S i 



Fill 9 



In the dumpy level the vertical axis has always been considered 

 at the basis of adjustments. Fig. 9 is intended to show an exag- 

 gerated state of affairs in which the upright, A, is shorter than the 

 upright, B, and in which the sight-line, SL, passing through the 

 nodal point of the objective and the intersection of the cross wires 

 at x, is slightly inclined in the telescope tube on this account, in 

 order that it shall be made parallel to the horizon of the bubble, 

 HO, and perpendicular to the vertical axis. 



Such a sight-line, which is coincident with neither the geo- 

 metrical nor the optical axes, cannot be relied upon to remain in 

 the horizon when focused for all distances in the field. Thus, in 

 the sketch, if the ocular-draw is run outward for focusing on a 

 nearby object, the cross wires will drop slightly below the original 

 position and so impair the adjustment for parallelism. The respon- 

 sibility with the maker, therefore, is to produce an ideal condition 

 by building the telescope and vertical axis so that when the sight- 

 line is collimated it will be at right angles to the vertical axis. 



For the purposes in hand the Level Vial may be mounted on 

 a base bar or upon, brackets cast in one piece with the telescope 

 tube. We prefer the latter method because weight is reduced, 

 nothing is sacrificed to stability, and with a hinged mirror much 

 is accomplished in the convenience of checking the bubble at the 

 moment of observation. 



The bubble is protected against accident by an outer revolving 

 sheath, and its observation is unobstructed by any portion of the 

 instrument. One of the main objections to using the telescope 

 bubble of the transit for leveling purposes is due to its inaccessibility 

 behind the standards, the vertical circle, etc. 



The Reading Mirror \%as piv\ iously attached and adjusted to 

 a suitable angle on a ball joint, as shown in Fig. 8, but quite recently 

 we have changed the design in favor of a heavily constructed hinge, 

 the tension of which can be regulated in the threaded bolt with an 

 ordinary adjusting pin. When the mirror is closed down, it drops 

 into a spring clip that insures it against displacement and damage. 



17 



