STANDARD - Survey Instrument Antique Center!

95
The adjustment of the horizontal wire for coilimation may be made by selecting
one of the methods best adapted for a particular design of telescope.
These two adjustments should be repeated until both are correct.
To measure the excentricity of the telescope, set up the instrument as near to
a wall or other vertical object as possible. Draw a horizontal line upon the wall
at a convenient height. Point the telescope exactly at right angles to the wall,
mark where the vertical wire intersects the line just drawn, and read the verniers
of the horizontal limb. Turn the vernier plate exactly 180, revolve the telescope
and make a second mark where the vertical wire now intersects the line. The distance
between these two marks will be twice the excentricity of the telescope.
When using an instrument of this description for short sights, it is very convenient
to use sighting poles with excentric targets, or an offset at the foot of the
pole corresponding to the excentricity of the telescope.
The Auxiliary Top Telescope,
Now superseded by the interchangeable auxiliary telescope, see style I.
This auxiliary, as previously made by us, was mounted in adjustable wyes on
standards permanently fixed to the main telescope, so that both lines of sight could
be made parallel. The weight of the top telescope was balanced by a counterpoise
attached to a stem also permanently fixed to the cross-axis of the main telescope.
When the top telescope was not in use it was kept in the instrument box, while the
standards and counterpoise stem were permanently fixed to the main telescope so as
to avoid frequent and tedious adjustments. This feature made the instrument
troublesome and unwieldy for the more ordinary work in mines, and still less convenient
for surface work.
This improvement when first introduced by us superseded all other forms of top
telescopes as made by others whose main object seemed to be simply to straddle another
telescope above the main one (a mere commercial article, not an instrument
of precision) for the purpose of steep sighting. But since the line of sight of such a
telescope can never be placed truly at right angles to the cross-axis, the line of coilimation
does not move in a truly vertical plane, therefore horizontal angles measured
between points differing greatly in elevation or in distance are never correct.
It can also be readily seen that the telescope of a solar attachment as commonly
made, having no means of lateral adjustment to the main telescope, is insufficient in
this respect (even leaving aside its low power and aperture) and cannot meet tlie
requirements properly. The adjustment by which the line of coilimation of top
telescope is placed in the same vertical plane as that of the main telescope is just as
important as that of the main telescope itself.
A most convenient and practical device all
haying the advantages
of that former
style, i. e., means of adjusting the line of coilimation parallel to that of the main
telescope, so that after having been removed it will retain its adjustments when again
attached, is our new mounting of the top telescope by means of threaded studs. This
enables the engineer to read horizontal angles when the main telescope cannot be
used, obviating the making of corrections for the eccentricity of the telescope.
Patent Adjustable Top Telescope.
This device consists of an adjustable auxiliary telescope (see pages 189 and
193) and an open central pillar, which latter screws to a threaded stud cast on or
permanently secured to the cross-axis of the main telescope. When not needed,
the auxiliary telescope and its counterpoise may be returned to the box and the
instrument is free of incumbrances, save the stem for the counterpoise and the
stud to which the central pillar carrying the auxiliary telescope is attached, and is
ready for surface work. .
If
desired, the top telescope may be entirely unscrewed
from the central pillar, leaving the latter attached to the main telescope.
The Adjustment of the Auxiliary Telescope used as a Top Telescope:
It is assumed that all adjustments of the transit proper have been made, that is,
that the plate and telescope levels, the line of coilimation, the vertical plane, etc.,
have been verified and corrected, and that the verniers of the vertical circle read