92 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



vision ;* bring the intersection of the cross wires to coincide 

 with some well-defined part of the object, then turn the 

 telescope round on its axis as it lies in the Y's, and observe 

 whether the coincidence remains perfect during its revolu- 

 tion ; if it does, the adjustment is correct; if not, the wires 

 must be moved one -half the quantity of error, by turning 

 the little screws near the eye-end of the telescope, one of 

 which must be loosened before the opposite one is tightened, 

 which, if correctly done, Avill perfect this adjustment. 



To set the Level parallel to the Line of Collimation. 

 Move the telescope till it lies in the direction of two of the 

 parallel plate-screws, the clips which confine the telescope 

 in the Y's being laid open, and by giving motion to the 

 screws, bring the air-bubble to the middle of the tube, 

 shown by the two scratches on the glass. Now reverse the 

 telescope carefully in its Y's, that is, turn it end for end ; 

 and should the bubble not return to the centre of the level 

 as before, it shows that it is not parallel to the optical axis, 

 and requires correcting. The end to which the bubble 

 retires must be noticed, and the bubble made to return one- 

 half the distance by the parallel plate-screws, and the other 

 half by the capstan-headed screw at the end of the level, 

 when, if the halves have been correctly estimated, the air 

 bubble will settle in the middle in both positions of the tele- 

 scope. This and the adjustment for the coilimation gene- 

 rally require repeated trials before they are completed, on 

 account of the difficulty in estimating exactly half the 

 quantity of deviation. 



To set the Telescope perpendicular to the Vertical Axis. 

 Place the telescope over two of the parallel plate-screws, 

 and move them, unscrewing one while screwing up the 

 other, until the air-bubble of the level settles in the middle 

 of its tube ; then turn the instrument half round upon the 

 vertical axis, so that the contrary ends of the telescope may 

 be over the same two screws, and if the bubble again settles 

 in the middle, all is right in that position ; if not, half the 



* The eye-piece must first be drawn out until the cross wires are 

 perfectly well defined, then the object-glass moved till distinct 

 vision is obtained without parallax, which will be the case, if, on 

 looking through the telescope at some distant object, and moving the 

 eye sideways before the eye-glass, the object and the wires remain 

 steadily in contact; but it' the wires have any parallax, the object 

 will appear flitting to and from them. 



