The optical plumbing assembly, or plummet, is a device built into the alidade or the tribrach of some of the instruments to center the instrument over a point. Its working principle is shown in figure 11-42. The plummet consists of a small prismatic telescope with a cross wire or

marked circle reticle adjusted to be in line with the vertical axis of the instrument. After the instrument is leveled, a sighting through the plummet will check the centering over a point quickly. The advantages of the plummet over the plumb bob are that it permits the observer to center over a point from the height of the instrument stand, and it is not affected by the wind. The plummet is especially useful for work on high stands. A plumb bob requires someone at ground level to steady it and to inform the observer on the platform how to move the instrument and when it is exactly over the point. With the plummet, the centering and checking is done by the observer.