53 



Adjusting:. 



After the engineer has set up the instrument and adjusted the eye-piece for 

 parallax, as described under the engineer's transit, the horizontal cross-line had 

 better be made to lie in the plane of the azimuthal rotation of the instrument. Thig 

 may be accomplished by rotating the reticule, after loosening the capstan-headed 

 screws, until a point remains bi-sected throughout the length of the line when the 

 telescope is moved in azimuth. In making tins adjustment, the level tube Is to be 

 kept directly beneath the telescope-tube. When made, the small set screw 

 attached to one of the wyes may be set so that by simply bringing the projecting 

 pin from the telescope against it, the cross-lines will be respectively parallel ana 

 perpendicular to the motion of the telescope in azimuth. 



The first collimating of the instrument may be made using an edge of some 

 building, or any profile which is vertical. Make the vertical cross-line tangent 

 to any such profile, and then turn the telescope half-way round in its wyes. If the 

 vertical cross-line is still tangent to the edge selected, the vertical cross-line is 

 collimated. 



Select some horizontal line, and cause the horizontal cross-line to be brought 

 tangent to it. Again rotate the telescope half-way round in its wyes, and if the 

 horizontal cross-line is still tangent to the edge selected, the horizontal cross-line 

 is collimated. 



*Having adjusted the two wires separately in this manner, select some well defined 

 point which the cross-lines are made to bi-sect. Now rotate the telescope half way 

 round in its wyes. Jf the point is still bi-sected, the telescope is collimated. A 

 Very excellent mark to use is the intersection of the cross-lines of a transit instrument. 



Center the eye-piece by the four capstan-headed screws nearest the eye end. 

 This is done by moving the opposite screws in the same direction until a distant 

 object under observation is without the appearance of a rise or fall throughout an 

 entire rotation of the telescope in its wyes. The telescope is now adjusted. 



Next, bring the level bar over two of the leveling screws, focus the telescope 

 upon some object about 300 feet distant, and put on the sun-shade. These precau- 

 tions are necessary to a nice atjustment of the level tube. Throw open the two 

 arms which hold the telescope down in its wyes, and carefully level the instrument 

 over the two level screws parallel to the telescope. Lift the telescope out of its 

 wyes, turn it end for end and carefully replace it. If the level tube is adjusted, the 

 level will indicate the same reading as before. If it does not, correct half the 

 deviation by the two leveling screws and the remainder by moving the level tube 

 vertically by means of the two cylinder nuts which secure the level tube to the 

 telescope tube at its eye-piece end. Loosen the upper nut with an adjusting pin, 

 and then raise or lower the lower nut as the case requires, and finally clamp that 

 end of the level tube by bringing home the upper nut. This adjustment may require 

 several repetitions before it is perfect. 



The level is now to be adjusted so that its axis may be parallel to the axis of the 

 telescope. Rotate the telescope about 20 in its wyes, and note whether the level 

 bubble has the same reading as when the bubble was under the telescope. If it 

 has, this adjustment is made. If it has not the same reading, move the end of the 

 level tube nearest the object-glass in a horizontal direction, when the telescope is 

 in its proper position, by means of the two small capstan-headed screws which 

 secure that end of the level to the telescope tube. If the level bubble goes to the 

 object-glass end when that end is to the engineer's right hand, upon rotating the 

 telescope level toward him, then these screws are to be turned in the direction of a 

 left-handed screw, as the engineer sees them, and vice versa. Having completed 

 this adjustment, tfre level bar itself must now be made parallel to the axis of the 

 level. 



To do this, level the instrument carefully over two of its leveling screws, the 

 other two being set as nearly level as may be ; turn the instrument 180 in azimuth, 

 and if the leveiindicates the same inclination, the level bar is adjusted. If the level 

 bubble indicates a change of inclination of the telescope in turning 180, correct 

 half the amount of the change by the two level screws, and the remainder by the 

 two capstan-headed nuts at the end of the level bar, which is to the engineer's left 

 hand when he can read the firm's name. Turn both nuts in the same direction, an 

 qual part of a revolution, starting that nut first which is in the direction of the 

 desired movement of the level bar. Many engineers consider this adjustment of 

 little importance, prefering to bring the level bubble in the middle of its tube at 

 ach sight by means of the levelling screws alone, rather than to give any consid- 

 eration to this adjustment, should it require to be made. 



* See Note p. 49. 



