Argument 



In the stadia method of estimating distances we have a fixed 

 interval in the telescope corresponding to 1/100 of the focal length 

 of the objective and a variable interval on the rod depending upon 

 its distance from the instrument. The angular value of the field of 

 the average telescope, depending upon the focal length, aperture 

 and magnification, varies between about two and four degrees, but 

 the stadia interval, if accurately spaced, will occupy exactly 34' 23" 

 of said field. The apex of this angle is at the anterior focal point, 

 and the intercept on the rod constitutes the subtended base. The 

 sum of the angles at the base is therefore equal to 179 25' 37", and 

 considering the privileges we are at liberty to take with small angles, 

 one of those at the rod may be designated as a right angle. In this 

 case we have B = P cot a. The nat cot 34' 23" is 100, but any 

 other spacing that could be included in the field might be used and 

 distances, from the anterior focal point, calculated in the same way. 



Then let us mount a single wire on a special movable staging 

 in the focal plane, that can traverse the whole field, and measure. the 

 movement of the wire on the graduations of a drum head, the exact 

 angular value of which, for any particular telescope, may be deter- 

 mined by experiment. Fig. 31 is a modified reproduction from 

 the ?. News. Vol 65, No. 16, where R. H. Sargent, U. S. G. S. 

 gives expert testimony regarding the efficiency and adaptability of 

 this instrument for both detailed and reconnaissance surveys. The 



Fig. 31 



filar micrometer diaphragm consists essentially of a fixed staging, 

 with a horizontal wire at H and a twin vertical wire at V, so disposed 

 that the collirnation adjustment c:m be an omplished independently 

 by rotating the telescope in theaxis sheath and making the necessary 

 correction with the adjusting sc 



When the movable wire, F, occupies the middle space between 

 the double fixed vertical wires, the zero of the graduated drum 

 should coincide with the index. If not, the drum is turned on the 

 shank of the screw until these conditions are fulfilled. The movable 

 wire, F, can be made to traverse the entire field, as indicated by the 



49 



