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  Micrometer \Mi*crom"e*ter\, n. [Micro- + -meter: cf. F.
     microm[`e]tre.]
     An instrument, used with a telescope or microscope, for
     measuring minute distances, or the apparent diameters of
     objects which subtend minute 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 focus of the object glass of a telescope, and used to
        determine differences of right ascension and declination
        between stars by observations of the times at which the
        stars cross 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 Bifilar, micrometer. See under Bifilar.
  
     Micrometer caliper or gauge (Mech.), a caliper or gauge
        with a micrometer screw, for measuring dimensions with
        great accuracy.
  
     Micrometer head, the head of a micrometer screw.
  
     Micrometer microscope, a compound microscope combined with
        a filar micrometer, used chiefly for reading and
        subdividing the divisions of large astronomical and
        geodetical instruments.
  
     Micrometer screw, a screw with a graduated head used in
        some forms of micrometers.
  
     Position micrometer. See under 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.