A micrometer is an instrument, which is used with a telescope, for the purpose of measuring small angles. A great varie ty of micrometers have been contrived by various ingenious persons ; and they are more or less complicated, more or less ex pensive, as also more or less accurate.
See MICROMETER.
Achromatic Telescope," is a name given to the refracting telescope invented by Mr. John Dollond, and so contrived as to remedy the aberration arising from co lours, or the different refrangibility of the rays of light. The improvement made by Mr. Doliond in his teiesctipes, by mak ing two object-glasses of crown-glass, and one of flint, which was tried with success when concave eye-glasses were used, was completed by his son Peter Dtillond ; who; conceiving that the same method might be practised with success with convex eye glasses, found, after a few trials, that it might be done. Accordingly he finish ed an object glass of five feet focal length, with an aperture of 34 inches, composed of two convex lenses of crown.glass, and one concave of white flint glass. But ap prehending afterward that the apertures might be admitted still larger, he com pleted tine of 3i feet focal length, with the same aperture of 34 inches. In the 17 inch improved achromatic telescope, the object-glass is composed of three glasses, :viz. two convex of crown•glass, and one concave of white flint-glass : the focal distance of this combined object glass is about seventeen inches, and the diameter of the aperture two inches. There are four eyeglasses contained in the tube, to be used for land objects ; the Magnifying power with these is near fifty times ; and they are adjusted to different sights, and to different distances of the object, by turning a finger screw at the end of the outer tube. There is another tithe, containing two eye-glasses that mag nify about seventy times, for astronomical purposes. The telescope may be ed to any object by turning two screws in the stand on which it is fixed, the one giving a vertical motion, and the other a horizontal one. The stand may be in closed in the inside of the brass tube. - 'The object-glass of the 2i and 31 feet telescopes is composed of two glasses, one convex,of crown•glass, and the other concave, of white flint glass ; and the di. ameters of their apertures are two inches and 24 inches. t ach of them is furnished with two tubes ; one for land objects, containing four and another with two eye=glasses for astronomical uses. They are adjusted by buttons on.
the outside of the wooden tube : and the vertical and horizontal motions are give by joints in the stands. The magnifying power of the least of these telescopes, with the eye-glass fir land objects, is nearly fifty times, and with those for as tronomical purposes, eighty times ; and that of the greatest fin land objects is nearly seventy times, but for astronomi cal observations eighty and a hundred and thirty times; for this has two tubes, either of which may be used as occasion re quires This telescope is also moved by a screw and rack-work, and the screw is turned by means of a hook's joint.
We must now say something of the spe cula of telescopes, having referred to this place from the article SPECULUM. The metals of reflecting telescopes are gene rally composed of thirty-two parts of cop per and fifteen of grain tin, with the addi tion of two parts of arsenic, to render,the composition more white and compact. It has been ascertained, by a variety of ex periments, that if one part of brass, and one of silver, be added to this composi tion, and only one of arsenic used, a most excellent metal will be obtained, which is the whitest, hardest, and most reflective. l'The first composition is, however, for inexperienced persons, the best, as the easiest to cast, to grind, and polish. When this is employed, the copper and tin shbuld be' melted, and when mixed toge.: ther should be'poured into cold water, 'which will ,eparate the mass Into a num ber of small particles. These small pieces of metal are then to be collected and put into the crucible, along with the silver and brass : after they have been together in a separate crucible, the pro per quantity of is to be added, and a little powdered, rosin thrown idtp the crucible before the metal is poured AV to the flasks. For the particular methods of grinding and polishing, we refer CO Brewster's edition of Ferguson's Mecham. ics, vol. i.
TELEtHltJ I, in botany, a_ genus of the Pentandria Trigynia class and order. Natural order of Portulace2e, Jussieu. Miscellanem, Linnzeus.- Essential charac ter: calyx five-leaved ; petals five, in serted into the receptacle ; capsule one celled, three-valved. There are two species, viz. T. imperati, true orpine ; and T. oppositifolium, both natives of Barbary.