us 



The Equality of the Collars and Flexure of the Telescope. It has also been as- 

 sumed that the collars are of the same diameter. This is of prime importance since 

 inequality in diameter will cause large errors. If they are of unequal diatn.-t.-is 

 the line of collimation will not define a horizontal plane when the bubble of the 

 striding-level is in the center of its tube, although the axis <>1 the collars nuy 

 have been made parallel to the combined geometrical and optical axis of the 

 telescope. The equality of the collars will always exist in our instruments when 

 new, since the auxiliary and collimator apparatus used by our firm enables us to 

 detect the least deviation of the collars from a circular form or from equality in 

 diameter, and, consequently, the collars will be of unequal diameters only from 

 the effects of wear and careless use. 



A similar deviation from a horizontal line will exist, however, although the col- 

 lars are of equal diameter, owing to the flexure of the telescope by its own weight, 

 when in a horizontal position. This flexure of the telescope is greater in the tele- 

 scopes of greater length. It is also aggravated when the telescope is not symmet- 

 rically mounted upon its transverse axis of revolution. This is the case to a certain 

 extent in all geodetic instruments, owing to the changes in the position of the center 

 of gravity of the telescope when focussing the object-slide for sights at different dis- 

 tances, or when the sun-shade is removed, slight as the effect of the weight of the 

 latter may be. 



To test the combined effect of inequality of the collars, if any, and the flexure 

 of the telescope, a method very similar to that used for adjusting the fixed telescope 

 level, explained on pages 51 and 53, may be used. 



After the striding-level and horizontal wire have been adjusted, as explained 

 above, one may try the methods for adjusting the fixed-level, and see whether the 

 line of collimation is in fact horizontal when the bubble of the striding-level is in the 

 center of its tube. If the horizontal cross-wire bisects the target when the rod is 

 held on the distant stake (see page 53), then the line of collimation is horizontal, but 

 if there is any deviation, it may be owing to inequality of the collars or to a flexure 

 of the telescope, or to both causes combined. To eliminate this error, the operator 

 may move the telescope by the vertical tangent-screw until the horizontal wire bi- 

 sects the target of the distant rod. The reading of the striding-level will then be 

 noted by taking the mean of the readings of the two ends of the bubble, and the 

 amount of displacement of the bubble from its central position will be the constant 

 error of the instrument, which should be applied as a correction in fine leveling. 

 If this error is caused solely by a flexure of the telescope, it may be corrected by 

 actually making the collars unequal in diameter to an amount which will cause 

 the line of collimation to be horizontal when the bubble is in the center of its 

 tube. 



It is thus seen that when the telescope is supported in the center only, a striding- 

 level is no more accurate than a fixed level of equal sensitiveness, unless all of the 

 above conditions are attained. Also, that the methods explained, for adjusting the 

 telescope with a fixed-level, being direct and avoiding the use of collars and elimi- 

 nating the effect of flexure, etc., preclude the existence of these errors. The fixed- 

 level has therefore its specific advantages and a legitimate place even in the best 

 surveying instruments. 



The Adjustments of the Telescope's Axis of Revolution of the En- 

 gineer's Transit, by means of the Transverse Stridiii^-KcvH, so 

 that the Line of Collimation shall describe a Vertical Plane. 



These adjustments consist, firstly, in adjusting the striding-level; secondly, 

 in making the transverse axis of the telescope horizontal, by placing it truly at 

 right angles to the vertical axis of revolution of the instrument by means of the 

 transverse striding-level; thirdly, in adjusting the vertical wire for collimation so 

 that it shall describe a vertical plane. 



The adjustment oj the transverse striding -level. In our instruments for general 

 work wh^re the telescope is reversible ou\y through the standards, as is the case 

 in the Tachymeters, the striding level will rest upon special collars of equal diame- 

 ters on the transverse axis of revolution, as it may then be attached to the telescope 





