Figure 11-14.-Theodolite reticle.

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Figure 11-13.-Coincidence- type level. shown in figure 11-13. The TELESCOPE LEVEL, mounted below the telescope, uses a prism system and a 45⁰mirror for leveling operations. When the telescope is plunged to the reverse position, the level assembly is brought to the top. TELESCOPE.— The telescope of a theodolite can be rotated around the horizontal axis for direct and reverse readings. It is a 28-power instrument with the shortest focusing distance of about 1.4 meters. The cross wires are focused by turning the eyepiece; the image, by turning the focusing ring. The reticle (fig. 11-14) has horizontal and vertical cross wires, a set of vertical and horizontal ticks (at a stadia ratio of 1:100), and a solar circle on the reticle for making solar observations. This circle covers 31 min of arc and can be imposed on the sun’s image (32 min of arc) to make the pointing refer to the sun’s center. One-half of the vertical line is split for finer centering on small distant objects. Figure 11-14.-Theodolite reticle. The telescope of the theodolite is an inverted image type. Its cross wires can be illuminated by either sunlight reflected by mirrors or by battery source. The amount of illumination for the telescope can be adjusted by changing the position of the illumination mirror. TRIBRACH.— The tribrach assembly (fig. 11-15), found on most makes and models, is a detachable part of the theodolite that contains the leveling screw, the circular level, and the optical plumbing device. A locking device holds the alidade and the tribrach together and permits interchanging of instruments without moving the tripod. In a “leapfrog” method, the instrument (alidade) is detached after observations are completed. It is then moved to the next station and another tribrach. This procedure reduces the amount of instrument setup time by half. CIRCLES.— The theodolite circles are read through an optical microscope. The eyepiece is located to the right of the telescope in the direct position, and to the left, in the reverse. The microscope consists of a series of lenses and prisms that bring both the horizontal and the Figure 11-15.-Three-screw leveling head. 11-22

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