In interferometers employing fringes of constant thickness, where the incidence is normal or very nearly so, the fringe spacing obtained is accepted as equivalent to half the wavelength of the monochromatic light employed. However, in the case of interference microscopes, with high-powered objective lenses producing strongly convergent incident light, experiments show that the equivalent value of the fringe spacing is approximately 10% greater than half the wavelength, a discrepancy undoubtedly due to the obliquity of the rays. The effect is of importance when using interference microscopes for accurately measuring the heights and depths of shallow features on surfaces.