MANIPULATION OF THE TELESCOPIC ALIDADE 
139 
rotate the telescope as before. The line of sight should now 
remain fixed upon the distant point; if it does not do so, correct 
half the apparent error as before. Repeat until the hairs are 
properly centered. 
The test for collimation should be frequently made. No 
. important triangulations should be begun until one is certain 
that the cross-hairs are properly located. Should the instru- 
ment be subjected to any unusual jar, it must be collimated 
before it is again used. In the normal routine of field work the 
position of the cross-hairs should be examined at least once each 
week. 
Striding level. The line of sight when correctly collimated 
should be in absolute parallelism to the bubble axis, which is a 
line tangential to the curved surface of the striding level vial at 
the center of its scale. The two “red metal’^ collars which 
support the striding level are trued in the factory to the axis of 
rotation defined by the axis-sleeve within which the telescope 
rotates. There is very little chance for wear in the sleeve and 
the collars themselves are subject to little or no wear, so that 
this adjustment is a fairly permanent one. The customary test 
of parallelism is therefore simple and rapid. Level the telescope 
by the striding level, then turn the level end for end on the 
collars. If the bubble does not come to rest in the same position 
as before, correct one half of the indicated error with the tan- 
gent screw and the other half in the striding level by turning the 
set screw in the crotch of one of the wyes with a screw driver. 
This will secure parallelism between the bubble axis and the 
contact points on the collars, but does not guarantee parallelism 
with the line of sight although that has supposedly been pro- 
vided for by the maker of the instrument. The reliable test is 
that of the peg-method described in most surveying manuals. 
Stadia constant. In alidades of the type customarily used by 
geologists, the distance between the stadia hairs is fixed in manu- 
facture and may not be adjusted in the field. Occasional test 
should be made to ensure a close approximation to the fixed 
Tracy, Plane Surveying. New York, 1907, pp. 597-600. Metro Manual, 
Bausch and Lomb Optical Company, Rochester, N. Y., 1915, pp. 19-20.' 
