Source : Webster's Revised Unabridged Dictionary (1913)

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.