106 



The adjustment of the horizontal wire for oollimation may be made by selecting 

 one of the methods best adapted for a particular design of telescope, as described 

 on page 70 for a telescope provided wit h a fixed level. If the telescope has a longi- 

 tudinal striding-level this adjustment should be made by the method explained on 

 page 96. 



These two adjustments should be repeated until both are correct. 



To measure the excentricity of the telescope, set up the instrument as near to 

 a wall or other vertical object as possible. Draw a horizontal line upon the wall 

 at a convenient height. Point the telescope exactly at right angles to the wall, 

 mark where the vertical wire intersects the line just drawn, and read the verniers 

 of the horizontal limb. Turn the vernier plate exactly 180, revolve the telescope 

 and make a second mark where the vertical wire now intersects the line. The dis- 

 tance between these two marks will be twice the excentricity of the telescope. 



When using an instrument of this description for short sights, it is very con- 

 venient to use sighting poles with excentric targets, or an offset at the foot of the 

 pole corresponding to the excentricity of the telescope. 



The Auxiliary Top- Telescope, 



Jiow superseded by the interchangeable auxiliary telescope, see styles I and II. 



This auxiliary, as previously made by us, was mounted in adjustable wyes on standards permanently 

 fixed to the main telescope, so that both lines of sight could be made parallel. The weight of the top tele- 

 scope was balanced by a counterpoise attached to a stem also permanently fixed to the cross-axis of the main 

 telescope. When the top telescope was not in use it was kept in the instrument box, while the standards and 

 counterpoise stem were permanently fixed to the main telescope so as to avoid frequent and tedious adjustments. 

 This feature made the instrument troublesome and unwieldy for the more ordinary work in mines, and still 

 less convenient for surface work. 



This improvement when first introduced by us superseded all other forms of top telescopes as made by 

 others whose main object seemed to be simply to straddle another telescope above the main one (a mere com- 

 mercial article, not an instrument of precision) for the purpose of steep sighting. But since the line of sight 

 of such a telescope can never be placed truly at right angles to the cross-axis, the line of collimation does not 

 move in a truly vertical plane, therefore honzontal angles measured between points differing greatly in eleva- 

 tion orin distance are never correct. 



It can also be readily seen that the telescope of a solar attachment as commonly made, having no means 

 of lateral adjustment to the main telescope, is insufficient in this respect (even leaving aside its low power and 

 aperture) and cannot meet the requirements properly. The adjustment by which the line of collimation of top 

 telescope is placed in the same vertical plane as that of the main telescope is just as important as that of 

 the main telescope itself. 



A most convenient and practical device having all the advantages of that former style, i. e., means of 

 adjusting the line of collimation parallel to that of the main telescope, so that after having been removed 

 it will retain its adjustments ivhen again attached, is our new mounting of the top telescope by means of 

 threaded studs. This enables the engineer to read horizontal angles when the main telescope cannot be 

 used, obviating the making of corrections for the eccentricity of the telescope. 



Patent Adjustable Top Telescope. 



This device consists of an adjustable trivet and an auxiliary telescope (see page 

 165) and an open central pillar, which latter screws to a threaded stud cast on or 

 permanently secured to the cross-axis of the main telescope. When not needed, 

 the auxiliary telescope and its counterpoise may be returned to the. box and the 

 instrument is free of incumbrances, save the stem for the counterpoise and the 

 stud to which the central pillar carrying the auxiliary telescope is attached, and is 

 ready for surface work. If desired, the top telescope may be entirely unscrewed 

 from the central pillar, leaving the latter attached to the main telescope. 



The Adjustment of the Auxiliary Telescope used as a Top Telescope: 

 It is assumed that all adjustments of the transit proper have been made 1 , that is, 

 that the plate and telescope levels, the line of collimation, the vertical plane, etc., 

 have been verified and corrected, and that the verniers of the vertical circle read 

 zero when plates are leveled up and that the bubble of the telescope level is in the 

 center of its graduation. 



The adjustment of Line of Collimation of Auxiliary Telescope : First examine the 

 coincidence of the intersection of the cross wires with the optical axis. This may 

 be done by rotating the telescope in improvised wyes of wood (see p. 104), or by 

 rotating it in the socket of the pillar [as sometimes made by us] by unscrewing it 

 e.bout one turn, when the adjustment is made by moving the capstan headed 

 screws as described in footnote, p. 49. The telescope must now be screwed t<> its 

 bearing in such a manner that the cross-wires are parallel to those of the main 



