Zernike, F.
How I discovered phase contrast.  Science 121: 345-349 (1955).

Phase contrast was not discovered while I was working with a microscope, but while I was working in a different part of optics. It originated in my interest in diffraction gratings, which began about 1920. A diffraction grating consists of a plane or concave mirror with a large number of equidistant grooves ruled on its surface. Small nearly unavoidable imperfections in the location of the grooves show clearly in the optical behavior of the grating. The most conspicuous error is a periodic one that repeats itself after each revolution of the screw of the ruling engine. The regularly recurring displacement of the grooves causes corresponding changes of the optical path, just as if the mirror surface were wavy. Consequently the instrument behaves as if a coarse grating, with a constant of about 2 mm, were superimposed on it, with the result that each strong spectral line is accompanied to its right and left by a number of weak spurious lines, the so-called "Rowland ghosts." These lines have a remarkable effect if one looks down at the surface of the grating, placing the eye at the position of a spectral line. A perfect grating would in this case show an evenly illuminated surface in the color of the spectral line. In reality, however, one sees a strongly striped surface.