28 
HINTS TO TRAVELLERS. 
ment as A, and move the stake until the mark on it is accurately covered 
by the intersection of the wires. If the collimation is in adjustment the 
stake will be at B, but if not it will be in some other position, such as C. 
In order to test this unclamp the vernier-plate and turn the instrument 
half round, and, without turning the telescope over, sight to the mark on A, 
and clamp the instrument in azimuth, turn the telescope over, and if the 
collimation is out of adjustment it will point to the position D in the 
figure as far to the right of B as C was to the left. This shows that the 
collimation of the telescope is not perpendicular to its horizontal axis. 
In order to correct this, measure the distance from C to D and set up 
a stake at the middle point B, and another stake midway between B 
and D, at E. This will be one-fourth of the distance between C D, the 
amount of adjustment required, and must be made by moving the vertical 
wire to the right or left by the capstan-headed screws y y. The telescope 
will then be on the line E F, both of which points are respectively equi¬ 
distant from A and B, so that if the intersection of the cross-wires be 
accurately placed on a mark on the staff at B and turned over, it will 
strike the mark on the staff A, and the adjustment for collimation in 
azimuth will have been made; this is, however, seldom done at the first 
trial, and the operation has generally to be repeated. In both of these 
cases the adjustment has been made by the vertical wire. 
Adjustment of the Telescope Level.-—Level the instrument carefully on 
the azimuth axis H, by means of the levels d d on the horizontal limb 
A B ; next, take a pair of verticals, on faces right and left, to any well- 
defined terrestrial object; set the vertical circle R to the mean of these 
readings, and clamp it; now intersect the object, using the two screws z z, 
which clip the limb of the vertical circle x , to the stud in the frames N N, 
and not the tangent-screw W; then repeat the process as before. 
Remember that after each pair of readings the mean is to be taken, and 
the object intersected by the clip-screws 2 2 , and not hy the tangent-screiv 
W; and when the readings on the right face agree with the left face, 
the index error will be 0. Next clamp the vertical circle R at 0° O' 0", 
and bring the bubble of the telescope level to the middle of its run by 
means of its adjusting screws g, and the level will be in adjustment. 
With regard to the clips 2 2 , which keep the verniers s s in position, 
never unscrew loth after the adjustment has been made; but to release 
the yertical circle before putting the instrument into its box, unscrew 
