MICROMETER. An instrument ap plied to telescopes and microscopes for measuring very small distances, or the diameters of objects which subtend very small angles. A great number of con trivances of various kinds, and depend ing 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.
Wire Micrometer.—This instrument, when placed in the tube'of a telescope, at the focus of the object glass, presents the appearance represented in the an nexed figure. A a is a spider s web line, or very fine wire, fixed to the diaphragm; and B b and C c are similar wires stretched across two forks, each con nected with a milled headed screw. By means of these screws the two wires, Bb and C e, which are exactly parallel to each other, are movable in the direction perpendicular to A a; and, in order that the wire A a may be placed in any direc tion relatively to the meridian, there is an adjusting screw, which works into an interior toothed wheel, and turns the apparatus round in its own plane perpen dicular 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 telegraph being set and kept on the objects, the microme ter 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 b bisects one of the two stars, and C o bisects the other. The observation is now completed, and it only remains to ascertain the position and distance indi cated 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 movable wires, for instance C c, is turned until C c coincides with B b; 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 revo lution 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, andplace them in the direc tion of the meridian. Observe the tran sits of several stars of known declina tion over the wires ; then multiply each interval of seconds by 15, and by the cosine of the star's declination ; and, ta king the mean, you have the seconds of space which correspond to a known num ber of revolutions of the screw.
Circular Micrometer.—This instrument, which differs entirely from the above, was first suggested by Boscovich, in the Leipzig Acts for 1740, and used by Lacaille in observing a comet in 1742 ; but seems afterwards to have fallen into disuse, un til 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 celes tial bodies across the field may be con sidered as two parallel chords, which are given in terms of a circle of known di ameter. 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 from their lengths, gives the difference of the declination of the two bodies. The most approved construction of the annular micrometer is that of the hits Fraunhofer. It consists of a disk of par allel plate glass, having in its centre a round hole of about half an inch in di ameter, to the edges of which a ring of steel is cemented, and afterwards truly turned in a lathe. The disk being mount ed in a brass tube, so that it may be ac curately adjusted in the focus of the eye piece,. and applied to a telescope, the steel ring is alone visible, and appears as if suspended in the atmosphere, whence the instrument is called the suspended an nular micrometer.
The micrometer is an instrument of the utmost importance in astronomy, and one, in fact, to which that science is as much indebted as to the telescope itself. From a paper by Mr. Townley, in the Phil. Trans. for 1667, it appears certain that a micrometer with a movable wire was first constructed by an Englishman, Gascoigne, about the year 1640, and used by him for measuring the diameters of the moon and some of the planets ; but as Gascoigne, who was killed in the civil wars in 1644, published no account of his invention, the instrument was entirely forgotten, and the merit of reinventing it, and bringing it into general use, be longs to the French astronomer Azout, who published a description of it in 1667. Huygens, a few years previously, had con trived to measure the diameter of a planet by inserting in the tube of a telescope, at the focus of the object-glass and eye glass, a slip of metal which covered ex actly the image of the planet, and then deducing the diameter from the breadth of the slip, compared with the diameter of the field ; and Malvasia had employed for the same purpose a reticle or net work of fine silver wires, crossing each other at right angles, and dividing the field of the telescope into a number of equal squares.