The principle of the telescope is then simply this : the object-glass forms in its focus a distinct image or picture of the object which, though very much smaller than the object, is yet seen under a much greater angle, or magnified, and this im age so magnified is seen under a still greater angle, or still farther magnified, by the eye-glass, which enables the eye to see it distinctly at a distance less than six inches. The terrestrial telescope differs from the astronomical in having two ad ditional lenses placed in the tube of the eye-glass, for the purpose of restoring the inverted image to its erect position and thereby accommodating the telescope to terrestrial objects ; the focal lengths of these additional lenses being usually the same as that of the eye-glass. The per formance of these refracting telescopes depends most essentially on the goodness of the object-glass, for if the first image be bright and distinct and perfectly ach romatic, or without the prismatic colors at its edge, there is little difficulty in forming eye-pieces to magnify it without causing it to undergo any sensible alter ation. When suitable lenses are obtain ed, it is only necessary to adjust them at proper distances in the tube to complete the modern telescope : the tube keeping off the external rays which would other wise cross and interfere with the parallel rays coming from the object. In reflect ing telescopes a speculum or mirror per forms the same office which the object glass does in those of the refracting kind : it is therefore called the object-mirror. This telescope is constructed in various forms, differing from each other chiefly in reference to the contrivances 'which have been adopted for bringing the focal image into a convenient situation.for be ing viewed by the eye-piece. The chief forms are the Newtonian, Gregorian, Cassegranian and Ierschelian. In the Newtonian telescope the mirror was at the bottom of the tube and the image re ceived on a small diagonal plane specu lum, which threw the rays to the side of the tube and formed the image there, when itcould be viewed by the eye-piece.
In the Gregorian telescope the incon venience of taking a lateral view is avoid ed. It consists in a concave speculum A, B, fixed in a tube, but pierced in the centre with a hole, through which, by means of a lens or a combination of len ses, the image of the object is viewed. The rays forming the image of the object, are incident on a small concave mirror C, previous to which the rays have crossed themselves at the focus e, the image therefore at C is an inverted one : this image is viewed through the aperture in or trunions at the centre of gravity, through one of which in a tubular form the rays from the reflector within are thrown into the eyes thus placed, as in the Newtonian telescope, at the side : and the advantage of this arrangement is that the eye does not require to move on a movement of the telescope. In order
that the telescope may be accommodated to objects at different distances, it is ne cessary that the tube should be made to slide backward and forward, and the ob ject will always be inverted from the in tersection of the rays by refraction, and its use will be thus limited. An erect im age may always be obtained by adding two other convex lenses behind C, in the illustration, and of the same focal length: but a loss of light is necessarily produced by their use. Spherical aberration may be prevented in telescopes in the same way in which it is prevented in micro scopes, namely, by giving to the reflect ing surface such a configuration as will enable it to reflect all the rays incident upon it to one focus. The parabola and ellipse possess this property, and nothing but the mechanical difficulty of construct ing mirrors of these figures prevents their being employed instead of spherical mir rors. If a concave eye-glass be substi tuted for the lens C in the simple refract ing telescope, we have the Galilean teles cope, which' exhibits objects in an erect position and with very great clearness. TELL-TALE. The dial plate at the wheel, showing the position of the tiller. TELLURIUM. This rare metal has .• only been found in small quantities in the gold mines of Transylvania : it occurs in the metallic state, combined with gold or silver. It is white, brilliant, brittle, and easily fusible. Its specific gravity is about 6.25. It is combustible, and often exhales a peculiar odor, like horsc-radish, which Berzelius ascribes to the presence of minute portions of selenium. It forms a protoxide and a peroxide, often called telluroue and telluric ad.& Its equivalent is either 32 or 64. Tellurium fbrms a gaseous compound with hydrogen, which has been called hydrotelluric acid. TENACITY OF THE METALS. The power which metallic wires possess of sustaining, without breaking, the action of a suspended weight. See COHESION, STRENGTH Or MATERIALS.