ASTRONOMICAL INSTRUMENTS. 129 



will then be perfect. The collimation adjustment may 

 likewise be examined from time to time, by observing the 

 transit of Polaris, or any other close circumpolar star, over 

 the first three wires, which gives the intervals in time from 

 the first to the second, and from the first to the third wire ; 

 and then reversing the axis, observe the same intervals in a 

 reverse order, as the wires which were the three first, in the 

 former position, will now be the three last : if the intervals 

 in the first observations are exactly the same as the intervals 

 in the second, the collimation adjustment is correct; but 

 should the corresponding intervals differ, such difference 

 points out the existence of an error, which must be removed 

 as before described, one half by the collimating screws, and 

 the other half by the azimuthal motion of the instrument. 



It is desirable that the central, or middle wire (as it is 

 usually termed), should be truly vertical ; as we should then 

 have the power of observing the transit of a star on any 

 part of it, as well as the centre. It may be ascertained 

 whether it is so, by elevating and depressing the telescope : 

 when directed to a distant object, if it is bisected by every 

 part of the wire, the wire is vertical ; if otherwise, it should 

 be adjusted by turning the inner tube carrying the wire- 

 plate, until the above test of its vertically be obtained, or 

 else care must be taken that the observations are made near 

 the centre only ; the other vertical wires are placed by the 

 maker equidistant from each other and parallel to the middle 

 one therefore, when the middle one is adjusted, the others 

 are so too ; he also places the two transverse wires at right 

 angles to the vertical middle wire. These adjustments are 

 always performed by the maker, and but little liable to de- 

 rangement. When, however, they happen to get out of 

 order, and the observer wishes to correct them, it is done 

 by loosening the screws which hold the eye-end of the tele- 

 scope in its place, and turning the end round a small quantity 

 by the hand until the error is removed. But this operation 

 requires very delicate handling, as it is liable to remove the 

 wires from the focus of the object-glass. 



The axis on which the telescope turns must next be set 

 horizontal. To do this, apply the level to the pivots, bring 

 the air-bubble to the centre of the glass tube, by turning 

 the foot-screw, b, which raises or lowers that end of the 

 axis, and consequently the level resting upon it ; then re- 

 verse the level by turning it end for end, and if the air- 



