The Collaborative International Dictionary of English v.0.48:
Micrometer \Mi*crom"e*ter\, n. [Micro- + -meter: cf. F.
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 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
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
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
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