. Brashear Company of Pittsburgh, from a disc of glass cast in Belgium and shipped out of that country two days before the war broke out. The mirror is parabolic, 13 inches thick at the rim with a hole 10% inches in diameter through its centre. It weighs 4,340 pounds and has a focal length of 30 feet. Observations may be made from the side of the upper part of the tube, from the side at the lower end or through the opening at the centre of the mirror. The telescope with all its fittings weighs 55 tons and is moved by an amount of electric current barely sufficient to light a 16-candle power electric lamp. The mounting was built by Warner and Swazey. The only other telescope of this size in existence is that of Lord Rosse set up upon his estate in Ireland in 1842. Its first mirror was of specu

lum metal and was replaced by a 72-inch silvered glass mirror in after years. For a long period it was the largest telescope in the world; but was abandoned for optical reasons.

There are three 60-inch reflectors in the Western Hemisphere, the newest one erected at the National Observatory of the Argentine Republic near Cordoba. The mirror was made by the John A. Brashear Company in 1916. In 1904 Harvard University secured the 60-inch reflector made by Dr. A. A. Common of Ealing, England. This instrument is of the Cassegrainian type and has a rectangular tube. The mounting is peculiar, the telescope being supported in position upon the end of a hollow cylinder which floats in a well or deep basin of water. The cylindrical float is 18 feet long and seven and two-thirds feet in diameter and arranged to float constantly at an angle with the horizon equal to the elevation of the celestial pole at Cambridge-about 45 degrees. The instrument weighs over 20 tons, but is so delicately balanced that it may be moved in any direction with the greatest ease by its electrical controls. The eye-piece of the instrument is detached from the telescope and housed in the second story of an adjacent building, the light from the principal mirror being directed to it by accessory mirrors.

The 60-inch reflector of the Mount Wilson Observatory, constructed by Dr. Ritchey in 1908, was placed in commission in December of that year. The instrument complete weighs 211⁄2 tons, nearly all of which is ingeniously supported upon a float in a basin of mercury, of which it displaces the equivalent of 50 cubic feet, although the entire amount of mercury in the basin is but 635 pounds. The controls are electric and may be manipulated from several stations about the instrument.

Another fine telescope which should be mentioned in this connection is the 48-inch reflector built by Sir Howard Grubb and located at Melbourne, Australia.

For more detailed descriptions of these wonderful instruments and their achievements the files of the astronomical journals are recommended.

WORCESTER REED WARNER, F.R.A.S., Of the Warner and Swasey Company, Cleveland, Ohio.

TELFORD, těl fōrd, Thomas, Scottish_engineer: b. Eskdale, Dumfriesshire, 9 Aug. 1757; d. Westminster, 2 Sept. 1834. At 14 he was apprenticed to a mason and on the expiration of his time worked as a journeyman at that trade, but subsequently removed to Edinburgh and there applied himself to the study of architecture. In 1782 he went to London, where he was befriended by Sir William Pulteney, through whom he was appointed surveyor of the public works in Shropshire. He now became a civil engineer and in 1793 was entrusted with the construction of the Ellesmere Canal, to connect the Mersey, Dee and Severn. 1803 and 1804 the Parliamentary commissioners for making roads and building bridges in the Highlands of Scotland, and also for making the Caledonian Canal, appointed him their engineer. Under the former board 1,200 bridges, two of 150 feet span, were built and 1,000 miles of new road were made; and under the latter board the Caledonian Canal was constructed.

In

Under other commissioners he built over 30 harbors, some of which, as at Aberdeen and Dundee, are upon an extensive scale. He was also employed in England, superintending the construction of five large bridges over the Severn, of eight canals, and the execution of numerous important works for the metropolis. In 1808 he was employed by the Swedish government to lay out a system of inland navigation through the central parts of Sweden and to form a direct communication by water between the North Sea and the Baltic. He also built the road between Warsaw and BrestLitovski in Poland. The greatest monument of his engineering skill is the Menai Suspension bridge, connecting Caernarvonshire with the island of Anglesea, which was opened on 30 Jan. 1826. In 1828-30 he superintended the drainage of nearly 50,000 acres of the Fen country. He invented the Telford pavement. See ROADS, IMPROVEMENT of.

