95 



izonral wire. Then turn the instrument on its vertical center and find or mark a 

 pomt, distant about 50 feet, on the house or tree, carefully focussing the telescope 

 upon it. Now unclamp, reverse the telescope, clamp it again and bisect the nearer 

 mark with the horizontal wire. Then turn the instrument again on its vertical axis 

 and see whether the wire also bisects the distant point ; the telescope having been 

 carefully re-fooussed. If the wire does not bisect it, correct the error by moving 

 the wires, by means of the capstan-headed screws, in the direction explained in the 

 foot-note on page 49. 



The amount that the wire should be moved can be determined, by this method, 

 only by successive approximations. If the horizontal wire is considerably out, move 

 it an amount equal to two or three times the apparent error, and again test the 

 adjustment. As the adjustment approaches completion, the wire should be moved 

 less and less, until the correct position is attained. 



As before stated, this method can only be relied on when the principal optical 

 axis passes through the geometrical horizontal axis of revolution. When these two 

 axes do not intersect, the amount of error is doubled and multiplied by reversing 

 the telescope, and causes an eccentric position of the wire when the adjustment 

 appears to be perfect. It is extremely rare that an instrument fulfills this condition 

 exactly, the maker having to rely solely upon mechanical methods of construction ; 

 and it is not wise for the engineer to place much confidence in this method of ad- 

 justing the horizontal wire. 



The adjustment of the vertical wire should now be made, as explained on page 

 49, by reversing the telescope through the standards, or over the bearings of the 

 horizontal axis of revolution, as explained on page 100. The adjustment of the hori- 

 zontal wire should then be examined again, and if found to be disturbed, the two 

 wires should be adjusted again, in turn, until the position^ of each is found to be 

 correct. 



The Adjustment of the Fixed-Level of the Telescope. This is to be done by means 

 of stakes set on opposite sides of the instrument, as described on pages 48, 50 and 

 54. By this method the axis of the spirit-level is placed parallel to the line of col- 

 limation directly, without depending on the accuracy of collars or other mechanical 

 arrangements ; thus ensuring great accuracy in the adjustment. 



2. A Striciing-Lievel Parallel to the Telescope. 



The adjustment of a transit for leveling, when it has a striding-level parallel to 

 the telescope, is similar in some respects to the telescope adjustment of the ordinary 

 wye level. The telescope of the transit is provided with collars, or rings, upon 

 which the striding-level rests. It may be placed upon them when the 

 telescope is either in a direct or in a reversed position, and it can be re- 

 versed end for end upon them. In a theoretically perfect instrument it is assumed 

 that the collars are cylinders of truly circular form and of exactly the sauie diame- 

 ter ; that they have a common axis which is parallel to or coincident with the geo- 

 metrical and optical axis of the telescope, so that the line of collimation shall remain 

 true for all focussing positions of the object-slide. It is also assumed that the 

 combined geometrical and optical axis intersects the geometrical transverse axis of 

 re volution of the telescope. The tests of these assumed conditions will be given 

 below. 



The following explanations will render the description of the adjustments more 

 simple : 



The interior surface of the glass tube or vial is ground so that it has a slight 

 curvature in the direction of its length, the radius of the curvature depending upon 

 the degree of sensitiveness required for any particular instrument. This grind- 

 ing gives a symmetrical barrel-shape to the interior surface. Consequently, if the 

 tube be rotated slightly to one side, the bubble will still indicate a horizontal plane. 

 The vertical plane which bisects the bubble longitudinally maybe called the vertical 

 axial plane of the bubble. The line tangent to the upper longitudinal element of the 

 ground interior of the tube, at the central point of the graduations, is called the 

 axis of the spirit-level. It is situated in the plane of curvature of the element, that 

 is, in the vertical axial plane, and is horizontal when the bubble has settled normally 

 in the center of its graduations. 





