Abstract
Slight optical non-homogeneities in certain transparent specimens are usually rendered visible by putting the specimen out of focus. One then sees not the non-homogeneities themselves, but the focal spots and lines they produce. Zernike's method of phase-contrast illumination, on the other hand, provides the maximum contrast when the specimen is accurately in focus, so that what is seen bears a closer resemblance to what is "really there". A simple method of adapting an ordinary microscope for phase-contrast is described, together with the preparation of a test-slide to check its performance. The method has proved helpful in studies of the interaction between leucocytes and mobile organisms.