of the object MN. This image m'n' is seen by the eye at E through the lens GH, as the rays diverging from m' and n' in the focus of GH enter the eye in parallel pencils. When the first three lenses are equal, the magnifying power is the same as that of the astronomical telescope, whose object and eye glasses are the same as A B and CD.

The performance of refracting telescopes depends most essentially on the perfection of the object-glass; for if the first image is bright

object. Now, if a person attempted to view the image in its place at a by placing himself directly before the mirror, he would necessarily intercept the rays of light from the object

Fig. 5.

E

D

passing down the tube to the mirror, and consequently there would be no image to view. Sir Isaac Newton overcame this difficulty by introducing a small diagonal plane speculum d between AB and a, which, itself intercepting but a small portion of the light, reflects towards the side of the tube the rays converging from A B, and causes the image which would have been formed at a to be formed at b, where it can be conveniently viewed by the eye-piece E attached to the side of the tube. The small mirror is of an oval form, and is fixed on a slender arm c connected with a slide, by means of which it may be made to approach or recede from the large speculum A B, as the image approaches to or recedes from it. In this telescope the magnifying power is equal to the focal length of the object-mirror A B divided by that of the eye-glass.

Gregorian Telescope.-In this construction the object-mirror A B (fig. 6) is perforated in the middle, and the rays of light from a dis

A

A

Fig. 6.

tant object being reflected from the surface of A B cross each other in the focus, where they form an inverted image a, and are then intercepted by a small concave mirror d, which causes them again to converge to a focus at b, near the perforation of the object-mirror, where they form a reinverted or direct image, which is viewed by an eye-pi ce E screwed into the tube behind A B. The curvature of the small speculum should be elliptical, having the foci at a and b; but it is generally made spherical. In this case the great speculum should be slightly hyperbolic, to counteract the aberration of the small mirror.

Cassegrainian Telescope.-The great speculum of this instrument is perforated like the Gregorian; but the rays converging from the surface of the mirror A B (fig. 7) towards the focus a are intercepted before they reach that point by a small convex mirror d, not sufficiently convex to make the rays divergent, but of such a curvature as to prevent them from coming to a focus till they are thrown back to b, near the aperture in A B, where they form an inverted image which is viewed by the eye

The

The reflecting telescope was invented by James Gregory, and is described by him in his Optica Promota (1663); but the first telescope of the kind was executed by Newton. celebrated instrument of Sir William Herschel, erected at Slough in 1789, was 40 feet in length. Its great speculum had a diameter of 49 inches; its thickness was about 3 inches, and its weight when cast was 2,118 ĺbs. Its focal length was 40 feet, and it admitted of a power of 6,450 being applied to it. The essential advantage of large telescopes of this kind consists in the immense quantity of light which they collect, thus enabling the observer to perceive faint nebula and stars which are altogether invisible in ordinary instruments. For the more accurate kinds of work they are inferior to refractors, as it is almost impossible to obtain a perfect figure.

Reflecting telescopes are used only for observing phenomena. In order to derive full benefit from them, they must be used in the open air, and must either be mounted equatorially [EQUATORIAL] or else in such a manner

as to be capable of a smooth motion both in a vertical and horizontal direction.

Lord Rosse's 50-ft. Reflecting Telescope.This enormous telescope, which has two mirrors of 6 ft. diameter, and is 53 feet in focal length, was begun in 1842. The metal used for the specula is an alloy of tin and copper in atomic proportions, the weight of the copper being something more than double that of the tin. The weight of metal in one of the specula is 33 tons, and that of the other 4 tons. For fusing these immense masses of metal it was placed in three iron crucibles cast expressly for the purpose, each crucible weighing of itself about 1 ton. The crucibles were placed in as many furnaces, whose mouths were level with the ground, and with flues opening into one common stack or chimney. The metal for its complete fusion required to be kept in the furnace about twelve hours, and when it was in the state proper for casting, the crucibles were withdrawn from the furnaces by means of a powerful crane to the iron cradles or pouring frames arranged round the mould at intervals of ninety degrees.