TELHARMONIUM, an electrical instrument devised to produce music at any distance from a central station. The device was invented by Dr. Thaddeus Cahill (q.v.) and operates on the principle of the telephone, but the receiving instrument is not held to the ear. The music is produced through the medium of dynamos transmitting vibrations to any number of receiving stations. The operator plays on a keyboard similar to that of an organ, the keys controlling electric currents at varying speeds of alternation and producing the notes of practically any instrument with great purity and sweetness.

TELL, těl, William, Swiss peasant of Bürglen, near Altorf, celebrated in legend for his resistance to the tyranny of the Austrian governor, Gessler. The stories connected with him, with those relating to the origin of the Swiss Confederation, first appear in the 15th century. According to them, Gessler, the tyrannical Austrian bailiff of Uri, one of the forest cantons, pushed his insolence so far as to require the Swiss to uncover their heads before his hat (as an emblem of the Austrian sovereignty), and condemned Tell, who refused to comply with this mandate, to shoot an apple from the head of his own son. Tell was successful in his attempt, but confessed that a second arrow, which he bore about his person, was intended, in case he had failed, for the punishment of the tyrant, and was, therefore, retained prisoner. While he was crossing the Lake of the Four Cantons, or Lake of Lucerne, in the same boat with Gessler, a violent storm threatened the destruction of the skiff. Tell, as the most vigorous and skilful helmsman, was set free, and he conducted the boat successfully near the shore, but seized the opportunity to spring upon a rock, pushing off the bark. He had fortunately taken his bow with him, and when the governor finally escaped the storm, and reached a rocky defile on the road to Küssnacht, Tell shot him dead. The death of Gessler was the signal for a most obstinate war between the Swiss and Austrians, which was not brought to a close until 1499. Tell was present at the battle of Morgarten, and is supposed to have lost his life in an inundation in 1350 while attempting to save a friend. Such is the legendary story of William Tell. Inves tigation has broken down the proofs of his ex

istence. There is no mention of him by any contemporaneous historian; his name is first met with in the chronicles of the second half of the 15th century, and none of the Tell ballads are of an earlier date. Similar stories in regard to the shooting of the apple occur in Saxo Grammaticus, the Danish historian, and in Icelandic literature, not to mention the old English ballad of Adam Bel, Clym of the Cloughe and Wyllyam of Cloudeslè. Besides, the many contradictions between the various personages, dates and places, and the widely differing representations of the event, show the gradual development of the legend. The untiring industry of historical scholars has not been rewarded by the finding of the name of Tell in the archives and church registers of Uri, and although an uninterrupted series of charters exists relative to the bailiffs or governors of Küssnacht in the 14th century, there is no Gessler among them. The Tell chapels were erected or called by his name generations after his death; the document which speaks of the assemblage in 1388 of 114 persons who knew him personally, and of the erection at that time of a Tell chapel on the shore of the Lake of Lucerne, was not known until 1759. Consult Hisely's 'Recherches Critiques' (1843); Rochholz's 'Tell und Gessler in Sage und Geschichte (1877); Gisler's 'Die Tellfrage: Versuch ihrer Geschichte und Lösung' (1895). See SWITZERLAND, History.

TELL CITY, Ind., city in Perry County, on the Ohio River, 125 miles southwest of Indianapolis, on the Southern Railroad. There are deposits of coal and clay in the vicinity, and manufactures include furniture, iron products, woolens, flour and tobacco. The city was founded by the Swiss Colonization Society in 1857. Pop. (1920) 4,086.

