Mī-crōm'e-ter, n.

n. [Fr. micromètre, It. & Sp. micrometro, from Gr. μικρός, small, little, and μέτρον, measure.] An instrument, used with a telescope or microscope, for measuring very small distances, or the apparent diameters of objects which subtend very small angles. The measurement given directly is that of the image of the object formed at the focus of the object-glass.

Circular or ring micrometer, a metallic ring fixed in the field of a telescope, and used to determine differences of declination between stars from the differences of time occupied by them in traversing different chords either of the inner or outer periphery of the ring.—Double-image micrometer, a micrometer in which two images of an object are formed in the field, usually by the two halves of a bisected lens which are movable along their line of section by a screw, and distances are determined by the number of screw revolutions necessary to bring the points to be measured into optical coincidence. When the two images are formed by a bisected object-glass, it is called a divided-object-glass micrometer, and when the instrument is large and equatorially mounted, it is known as a heliometer.—Double refraction micrometer, a species of double-image micrometer, in which the two images are formed by the double refraction of rock crystal.—Filar or wire micrometer, a micrometer in which spider-lines or fine wires are made to traverse the field by the action of delicate screws, which have graduated heads for noting fractional parts of a turn.—Micrometer microscope, a compound microscope combined with a filar micrometer, used chiefly for reading and subdividing the divisions of large astronomical and geodetical instruments.—Position micrometer. See POSITION.—Scale or linear micrometer, a minute and very delicately graduated scale of equal parts used in the field of a telescope or microscope, for measuring distances by direct comparison.