The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Circular micrometer, 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 micrometer, or Bifilar micrometer. See under
      Bifilar.

   Micrometer caliper or Micrometer 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; turning the head one full
      revolution advances the position of the tip of the screw
      only by a little.

   Position micrometer. See under Position.

   Scale micrometer, 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.
      [1913 Webster] Micrometric

The Collaborative International Dictionary of English v.0.48:

Bifilar \Bi*fi"lar\, a. [Pref. bi- + filar.]
   Two-threaded; involving the use of two threads; as, bifilar
   suspension; a bifilar balance.
   [1913 Webster]

   Bifilar micrometer (often called a bifilar), an
      instrument form measuring minute distances or angles by
      means of two very minute threads (usually spider lines),
      one of which, at least, is movable; -- more commonly
      called a filar micrometer.
      [1913 Webster]