TELL EL AMARNA, Egypt, a district near the east bank of the Nile, 190 miles above Cairo, comprising the site and environs of the ancient city of Akhenaton, known also as Ekhaton and Akhet-Aton, built by Amenophis IV, who later was known as Akhenaton or Ikhnaton. The city was built by Amenophis about 1360 B.C. as a new capital of the empire in place of Thebes after he had abandoned the religion of Ammon for that of Aton. It grew rapidly but was abandoned upon the death of Amenophis, the court returning to Thebes as its capital and to the worship of Ammon. The most important ruins are those of the palace and the House of Rolls, and there are remains of the temple. About 300 clay tablets containing valuable records of the time of Amenophis and his father were discovered in the House of Rolls in 1887. To the northeast and to the south of the ruined city are tombs hewn in the sides of the hills which abound in sculptured scenes picturing mainly the worship of the sungod. The tomb of Meri-Ra, high priest of the sun, has two spacious chambers and a façade nearly 100 feet long. The tomb supposed to be that of Amenophis is in a ravine about midway between the north and the south tombs. Consult Petrie, F., 'Tell el-Amarna' (1894); Davies, N. de G., 'Rock Tombs of Tell el Amarna) (1903-08); Petrie, W. M. F., (Syria and Egypt from the Tell el Amarna Letters' (1898).

VOL. 26-26

TELL EL KEBIR, Egypt, village in the northeastern portion of the country, on the Sweetwater Canal, 18 miles southeast of Zagazig. It was the scene 13 Sept. 1882 of a battle between the English forces commanded by Sir Garnet (later Lord) Wolseley and the Egyptians under Arabi Pasha, which resulted in the utter defeat of the Egyptians.

TELLER, těl'ér, Henry Moore, American senator: b. Granger, Allegany County, N. Y., 23 May 1830; d. Denver, Colo., 23 Feb. 1914. He was educated at Alfred University, New York, taught school, and after admission to the bar practised law in Illinois and Colorado. He was United States senator from Colorado 1876-82, Secretary of the Interior 1882–85, and was a member of the national Senate from 1885, except for a brief interval 1896-97, up to 1909. He was especially prominent as a silver advocate, and had the unusual experience of serving his constituents as a nominee of the Republican and later of the Democratic party.

TÉLLEZ, těl'yǎth, Gabriel, Spanish dramatic author, better known by his pseudonym, EL MAESTRO TIRSO DE MOLINA: b. Madrid, between 1570 and 1572; d. Soria, 12 March 1648. He studied at Alcala and remained for some time at Toledo, whence some of his works are dated, also in Galicia and in Seville. The date of his profession as a Brother of Charity is unknown, but we know that he had become superior by 1619. In 1634 he was named Definidor general of Castille. His first poetical work, Los Cigarrales de Toledo) (1624) is a collection of tales in which there is a semblance of the influence of Boccaccio. This influence is clearer in 'Los tres maridos burlados,' which is an admirable adaptation of the 'Decameron.' Instead of a second part of the 'Cigarrales' promised by the author there appeared in 1635 a new collection (Deleitar aprovechando') of religious stories mixed with 'Autos, of which 'El Colmenero divino' is one of the best efforts in religious drama. For a long time Téllez devoted himself to this species of composition. In 1620 he dedicated to his friend Lope de Vega La Villana de Vallecas and four years later he stated that he had written well nigh 300 comedies. He excelled in the religio-theological dramas of his period and also in historic dramas, farces and comédies d'intrigue. He had a penchant for epigram but was capable of reaching the highest conceptions and frequently sounded tragic depths. Some of his works are equal to Calderon's or Lope's best and in recent years critics have begun to do him full justice. An eloquent proof of his merit is that some of his works have been attributed for centuries to Lope or to Calderon. Such was the case of 'El Burlador de Sevilla y Convidado de Piedra, an admirable scenic portrayal of the legend of Don Juan, which, although universal as proved by Farinelli, has taken on the character of a purely Spanish legend through this work of Téllez, which has been widely imitated in other literatures. It is his best work and in order of importance may be mentioned La prudencia en la mujer); Marta la piadosa'; 'El vergonzoso en Palacio'; 'Don Gil de las calzas verdes'; 'El amor y la amistad,' and 'La villana de Vallecas. It has been said that his feminine characters are