INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 13 



of the prism makes an angle of 45° with the incident beam of light, the 

 deflection is no longer at right angles, but may be either greater or 

 less than 90°, depending upon the relative position of the prism. 



27. Before attempting to adjust the prism, first determine whether 

 the angle of deflection is greater or less than 90°. This may readily 

 be done from the above test. If point C, the last point set, falls to 

 the left of point B (the observer facing the points), the angle of 

 deflection is apparently less than 90°. If it falls to the right of 

 point B, the angle of deflection is apparently greater than 90°. The 

 reverse of the above is actually true, however, because of the fact 

 that the theodolite inverts the objects. In the first case the angle 

 of deflection is in reality greater than 90°, and the prism must be 

 moved so as to mcrease the angle made by its reflecting surface to 

 that of the incident beam of light. In the second case the angle of 

 deflection is in reality less than 90°, and the reflecting surface must 

 be moved so that it will make a smaller angle with the incident 

 beam. In figure 6 (neglecting the effects of refraction of light in 

 the glass) (a) shows position of prism with reference to mcident beam 

 of light to cause deflection greater than 90° and (6) position to cause 

 deflection less than 90°. 



28. To make this adjustment, the prism must be removed from 

 the telescope. This is accomplished by removing the small brass 



Figure ".—Horizontal axis adjustment . 



screws from the plate covering the cube at the axis of the telescope. 

 The prism is attached to this plate and is removed with it. Two 

 set screws hold the prism in position on the plate. Its reflecting 

 surface may be moved with reference to the line of sight by loosening 

 one set screw and tightening the other. Care should be taken not 

 to overadjust the prism, for a glance at figure 6 will show that any 

 movement in the reflecting surface to the incident beam of light will 

 be doubled in the reflected rays. In this instance, the assumed move- 

 ment of 5° in the reflecting surface produces 10° difference in the 

 angle of deflection. Insofar as the deflection of the line of sight is 

 concerned, the 45° prism produces the same eft'ect as a plane mirror 

 placed in the position of the reflecting surface of the prism. Any 



431307 O - 42 - 2 



