This Calibrator-Compensator System was developed at Farrand Controls to satisfy a need for a practical measuring system with a long term accuracy of a few parts in ten million over distances up to 1.9 meters (75 inches). The inherent high accuracy of laser interferometers is limited by changes in the wavelength of light as a function of atmospheric temperature, pressure, and composition. The errors can exceed 13 PPM in air under constant temperature conditions. The best commercially available automatic compensator measures atmospheric variations with separate transducers, digitizes the data, and calculates the required correction factor. Since the remaining errors, typically 3 PPM, did not meet requirements, an improved compensation technique had to be developed. A reference optical path of known fixed length is used to determine the initial wavelength and to track changes in wavelength as they occur. The true wavelength of light is determined by evacuating the optical path to a modest vacuum and measuring the apparent path length change. The same optical path, open to the atmosphere, monitors the aggregate effect of all atmospheric changes without the use of separate transducers. The relative velocity of light is servoed to the reference path length, and the outputs of all axes of an Option 450 Hewlett-Packard laser interferometer are corrected. There is no cumulative error, and full accuracy is maintained over indefinitely extended intervals. Test runs of 50 to 90 hours have consistently shown total errors less than 3 parts in 10 million over path lengths of 0.25 to 1.9 meters (10 to 75 inches).