96 



The adjustment of such a transit for leveling consists in placing the axis of the 

 striding-level parallel to the axis of the collars, and then, in placing the horizontal 

 wire in the combined geometrical and optical axis of the telescope by means of the 

 striding-level. 



The adjustment of the striding-level consists of two separate operations, as 

 follows : 



The lateral adjustment of the Stridiny-Leyel. This adjustment is the first to bo 

 made. Its object is to place the vertical axial plane of the spirit-level punilU'l to, or 

 coincident with the axis of the collars, and is necessary to avoid errors in the posi- 

 tion of the bubble, should the striding-level be slightly moved on its collars later- 

 ally during the manipulation of the instrument. The telescope should be clanipdl in 

 a position neaHy horizontal, and the striding-level should be placed with its wyes 

 resting on the collars. The bubble must now be brought approximately to the 

 center of its tube by the tangent-screw of the telescope. The striding-level should 

 then be detached from its fastenings on the telescope, and, while resting on tho 

 collars, be inclined to one side of the telescope, say about 10 from its vertical posi- 

 tion, and the reading of the bubble be noted. Next, it must be rocked on the collar 5 

 an equal amount to the opposite side of the telescope, and the reading be noted. 

 If both readings are the same, the adjustment is correct, but if the bubble moves 

 towards one end of its tube when the striding-level is in one of these positions, and 

 towards the other end of the tube when it is in the second position, it indicates 

 that the vertical axial plane of the bubble is not parallel to the axis of the collars, 

 and that the end towards which the bubble moves is elevated with respect to the 

 axis of the collars, causing the movement of the bubble from the center of the tube. 

 The adjustment to be made is to bring the bubble again to the center by means of 

 the capstan-headed screws at the side of one leg of the striding-level. They are op- 

 posing butting screws, and the one towards which the tube must be moved to bring 

 the bubble again to the center must first be slightly loosened. The opposite screw 

 must then be tightened until it is perceived that a perfect metal contact has been 

 established. This operation must be repeated until the reading of the bubble re- 

 mains the same in both positions of the striding-level relative to its normal posi- 

 tion. This will also indicate that the ground portion of the tube is truly barrel- 

 shaped. 



If the level be extremely sensitive, it is sometimes found that the bubble will 

 run slightly towards the same end of the tube, as the lateral adjustment approaches 

 completion, although the striding-level may have been inclined at the same angle 

 when in each inclined position. This indicates that the ground portion of the tube 

 is funnel-shaped, and then the adjustment is completed by making the bubble run 

 to the same amount when the striding-level is rocked through equal angles later- 

 ally. The effect of this deviation of the glass vial from the barrel form is generally 

 within the limits of error of observation with the telescope, but aside from this, it is 

 imperceptible in most cases, as the striding-level can rock only one or two degrees 

 laterally when it is fastened to the telescope. The lateral adjustment is necessary 

 in order to avoid errors in leveling, should the striding-level be moved laterally on 

 its collars during the manipulation of the instrument. 



The horizontal adjustment oftJie Striding- Level. This, the principal adjustment, is 

 to make the horizontal axis of the spirit-level parallel to the axis of the collars. 

 To do this, bring the bubble to the center of its tube by means of the tangent screw 

 of the telescope, then reverse the striding-level end for end, and note whether the 

 bubble returns to the center. If it does, the adjustment is correct. If not, correct 

 half the error by means of the vertical opposing capstan-headed screws at the other 

 leg of the striding-level, moving them as has been explained for the lateral adjust- 

 ment, and correct the other half by the tangent screw of the telescope. If, upon 

 again reversing the striding-level end for end, the bubble remains in the center of 

 its tube, all the adjustments of the striding-level are made. But it is well to verify 

 the correctness of the lateral adjustment again, especially if the amount of correc- 

 tion required to adjust the horizontal axis of the level wa* large, and to repeat 

 both adjustments in the order given above until assured of their correctness. 



The adjustment of the horizontal cross-wire. After the striding-level has been fully 

 adjusted, the adjustment of the horizontal wire of the telescope should next be iu.i<l\ 

 This adjustment is to bring the horizontal wire into the combined geometrical and 

 optical axis of the telescope, as follows. First make the vertical cross-wire perpen- 



