46 ANGULAR SURVEYING 



to the transverse axis of the telescope, this point will be in the 

 prolongation of BA. In order to ascertain whether this is the 

 case, loosen either clamp, turn the instrument in azimuth 

 through 180, set the clamp and, by means of the tangent 



-D 

 -C 

 -F 



FIG. 5 



screw, direct the line of sight again to B, and plunge the tele- 

 scope again. If the line of sight strikes the same point as before, 

 it is perpendicular to the transverse axis, and no adjustment is 

 necessary. 



But, suppose that the point set after the first plunging is 

 at D, and that the point set after the second plunging is at E, 

 to one side of D, This will show that the line of sight must 

 be adjusted. In order to make this adjustment, measure the 

 distance DE (the points D and E are set at the same distance 

 from A, as nearly as can be estimated by the eye), and set a 

 mark at F, making the distance EF equal to one-fourth DE. 

 Move the cross-hairs by means of the capstan-headed screws 

 until the vertical hair exactly covers the mark at F, being 

 careful to move them in the opposite direction to that in 

 which it would appear they should move. In order to move 

 the cross-hairs, loosen the screw on the side of the telescope 

 tube away from which they are to be moved, and then tighten 

 the screw on the opposite side. Bring the screws to a firm 

 bearing, but do not turn them so tight as to cause any strain. 

 The cross-hairs having been thus moved and the telescope 

 plunged back, the line of sight will not fall on the point B, 

 but on a point C, at a distance from B equal to EF. By 

 means of either tangent screw, bring the line of sight again 

 on the point B, then plunge the telescope. If the adjust- 

 ment is perfect, the line of sight will strike the point C which 

 is in the prolongation of the line BA and midway between 

 D and E. The adjustment should be tested by reversing the 

 instrument again in azimuth, then plunging the telescope 

 and sighting forwards as before. It may be necessary to 





