A capacitive-strainmeter transducer is calibrated using a variable-spacing, Fabry-Perot interferometer. Optical flats are mounted on a spring parallelogram. Thermal expansion of a steel tube deforms the parallelogram, smoothly displacing one of the optical flats. This modulates the intensity of the 5016 Å line, observed at the central zone of the interference pattern, and thus produces sharp peaks on a pen recorder. A theoretical analysis of the peak width, based on the work of Chabbal (1958), agrees with that recorded by the interferometer. This calibration permits earth-strain measurements to an accuracy of 1 per cent.