The mould had for its base a framework of hoop iron six inches in depth, placed edgewise, packed in a strong frame, and supported by strong transverse bars beneath. The upper edges of these were ground into a slightly convex surface of about 108 feet radius, and thus formed the base of the mould; allowing the air to escape through the interstices, though, from its viscosity, no particle of the alloy could escape through them. The metal also by being poured upon the iron became chilled immediately into a dense sheet of about half an inch thick, and thus secured one of the conditions which have been found to be most important, if not indispensable, in the casting of speculum metal. After becoming tolerably solid, the speculum was withdrawn to the annealing furnaces, where it remained for about sixteen weeks, when it was considered to be ready for the process of grinding and polishing.

A full account, given by Lord Rosse, of the polishing machinery will be found in the Phil. Trans. for 1840 and for 1861. It is sufficient to state that the beam carrying the polisher is moved by a small steam engine, by which a rotatory motion round a vertical spindle is given to a crank, which by a connecting rod acts upon a sliding rod which moves the grinder or polisher backwards and forwards. By such means the polisher makes strokes backwards and forwards very nearly in the manner in which the hand would make them, if the polishing were performed by hand, and at the same time another part of the machinery causes the frame carrying the speculum, and therefore the speculum itself, to revolve with a slow motion. We must remark further, in connection with this part of the subject, that it was necessary to cast and polish the speculum upon the same fixed frame (distinct from the rotatory frame before mentioned) that would support it when in the telescope. For this purpose the fixed

frames are provided with small wheels, by which, after being conveyed in a carriage to a railroad running into the lower part of the tube of the telescope, the speculum is deposited in its proper position for use.

For the support of the mirror in such a way as to avoid strain and flexure, a very ingenious system of levers is employed. This consists of a combination of three similar systems, resting on three points under the centres of gravity of the three equal sectors into which the speculum may be supposed to be divided. Each system consists of one triangle with its point of support directly under its centre of gravity, upon which it freely oscillates. This triangle carries at its angles three similar points of support for three other triangles, under their centres of gravity, and they again at their angles carry in a similar way cast-iron platforms formed of three ribs, so as to make a kind of irregular open-work grating, supported under their centres of gravity. There are thus twenty-seven platforms, which are coated with greased cloth, and upon these the speculum is supported, each bearing of the weight.

The above is the construction first adopted by Lord Rosse. But, as the surface of the speculum by the pressure on its edge at dif ferent elevations was found to be distorted, instead of the platforms in contact with the back before mentioned there were adopted eighty-one brass balls capable of revolving freely, i.e. three at the corners of each triangle, occupying the place of the platform in the former construction.

We now come to the mounting of this immense speculum. The tube of the telescope is made of wood, and is at the middle about 7 ft. i diameter; the interior exceeding 6 ft. in every part. This is fixed to a cube of 10 feet, which has folding doors in one of its sides for admis sion of the speculum, and which carries the fixed frame supporting the mirror on the side opposite to the mouth of the telescope. To this side of the cube is attached a universal joint by which the lower end of the telescope connected with a fixed support, the joint being a few feet below the general surface of the ground.

On the east and west sides of the telescope are immense piers about 70 feet long and nearly 50 feet high, of which the eastern carriesa large iron arc of a circle, in which moves a slider attached to a racked bar working by means of a pinion carried by the telescope tube, while the other carries the stairs and galleries necessary for the observers. Near the tops of the piers in the east and west plane passing through the universal joint are two cranes with pulleys, over which pass chains attached to the telescope. To the ends of these chains, after they have passed through fixed pulleys in the walls, are attached counterpoises, weighing about four tons each. There is also a contrivance connected with the chains for equalising the action of the counterpoise-weights in different positions of the telescope.

TELESCOPE

TELL-TALE

At the south end of the piers there are strong ladders, and upon these (assisted by counterpoises) there slides a stage upon which a small observing gallery travels backwards and for wards. For great elevations, curved galleries are mounted upon the curved slope of the upper part of the western pier, carried by beams running above and below pulleys fixed to the top of the pier; they are run out by rack-and-pinion work to approach the side of the telescope.

A quick motion in declination is given by the windlass below, and a slow motion is given by hand above for measurements: the quick motion in right ascension is given also below by a wheel turned by a workman, and the slow motion by hand above.

