A specific interterometric method has been developed. The method uses monochromatic light of continuously variable wavelength through the visible spectrum, and thus is referred to as variable-wavelength interferometry (VAWl). In some special circumstances, the VAWI method takes a specific form, referred to as object-adapted (or adaptive) variable-wavelength interferometry (AVAWI), which enables the optical path difference or phase retardation and derived quantities to be measured very precisely by relatively simple means. In practice, however, the most frequent interlerometric situations at the microscopic level can be qualified as the quasi-object-adapted ones. Therefore, a modified approach to quasi-object-adapted variable-wavelength interferometry (QAVAWI), its features, and performance is presented. In general, the VAWI method is much more accurate than conventional visual two-beam interferometry techniques. In particular, the AVAWI version enables the measuring accuracy of the optical path difference to be improved by two orders of magnitude, while the QAVAWI procedure offers better accuracy by at least one order of magnitude.