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the others, found a focus of its own. The red portion in the case of very large discs. The ray, being most refracted, reached its focus reflecting surface is ground and polished, with first; next came the yellow ray, then the green great precision, to a parabolic form of the focus and last and nearest of all to the eye-piece, the required, and then, by a chemical process, coated blue ray made its image. When a star is with silver, which may be easily renewed when viewed with such a telescope the image seen tarnished or otherwise injured. consists of a yellowish point at the centre, sur- The making of the optical parts of telerounded by a mixture of green and blue light, scopes is a rare art, which, however, has been with red outside. This difficulty in refracting cultivated with peculiar success in America. telescopes, called chromatic aberration, checked Alvan Clark and Sons of Cambridge, Mass., the progress of astronomy for 150 years, until, attained world-wide fame in this connection in 1750, the English optician, Dolland, invented during the lifetime of the gifted men composing the achromatic objective as shown in Fig. 1. the firm. At the present time the largest repuIn this diagram E is a double-convex lens of tation as a maker of large telescopic objectives crown glass and D a Aint-glass lens of nearly belongs to John A. Brashear of Pittsburgh, Pa. plano-concave form. The difference between The telescope tube is usually carried by an the crown and fint material in light refraction equatorial mounting. This form of instrument and dispersion, together with the compensating has its principal or polar axis set parallel to form of the two lenses, results in clear and the axis of the earth, its inclination, therefore, distinct definition, the image of the star being corresponding to the latitude of the observasharply, outlined and colorless. Optical glass tory. At right angles to the polar axis is the for such lenses is of special manufacture. The declination axis, which, in turn, carries the world's supply comes from three makers, one telescope tube at right angles to itself. Each each in England, France and Germany. The axis is supplied with a graduated circle, indinewer objectives are made of several thin len- cating, respectively, the position of the star in ses cemented together, usually four in number hours, minutes and seconds of right ascension, and alternately crown and Aint glass. Some and in degrees, minutes and seconds of declinamakers are using five lenses successfully, tion. It will be evident that when the tube is though which much difficulty. The mirror or pointed to a star in any part of the visible heav
ens, 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 El
so placed that the light it gathers from the star
is reflected by the mirrors through the tube to LE
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 teleFIG. 6.—Universal Prism Pield Glass (sectional view). scope with its various modifications as above de
scribed, while giving, facilities for examining speculum of the reflecting telescope is made and photographing the heavenly bodies, does from a casting of ordinary glass of sufficient not enable the astronomer to determine with rethickness to be handled without flexure, pref- quired accuracy the positions of the stars and erably one-sixth of the diameter of the disc, planets. These fine measurements are secured although one-eighth is a commonly accepted pro- only by special forms of telescopes. The Me
ridian Circle is one of the most approved in- Tesults of his observations were compared with struments for this purpose. From the middle the Airy observations of a half century before, of the tube trunnions extend on either side, car- and the supposed errors of the “Reflex Zenith rying 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
විට 1 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.,
Fig. 7.—Reflex Zenith Telescope. but the illustrations given will suffice.
Even with all the caution used in the con- Tube) were at once traced to the variation in struction of these delicate instruments, errors latitude. The old instrument which had been are sure to develop, due to refraction, flexure condemned is thus proven to be correct both in of the tube, variation resulting from changes in theory and practice. It, therefore, represents temperature and other contributing causes, for the latest development in_astronomical teleall of which allowance must be made in the scopes, and a large Reflex Zenith Tube is now final reduction of the observations. About the in the service of the University of Pennsylmiddle of the last century Professor Airy, then vania. Astronomer Royal at Greenwich, designed and In recent years the mounting of great equaconstructed a vertical telescope, which he be- torials has passed from the domain of the inlieved would eliminate the errors so manifest in strument-maker to that of the engineer, who his other instruments. He named it the "Reflex finds abundant scope for ingenuity and technical Zenith Tube. The principle is shown in Fig. expertness in combining very massive and so, 7. Every part of the instrument is stationary rigid, construction with very delicate mechanism. and no part need be touched when in use by the At the present time the largest telescopes in the astronomer. The light from the star as it world are owned and made in America. passes the zenith is concentrated by the objec- The table on following page gives a list of tive upon a surface of mercury in the base of the larger refracting telescopes in the equipment the column, by which it is reflected back through of the more important American observatories. a hole in the objective; the cone of rays then While the refracting telescope still holds its meets a diagonal prism, is reflected at right advantages for photographic astronomical work angles and enters the eye-piece to be observed the tendency of late years has reverted to the as in other instruments. Contrary to the ex- reflecting telescope for visual work. Astronopectations of the Astronomer Royal, errors in mers prefer it because of the much clearer the observed position of the stars were still images obtained, due in large part to the entire manifest and the most careful investigations absence of the secondary spectrum. The failed to trace them to their source. The in- images formed by the best refractors have a strument was, therefore, discarded. Fifty years turbid quality as compared with the clean later, Professor Chandler, of Cambridge, Mass., crispness of the reflector. As in the case of discovered that the pole of the earth “wobbles.” the refracting instruments, America boasts the slightly, causing a variation in latitude. The largest reflectors. The largest of all is the
