STANDARD - Survey Instrument Antique Center!

94
gent screw of the main telescope and now, by turning the instrument on its vertical
center see if the horizontal wire of the side-telescope bisects the object or target
also. If so, this adjustment is made, but if not, it must be completed by moving the
vertical capstan-headed screws as explained on page 58.
To verity this adjustment, the side-telescope may be reversed on its horizontal
axis of revolution and clamped to its hub when nearly in the same level plane.
Then turn the instrument a little more than 180 on its vertical center, place the
auxiliary level on the side-telescope, same as before, and bring the bubble to the
center of its tube by means of the vertical tangent screw. If now, when the sidetelescope
is in the reversed position the horizontal wire bisects the object also, this
adjustment is completed, but if it does not then the horizontal wire must be moved
again to a point half-way between the two readings.
This adjustment may also be made by the auxiliary level alone or by means of
a striding-level without the aid of the main telescope (see page 140).
Adjustment of the vertical wire. Select a well defined object, as a church spire,
distant 5 or 6 miles. Bisect it with the vertical wire of the main telescope, and
without moving the instrument, look through the side-telescope and note whether
the object is also bisected by its vertical wire. If not, make the adjustment by
moving its vertical wire by the horizontal capstan-headed screws, until the object
is bisected also. The distance between the two telescopes being only a few
inches, the vertical wires will cover so great a width, if the object be sufficiently
distant, that the effect of the excentricity of the side-telescope will be almost imperceptible
and the same distant point may be used for each telescope.
(c) When a distant object is not available, measure with a pair of dividers the excentricity
of the side-telescope, which is the distance between the centers of the two
telescopes. Then transfer it to the face of a wall as far distant as practicable and
make two marks whose horizontal distance apart is equal to this excentricity. Bisect
one of these marks by the vertical wire of the main telescope and then look through
the side-telescope and note whether the other mark is bisected by its vertical wire. If
not, make it do so by moving the cross-wires of the side-telescope as described on page
58. The direction of the lines of sight should be at right angles to the surface upon
which the two marks are made.
The position of the side-telescope with respect to the main telescope should be
assured whenever the former is to be used. This may be done as follows : find a mark
that is bisected by the horizontal wire of the main telescope.
Then turn the instru-
ment on its vertical axis and notice whether the horizontal wire of the side-telescope
bisects the same mark. If so, firmly clamp the side-telescope to its hub. If not,
gently tap one end of the side-telescope, which hitherto has only been loosely clamped,
until its horizontal wire coincides with the mark and then clamp the side-telescope to
its hub. The telescopes are now set to correspond with the zero of the vertical circle.
To place the telescopes at an angle with each other. Level up and fix a mark
when the main telescope is level. Then raise or depress the main telescope the required
angle and clamp the horizontal axis. Now move the side-telescope until its horizontal
wire bisects the mark and clamp it firmly to its hub. During an extended operation with
the side-telescope, the relative position of the two telescopes should be verified from
time to time to detect any disturbance of the side-telescope.
Transits having the telescope mounted at the end of the horizontal axis of revolution
are sometimes used in mines ; or, as shown in the Alt-Azimuths Nos. If a and 16b,
this construction is used in some instruments for geodetic and smaller astronomical
work. The adjustment of such a telescope for cpllimation may therefore be explained
in this connection. The following method is as simple as any :
Select a well-defined object, as a church-spire, distant at least 5 or 6 miles. The
^istrument being leveled, bisect the object with the vertical wire and read the verniers
of the horizontal limb. Then turn the vernier plate so as to read exactly 18u different
from the previous reading, and revolve the telescope. If the vertical wire is adjusted
for collimation it will again bisect the distant object, since the space covered by the
cross-wires on an object at such a distance will be much greater than the change in the
position of the telescope as caused by its excentricity from the center of the instrument.
If it does not again bisect the object, correct one-half the error by means of the hori
zontal capstan-headed screws as explained on page 58.