METER

(Yunanca köken — orijinale bakınız.) An instrument applied to telescopes and microscopes for measuring very small distances, or the diameters of objects which subtend very small angles. A great number of contrivances of various kinds, and depending on different prin ciples, have been employed for this purpose; but it will be sufficient to cive ■ general description of some of the most useful or remarkable o (10 Hire Micrometer. — This instrument, when placed in the tube of a tele so pe, at the focus of the object gla , presents the appearance represented in the annexed figure (fig. 1) A a is a spider's web line, or very fine wire fixed to the diaphragm; and B6 and C c are similar wires stretched acro two forks, each connected with a milled-headed screw. By means of these screws the two wires, fj b nnd C c which are exactly parallel to ench & other, are movable in the direction perpendicular to A a: and, in order that the wire A a may be placed in any directionre.; an adjusting screw, which works into an interior toothed wheel, nnd turns the apparatus round in its own plane per pendicnlar to the axis of the telescope. The method of using the micrometer is as follows; Suppose the object to be accomplished were the measurement of the angle of position and distance of two very close stars; the telescope being set and kept on the objects, the micrometer is turned by its adjusting screw until the spider line A a coincides with the line joining the two stars, or threads them both at the same moment The milled heads of the screws, which carry the two movable wires, are then turned until B h bisect! one of the two stars, and C c bisects the other. The observation is now completed, and it only re mains to ascertain the position and distance indicated by the micrometer. For the first of these purposes, the circumfer ence of the micrometer is divided into degrees and minutes. and read by two verniers: this reading gives the position of A a in respect of the horizontal nnd vertical planes, and consequently the angle of position of the two stars. To find their distance, the head of the screw which carries one of the movable wires, for instance C e, is turned until C c coin cidee with B A; and the number of revolutions, nnd parts of a revolution, required to effect the coincidence, gives the distance of the stars when the value of the scale of the micrometer is known; that is to say, when the number of seconds of space which correspond to one revolution of the screw is known. The screws must be made with great accuracy, and their hends are usually divided into GO equal pans,The representing value of the seconds.scale, or of a revolution of the screw, is obtained in the folk wing manner: Set the two wires, B b and C c, apart to a certain number of revolutions, nnd place them in the direction of the meridian. Observe the transits of several stars of known declination over the wires; then multiply each interval of seconds by 15, and by the cosine of the star's declination;.and, taking the mean, you have the seconds of space which correspond to a known number of revolutions of the screw. (See Appendix, by the Rev. R. Sheepshanks, to Profe or De Morgan's Explanation of the Onomonic Projection of the Sphere.) Circular Micrometer.— This instrument, which differs entirely from the above, was first suggested by Boscovich, in the Leipzic Jlcts for 1740, and used by Lacnille in observing a comet in 1742; but seems afterwards to have fallen into disuse, until it was revived by Dr. Olbers, about 1798. The principle maybe explained as follows: If the field of a telescope be perfectly circular (which may be effected by means of a dinphragm turned in a lathe), and if its diameter be determined from observation, the paths of two celestial bodies acro the field may be considered as two parallel chords, which are given in terms of a circle of 743 [s. 757]