ADJUSTMENTS OF THE THEODOLITE. 
27 
vernier-plate A, direct the telescope on some well-defined object, 
and bring it into coincidence with the point of intersection of the 
threads of the diaphragm; take the reading on the horizontal limb A B, 
suppose it to be 20°, then move the vernier-plate, A, half-round, turn 
the telescope over, and again intersect the object, taking the reading on 
the horizontal limb, suppose 200° 2' 30", take the difference between this 
and the first reading + 180° (which in the present case would be 200°), 
and the difference would be 2' 30" ; halve this difference, and subtract it 
from the second reading, when it is greater than the first reading + 180°, 
and add it when it is less; this is the mean reading ( = 200° 1' 15"); set 
and clamp the instrument to this mean reading, and intersect the object 
by means of the capstan-headed screws y y } which move the diaphragm, 
taking care to loosen one before moving the other. Bepeat this operation 
until the readings taken with the instrument in these two different 
positions, face right and face left, differ from one another by 180°. 
2nd Method.—-Set up the theodolite as at T (see figure below) and level 
it carefully. Set up a stake, with a mark on it, at such a distance that the 
mark is distinctly visible, as at A. Turn the telescope on it and accurately 
cover the mark with the intersections of the cross wires in the diaphragm, 
and clamp it in azimuth. Next turn the telescope over and set up 
another stake, with a mark on it, at the same distance from the instru- 
cardboard, and the wires will be plainly seen. The intensity of the illumination 
will be increased or decreased according to the distance at which the lantern is 
held from the strip of cardboard. A piece of copper wire about eighteen inches 
long, with a small piece of tin soldered to one end, can be used for the same 
purpose if w T ound round the object end of the telescope and bent over the object 
glass to the required angle ; it can be kept in the theodolite box, and is always 
ready for use. This method of illuminating the wires can be used with a 
theodolite which has not a hollow axis, 
