113 



TELESCOPE, HISTORY OF THE. 



TELLUROUS ACID. 



114 



cryolite and fluate of lime is only two-thirds' that of plate glass, holding 

 nearly the same place with respect to this glass that the latter does to 

 flint. The fluoride of strontium is also suggested as a promising 

 material. It is also stated that M. Jam in, by vising oxide of zinc 

 instead of that of lead in flint glass, has succeeded in forming a zinc 

 flint glass of exquisite limpidity, of low refractive and dispersive power, 

 and capable of being wrought into discs of any size. 



Attempts have been made by M. Chevalier to diminish the aberra- 

 tions by means of two achromatic object-glasses placed at a certain 

 distance from each other in the tube ; and by Mr. Rogers of Leith, by 

 a single convex lens of plate-glass, in combination with a double achro- 

 matic lens, the convex lens being of plate-glass, and the concave lens of 

 flint-glass. This last gentleman proposes to unite the red and violet 

 rays at the image of the object by a proper distance between the single 

 and the double lens, and to correct the spherical aberration either by 

 giving proper curvatures to the surfaces of the compound lens, or by 

 placing the two lenses at a small distance from each other in the 

 narrower part of the converging cone of rays. (' Memoirs of the Astron. 

 Soc.,' vol. iii.) 



This was reduced to practice in 1839 by M. Plossl, of Vienna, 

 under the name of the Dialytic Telescope, which is characterised by 

 Sir John Herschel as " a very artificial and beautiful invention, highly 

 deserving further trial." Somewhat resembling this contrivance is 

 Mr. Peter Barlow's plan of placing in the narrowing cone of rays from 

 a plate-glass object-lens a concave lens formed of two plate-glass cap- 

 sules of equal thickness, inclosing between them a highly dispersive 

 fluid, namely, the bisulphide of carbon, of which the refractive index 

 is 1'678, and the dispersive power O'llS, or more than double that of 

 flint-glass. This fluid lens is concavo-convex. By proper curves the 

 spherical aberration can be destroyed; and by varying the distance 

 between the two lenses, exact achromaticity can be produced. The 

 principle was tested in a telescope of 8 inches aperture and 12 feet 

 focal length, with success : as Mr. Barlow remarks, " less than an ounce 

 of sulphuret of carbon, value three shillings," was made to perform the 

 office of a very costly flint disc of 8 inches. (' Phil. Trans.,' 1828, 1829. 

 1831.) 



Sir D. Brewster has suggested (' Treatise on New Phil. Inst.,' p. 400) 

 that it may be possible to remove, or at least very much diminish, the 

 unconnected colour in the image by the use of two lenses of the same 

 kind of glass with the same or different dispersive powers. He pro- 

 poses that the exterior lens should have the meniscus form, the convex 

 side being outwards, in order, from the obliquity of the incident rays 

 to the surface, that the dispersion produced by that lens may increase 

 in a higher ratio than its refraction, so that the dispersion produced by 

 the other lens may be corrected ; while in each pencil the rays, after 

 refraction through both, may be convergent. 



It would be improper to omit here to mention that M. Amici, at 

 Modena, some years since, invented a species of achromatic telescope 

 by a combination of four prisms, all of the same kind of glass : the 

 refracting edges of one pair of the prisms were parallel to one another, 

 and those of the other pair were also parallel to one another, 

 but perpendicular to the edges of the first pair ; and each pair formed 

 an achromatic combination. By the refraction in the first pair, the 

 breadth of the object is magnified, and by that in the second pair the 

 length is magnified in the same ratio : thus the result is an image 

 undiatorted and magnified. Sir John Herschel states that, in 1826, he 

 saw in the hands of its inventor one of these telescopes, which magnified 

 about four times. 



The success which Sir W. Herschel obtained in the construction of 

 reflecting telescopes was unrivalled during many years ; but at length 

 has been surpassed by Earl Rosse, who has erected in the grounds of 

 BUT Castle, Parsonstown, Ireland, two telescopes, in the lesser of which 

 the speculum is of three feet aperture, and in the greater telescope 

 6 feet, or a reflecting surface of 28'274 square feet, being greater 

 than that of Herschel's large telescope in the ratio of 7 to 3. Its focal 

 length is 53 feet. It can be used either on the Newtonian or the 

 Herschelian principle. The great tube is of wood hooped with iron, 

 and is 7 feet in diameter and 52 feet in length. It is suspended 

 between two lofty meridional walls of solid masonry, between which 

 its upper end is allowed a considerable amount of lateral motion, so as 

 to admit of taking up the view of a celestial object some time before 

 its arrival on the meridian, and following it for some time after, with- 

 out Misplacing the lower end. This lower end is supported on a massive 

 universal joint of cast iron, resting on a stone pier buried in the 

 ground, and so counterpoised as to be easily moved in declination. The 

 preparation of the speculum, which weighs 4 tons, is described under 

 I.UM. The stairs and galleries for the observers are supported 

 by the western pier. The first gallery commands a view of objects at 

 an altitude of 42. It consists of a strong light prismatic framing 

 ali'liug between two fixed ladders ; it is counterpoised, and can be 

 raised to the required position by means of a windlass. There are 

 three other galleries at the summit of the western pier, which com- 

 mand the heavens to five degrees below the pole. Each gallery is 

 supported by beams which run between grooved wheels. For some of 

 the achievements of this splendid instrument we may refer to a 

 i by Lord Rome, ' Phil. Trans.' 1850. See also NEBUL& 



Mr. Lassells' reflector has a clear diameter of 2 feet, and 20 feet focal 

 length ; it was originally erected at his residence near Liverpool, and in 



ARTS AXD SCI. DIV. VOL. VIII. 



