1. Use the vertical circle tangent screw to move the collimation level until the ends of its bubble appear in coincidence (fig. 11-23) in the collimation level viewer on the left standard.

2. Read the vertical circle and micrometer scale as described before. Be sure to have proper coincidence before you take the reading.

3. The vertical circle graduations are num-bered to give a 0⁰ reading with the telescope pointing to the zenith. Consequently, the vertical circle reading will be 90⁰ for a horizontal sight with the telescope direct and 270⁰ for a horizontal sight with the telescope reversed. Figure 11-23 shows the view in the circle-reading microscope for direct and reversed pointings on a target.

These readings are converted to vertical angles as follows:

There are two separate occasions for setting the horizontal circle of the theodolite. In the first case, the circle is set to read a given value with the telescope pointed at a target. With the theodolite pointed at the target and with the azimuth clamp tightened, the circle is set as follows: Set the micrometer scale to read the unit minutes and the seconds of the given values. Then, with the circle-setting knob, you turn the circle until coincidence is obtained at the degree and tens of minutes value of the given reading. This setting normally can be made accurately to plus or minus 5 sec. After the circle is set in this manner, the actual reading should be determined.

In the second case, the circle is set to a value of a given angle. When a predetermined angle is measured, you first point the instrument along the initial line from which the angle is to be measured and read the circle. Add the value of the angle to the circle reading to determine the circle reading for the second pointing. Set the micrometer scale to read the unit minutes and the seconds of the value to be set on the circle. Then, you turn the instrument in azimuth and make coincidence at the degrees and tens of minutes value that is to be set. The predetermined value can usually be set on the circle in this way to plus or minus 2 sec.