Subtract KF from FC to get the position of K, and, while the 

 bubble is still centered, adjust the diaphragm upward or downward, 

 as required, until the horizontal wire reads on K. A correction is 

 made for the entire indicated error because the test is not made by 

 the process of reversion. 



Following out our directions for collimating the sight-line, 

 it will not be permissible to change the position of the wires, as here 

 indicated and as generally directed in text books. If the instru- 

 ment is properly constructed, the vertical axis will be perpendicular 

 to the collimated sight-line, and at this final stage in the process of 

 adjustment the sight-line must cut the point K. If not, the only 

 recourse will be to move the sight-line up or down, as suggested in 

 Fig. 9, or report the matter to the manufacturer, for in this event the 

 vertical axis itself needs readjustment. 



In adjusting a Y-Level by this method, the collimated 

 sight-line should also remain untouched but should be made to read 

 on the artificial horizon at K with the leveling screws, after which 

 the bubble should be adjusted to the center of its run with its own 

 adjusting screws. The sight-line and the bubble-axis being parallel, 

 they can then be adjusted to right angles with the vertical axis by 

 regulating the height of one of the wyes, as directed on page 9. 



Either the dumpy or Y-level may also be tested and adjusted 

 by the process of reciprocal leveling described on page 37. 



Still another routine for making the adjustment by the peg 

 method is outlined in our catalog, Metro III, on page 102. Set up 

 outside of each peg in turn, so that the eyepiece just touches the 

 rod. Take the first reading in each case by looking at the rod 

 through the objective. The cross wires will not be visible, but a 

 very accurate estimate of the center of the field can be made. Sight 

 the distant rod at each set-up in the usual manner and, if 250 ft. or 

 more apart, subtract the tabular correction ( page 36 ) for retrac- 

 tion before comparing with the first observation. Take half of the 

 sum of the differences between the two sets of observations, so 

 computed, and the result will be the true difference in elevation. 



Being at the upper peg, set the target at this elevation plus the 

 H.I. determined by looking through the objective in the last set of 

 readings. The bubble being previously adjusted to the vertical 

 axis, and now properly centered, bring the horizontal cross wire to 

 read on the target, \\ith the diaphragm screws, and the adjustment 

 is considered as complete. 



Note Any of these methods may be used for the adjustment of 

 the telescope bubble of a transit. See page 60. 



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