Hooker reflecting telescope of the Mount Wil- elevator into the pier, where all apparatus and son Observatory, Pasadena, Cal., with a clear material is at hand and where the temperature aperture of 10038 inches (2.549 meters) and a may be controlled. The telescope has motorprimary focal length 507.5 inches (12.89 driven fast and slow motions, whil the diu al meters); it can be used directly on the axis motion is supplied by the typical form of driveither (a) as a focal plane instrument, or (b). ing clock having a conical pendulum isochroin the Newtonian form; at its secondary foci nously governing a falling weight. This operates it may be used (c) as a Cassegrain instrument through a worm and a worm wheel (17 feet in with a focal length of 1,606 inches (40,792 diameter) cut and ground with high precision. meters), or (d) as a Coudé with focal length of The instrument is controlled, settings are made 3,011 inches (76.480 meters). Celestial objects and the dome is turned from a station on the from 65° north to 53° south declination can be pier at which the readings of the circles are observed at the principal focus. The telescope made; the "remote system of electrical conis mounted after the English fashion, the trol is used throughout, and most of the operaskeleton tube containing the mirror at its tions are duplicated by auxiliary controls at the lower end swinging between the sides of the three observing stations or foci. To reach the open polar axis yoke which has a bearing at principal focus an observing platform is proeither end resting on cast-iron pedestals built vided which travels along the shutter opening. up from the main concrete pier below. The The dome is of structural steel throughout, 100 mounting was designed by Mr. Pease and other feet in diameter, 100 feet high and weighs members of the observatory staff, assisted by 730 tons. The rotating portion weighing 500 Prof. Peter Schwamb of the Massachusetts tons is carried on 28 trucks and is traction Institute of Technology. The pier is 33 feet driven by two motors at opposite sides. The high while the intersection of the axes is 50 trucks and rails are carefully machined to conifeet above ground. Both the declination and cal surfaces to eliminate vibration at the teleright ascension bearings are composite; in part, scope when the dome is moved. The shutter of spherical type, serving to define the axes, is in halves which open sideways and permit while the remainder carries the load. The free working of the telescope from the zenith declination load is carried by means of counter- to the horizon. The walls and dome are of weight systems while the polar axis is sup- double construction and are ventilated at the ported by means of steel drums built integral top to prevent excess heating during the sumwith the axis and floating in cast-iron tanks mer. A 10-ton crane is provided to assist in filled with mercury. The tube complete weighs the erection and in the transfer of the various 35 tons and the total moving parts weigh 100 auxiliary sections of the tube. tons. They are constructed of cast and struc- The second largest reflector in the world is tural steel throughout and in as large units as the 72-inch instrument set up in 1918 on Little could be machined as a whole. The larger por- Saanich Mountain Vancouver Island, tions of the mounting, some weighing 10 tons, British Columbia, in the Dominion Observatory were made in Quincy, Mass., and their trans- there. The mirror of this instrument was portation up Mount Wilson to an elevation of ground by the John A. Brashear Company of 5,700 feet was a difficult feature in the erection Pittsburgh, from a disc of glass cast in Belof this instrument. The mirror is 101.2 inches gium and shipped out of that country two days diameter, 1234 inches thick and weighs about before the war broke out. The mirror is par9,000 pounds. It was cast in Saint Gobain, abolic, 13 inches thick at the rim with a hole France, and figured by G. W. Ritchey in the 10% inches in diameter through its centre. It observatory optical shops. It is mounted on a weighs 4,340 pounds and has a focal length of counterweight support system in the rear and 30 feet. Observations may be made from the on four edge arcs at the sides. To control its side of the upper part of the tube, from the temperature it is surrounded with a lining of side at the lower end or through the opening cork board built integral with its cell, and a at the centre of the mirror. The telescope with system of piping is installed within this through all its fittings weighs 55 tons and is moved by which liquid at any desired temperature may an amount of electric current barely sufficient be circulated from tanks in the pier below. to light a 16-candle power electric lamp. The Fans serve to equalize the temperature around mounting was built by Warner and Swazey. mirror and coils. When it becomes necessary The only other telescope of this size in existence to resilver the mirror it is removed from the is that of Lord Rosse set up upon his estate in tube and lowered in its cell by an electric Ireland in 1842. Its first mirror was of specu
lum metal and was replaced by a 72-inch Under other commissioners he built over 30 silvered glass mirror in after years.
