For the use of color phase contrast in microscopy, which was first discussed in the Spring of 1948, several difficult requirements must be met for the attainment of maximum brilliance and usefulness. The system itself is one in which a controlled chromatism of the phase plate is sought instead of the customary control of phase difference at one wave-length only. The thickness and dispersion of the phase ring in relation to the general aperture can be such that zero phase contrast will occur at one or more wave-lengths of transition in color sensation. If the phase contrast is made high but opposite in several spectral zones of relatively pure color sensation, striking and beautiful microscopical images are formed with objects of low inherent contrast. The greatest advantage of the system lies not in the beauty of the images, but in the clear distinction which is afforded between effects caused by scattering or general absorption and those caused by small differences of refractive index or thickness.
CHARLES PROFFER SAYLOR, ARTHUR T. BRICE, and F. ZERNIKE, "Color Phase-Contrast Microscopy: Requirements and Applications," J. Opt. Soc. Am. 40, 329-334 (1950)