Phase contrast microscopy is by now a familiar subject in the field of transmitted light microscopy. Its application in reflected light is more recent. Early systems for reflected light phase contrast either located the phase annulus directly in the objective focal plane, or else compromised on this desired location and placed it beyond the vertical illuminator, somewhere between aperture and field planes in the microscope. This paper describes a different approach to the problem, resulting in an instrument design which obviates the elaborate equipments required by the above-mentioned systems, requires only one phase annulus and one illuminating annulus for all objectives, and in which the phase annulus is located in the ideal aperture plane of the microscope.
JAMES R. BENFORD and RICHARD L. SEIDENBERG, "Phase Contrast Microscopy for Opaque Specimens," J. Opt. Soc. Am. 40, 314-316 (1950)