48 ANGULAR SURVEYING 



of sight strikes the same point as before. If it does, the 

 transverse axis of the telescope is horizontal. If the point 

 first set is the point B, and the second line of sight passes 

 through D, instead of B, the transverse axis is not horizontal, 

 and must be adjusted. The adjustment is made by raising or 

 lowering one end of the transverse axis (in this case the right- 

 hand end would have to be lowered), and again repeating the 

 test, until the points B and D coincide; that is, until the line of 

 sight, when the telescope is depressed, strikes the same point, 

 as C, both before and after reversal. 



Fourth Adjustment. To make the bubble of the telescope 

 level stand in the middle of its tube when the line of sight is 

 horizontal. 



This adjustment makes the transit adapted to leveling work. 

 It is the same as that of a regular level, and is described in 

 connection with the level. 



Fifth Adjustment. To make the vernier of the vertical arc 

 or circle read zero when the line of sight is horizontal. 



To perform this adjustment, level the instrument and turn 

 the telescope on its transverse axis until the bubble in the 

 attached level is nearly in the middle of its tube; clamp the 

 telescope, and center the bubble of the attached level exactly 

 by means of the gradienter screw g, Fig. 1. If the vernier of the 

 vertical limb does not read zero, set it so that it will read zero 

 by means of the capstan-headed screws that control it. 



This adjustment is not strictly necessary, provided the 

 reading of the vernier when the telescope is horizontal is 

 observed and noted. This reading is called the index error 

 of the vertical circle or vernier and should be allowed for in 

 reading vertical angles. 



Adjustment of the Cross-Hairs. For convenience in direct- 

 ing the telescope to a signal, it is desirable that the vertical 

 cross-hair should be truly vertical, and the other truly horizon- 

 tal. ' The two cross-hairs are attached to an adjustable dia- 

 phragm exactly at right angles to each other, so that when one 

 is vertical the other is horizontal. In order to test the vertical 

 cross-hair, sight on any sharply defined point, focusing the 

 telescope perfectly and bringing the point exactly in range with 

 either end of the vertical cross-hair. Then turn the telescope 



