78 LEVELING 



adjusting screws at one end of the level tube, which regu- 

 late its lateral movement, and repeat the operation until the 

 bubble will remain centered during the partial revolution of 

 the telescope. 



To adjust the level tube vertically, oenter the bubble accu- 

 rately, take the telescope out of the wyes, turn it end for end, 

 and replace it in the wyes very carefully so as not to disturb 

 their position. If the bubble remains in the center of the tube, 

 the adjustment is perfect. If the bubble runs to one end, bring 

 it half way back by means of the capstan-pattern adjusting 

 nuts at one end of the level tube, by which it can be raised or 

 lowered, and then bring it to the middle of the tube by means 

 of the leveling screws. Repeat the operation until the bubble 

 will remain truly centered when the telescope is reversed in the 

 wyes. 



Third Adjustment. Level up the instrument, using each pair 

 of leveling screws. Having centered the bubble carefully with 

 the telescope over one pair of leveling screws, reverse the tele- 

 scope or turn it end for end over the same pair of leveling 

 screws. If the bubble runs toward one end, bring it half 

 way back by means of the capstan-pattern nuts at the end of 

 the level bar; then center it perfectly with the leveling screws. 

 Repeat the operation over each pair of leveling screws alter- 

 nately until the bubble will remain perfectly centered through- 

 out an entire horizontal revolution of the telescope. 



Adjustment of the Wye-Level Cross-Hairs. Besides the 

 preceding adjustments, it is convenient in leveling to have the 

 horizontal cross-hair truly horizontal so as to be able to sight 

 with any portion of it. To test this, sight upon any sharply 

 defined point, focusing the telescope perfectly and bringing the 

 point exactly in range with the horizontal cross-hair near 

 either end; that is, near the right-hand or left-hand edge of the 

 field of view. Then, revolve the telescope slowly on its vertical 

 axis and notice whether or not the point sighted is cut exactly 

 the same by the cross-hair throughout its entire length. If any 

 deviation is discernible, it should be corrected by carefully 

 rotating the cross-hairs in a direction opposite to that in which 

 it appears they should be rotated, until the horizontal cross- 

 hair will cut the point exactly the same throughout its length 



