leveling. An eyepiece, located to the left of the
telescope, is used for viewing the bubble through
the prism system that brings both ends of the
bubble (fig. 11-13) into coincidence.
The level vial is located directly under the
telescope, but to the left and below, directly in
line with the capstan screws under the bubble-
viewing eyepiece. The level vial’s sensitivity is
given as 30 sec per 2-mm spacing. A circular
bubble that is viewed through a 450 mirror is
provided for the first approximate leveling before
the long level vial is used. For night work,
battery-powered electric illumination lights the
long bubble, the reticle, and the circular level. The
clamping screw and the horizontal motion tangent
screw are located on the right-hand side; the
former near the spindle and the latter below the
objective lens. The instrument has a three-screw
leveling head. The tripod for this level has a
non-extension leg to add rigidity and stability to
the setup.
Self-Leveling Level
The self-leveling level (also called auto-
matic level) shown in figure 11-27 is a precise,
time-saving development in leveling instruments.
It did away with the tubular spirit level, whose
bubble takes time in centering as well as in
resetting its correct position from time to time
during operation.
The self-leveling level is equipped with a small
bull’ s-eye level and three leveling screws. The
leveling screws, which are on a triangular foot
plate, are used to center the bubble of the
bull’s-eye level approximately. The line of sight
automatically becomes horizontal and remains
horizontal as long as the bubble remains approxi-
mately centered. A prismatic device called a
compensator makes this possible. The com-
pensator is suspended on fine, nonmagnetic wires.
The action of gravity on the compensator causes
the optical system to swing into the position that
45.749A
Figure 11-27.-An automatic level.
11-31
