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FRITS ZE R N I K E

How I discovered phase contrast
Nobel Lecture, December 11, 1953

« Phase contrast » was not discovered while working with a microscope, but in a different part of optics. It started from my interest in diffraction gratings, about from 1920 on. Such a (reflecting) grating consists of a plane or concave mirror with a large number of equidistant grooves ruled on itssurface. Small nearly unavoidable imperfections in the location of the grooves show clearly in the optical behaviour of the grating. The most conspicuous error is a periodic one which 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 millimeters, were superimposed on it, so that each strong spectral line is accompanied to right and left by a number of weak spurious lines, the so-called « Rowland ghosts ». These 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 perfectgrating would in this case show an evenly illuminated surface, in the colour of the spectral line. In reality, however, one sees a strongly striped surface. At the end of a 1902 paper, H. S. Allen remarked that these stripes were nothing real, but simply the effect of the interference between the principal line and its ghosts. Indeed the stripes disappear when the ghosts are covered up. I rememberstrongly objecting against his conclusion of unreality. On the contrary I was convinced that the striped surface gave more information about the periodic ruling errors than that obtainable by photographing the ghosts, because in the first case the relative phases of the gosts come into play, whereas these are lost in the second case. I kept the question in mind, planning to look further into it assoon as an opportunity would arrive. About 1930 our laboratory had obtained a large concave grating and set it up in a Runge-Paschen mounting. The striped appearance of the surface was soon found, but as the grating was 6 meters from the eye, I tried a small telescope pointed at the grating. Then the unexpected happened. The stripes were seen very clearly, but disappeared as the telescope wasexactly focussed

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on the surface of the grating! By a succession of experiments and calculations I soon succeeded in explaining this. On looking back to this event, I am impressed by the great limitations of the human mind. How quick are we to learn, that is, to imitate what others have done or thought before. And how slow to understand, that is, to see the deeperconnections. Slowest of all, however, are we in inventing new connections or even in applying old ideas in a new field. In my case the really new point was that the ghosts differed in phase from the principal line. Now it is common knowledge that in all interference phenomena differences of phase are all-important. Why then had phases never been considered before in this case, nor in the correspondingone in the microscope? Some excuse may be found in the difficulty to define them exactly. Let me explain this for a more simple case, the diffraction image of a slit. The way to observe this may be as follows. A telescope is pointed at a vertical line-source of light, such as the filament of an incandescent lamp. A vertical slit of say 2 mm width is placed closely behind the objective of thetelescope. This causes the image of the source to be broadened out into a diffraction pattern: a bright central stripe (order zero) is accompanied on both sides by weaker and weaker secondary maxima (orders one, two, etc.). The formula for this diffraction pattern is given in the textbooks, the amplitude being determined by the function sin x/x. In the few cases where the phases are mentioned in the...