{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.