26 
HINTS TO TRAVELLERS, 
on the side opposite to the vertical limb *; g g are capstan-headed screws 
for adjusting the telescope level. The telescope is brought to focus by a 
milled screw (not shown in drawing) near the object-glass; a diagonal 
eye-piece is also supplied with the instrument, and is extremely useful 
in astronomical observations; t is a capstan-headed screw used in ad¬ 
justing the axis of the telescope. 
A very useful addition to the transit theodolite is to provide it with a 
pair of micrometers in the eye-piece, by means of which the distance 
between the observer and staff of known length can be measured in 
the manner shown (pp. 37 to 40), in addition to which they increase 
the efficiency of the instrument for astronomical observation. 
Adjustments of the Theodolite . 
Parallax .—This adjustment is made by moving the sliding tube of the 
eye-piece until the threads of diaphragm are seen sharply defined against 
the sky, and then by pointing the telescope 0 at some object, and bringing 
it to the proper focus by the milled-head screw near the object-glass. To 
test the accuracy of this adjustment direct the telescope on some well- 
defined object, about as far distant as the points to be fixed. Intersect 
this object accurately by using the tangent screws, with the centre of the 
threads in the diaphragm. Now move the head laterally as far as the field 
of view wfill admit, at the same time watching the intersection of tbe 
object with the threads. If the object remains stationary on the threads, 
parallax has been eliminated; but if it does not, the parallax must be 
removed by turning the focussing-screw until the object remains stationary 
in whatever position the head of the observer may be. 
Adjustment for Collimation .—Level the instrument as carefully as 
possible, then clamp the lower plate B, and, having unclamped the 
* As generally supplied by the maker, these lanterns are a constant source of 
trouble. If there is much wind, it is almost impossible to keep them alight, and 
even when this lias been accomplished, the flickering light they give makes it 
most difficult to take accurate observations. In practice, except on very calm 
nights, it is better to dispense with this lantern altogether, and illuminate the 
wires by fixing a strip of thin white cardboard or thick paper at the object end 
of the telescope, and bending it over at an angle of about 45° in front of the 
object glass, then make an assistant throw the light of a lantern on the strip of 
