SURVEYING INSTRUMENTS. 85 



of collimation ; that is, to make the intersection of the cross 

 wires coincide with the axis of the cylindrical rings on which 

 the telescope turns : it is known to be correct, when an eye 

 looking through a telescope observes their intersection con- 

 tinue on the same point of a distant object during an entire 

 revolution of the telescope. The usual method of making 

 this adjustment is as follows : 



First, make the centre of the horizontal wire coincide 

 with some well-defined part of a distant object ; then turn 

 the telescope half round in its Y's till the level lies above 

 it, and observe if the same point is again cut by the centre 

 of the wire ; if not, move the wire one half the quantity of 

 deviation, by turning two of the screws at in (releasing one, 

 before tightening the other), and correct the other half by 

 elevating or depressing the telescope ; now if the coinci- 

 dence of the wire and object remains perfect in both positions 

 of the telescope, the line of collimation in altitude or depres- 

 sion is correct, but if not, the operation must be repeated 

 carefully, until the adjustment is satisfactory. A similar 

 proceeding will also put the vertical line correct, or rather 

 the point of intersection, when there are two oblique lines 

 instead of a vertical one. 



The second adjustment is that which puts the level at- 

 tached to the telescope parallel to the rectified line of colli- 

 mation. The clips, ii, being open, and the vertical arc 

 clamped, bring the air-bubble of the level to the centre of 

 its glass tube, by turning the tangent-screw, P ; which done, 

 reverse the telescope in its Y's, that is, turn it end for end, 

 which must be done carefully, that it may not disturb the 

 vertical arc, and if the bubble resume its former situation in 

 the middle of the tube, all is right ; but if it retires to one 

 end, bring it back one half, by the screw/, which elevates 

 or depresses that end of the level, and the other half by 

 the tangent-screw, P ; this process must be repeated until 

 the adjustment is perfect ; but to make it completely so, the 

 level should be adjusted laterally, that it may remain in the 

 middle of the tube when inclined a little on either side from 

 its usual position immediately under the telescope, which is 

 effected by giving the level such an inclination, and if neces- 

 sary turning the two lateral screws at g ; if making the 

 latter adjustment derange the former, the whole operation 

 must be carefully repeated. 

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