SURVEYING INSTRUMENTS. 03 



error must be corrected by turning the screw, B, and the 

 other half by the two parallel plate-screws over which the 

 telescope is placed. Next turn the telescope a quarter 

 round, that it may lie over the other two screws, and make 

 it level by moving them, and the adjustment will be 

 complete. 



Before making observations with this instrument, the 

 adjustments should be carefully examined and rectified, 

 after which the screw, B, should never be touched ; the 

 parallel plate-screws alone must be used for setting the 

 instrument level at each station, and this is done by placing 

 the telescope over each pair alternately, and moving them 

 until the air-bubble settles in the middle. This must be 

 repeated till the telescope can be moved quite round upon 

 the staff-head, without any material change taking place in 

 the bubble. 



A short tube, adapted to the object-end of the telescope, 

 will occasionally be found useful in protecting the glass 

 from the intensity of the sun's rays, and from damp in wet 

 weather. Price, $80 and $100. 



Troughtons Level (Fig. 106.) The telescope, A B, 

 rests upon the horizontal bar, a b, which turns upon the 

 staff-head (similar to the one employed in the Y level and 

 the theodolite). On the top of the telescope, and partly 

 imbedded within its tube, is the spirit-level, c d, over which 

 is supported the compass-box, C, by four small pillars ; thus 

 admitting the telescope to be placed so close to the horizon- 

 tal bar, a b, that it is much more firm than in the former 

 instrument. The bubble of the level is sufficiently long for 

 its ends to appear on both sides of the compass-box ; and 

 it is shown to be in the middle by its coinciding with scratches 

 made on the glass tube as usual. 



The wire plate (or diaphragm) is generally furnished with 

 three threads, two of them vertical, between which the* 

 station-staff may be seen ; and the third, by which the ob- 

 servation is made, is placed horizontally. 



The telescope is generally constructed to show objects 

 Inverted ; and as such a telescope requires fewer glasses than 

 jne which shows objects erect, it has the advantage in point 

 jf brilliancy ; and when an observer is accustomed to it, the 

 apparent inversion will make no difference to him. A diago- 

 nal eye-piece, however, generally accompanies the instru- 



