MICROMETER

(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 principles, have been employed for this purpose; but it will be sufficient to give a general description of some of the most useful or remarkable ones. JVire Micrometer. — This instrument, when placed in the tube of a telescope, 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 /J s wire fixed to the diaphragm; and ^ '^ B b 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, B b and C c, which are exactly parallel to each other, are moveable in the direction perpendicular to A a; and, in order that the wire A a may be placed in any direction relatively to tlie meridian, there is an adjusting screw, which works into an interior toothed wheel, and turns the apparatus round in its own plane perpendicular 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 moveable wires, are then turned until B b bisects one of the two stars, and C c bisects the other. The observation is now completed, and it only remains to ascertain the position and distance indicated by the micrometer. For the first of these purposes, the circumference 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 and 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 moveable wires, for instance C c, is turned until C c coincides with B bj and the number of revolutions, and 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 heads are usually divided into 60 equal parts, representing seconds. The value of the scale, or of a revolution of the screw, is obtained in the following manner: — Set the two wires, B b and C c, apart to a certain number of revolutions, and 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 Gfiotnonic Projection of the Sphere.) Circular Micrometer. — This instrument, which differs entirely from the above, was first suggested by Boscovich, in the Leipxic Acts for 1740, and used by Lacaille 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 may be explained as follows: If the field of a telescope be perfectly circular (which may be effected by means of a diaphragm 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 arc given in terms of a circle of known diameter. The differences of the times at which two stars arrive at the middle of their paths will be their ascensional differences; and the distance between the chords, which is readily computed [s. 751]