STANDARD - Survey Instrument Antique Center!

On slightly rising ground locate four points a, 6, c and d, on the same line, nearly,
making be = ca, and ad any convenient distance, preferably not much less than ca, and
in some simple ratio with it, for ease of calculation afterwards. Set the instrument at
c ; take readings A' and B' on a and b respectively, carefully leveling before each sight.
Then, unless the instrument is otherwise much out of adjustment, (B' A') is the true
difference of level of a and b.
Next set up at d, level carefully, and take readings A" and B'" on a and b respectively.
[In strictness the centre of the instrument should not be set over d, but beyond,
by an additional distance = principal focal length of the object-lens -j- the distance from
that lens to the centre of the tripod. (See the Manual, page 87, Fig. 2.) ] Then (B"
A") (B' A') = B'"= error of collimation in the distance ba, that is the error due to
the vertical angle between the line of sight and axis of spirit-level. Now, by similar
triangles, we have
B/ ,, . 6a = BIV : ^ ...
B " = B/// x bd ,
ba
which is the error in the distance bd, and is to be applied to the reading ~B ff . Set the
rod to read (B" Then :
B 1V
).
For Adjustment of a Dumpy Level.
Having first adjusted the spirit-level so that it remains true in all positions about
the vertical axis, point the telescope on the rod, properly held at 6, with target set to
read (B" B lv ). By means of the capstan-headed screws, raise or lower the horizontal
line until it bisects the target. To test the adjustment, set the rod over a, with index
reading (B"
B IV
) (B' A'), and see if the target is still bisected.
Adjustment of Attached Level of Transit Telescope.
The rod being held plumb at b, with target set to read (B" B iv ), move the telescope
by vertical tangent-screw until the line of sight bisects the target ; clamp securely.
Then bring the bubble to the middle of the tube by means of the level- adjusting nuts
alone. Test as in the other case.
KEMARKS. The diagram shows a special case, viz., when (B" A") > (B' A'),
or the angle subtended by B vi is one of elevation. If (B" A") = (B' A) the line of
sight is already level and no adjustment is needed. If (B" A") < (B' - A?), B* subtends
an angle of depression, and is to be added to B". In the latter case, if the slope
of the ground is slight, the difference (B" A") may be either zero or a very small
quantity, positive or negative ; but in all cases it is added algebraically to (B' A') to
obtain B" 7 .
As in all other methods of adjustment, we assume that the maker has done his part
so well that the line of collimation will not be disturbed in any movement of the objective
for focusing. Let us suppose that the line of collimation is made truly horizontal,
and that in its prolongation we have set the centres of two targets, one over a
and one over 6, the instrument being at d. If now we focus upon the farther target, the
image will be bisected by the horizontal spider-line. Then change the focus so as to
view the nearer target. If the centre of the objective has not moved truly in the line of
collimation, the new image will not be bisected at the focus, and the nearer target will
appear to be out of level, when in fact it is not. Hence, since this adjustment requires
change of focus, it cannot be made if the above defect, in the movement of the objective,
exists. If, however, the distance ad be not too small and the defect alluded to be only
slight, the error in changing focus for b and a may be hardly appreciable. The adjustment
once made approximately, we n^ed not remark that, in the field work, any further
error of objective is avoided when taking equidistant sights.