harbors, some of which, as at Aberdeen and long period it was the largest telescope in the Dundee, are upon an extensive scale. He was world; but was abandoned for optical reasons. also employed in England, superintending the
There are three 60-inch reflectors in the construction of five large bridges over the Western Hemisphere, the newest one erected at Severn, of eight canals, and the execution of the National Observatory of the Argentine Re- numerous important works for the metropolis. public near Cordoba. The mirror was made by In 1808 he was employed by the Swedish govthe John A. Brashear Company in 1916. In ernment to lay out a system of inland naviga1904 Harvard University secured the 60-inch tion through the central parts of Sweden and reflector made by Dr. A. A. Common of Ealing, to form a direct communication by water beEngland. This instrument is of the Casse- tween the North Sea and the Baltic. He also grainian type and has a rectangular tube. The built the road between Warsaw and Brestmounting is peculiar, the telescope being sup- Litovski in Poland. The greatest monument of ported in position upon the end of a hollow his engineering skill is the Menai Suspension cylinder which floats in a well or deep basin of bridge, connecting Caernarvonshire with the water. The cylindrical float is 18 feet long island of Anglesea, which was opened on 30 and seven and two-thirds feet in diameter and Jan. 1826. In 1828–30 he superintended the arranged to float constantly at an angle with drainage of nearly 50,000 acres of the Fen the horizon equal to the elevation of the celes- country. He invented the Telford pavement. tial pole at Cambridge—about 45 degrees. The See Roads, IMPROVEMENT OF. instrument weighs over 20 tons, but is so TELHARMONIUM, an electrical instrudelicately balanced that it may be moved in any ment devised to produce music at any distance direction with the greatest ease by its elec- from a central station. The device was intrical controls. The eye-piece of the instrument
vented by Dr. Thaddeus Cahill (q.v.) and is detached from the telescope and housed in
operates on the principle of the telephone, but the second story of an adjacent building, the the receiving instrument is not held to the ear. light from the principal mirror being directed to The music is produced through the medium of it by accessory mirrors.
dynamos transmitting vibrations to any number The 60-inch reflector of the Mount Wilson
of receiving stations. The operator plays on a Observatory, constructed by Dr. Ritchey in keyboard similar to that of an organ, the keys 1908, was placed in commission in December of
controlling electric currents at varying speeds that year. The instrument complete weighs of alternation and producing the notes of 21% tons, nearly all of which is ingeniously practically any instrument with great purity supported upon a float in a basin of mercury, and sweetness. of which it displaces the equivalent of 50 cubic feet, although the entire amount of mercury in
TELL, těl, William, Swiss peasant of the basin is but 635 pounds. The controls are
Bürglen, near Altorf, celebrated in legend for electric and may be manipulated from several
his resistance to the tyranny of the Austrian stations about the instrument.
governor, Gessler. The stories connected with Another fine telescope which should be men
him, with those relating to the origin of the tioned in this connection is the 48-inch re
Swiss Confederation, first appear in the 15th flector built by Sir Howard Grubb and located
century. According to them, Gessler, the tyat Melbourne, Australia.
rannical Austrian bailiff of Uri, one of the For more detailed descriptions of these
forest cantons, pushed his insolence so far as to wonderful instruments and their achievements
1 equire the Swiss to uncover their heads before the files of the astronomical journals are rec
his hat (as an emblem of the Austrian soverommended.
eignty), and condemned Tell, who refused to WORCESTER REED WARNER, F.R.A.S.,
comply with this mandate, to shoot an apple
from the head of his own son. Tell was sucOf the Warner and Swasey Company, Cleveland, Ohio.
cessful in his attempt, but confessed that a sec
ond arrow, which he bore about his person, TELFORD, těl' förd, Thomas, Scottish en- was intended, in case he had failed, for the gineer: b. Eskdale, Dumfriesshire, 9 Aug. 1757; punishment of the tyrant, and was, therefore, d. Westminster, 2 Sept. 1834. At 14 he was ap- retained prisoner. While he was crossing the prenticed to a mason and on the expiration of Lake of the Four Cantons, or Lake of Lucerne, his time worked as a journeyman at that trade, in the same boat with Gessler, a violent storm but subsequently removed to Edinburgh and threatened the destruction of the skiff. Tell, there applied himself to the study of architec- as the most vigorous and skilful helmsman, was ture. In 1782 he went to London, where he set free, and he conducted the boat successfully was befriended by Sir William Pulteney, near the shore, but seized the opportunity to through whom he was appointed surveyor of spring upon a rock, pushing off the bark. He the public works in Shropshire. He now be- had fortunately taken his bow with him, and came a civil engineer and in 1793 was entrusted when the governor finally escaped the storm, with the construction of the Ellesmere Canal, and reached a rocky defile on the road to to connect the Mersey, Dee and Severn. In Küssnacht, Tell shot him dead. The death of 1803 and 1804 the Parliamentary commissioners Gessler was the signal for a most obstinate war for making roads and building bridges in the between the Swiss and Austrians, which was Highlands of Scotland, and also for making not brought to a close until 1499. Tell was the Caledonian Canal, appointed him their en- present at the battle of Morgarten, and is supgineer. Under the former board 1,200 bridges, posed to have lost his life in an inundation in two of 150 feet span, were built and 1,000 miles 1350 while attempting to save a friend. Such of new road were made; and under the latter is the legendary story of William Tell. Inves board the Caledonian Canal was constructed. tigation has broken down the proofs of his ex