The tube is slung by chains, and is perfectly steady in a gale of wind. It carries at the upper extremity the apparatus for the Newtonian small mirror, which is itself of considerable size and weight, the minor axis of its ellipse being about 6 inches; but Lord Rosse has made provision in the construction of his observing galleries for using it as a Herschelian telescope, if it is found necessary or desirable to obtain an increase of light.

In the use of this wonderful instrument, Lord Rosse has hitherto confined himself chiefly to the observations of nebulæ.

1

12,000,000

Reflecting Telescopes with silvered Glass Specula. Recently, Steinheil of Munich and Foucault of Paris have successfully constructed specula of glass which, after polishing and figuring, receive a film of pure silver, deposited by chemical means upon their figured surfaces. This film does not exceed of an inch in thickness, but according to Steinheil it reflects, when polished with rouge, 90 per cent. of the light falling upon it, whilst speculum metal reflects only about 63 per cent. Another advantage connected with glass specula is that a good figure when once produced is permanent, since it is not altered either by the deposition or subsequent polishing of the silver film, whereas the figure of a metallic speculum is liable to serious alteration at every polishing. Thus there is every encouragement to bestow great labour upon the figuring of a glass speculum. Several small telescopes upon this principle have been constructed in this country. One of them, of 7 inches aperture, and with a power of 270, divides with ease and certainty the very close double star n Coronæ.

Eye-pieces of Telescopes.-When the image formed by the object-glass or mirror is viewed with a single lens or eye-glass, whether concave or convex, it is only in the centre of the field that distinct vision is obtained, all towards the margin being hazy and distorted. To remedy this defect, Boscovich and Huygens independently proposed the construction of an eye-piece formed of two lenses, placed at a distance from VOL. III.

737

Boscovich recommended two similar lenses: Huygens, that the focal length of the one should be twice that of the other: and as this construction is found to answer best in practice, it is that which is most commonly used.

The two lenses are usually plano-convex, with the convex faces towards the object-glass; the larger lens, called the field-glass, is innermost, or nearest the object-glass and a diaphragm cutting off the marginal rays is usually placed between them near the focus of the eye-lens, where the image is formed. This eye-piece is usually called the negative eye-piece, from its having the image seen by the eye behind the field-glass; and is that which is commonly supplied with telescopes intended only for the purpose of seeing objects, without reference to measurement.

Another modification of the two-lens eyepiece was proposed by Ramsden, and is called the positive eye-piece, because the image observed is before both lenses. The lenses are plano-convex, and nearly of the same focal length; but their distance from each other is less than the focal distance of the lens nearest the eye, two lenses thus placed acting as a compound simple lens. This eye-piece is the most convenient when micrometer wires are placed in the focus, because it can be taken out without injuring the wires; and it has also this advantage, that the measure of an object" given by one eye-piece is not altered when it is changed for another of a different magnifying power.

In both the eye-pieces now described, the image is seen inverted; and though this is of no importance in astronomical observations, it is inconvenient when the telescope is used for looking at terrestrial objects. By placing an additional pair of lenses in the tube of the eyepiece, the image is repeated and reinverted, and consequently seen erect. By this means, as explained above, the terrestrial telescope is obtained.

The name of diagonal eye-piece has been given to eye-pieces furnished with a diagonal reflecting mirror, the object of which is to give a more convenient direction to the rays emerging from the eye-piece when the telescope is pointed high. [EYE-PIECE.]

Telescopes are generally supplied with eyepieces of different powers, which are all fitted to enter the same tube: and the focal adjustment is commonly effected by a rack-andpinion motion acting on the tube which carries the eye-piece.

For full information respecting the construction of telescopes, and the best modes of mounting them, to secure steadiness and allow of the requisite motion, the reader is referred to Pearson's Practical Astronomy, vol. ii. [ALTAZIMUTH; EYE-PIECE; OBJECT-GLASS; TRANSIT INSTRUMENT.]

Tell-tale. On Shipboard, the dial plate at the wheel, showing the position of the tiller.

