14 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



refraction that is produced at the entrant face of the prism is nullified 

 by corresponding refraction on emergence of the ray of light. In the 

 later type of instruments, equipped with a 45° prism, adjustment of 

 the prism can be accomplished by means of outside screws, which 

 make it unnecessary to remove the prism from the telescope. With 

 such instruments it is usually more convenient to make even sUght 

 collimation adjustments by moving the prism rather than the reticle. 

 Most of the newer theodolites are equipped with pentagonal prisms 

 which do not require adjustment as they always give a 90° angle of 

 deflection of the line of sight. 



29. (3) The standard adjustment: To make the horizontal axis 

 of the telescope perpendicular to the vertical axis of the instrument, 

 carefully level the theodolite and sight on some high point, as a steeple 

 S, figure 7, lower the telescope and set a point R, below S, on about 

 the same level as the instrument. Revolve the telescope about its 

 horizontal axis and turn the instrument upon its vertical axis and 

 again sight at S. Lower the telescope as before and set a point L 

 opposite R. A point S' midway between R and L must be in the 

 same vertical plane with S. If the telescope is now set on S' and 

 raised to the elevation of S, the line of sight will fall on a point directly 

 above the first point R. The low end of the axis is on the same 

 side of S *S' as the point R. Now adjust the horizontal axis until 

 the line of sight faUs on point S. This is accomplished by raising 

 or lowering the adjustable end of the horizontal axis by means of the 

 capstan-headed screws. The test should be repeated until the line 

 of sight coincides with S S'. Care should be taken to leave the cap 

 screws tight enough to insure that the axis rests on its bearing but 

 not tight enough to cause friction in turning the axis. 



30. (4) The vernier adjustment: To make the vernier read zero 

 when the line of sight is horizontal. This adjustment is usually made 

 by one of the peg methods. The following is perhaps the simplest: 



Figure 8.— Peg adjustment. 



The instrument is set up exactly midway between two pegs N and 

 S, figure 8. Care should be taken that the vernier setting is not 

 disturbed while making this test. With the vernier set on zero, the 

 instrument is sighted on a rod placed on A^, and the point marked 

 1 . Then it is sighted on the rod placed on S, and the point marked 2. 

 Even if out of adjustment the points 1 and 2 are in the same hori- 

 zontal plane. The instrument is next set up near S so that look- 

 ing through the telescope with the eye of the object end, a point 3 

 is marked on the exact center of the small field of view. Now a point 

 4 is placed on the rod the same distance above or below point 1 as 

 point 3 is above or below point 2. The instrument is now sighted on 



