adjustments of the Theodolite. 
20 
Only one of the clips, and mark it so that it may be known, and use this 
same screw when setting up the instrument again. The other clip-screw 
should never be touched; and, indeed, it would be an improvement if one 
of the clip-screws were fitted with a lock-nut, by which it would be kept 
in its proper place, and at once be distinguished from the working screw. 
To make the vertical and horizontal wires respectively vertical and 
horizontal .—As these wires are fixed in the diaphragm by the maker so 
as to cut each other at right angles, it follows that to adjust one wire is 
to adjust both, and this may be done by the following methodLevel 
the instrument with care, and intersect any small, well-defined point 
with the vertical wire, and see if it continues bisected along the wire 
when the telescope is moved in a vertical plane. If this is not the case 
the capstan-headed screws y y must be slackened sufficiently to allow the 
diaphragm to be revolved until this condition is secured, when they 
must again be tightened. It will now be found that the horizontal wire, 
if properly fixed by the maker, will continue to bisect an object on which 
it has been placed when the instrument is turned in azimuth. 
Adjustment of the Horizontal Limb.— Tighten the clamp-screw E, unclamp 
the vernier-plate A, and turn it round until the telescope is immediately 
over one of the parallel plate-screws I I; bring the bubble in the tele¬ 
scope level P to the middle of its run by turning the tangent-screw W ; 
turn the vernier-plate 180°, so as to bring the telescope again over the 
same screw, but with its ends in a reverse position. If the bubble of 
the telescope level does not remain in the middle of its run, bring it 
back to that position, half by the parallel plate-screws 11, and half by 
the tangent-screw W.* This operation must be repeated until the bubble 
remains accurately in the centre of its run in both positions of the tele¬ 
scope; now turn the vernier-plate A until the telescope is at right 
angles to its former position, and bring the bubble to the middle of its 
run half by the tangent-screw and half by the pair of foot-screws with 
which the telescope is parallel, reversing it as before until the bubble 
remains in the middle of its run in both positions.f The bubble should 
now retain its position, while the vernier-plate is turned completely 
* When the level is carried on the vernier arms, the clip-screws must be 
used, and not the tangent-screiv. 
f If the theodolite is furnished with four parallel plate-screws, they must 
always be used in pairs diagonally opposite to each other. 
