MICROMETER

n. Mi2k-ro2m'e2t-u6r. From /mi/cpds, small, and fitrpov, a measure. Fr., micrometre. Ger., Mikrometer. An instrument for measuring microscopical objects or very small distances. The ordinary form consists of a fine scale in fractions of a millimetre or inch ruled on gla or metal. Those most commonly used with the microscope are on gla . [J, 200; a, 48.] Cobweb in., Filar m. Syn.: ocular screw m. An ocular m. originally devised by Ramsden for the telescope, but frequently applied to oculars for use with the microscope. It consists of a fixed point or thread and a movable thread or cro . The relation of the fixed and movable parts are first noted and then, by means of a m. screw with a large graduated head, the movable hair or cro is moved from edge to edge of the object to be measured. The whole and partial revolutions of the screw required to measure the object are then read off and multiplied by the valuation for each revolution found by means of a stage m. [J, 210.] Objective m. See Stage m. Ocular m. Fr., micrometre oculaire. A m. placed in the ocular of a microscope and serving to measure the size of the real image of a microscopic object; then, by knowing the valuation or ratio of the ocular m., the actual size of the object may be determined. The use of the ocular m. is the most convenient and accurate method of measuring microscopic objects. It is nece ary, however, to have all the conditions affecting the size of the real image exactly the same as when determining the valuation or ratio of the ocular m. [J. 200.] Screw in. 1. A m. moved by a screw so that it may be made to exactly cover a delicate object. 2. A m. in which the object is moved over the field of the microscope or a cro hair by a screw with a graduated head. The size may then be read off directly by noting the number of revolutions of the screw. [J.] Cf. Cobweb m. Stage m. Ger., Objective-Mikrometer. A m. for use on the stage of the microscope as an object. It is ordinarily a scale ruled in fractions of a millimetre or of an inch on gla . [J, 200.] [s. 779]