The Vernier Scale is to be adjusted to the circle by moving it 

 slightly in the groove provided, with the small capstan-head screws. 

 The scale is fixed by the larger capstan screws which hold it to the 

 guard in approximate position, as suggested in Fig. 37. Only the last 

 minute or two are to be controlled by the smaller screws which oper- 

 ate against the side of the standards. Turn these in the same direction 

 as in adjusting a diaphragm, first, by slightly loosening the one, 

 then by tightening the other. If a large correction is to be made, 

 the whole vernier frame is to be readjusted by loosening the tension 

 in the screws at the back. 



Opposite Verniers for the Vertical Circle must be built by 

 the manufacturer so that the indices will be diametrically opposed, as 

 in the horizontal limb. In this case they are fixed to a frame work 

 which also forms a guard against accident to the reading edge. This 

 arrangement of details makes it convenient to mount the guard on 

 the same axle with the circle so that it will revolve in short arcs 

 concentrically with it. Generally, the indices of opposite verniers 

 are to be adjusted to the zero of the limb by capstan head studs 

 operating against a nose-piece set into the standard leg nearest the 

 ocular. In the Tachy meters and Theodolites of our manufacture 

 we supply an Index Adjuster with a check nut which is to be 

 operated, to accomplish this adjustment, like any other tangent 

 screw. (See Fig. 38 p. 76). 



VIII. The Control Bubble. See p. 94 . 



IX. The Standard Adjustment. 



This adjustment seeks to make the line of collimation revolve 

 in a vertical plane that passes through the vertical axis. The 

 collimation adjustment makes the sight line describe a plane in- 

 stead of a cone and the standard adjustment brings that plane into 

 a truly vertical position. It will not necessarily pass through the 

 vertical axis, however, unless the maker has built a telescope whose 

 optical axis is midway between bearings that are spaced equi-distant 

 from the center. 



The test points for collimation adjustment should properly 

 occupy positions at extreme localities in the horizon, but the 

 theoretically correct position for the test points in the standard 

 adjustment is in the zenith and nadir. The object of the ad- 

 justment is to insure correct horizontal angles as read between 

 points widely separated in elevation. The adjustment is of par- 

 amount importance in mining theodolites on this account. Recog- 

 nizing this fact, in a recent work on this topic, the author has stood 

 up so straight, however, that he has fallen over backward. The 

 argument therein advanced, providing for a succession of taut wires 

 and plumb lines, is not sustained or recommended. 



The plate bubbles being in perfect adjustment, particularly the 

 one lying transversally under the line of the telescope, and the 

 vertical axis being truly vertical, proceed as follows: 



72 