3 B

TELLER OF THE EXCHEQUER

Teller of the Exchequer. A very lucrative sinecure office, abolished by stat. 4 & 5 Wm. IV. c. 15. In 1780, the emoluments of each teller for the exchequer, exceeded 7,000l. a year, and the offices became still more valuable in subsequent years.

Tellina (Gr. TEXλív, a species of muscle). A genus of cockles (Cardiaceous Bivalves in the Cuvierian system), characterised by the hinge of the shell having one tooth on the left and two teeth on the right valve, often bifid; in the right valve there is a plate which does not enter a cavity in the opposite valve. There is a slight fold near the posterior extremity of both valves which renders them unequal at that part, where they gape a little. The soft parts, or animal of the Tellina, called peronea by Poli, has two long tubes for respiration and excretion, which can be withdrawn into the shell, and are concealed in a fold of the mantle. Cuvier separated from the Linnæan Telline the genus Loripes, distinguished by the feebly developed cardinal teeth, and by a long cylindrical foot. Other genera have since been detached from the Cuvierian Telline, which now form a family, Tellinidæ, in the system of Lamarck.

Tellurbismuth. A native telluride of lead and bismuth, found only at Field's mine in Georgia, and at the Tellurium mine, Virginia, U.S.

Tellurism (Lat. tellus, the earth). A name given to the system of magnetism put forth by Kieser, who substituted the idea of a telluric spirit in place of the universal fluid of Mesmer and the nervous atmosphere of Kluge. This tellurism was not confined to the substance of the earth, but extended to other bodies also. Thus, the moon was said to magnetise the inhabitants of the earth by night, while the sun demagnetised them in the morning. [MAGNETISM, ANIMAL; SOMNAMBULISM.]

Tellurite or Telluric Ochre. Native oxide of tellurium. It occurs in small spherical masses with a fibrous radiated structure, at Facebay and Zalathna in Transylvania.

TEMPERANCE SOCIETIES

name to a fountain near Haliartos. But in the so-called Homeric hymn to Apollo, she appears rather as the fountain itself than as the nymph of the fountain. In that hymn Apollo is represented as exciting her anger by marking out a place for a temple to be built to himself, and laying its foundations: and Telphusa, to avert the danger threatening her waters, advises him to go on to Crisa, where he slew the dragon Python who nursed Typhaon, the child of HERA.

But Apollo, finding that he had been lured into a desolate land, returned full of wrath to Telphusa, and, hurling down great stones, choked the waters of the fountain for ever.

Tempera Painting. [DISTEMPER.] Temperament (Lat. temperamentum). In Music, the adjustment of the notes in musical instruments whose sounds are fixed, such as organs, pianofortes, &c. The defect to be remedied arises from the single short keys between the two larger ones serving for flats as well as sharps. It is necessary to observe, that in the theory of harmonies the interval of a tone is not always the same; for instance, that lying between the fourth and fifth of the scale contains nine small parts, called commas, whereas that between the fifth and sixth of the major scale contains only eight commas. Again, the diatonic semitone contains five commas, and the chromatic semitone three or four, according to the magnitude of the tone. Hence it is that the different situation of these elements with regard to each other causes intervals of the same names to consist of different degrees or elements. To improve them, therefore, musicians temper them so as to reduce the whole more to mean distances from each other, necessarily producing a new division of the octave. Pianofortes are generally tuned on what is called equal temperament, i.e. there being eleven notes in the octave, an equal value is given to each interval between them representing a semitone. The conse quence of this is that although all the intervals, the thirds, fifths, &c., are slightly imperfect, yet they all approach perfection in the same degree; whereas if some of them were made accurate, others must be very far wrong.

TEMPERAMENT. In Physiology, temperament has been defined as that peculiarity of organisation which to a certain extent influences our thoughts and actions. The ancient physicians enumerated four temperaments; viz. the bilious or choleric, the phlegmatic, the sanguine, the melancholic. To these some have added the nervous; and these terms are still in use among medical writers.

Temperance Societies. According to a statement made at the meeting of the 'World's Temperance Convention' at New York in 1853, the first National Organisation against Alcohol,

Temperance, was established in 1826. Societies of persons pledged by mutual agreement to abstain from intoxicating drink were formed about the same time both in Britain and