The history of the optical micrometer is briefly traced from its invention by James Watt about 1770. Attention is drawn to errors that can arise when it is used to produce calibrated displacements to be observed with a photoelectric detection system. These errors are caused mainly by stray reflections, and can be much reduced by antireflection treatment of the optical micrometer plate and other components. For a tungsten filament source at 2900°K and with silicon photocells the effective refractive index to be used in the optical micrometer formula is that of the micrometer plate at about 760nm; this should then be made the peak of the antireflection coating. Displacements calibrated to 1 nm can be reliably made. The optical micrometer may also be used in a servo-control system to stabilize the position at which a laser beam crosses any desired plane.