105 



gent screw of the main telescope and now, by turning the instrument on its vertical 

 center see if the horizontal wire of the side-telescope bisects the object or target 

 also. If so, this adjustment is made, but if not, it must be completed by moving the 

 vertical capstan-headed screws as explained in foot-note, page 49. 



To verity this adjustment, the side-telescope may be reversed on its horizontal 

 axis of revolution and clamped to its hub when nearly in the same level plane. 

 Then turn the instrument a little more than 180 on its vertical center, place the 

 auxiliary level on the side-telescope, same as before, and bring the bubble to the 

 center of its tube by means of the vertical tangent screw. If now, when the side- 

 telescope is in the reversed position the horizontal wire bisects the object also, this 

 adjustment is completed, but if it does not then the horizontal wire must be moved 

 again to a point half-way between the two readings. 



This adjustment may also be made by the auxiliary level alone or by means of 

 a striding-level without the aid of the main telescope. (See adjustment of the 

 horizontal wire of the Engineer's transit telescope by means of the longitudinal 

 striding-level, page 95.) 



Adjustment of the vertical wire. Select a well defined object, as a church spire, 

 distant 5 or 6 miles. Bisect it with the vertical wire of the main telescope, and 

 without moving the instrument, look through the side-telescope and note whether 

 the object is also bisected by its vertical wire. If not, make the adjustment by 

 moving its vertical wire by the horizontal capstan-headed screws, until the object 

 is bisected also. The distance between the two telescopes being only a few 

 inches, the vertical wires will cover so great a width, if the object be sufficiently 

 distant, that the effect of the excentrieity of the side-telescope will be almost im- 

 perceptible and the same distant point may be used for each telescope. 



(c) When a distant object is not available, measure with a pair of dividers the ex- 

 centrieity of the side-telescope, which is the distance between the centers of the two 

 telescopes. Then transfer it to the face of a wall a-s far distant as practicable and 

 make two marks whose horizontal distance apart is equal to this excentrieity. Bisect 

 one of these marks by the vertical wire of the main telescope and then look through 

 the side-telescope and note whether the other mark is bisected by its vertical wire. If 

 not, make it do so by moving the cross-wires of the side-telescope as described on page 

 49. The direction of the lines of sight should be at right angles to the surface upon 

 which the two marks are made. 



The position of the side-telescope with respect to the main telescope should be 

 assured whenever the former is to be used. This may be done as follows : find a mark 

 that is bisected by the horizontal wire of the main telescope. Then turn the instru- 

 ment on its vertical axis and notice whether the horizontal wire of the side-telescope 

 bisects the same mark. If so, firmly clamp the side-telescope to its hub. If not, 

 gently tap one end of the side-telescope, which hitherto has only been loosely clamped, 

 until its horizontal wire coincides with the mark and then clamp the side-telescope to 

 its hub. The telescopes are now set to correspond with the zero of the vertical circle. 



To place the telescopes at an angle with each other. Level up and fix a mark 

 when the main telescope is level. Then raise or depress the main telescope the required 

 angle and clamp the horizontal axis. Now move the side-telescope until its horizontal 

 wire bisects the mark and clamp it firmly to its hub. During an extended operation with 

 the side-telescope, the relative position of the two telescopes should be verified from 

 time to time to detect any disturbance of the side-telescope. 



Transits having the telescope mounted at the end of the horizontal axis of revolu- 

 tion are sometimes used in mines ; or, as shown in the Alt-Azimuths Nos. loa and lob, 

 this construction is used in some instruments for geodetic and smaller astronomical 

 work. The adjustment of such a telescope for collimation may therefore be explained 

 in this connection. The following method is as simple as any : 



Select a well-defined object, as a church-spire, distant at least 5 or 6 miles. The 

 ^istrument being leveled, bisect the object with the vertical wire and read the verniers 

 of the horizontal limb. Then turn the vernier plate so as to read exactly 180 different 

 from the previous reading, and revolve the telescope. If the vertical wire is adjusted 

 for collimation it will again bisect the distant object, since the space covered by the 

 cross- wires on an object at such a distance will be much greater than the change in the 

 position of the telescope as caused by its excentrieity from the center of the instrument. 

 If it does not again bisect the object, correct one-half the error by means of the hori- 

 zontal capstan-headed screws as explained in the footnote on page 49. 



