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HORIZONTAL CIRCLE. To
read the HORIZONTAL CIRCLE, turn
the INVERTER or CIRCLE-SELECTOR
KNOB until its black line is
horizontal. Adjust the illuminating mirror to
give uniform lighting to both sections of the horizontal
circle; the micrometer scale is viewed through
the circle-reading microscope. Focus the microscope
eyepiece so that the graduations are sharply
defined. The view through the microscope should
then be similar to figure 11-22, view A. From
this point, continue in the following way: 1. Turn the coincidence knob until the images of the opposite sides of the circle are moved into coincidence. Turning this knob also moves the micrometer scale. The view through the microscope now appears as shown in figure 11-22, view B.2. Read the degrees and tens of minutes from the image of the circle. The nearest upright number to the left of the index mark is the number of degrees (105). The diametrically opposite number (the number 1800) is 285. The number of divisions of the circle between the upright 105 and inverted 285 gives the number of tens of minutes. In figure 11-22, view B, there are five divisions between 105 and 285; and the reading, therefore, is 1050 50. The index may also be used for direct reading of the tens of minutes. Each graduation is treated as 20 min. Thus, the number of graduations from the degree value to the index mark multiplied by 20 min is the value. If the index falls between graduations, another 10 min is added when the tens of minutes is read directly.3. Read the unit minutes and seconds below from the image of the micrometer scale. This scale has two rows of numbers below the graduations; the bottom row is the unit minutes and the top row, seconds. In figure 11-22, view B, the unit minutes and seconds are read as 7'23.5''4. Add the values determined in Steps 2 and 3 above. This gives 1050 57'23.5''as the final reading.
Figure 11-22.-View of a 1-second theodolite circle. |
 
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