138 
KIRTLEY F. MATHER 
This line is correctly collimated when it coincides with the opti- 
cal axis of the objective. That is, the intersection of the cross- 
hairs should remain stationary in the field of vision when the 
telescope is rotated on its horizontal axis. The telescope is 
mounted between 180-degree stops in the axis-sleeve for this 
purpose. 
Sight some distant fixed object of small size and center the 
cross-hair exactly upon it. The telescope need not be hori- 
zontal. Rotate the tube carefully half way round and twist the 
prismatic eye-piece back into position. Note whether the cross- 
hairs are still centered upon the object. If not, correct half the 
discrepancy by means of the dia- 
phragm adjusting studs, which may 
or may not be concealed beneath a 
ferrule which forms a guard against 
accident or tampering. In figure 13, 
let the original position of the cross- 
hairs be represented by the lines 
passing through the point A, their 
position after rotation of the tele- 
scope by the lines passing through 
the point B and their collimated 
position by the lines passing through 
the center of the circle. Move the 
vertical hair to left or right by turn- 
ing both lateral studs in the same direction, first slightly loosening 
the one, then tightening the other. If the alidade is of the erect- 
ing type with field reversed from right to left, as is commonly 
the case, loosen the screw away from which the vertical hair must 
apparently be moved, and tighten the opposite screw. Move 
the horizontal hair up or down by turning top and bottom studs 
in the same direction, first slightly loosening the one and then 
tightening the other. If the eyepiece is of the erecting type, 
loosen the screw towards which the horizontal cross-hairs must 
apparently be moved and tighten the opposite screw. Having ' 
corrected half the discrepancy in this way, shift the alidade until 
the cross-hairs are again centered upon the distant object, and 
Fig. 13. Diagkam Illustrating 
THE Adjustment for 
COLLIMATION 
