the original position. Correct the error by halves as before and 

 continue the test for each bubble until they remain centered during 

 an entire revolution. It is customary to adjust one bubble over 

 one set of leveling screws and the other over the second set. Both 

 sets of leveling screws must be brought into commission or the 

 vertical axis will not be adjusted to vertically in both planes. 



(b) A second method of adjusting the plate bubbles, and 

 indeed a more accurate one, may be accomplished with the more 

 sensitive bubble suspended beneath the telescope. Swing the tele- 

 scope vial over either set of leveling screws; set the vernier plates to 

 zero and clamp both the vertical and a/imuth axes. Level the 

 telescope bubble with the leveling screws, after having set the ver- 

 tical arc to the index of the vernier. 



Revolve 180 on the vertical axis; correct half the error indicated 

 by the telescope bubble with the leveling screws and the other half 

 with the tangent screw which controls the telescope. Turn the 

 instrument back 90 and, using the other set of leveling screws, 

 bring the bubble to the center of its scale. If the first test and 

 correction were perfectly made, the bubble will have been made 

 normal to the vertical axis and a further adjustment of the bubble 

 should not be required; but revolve 180 again and correct half 

 errors as before if necessary. 



By repeated tests over both sets of leveling screws the telescope 

 bubble will ultimately remain centered during an entire revolution. 

 The final careful test will be made over either set of leveling screws, 

 and when verifying over the other set half corrections should not 

 be necessary unless eccentricity of centers is evident, in which case 

 the bubble cannot be reconciled to both sets of leveling screws. 



The vertical axis, in the manner prescribed, has been adjusted 

 to perfect verticality by temporary use of the telescope bubble, 

 whose relationship with the sight line has no influence or bearing 

 whatever in this case. If the plate bubbles are now adjusted to the 

 center of their scales, the adjustment is complete. 



Clamp the vertical axis and open the azimuth axis. Revolve 

 180", as nearly as may be judged by the eye, and note the position 

 of the plate bubbles. If there is a noticeable deviation, eccentricity 

 of i enters ise\ident. In all our portable instruments we aim to 

 keep this error of centering in the compound spindles down to a 

 few seconds of arc and in instruments intended for finer reading and 

 accurate triangulation, this error must be reduced to zero. Plate 

 bubbles cannot be kept in adjustment if the compound centers do 

 not revolve about the same axial line. 



II. To Test for Eccentricity of Centers. 



Open both vertical and azimuth clamps; tie the clamping 

 mechanism at the edge of the plates with a cord to some fixed object, 

 and with the telescope bubble still centered turn the lower plate 

 with the tips of the fingers in a direction against the pull of the cord. 



63 



