LEVELING 77 



telescope is turned bottom side up, the line of sight denned by 

 the intersection of the cross-hairs is no longer on the point, 

 move the cross-hairs by means of the capstan-headed adjust- 

 ing screws so as to correct one-half the apparent error, being 

 careful to move them in the opposite direction to which it 

 would appear they should be moved. The apparent error 

 shown by reversing the telescope is double the real error, as is 

 illustrated in Fig. 2. 



Suppose that with the instrument at A the line of sight 

 given by the intersection of the cross-hairs is directed to the 

 point B, and that when the telescope has been revolved or 

 turned upside down in the wyes, the line of sight strikes the 

 point C; then the distance BC will be double the real error, 



FIG. 2 



and the true line of sight will be at D, half way between B 

 and C. Sometimes both the horizontal and the vertical 

 cross- hairs are out of adjustment, in which case they should 

 be moved alternately until their intersection will coincide 

 with the same point throughout a complete revolution of the 

 telescope. 



Second Adjustment. The second adjustment consists of two 

 parts, one lateral and the other vertical. 



To adjust the level tube laterally, level up the instrument, 

 remove the pins from the wyes, and open the clips; place the 

 telescope over a pair of leveling screws and clamp the spindle. 

 Bring the bubble exactly to the middle of the tube by means of 

 the leveling screws and revolve the telescope in the wyes, first 

 in one direction and then in the other, through about an eighth 

 of a revolution. If the bubble runs toward one end of the tube 

 when in the first position and toward the other end when in the 

 second, it shows that the longitudinal axis of the bubble tube 

 and the line of collimation, or longitudinal axis, of the tele- 

 scope do not lie in the same plane. To correct the error, bring 

 the bubble nearly to the center by means of the capstan-headed 