1852 was removed to Malta, for the advantage of a clearer atmosphere 

 and a lower latitude. By means of this instrument, Mr. Lassells 

 discovered two of the satellites of Uranus, one of Satum, and one of 

 Neptune. This instrument is erected under a revolving cupola 30 feet 

 in diameter, carrying a stage for the observer. The image may be 

 deflected towards the eye-glass, either by a small two-inch speculum, 

 or by a prism. To prevent dew forming on the latter, a small piece of 

 heated lead is placed in a case near it. Metal does not contract dew, 

 but glass is peculiarly liable to deposit it; hence achromatics are 

 generally furnished with a dew-tube, which is merely a brass cylinder 

 blackened within. 



In Mr. Nasmyth's reflector, the rays from the great speculum are 

 received on a small speculum or prism, placed in the axis of the tube, 

 between the focus and the great speculum, by which they are reflected 

 at right angles, and the image is formed in a tube inserted in one of 

 the trunnions, on which the instrument turns. The image is then 

 viewed in the usual way by an eye-piece ; the advantage is, that while 

 the great tube is moved in altitude, the side-tube is fixed, and the 

 observer can survey the whole meridian or any other vertical circle 

 without changing his position. The instrument is moved in azimuth 

 by means of a turn-table, on which is the frame of the instrument and 

 the seat for the observer, who can thus command every required 

 motion, both in altitude and azimuth. The great tube has a length 

 of 28 feet, and a diameter of 54 inches. 



Telescopes for general astronomical purposes are now almost always 

 mounted equatorially, and great improvements have been made in the 

 mounting, the method of the Astronomer Royal being particularly 

 recommended for its lightness, combined with extreme stiffness to 

 resist both flexure and twist. 



The history of the telescope may be assisted by reference to the 

 following heads : EQUATORIAL ; TRANSIT ; SPECULUM ; ACHROMATIC ; 

 LIGHT; LENS; APLANATIC LENS; ABERRATION; MICROMETER; EYE- 

 PIECE ; ASTROLABE ; QUADRANT ; THEODOLITE ; HELIOSTAT ; HELIO- 

 METER ; OPTOMETER ; REFRACTION ; DISPERSION. 



TELESCO'PIUM (the Telescope), a constellation of Lacaille, in the 

 southern hemisphere, surrounded by Ara, Pavo, Sagittarius, and 

 Ophiuchus. Its principal star is as follows : 



Character. 



No. in Catalogue 

 of Piazzi. 



60 



No. in Catalogue 

 of British 



Association. 



6240 



Magnitude. 

 4 



TELLER-BISMUTH. [TELLURIUM.] 



TELLERS OF THE EXCHEQUER (tailler) were the holders of 

 an ancient office in the Exchequer. [TALLY.] They were four in 

 number; their duties were to receive money payable into the 

 Exchequer on behalf of the king, to give the clerk of the pells (skins 

 or rolls of parchment) a bill of receipt for the money, to pay all money 

 according to the warrant of the auditor of receipts, and to make 

 weekly and yearly books of receipts and payments for the lord 

 treasurer. (4 ' Inst', 108 ; ' Com. Dig.', tit. ' Court,' D. 4, 14, 15.) The 

 office was abolished by act of parliament (445 Wm. IV., c. 15), 

 together with that of the clerk of the pells and the several offices 

 subordinate thereto. 



TELLURETTED HYDROGEN. [TELLURIUM.] 



TELLURIC ACID. [TELLURIUM.] 



TELLURIUM (Te) is a rare element, very similar to SELENIUM, 

 and closely allied to SULPHUR. It is sometimes found in the free 

 state, but usually occurs in combination with metals forming till 

 [TELLURIUM, in NAT. HIST. DIV.] From the Nagyag ore, which con- 

 tains about thirteen per cent., it is obtained by dissolving in nitric acid 

 after the sulphides of lead and antimony have been separated hy 

 digestion in hydrochloric acid. The nitric acid solution is evaporated 

 to dryness, the residue treated with hydrochloric acid and filtered, and 

 tellurium thrown down from the filtrate by the addition of sulphite of 

 soda. 



Tellurium has a silver-white lustre, and is very brilliant : it is crystal- 

 line and brittle, of a lamellar fracture, easily pulverised, and a worse 

 conductor of electricity than antimony or bismuth. Its specific 

 gravity is 6'65. It is nearly as fusible as antimony, and at a high 

 temperature it boils, and may be distilled. When strongly heated in 

 contact with air, it burns with a lively blue flame, green at the borders, 

 and forms a white vapour, which has an acid odour. 



The equivalent of tellurium is 64'5. 



Tdlu.riu.rn and oxyyen form two compounds : 



Tcllurous acid TeO, 



Telluric acid TeO 3 



Their constitution is obviously similar to that of sulphurous and 

 sulphuric aci<ln. 



TELLURIUM-AMYL. [OROANO-METAI.LIC BOBIKS.] 

 TELLURIUM-ETHYL. [OROANO-METALLIC BODIKS.] 

 TELLURIUM-METHYL. [OROANO-METALLIC BODIES.] 

 TELLUROUS ACID (TeO,,). It has been already mentioned that 

 when tellurium is heated in contact with air, it burns, and a white 

 vapour is formed : this is oxide of tellurium, or tellurous acid. It 

 may also be obtained by the action of nitric acid on the metal ; by 



I 



