Quantitative microscope interferometry has, until recently, been limited to observations at small apertures with, consequently, limited image brighteness and resolution. Also, in the beam splitting comparison techniques, specimen shape and area are necessarily limited because of the requirement for an adjacent background reference area. We have previously described a quantitative finite aperture theory (FAT) for incident light interferometry which avoids these endemic problems and operates with cones of incident light having included angles between 30° and 105°. In addition to determining thicknesses, it is possible to derive film refractive index by making measurements at several different apertures, a unique feature among optical methods because it is capable of measuring over areas less than 0·05 wm2. We report here new measurements which confirm the correctness of finite aperture theory to a high level of precision.