STANDARD - Survey Instrument Antique Center!

62
The Adjustment of the Wye Level.
After the engineer has set up the instrument and adjusted the eye-piece for
parallax, as described under the engineer's transit, the horizontal cross-wire had
better be made to lie in the plane of the azimuthal rotation of the instrument.
This may be accomplished by rotating the reticule, after loosening the capstan-headed
screws, until a point remains bi-sected throughout the length of the wire when the
telescope is moved in azimuth. In making this adjustment, the level tube is to be
kept directly beneath the telescope-tube. When made, the small set screw attached
to one of the wyes may be set so that by simply bringing the projecting pin from the
telescope against it, the cross-wires will be respectively parallel and perpendicular to
the motion of the telescope in azimuth.
The first collimating of the telescope may be made using an edge of some
building, or any profile which is vertical. Make the vertical cross-wire tangent to any
such profile, and then turn the telescope half-way round in its wyes. If the vertical
cross-wire is still tangent to the edge selected, the vertical cross-wire is collimated.
To make the adjustment of the horizontal wire, select some horizontal line,
and cause the horizontal cross-wire to be brought tangent to it. Again rotate the telescope
half-way round in its wyes, and if the horizontal cross-wire is still tangent to the
edge selected, the horizontal cross-wire is collimated.
*
Having adjusted the two wires separately in this manner, select some well-defined
point which the cross-wires are made to bi-sect. Now rotate *,he telescope halfway
round in its wyes. If the point is still bi-sected, the telescope is collimated. A
very excellent mark to use is the intersection of the cross-wires of a transit instrument
using same as a collimator.
To center the eye-piece by the four capstan-headed screws nearest the eye end :
This is done by moving the opposite screws in the same direction until a distant object
under observation is without the appearance of a rise or fall throughout an entire rotation
of the telescope in its wyes. The telescope is now adjusted.
To adjust the spirit level to the telescope, bring the level bar over two of the
leveling screws, focus the telescope upon some object about 300 feet distant, and put
on the sun-shade. These precautions are necessary to a nice adjustment of the level
tube. Throw open the two arms which hold the telescope down in its wyes, and carefully
level the instrument over the two level screws parallel to the telescope. Lift the
telescope out of its wyes, turn it end for end and carefully replace it. If the level tube
is adjusted, the level will indicate the same reading as before. If it does not, correct
half the deviation by the two leveling screws and the remainder by moving the level
tube vertically by means of the two adjusting nuts which secure the level tube to the
telescope tube at its eye-piece end. Loosen the upper nut with an adjusting pin, and
then raise or lower the lower nut as the case requires, and finally clamp that end of
the level tube by bringing home the upper nut. This adjustment may require several
repetitions before it is perfect.
To make the lateral adjustment of the spirit level : The level is now to be adjusted
so that its axis may be parallel to the axis of the telescope. Kotate the tele-
scope about 20 in its wyes, and note whether the level bubble has the same reading
as when the bubble was wider the telescope. If it has, this adjustment is made. If it
has not the same reading, move the end of the level tube nearest the object-glass in a
horizontal direction, when the telescope is in its proper position, by means of the two
small horizontal capstan-headed screws which secure that end of the level to the telescope
tube. If the level bubble goes to the object-glass end when that end is to the
engineer's right hand, upon rotating the telescope level toward him, then these screws
are to be turned in the direction of a left-handed screw, as the engineer sees them, and
vice versa. This accomplished the vertical adjustment of the spirit level for parallelism
with the line of collimation of the horizontal wire must now again be verified. Having
completed this adjustment, the level bar itself must now be made parallel to the axis
of the level.
To make the adjustment of the level bar: Level the instrument carefully
over two of its leveling screws, the other two being set as nearly level as may be; turn
the instrument 180 in azimuth, and if the level indicates the same inclination, the
level bar is adjusted, If the level bubble indicates a change of inclination of the telescope
in turning 180, correct half the amount of the change by the two level screws,
and the remainder by the two capstan-headed nuts at the end of the level bar. Turn both
nuts in the same direction, an equal part of a revolution, starting that nut first which
is in the direction of the desired movement of the level bar. Many engineers consider
this adjustment of little importance, prefering to bring the level bubble in the middle
of its tube at each sight by means of the leveling screws alone, rather than to give any
great consideration to this adjustment, should it require to be made.
* See page 58.