12 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



cal axis, and (4) the vernier on the vertical circle to give true readings 

 of the angle of elevation of the line of collimation. These results may- 

 be brought about by the following adjustments: 



24. (1) The plate-level adjustment: To make the axis of each 

 plate level lie in a plane perpendicular to the vertical axis, bring one 

 of the level tubes in line with two of the leveling screws. Level the 

 levelmg screws, revolve the instrument 180° in azimuth, correct one 

 half the movement of the bubble on the leveling screws and the other 

 half by raising or lowering the adjustable end of the level tube. Now 

 level up again and revolve 180°, and the bubbles should remain in the 

 center. If not, adjust for one-half the amount, as before, and so 

 continue until the bubbles remain in the center for all positions. 



25. (2) The collimation adjustment: To make the Ime of sight 

 perpendicular to the horizontal axis of the telescope. When this 

 is done the line of sight will generate a plane when the telescope is 

 revolved on its horizontal axis. Set up the theodohte on level ground 

 where a view can be had in opposite directions. (If the reference 





A 



A N 



IW ■ N' 

 Figure 6.— Effect of prism on line of sight. 



points selected are not level with the theodolite, or nearly so, a small 

 error will be introduced into this test.) With the telescope pointing 

 to the left, set the line of sight on a definite pouit A, figure 5, a few^ 

 hundred feet away. Revolve the telescope about its horizontal axis 

 and set another point B in the opposite direction. Now rotate the 

 instrument in azimuth until the line of sight comes upon the first 

 point A. Revolve the telescope about its horizontal axis again and 

 fix a third point C on the line of sight beside the second •point B. 

 From the last point set, measure off one-fourth the distance between 

 these two points to a point D and bring the line of sight to this posi- 

 tion by moving the reticle laterally. This movement is reversed in 

 the theodolite, as it is an inverted instrument. This adjustment 

 should be repeated as a check. 



26. It is often found that the line of sight cannot be brought to 

 position without moving the reticle too far from the center of the tube. 

 In this case adjustment must be made on the 4.5° glass prism which 

 is placed in the cube at the axis of the telescope for the purpose of de- 

 flecting the line of sight at right angles. Unless the reflecting surface 



