When division-lines, on measures of great nicety, get too fine
to be read by the eye, we use the microscope. By its means we are
able to count 112,000 lines ruled on a glass plate within an inch.
The smallest object that can be seen by a keen eye makes an angle
of 40", but by putting six microscopes on the scale of the telescope
on the mural circle, we are able to reach an exactness of 0".1, or
1/3600 of an inch. This instrument is used to measure the declination
of stars, or angular [Page 64] distance north or south of the
equator. Thus a star's place in two directions is exactly fixed.
When the telescope is mounted on two pillars instead of the face of
a wall, it is called a transit instrument. This is used to determine
the time of transit of a star over the meridian, and if the transit
instrument is provided with a graduated circle it can also be used
for the same purposes as the mural circle. Man's capacity to measure
exactly is indicated in his ascertainment of the length of waves of
light. It is easy to measure the three hundred feet distance between
the crests of storm-waves in the wide Atlantic; easy to measure the
different wave-lengths of the different tones of musical sounds. So
men measure the lengths of the undulations of light. The shortest is
of the violet light, 154.84 ten-millionths of an inch. By the
horizontal pendulum Professor Root has made 1/36000000 of an inch
apparent.

The next elements of accuracy must be perfect time and perfect
notation of time. As has been said, we get our time from the stars.
Thus the infinite and heavenly dominates the finite and earthly.
Clocks are set to the invariable sidereal time. Sidereal noon is
when we have turned ourselves under the point where the sun crosses
the equator in March, called the vernal equinox. Sidereal clocks
are figured to indicate twenty-four hours in a day: they tick exact