This Optical Micrometer measures horizontal or vertical line displacement. Range of .200 inch. Reads to .001 inch. The mounting ring has four machined slots which fit over a pin in the telescope barrel for quick and accurate indexing of the micrometer to a horizontal or vertical position. Unit has a dust cover glass and a split clamp Available in single axis (WK-58-2100) and dual axis (WK-58-2200). Can be adapted to fi t various theodolites, transits, and levels.