2G 



permanent derangement of adjustment; such also will be the case when the temperature 

 of an instrument is greatly altered after the adjustments have been completed. A 

 similar result is caused if the bubble of a spirit level should not lengthen symmetrically 

 from the center of its graduated scale in varying temperatures. These imperfections, 

 however, seldom occur in instruments of modern make (or if they occur, they are gener- 

 ally caused because the principal constituents, glass and metal, are substances of 

 widely differing co-efficients of expansion), and are generally so slight in well m:nlo 

 instruments, as to be of little practical value, and may be overcome by adjusting the 

 instrument while at a mean temperature of an entire season. 



If an instrument does not remain in adjustment a reasonable length of time, the 

 cause that leads to the trouble, such as a loose object-glass or cell, loose object-slide, 

 worn out screws or bearings, etc., must be found and remedied. If this is beyond the 

 scope of the operator the corrections should be made by an instrument maker. 



Some Facts Worth Knowing. 



The Line of Collirnation. 



The expression "Line of Collimation," usually defined vaguely in treatises on 

 geodetic instruments, generally means any line of sight in a telescope given by the inter- 

 section of the cross-wires, whether they are in perfect adjustment or not. The term 

 " Line of Collimation/' should, however, be confined solely to the line of sight defined 

 by the cross-wires when they are in perfect adjustment, with reference to the optical 

 axis of the object glass ; and any difference existing between the optical axis of an ob- 

 ject glass and the actual line of sight as delineated by the geometrical axis of the instru- 

 ment is the " Error of Collimation." 



The principal optical axis of an object-glass is the line passing through the optical 

 centers formed by the curvatures and the thickness of the two lenses composing it. 

 Thus it will be seen that it is a well defined axis, giving direction to the light passing 

 through an object glass, and that, when the intersection of the cross-wires is placed in 

 its prolongation at the focus of the object glass, it becomes the axial or fundamental 

 line by and from which all measurements by telescopic sighting are made. It is the Line 

 of Collimation. 



To make a good instrument, therefore, it is necessary that the outer circumference 

 of the lenses composing an object glass shall be truly concentric with the optical cen- 

 ters. The aim of the maker is to so construct his instruments that this optical axis 

 shall be truly concentric with the geometrical axis of the telescope and that the latter 

 shall also occupy a normal position with regard to the geometrical axis of all other im- 

 portant parts : upon this depends the proper working of an instrument. 



In the larger geodetic and stationary astronomical instruments, the telescopes of 

 which are arranged only for distant sighting, this condition is readily obtained ; but it 

 becomes very difficult of attainment in the smaller geodetic instruments, since, owing to 

 the varying position of the focussing slide when set for different distances, the optical 

 axis may not always remain truly coincident with the geometrical axis of the telescope. 

 Hence in these instruments, carefully adjusted for distant sights, there is frequently an 

 error of Collimation when nearer sights are taken. In the latter case^the intersection 

 of the cross-wires remains no longer exactly in the optical axis, its displacement 

 being the cause of the error observed disregarding momentarily the other and more 

 complicated features of different instruments, upon which the line of Collimation also 

 depends. 



In the Engineer's transit, however, the line of Collimation must also lie exactly 

 at right angles to the axis of revolution of the telescope, so that when this axis 

 is placed in a horizontal position, the line of collimation shall describe a truly vertical 

 plane, whether the telescope be mounted in the centre of the instrument or outside of 

 the plates, or whether it be focussed for long or short sights. In the more common 

 instruments of this class, where the telescope is situated in the center of the instru- 

 ment, the intersection formed by the line of collimation and the horizontal axis of revo- 

 lution is also required to lie truly in the prolongation of the vertical axis of revolution, 

 so that there be no eccentricity between the vertical axis of revolution and the line of 

 collimation when sights are taken at objects nearer than 200 fe.-t. 



In transits of this latter type, and.in which the above conditions are fulfilled, the 

 sights taken would at once define the true angle, and no reversing of the telescope 

 would be necessary, were it not for other reasons. On account of the necessity for 

 eliminating the eccentricity and error of graduation and verniers, as well as for eliminat- 

 ing errors arising from an inaccurate adjustment of the line of collimation and of the 



