A proposed method for the precise realization of an absolute standard of dc voltage requires, inter alia, the measurement of a number of spacings, of order 2 to 5 mm, between a horizontal reflecting mercury surface and an upper semitransparent film. The accuracy with which these measurements are made will largely determine the overall accuracy of the voltage realization. An interferometry system is being developed in which the spacings are measured by white-light optical multiplication with respect to scanning Fabry-Perot measuring interferometers having nominal spacings of about 0.66 mm. The Fabry-Perot interferometers are calibrated using a stabilized He-Ne laser and are capable of measurements having an accuracy of the order of 10-4 fringe (X = 633 nm). The design and performance of the interferometers and the other major components of the optical system are discussed. Those uncertainties which can be assessed at this stage seem likely to amount in total to less than 10-3 fringe in spacing, or less than one part in 106 of voltage.