Zernike, F.
Phase contrast, a new method for the microscopic observation of transparent objects. Part I.  Physica 9: 686-698 (1942).

Every microscopist knows that transparent objects show light or dark contours under the microscope in different ways varying with change of focus and depending on the kind of illumination used. Curiously enough the wave theory of light has never been explicitly applied to the case of absolutely transparent objects, the details of which only differ in thickness or refractive index. This is carried through in the first part of the present paper, in which it is found that-the theory is capable of explaining every detail of the microscopic aspect of transparent objects. At the same time a new method of observation is at once suggested by the results of our discussion. In Part II the effect of this method, which we call Phase Contrast, will be explained on a different line, which unlike the first treatment is not restricted to objects of simple structure such as gratings. Finally the construction of the requisite optical parts will be described and the appearance of a few objects by the new method as compared to the older methods shown by photomicrographs.