This method of test and correction may be likened to the 

 process of ranging in a point on a hill with two flag poles in 

 alignment with two points at opposite sides. The exact amount of the 

 error cannot very well be measured, and the adjustment can only be 

 accomplished by repeated test and correction until the line of sight 

 assumes a stable condition for extreme positions in the field, in 

 both normal and inverted positions. The principal precaution to 

 observe is to guard against lost motion in the focusing rack. 



(b-2) The Astronomical Method of collimatingis the most 

 rapid and accurate known. It requires a special ocular, suggested 

 by Bohneberger in 1825, with an opening in the side between the 

 lenses for the admission of artificial illumination. The telescope is 

 focused for infinity and turned downward toward a vessel ot 

 mercury. As it approaches the nadir, a reflected image of the cross 

 wires will be thrown back on the plane of the diaphragm. A coin- 

 cidence cannot be made with the cross wires and their reflected image 

 unless they lie in the optical axis of the objective, and this is just as 

 true of the vertical as the horizontal wire. 



After a test in one position the telescope should be reversed on 

 the vertical axis to determine the error of horizontality in the 

 horizontal axis which must be corrected. Ultimately when the 

 cross wires are in the optical axis and directed to the absolute nadir, 

 the instrument will spin on its vertical axis without any indicated 

 deviation at the intersection of the cross wires. 



This method is probably the best one known for the deter- 

 mination of true vertical angles and has been occasionally used for 

 optical plumbing in mines in connection with specially constructed 

 instruments with an enlarged and perforated vertical axis. 



(c) The Adjustment of the Eyepiece is one which has no 

 influence upon the accuracy of the operation. If the cross wires do 

 not appear to be in the center of the field, when collimated as 

 directed, it indicates merely that the field of the eyepiece is not 

 concentric with the collimation axis. Some makers supply an extra 

 set of adjusting screws for this purpose but our eyepieces are suffi- 

 ciently centered in the process of construction. 



VI. (a) The Adjustment of the Telescope Bubble. 



This adjustment may be performed by any of the tests herein 

 described for the dumpy level. A transit cannot be expected to 

 perform even ordinary leveling unless the horizontal wire is properly 

 collimated to the equator of the field; so that we may rationally 

 conclude that any method prescribing a movement of the horizontal 

 wire to agree with the axis of a bubble, casually applied, is 

 essentially wrong. (Consult p. 



