V. Collimation. 



A line of sight may be anywhere in the telescope. The 

 stadia wires, for instance, mark: two sight-lines that are considerably 

 removed from the center of the field. A ^ne of sight does not be- 

 come a line of collimation until it occupies a \ery precise position 

 in the optical axis. 



Essentially, the collimation adjustment involves the fixation of 

 the intersection of the cross wires in the optical axis of the objective. 

 Incidentally it arranges a sight-line, or two intersecting planes 

 of vision, that should not only be fixed at right angles with the 

 horizontal axis in both planes but eventually, by the standard 

 adjustment, must revolve in a plane that passes through the 

 vertical axis. 



Necessarily this requires that the adjustment should be 

 conducted in two planes, although the position of the hori- 

 zontal wire has been quite generally treated to culpable negligence. 

 Some years ago Mr. S. P. Baird remarked * that "instrument makers 

 throw the errors of eccentricity into the vertical w ire of levels and 

 into the horizontal wire of transits. The average transit", he con- 

 cludes, "is, therefore, not intended for precise leveling". 



One of our contemporaries announces that their instruments are 

 "so mechanically perfect that the cross wires may be placed in the 

 optical axis by simply placing them in the center of the field of 

 view"; but we take issue with such impressions or assumptions. 

 These are matters with which we need not temporize, because they 

 are susceptible of examination and verification by the surveyor. 



(a-1) Adjustment of the Vertical Wire. 



The vertical wire marks a vertical plane of vision which must 

 be made normal to the horizontal axis, or in plunging the telescope 

 it w ill describe a cone, and straight lines may not be prolonged. In 

 reading deflection angles there will also be a cumulative discrepancy 

 due to collimation error. For reading horizontal angles, however, 

 a constant error of this kind, applied to each observation, would 

 not affect the net result if the focus of the telescope were not 

 required to be changed. This adjustment has been quite generally 

 called "the collimation adjustment", whereas in reality it is only 

 an important half. We may otherwise describe the collimation 

 line as a very particular element in the telescope whose termini 

 are marked by the nodal point in the objective and the intersection 

 of cross wires that have been placed in its focal plane and in its 

 optical axis. Obviously, any movement of either of these terminal 

 points in a direction that is not continuous with the line itself, 

 must destroy that relationship and affect the accuracy of both 

 the adjustment and the observation. 



En. AVrrv, / 'ol. L\\\ 1901, p. 377. 

 66 



