You must be capable of performing six commonly performed tests and adjustments of the transit. All tests and adjustments of the transit are made with the instrument mounted on its tripod and setup in the shade. You must make these tests periodically and in the sequence in which they are discussed in the following paragraphs. When one of the tests indicates that an adjustment is necessary, you must make this adjustment and then you must repeat all previous tests before proceeding with the next test.

Figure 6-4.Adjusting the plate bubbles.

ADJUSTING THE PLATE BUBBLES. The purpose of adjusting the plate bubbles is to make the axis of the plate-level tubes perpendicular to the vertical axis (fig. 6-4). This ensures that when the instrument is set up and leveled, the vertical axis is truly vertical. When this condition is met, horizontal angles are measured in a truly horizontal plane and vertical angle do not incur index error because of an inclined vertical axis. You should make the plate-bubble test every time you set up the instrument for use and always before making any other tests and adjustments of the transit. Make this test and adjustment using the following steps: 

 1. Rotate the instrument about the vertical axis and bring each level tube parallel to a set of opposite leveling screws. Bring both bubbles to the center of their tubes by turning the leveling screws (view A, fig. 6-4).

2. Rotate the instrument 180 about its vertical axis. If the bubbles remain centered, no adjustment is necessary. If the bubbles do not remain centered, note the amount of distance that the bubbles move from their center (view B, fig. 6-4) and proceed with Steps 3 through 5.

3. Bring each bubble half the distance back to the center of its tube by turning the capstan screws at the end of each tube.

4. Relevel the instrument using the leveling screws and rotate the instrument again. Make a similar correction if the bubbles do not remain in the center of the tubes.

5. Check the final adjustment by noting that the bubbles remain in the center of the tubes during the entire revolution about the vertical axis (view C, fig. 6-4).

NOTE: You can compensate for out-of-adjustment plate levels by leveling the instrument, rotating it 180 in azimuth, and bringing the bubbles halfway back using the leveling screws.

ADJUSTING THE VERTICAL CROSS HAIR. In a perfectly adjusted transit, the vertical cross hair should lie in a plane that is perpendicular to the horizontal axis. In this way, any point on the hair may be used when measuring horizontal angles or running lines.

To make the vertical cross hair lie in a plane perpendicular to the horizontal axis (fig. 6-5), you should follow the procedure below:

1. See that parallax is eliminated. Sight the vertical cross hair on a well-defined point; and with all motions clamped, move the telescope slightly up and down on its horizontal axis, using the vertical slow motion tangent screw. If the instrument is in adjustment, the vertical hair will appear to stay on the point through its entire length.

2. If it does not stay on the point, loosen the two capstan screws holding the cross hairs and slightly rotate the ring by tapping the screws lightly.

3. Sight again on the point. If the vertical cross hair does not stay on the point through its entire length as the telescope is moved up and down, rotate the ring again.

4. Repeat this process until the condition is connected.

NOTE: To compensate for the above maladjustment, use only that part of the vertical hair that is closest to the horizontal cross hair.