Phase microscopy has been adapted for observing specularly reflecting specimens with the vertical illuminator. The specimen is illuminated by light, preferably monochromatic, issuing from an annular opening in a diaphragm placed at the entrance pupil of an optical system consisting of the field lens, the microscope objective, and the reflecting surface of the specimen. The diaphragm is imaged on a diffraction plate placed at the exit pupil of this optical system. Stray light and illumination losses are minimized by locating the exit pupil between the beam splitter and the eyepiece. The method can be used advantageously to examine those surface irregularities which give rise to small differences in optical path or in phase between the reflected-ray bundles. Both bright and dark contrast with excellent definition may be obtained by the use of suitable diffraction plates. The advantage of this method disappears when the specimen diffuses the incident light.
HELEN JUPNIK, HAROLD OSTERBERG, and G. E. PRIDE, "Phase Microscopy with Vertical Illumination," J. Opt. Soc. Am. 38, 338-342 (1948)