ADJUSTMKNTS 



The Fiducial Edge should be straight. Draw a line against the 

 straight edge and turn the alidade end-for-end. If the straight 

 edge coincides perfectly with the test line, the requirements 

 are satisfied. 



The Base Rule Bubbles are to be made parallel to the plane of the 

 table or the under surface of the base rule. Set the alidade on 

 the planchette, mark a line along the fiducial edge and bring 

 the bubbles to the center of their scales. Turn the alidade 

 end-for-end along the guide line. If the bubbles remain 

 centered, the adjustment is complete. If not, do not undertake 

 to readjust the planchette but simply correct half of the error 

 in the adjusting screws of the level vials. 



The Bulls Eye Level has now generally superceded the grad- 

 uated vial because it is conceded to be accurate enough for the 



intended purpose. Where- 

 as level vials are ground to 

 cylindrical curves, the in- 

 ner surface of the circular 

 bubble is that of a sphere 

 of long radius. Usually 

 the normal position of the 

 bubble only is shown by a 

 small circle, but if gradua- 

 tions were supplied they 

 would occur in series of 

 concentric circles. These 

 bubbles are adjusted by three small screws, not shown in Fig. 33, 

 operating against a circular spring plate in the base. Once 

 justed in the factory these bubbles rarely have to be rectify 

 but in case of this necessity place the alidade upon a plane 

 surface, which is known to be level in all directions, and 

 tighten the screw toward which the bubble seems to creep. 

 The Telescope Axis must be adjusted to horizontally in order 

 that the vertical wire shall travel in a vertical plane. This 

 adjustment cannot be conducted with such care as in the 

 transit, for it is too difficult to level up the base with such 

 accuracy or stability, and no adjustment for the horizontal axis 

 can be conducted with any certainty unless the base rule is 

 perfectly level across its shortest dimension. The plumb 

 line test is therefore sufficient. Carefully level the planchette 

 and place the alidade along a well denned mark. Place a 

 plumb line in the field of view and revolve the telescope to 

 check against it. If the vertical wire deviates to the right, for 

 instance, reverse alidade along the guide line and test on 

 another plumb line. If in this case the sight line deviates an 



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