as follows: Level the instrument; sight some well 

 defined reference point with \ertical axes clamped; plunge the tele- 

 scope and make a mark as indicated l>y the vertical wire. If a lateral 

 error exists, this reversion of the telescope will double it. Reverse 

 the instrument on the vertical axis with telescope still inverted and 

 sight at the test point a second time. Plunge the telescope again 

 and mark the second position indicated by the vertical wire. If it 

 coincides with the first point, the adjustment is correct. If not, 

 bear in mind that the second reversal has doubled the error again in 

 the opposite direction. The distance between the points is, there- 

 fore, four times the total error. Correct back one quarter of the 

 distance between the test points. It the telescope is erecting, move 

 the diaphragm to the side opposite to the one which seems to be 

 nei essary, but do not under any circumstances strain the adjusting 

 sciews. The test points may be on the ground, but theoretically 

 they should both be in the horizon at distances each side of the 

 instrument equal to to the average field sight. To make u trial of 

 the accuracy of the draw-tube, however, first collimate on points 

 MM > ft. distant then check the adjustment on a new set not over 25 ft. 

 away. If the second test can not be reconciled with the first, either 

 the focusing tube does not travel in the optical axis or the standards 

 are not centered with respect to the vertical axis. 



In our 4>-in. Tachymeter the erecting telescope is so arranged 

 that the diaphragm and cross wires are set between the two lenses 

 of the Huyghens ocular, and in this case the wires should be drawn 

 in the direction apparently necessary. This will be true also for 

 any inverting telescope. 



(a-2) A second method of collimating the vertical wire may 

 be accomplished without a helper, by assistance only of the 

 graduated plates, on the assumption that the zero and the 180 lines 

 are diametrically opposite. 



Set Vernier A of the horizontal limb to zero and sight any 

 object, or a well illuminated scale, at any convenient distance with 

 both vertical and azimuth axes clamped. Plunge the telescope 

 backward, open the vertical axis, revolve the instrument 180 and 

 very carefully set the index of Vernier B to the zero of the limb. 

 Now observe the relationship of the vertical wire with the original 

 trst point. If coincident, the test is satisfied. If there is a sepa- 

 iation,move the cross wires back over half the distance. 



(b-1) Adjustment of the Horizontal Wire. 



If the transit is to be used for leveling, the same care in center- 

 ing the horizontal wire should be exercised as in any leveling instru 

 ment. This process is facilitated in a Y-le\i>l by having collars at 

 hand and wye bearings in which to invert the telescope. \\'e lia\e 

 shown also how this test can be accomplished in a dumpy level, on 

 l IS, but neither of these mechanical conditions usually exists in 

 the Ameiican Transit. 



