16 



HYDROGRAPHICAL SURVEYING 



[chap 



Adjust- 

 ment for 

 Parallax. 



Adjust- 

 ment for 

 Collima- 

 tion. 



in case the former work should not be at hand. The adjust- 

 ments are — 



1. Adjustments of the telescope, viz., for parallax and for 

 collimation. 



2. Adjustment of horizontal limb, viz., to set the levels on 

 the horizontal limb to indicate the verticality of the azimuthal 

 axis. 



3. Adjustment of the vertical limb, viz., to set the level 

 beneath the teL?scope to indicate the horizontality of the line 

 of collimation. 



Commence operations by setting up the theodolite as level 

 as you can by eye, by moving the legs. See that the legs are 

 firm, and everything tight. Set all hvels as true as you can, 

 by the parallel plate screws and vertical arc tangent screw. 



Parallax is occasioned by the image formed by the object 

 glass not falling exactly on the cross-wires. 



First adjust the movable eye-piece until the cross-wires are 

 sharply defined. Then obtain the proper focus for the object 

 by moving the milled head on the telescope. 



This will throw out the image of the cross-wires, and the eye- 

 piece must be again adjusted, until cross-Avires and object are 

 both truly in focus. 



This has to be done each time the theodolite is set up, and is 

 therefore only a temporary adjustment. The others are more 

 permanent. 



Collimation is effected by directing the telescope on some 

 well-defined point, and bringing it to coincide with the inter- 

 section of the wires, with the level downwards. 



Turn the telescope in the Y's, until the level is uppermost. 

 If the object is still at the intersection of the wires, the collima- 

 tion in altitude is correct. 



If not, bring the wires half-way towards the object by turning 

 the screws holding the diaphragm. Then reset the telescope 

 by the tangent screw for the object, and bring the telescope 

 round in the Y^'s to its former position, when any displacement 

 still existing must be corrected in the same way, half by the 

 diaphragm screws, and half by the tangent screw. After a few 

 trials the error should be corrected. 



Do the same mth the telescope with level right, and level 

 left, at right angles to its former positions in the Y's, for 



