It is apparent that in fixing the cross \vires in the optical axis 

 it makes no difference whether we revolve the objective (in wyes) 

 or revolve the cross wires as directed. The chances for permanency 

 of adjustment are better than in a wye level because the weight and 

 "overhang" of the focusing tube are not so great. 



The Bubble Tube is adjusted to right angles with the ver- 

 tical axis by centering over either set of leveling screws. If using 

 the three-screw base, swing the bubble parallel to any two screws. 

 Revolve the telescope 180; correct half the displacement with the 

 leveling base and the other half with the adjusting nuts at the end 

 of the vial tube. Previously we made this a permanent adjustment 

 in our works, but in more recent years we have provided an adjust- 

 ing screw at one end counteracted by a spring beneath the lug. 



The Sight-Line is adjusted to parallelism with the bubble 

 axis, as established above, by centering the instrument between two 



pegs, C and D, say 300 feet apart. Take two readings as AC and 

 HU. Assuming the sight line to be inclined upward from the 

 bubble horizon, HO, the error OA = HB if LO = LH. AC 

 BD therefore will be the exact difference in elevation between the 

 pegs, without regard to the adjustments of the instrument. 



Place the instrument 10 feet back of one rod at T, so that TDC 

 forms nearly a straight line. Take the reading ED and FC. As- 

 suming that the bubble horizon would have cut through GK, the 

 total error would be KF and the error for the distance between the 

 pegs is JF. From similar triangles \-e have: 



KF = T [ (FC ED) (AC 1 BD) ] 



in which FC ED is the apparent difference in elevation and AC 

 BD is the exact difference in elevation. 



19 



