\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.