The 1-sec theodolite is a precision direction type of instrument for observing horizontal and vertical directions. This instrument is similar to,

Figure 11-20.-A 1-second theodolite.

but slightly larger than, the 1-min theodolite. The WILD theodolite shown in figure 11-20 is compact, lightweight, dustproof, optical reading, and tripod-mounted. It is one spindle, one plate level, a circular level, horizontal and vertical circles read by an optical microscope directly to 1 sec (0.002 roil), clamping and tangent screws for controlling the motion, and a leveling head with three foot screws. The circles are read using the coincidence method rather than the direct method. There is an inverter knob for reading the horizontal and vertical circles independently. The essential parts of a l-see theodolite are very similar to that of the 1-min theodolite, includ-ing the horizontal and vertical motions, the levels, the telescope, the tribrach, and the optical system shown in figure 11-21. The main difference between the two types, besides precision, is the manner in which the circles are read.

The CIRCLE to be viewed in the 1-see theodolite is selected by turning the inverter knob on the right standard. The field of the circle-reading microscope shows the image of the

Figure 11-21.-Circle-reading optical system.

circle (fig. 11-22) with lines spaced at 20-min intervals, every third line numbered to indicate a degree, and the image of the micrometer scale on which the unit minutes and seconds are read. The numbers increase in value (0⁰ to 360 ⁰ , clockwise around the circle. The coincidence knob on the side of, and near the top of, the right standard is used in reading either of the circles. The collimation level and its tangent screw are used when the vertical circle is read. The circles of the theodolite are read by the COINCIDENCE METHOD in which optical coincidence is obtained between diametrically opposite graduations of the circle by turning the MICROMETER or COINCIDENCE KNOB. When this knob is turned, the images of the opposite sides of the circle appear to move in opposite directions across the field of the CIRCLE-READING MICROSCOPE. The graduations can be brought into optical coincidence and appear to form continuous lines crossing the dividing line. An index mark indicates the circle graduations that are to be used in making the coincidence. The index mark will be either in line with a circle graduation or midway between two graduations. The final coincidence adjustment should be made between the graduations in line with the index mark or when this index mark is halfway between the two closest graduations.