top, sight some object or candle light within convenient range, 

 with the vertical wire of the main telescope, all clam ps set. While sight- 

 ing through the auxiliary, bring its u-rtic:il wire to bear on the same 

 point with the small clamp -and-tangent which operates on its 

 mechanical axis. If at the side, the same general procedure is 

 followed, but what was the vertical wire at the top has become the 

 horizontal wire at the side. One wire then in ihe auxiliary, prop- 

 erly collimated to the equator of the field, would meet every 

 requirement for mine surveys. 



To attach the Auxiliary to the side of the instrument, 

 unscrew the swivel distance piece which is used when applied at the 

 top. This reveals a threaded hub which may be securely screwed 

 into either socket at the extremities of the horizontal axis. For 

 reading vertical angles in this position it is not necessary to attempt 

 to carry the instrument into the mine after having adjusted the side 

 telescope for parallelism at the surface. 



Centering is accomplished by the maker by so placing the 

 mount that, when the bulls-eye target is screwed on, it may be used 

 for overhead plumbing as in Fig. 101. This test, however, may be 

 conducted precisely as designated for the solar attachment, (see p. 

 163). Importance of centering for very steep sights is manifest. An 

 eccentricity of Vioo of an inch on a horizontal sight of 100 ft is a 

 negligible quantity, but when the inclination of the sight reaches 

 75, or thereabouts, the horizontal component is reduced to 25 ft. and 

 the error is increased to about seven seconds. 



The Illuminator Tube 



When looking into a dense background, as in mining or 

 stellar observations, there is scarcely ever sufficient illumination 

 about the object sighted to makfi the cross wires visible. In cases 



Pig. 109 



of this kind a convenient and satisfactory means of illumination is 

 secured by projecting a little diffused light back into the telt- 

 tube by means of a rehYctor ring that is inseitt-d diagonally in a 

 tube, as suggested in Fig. 10V. See also Figs. 105 and 108. 



187 



