The variable phase system described is characterized by a wide separation of the deviated and the undeviated light bundles so that phase-modifying or absorbing elements can readily be inserted. The path separation is accomplished by a partially aluminized plate inserted in a direction normal to the optical axis, thus facilitating the necessary alignment. Geometrical coincidence has been obtained by introducing into one of the pathways a penta-type mirror system.
JURGEN R. MEYER-ARENDT, "Novel System for Obtaining Variable Phase Contrast," J. Opt. Soc. Am. 50, 163-165 (1960)