Bennett, A. H., Jupnik, H., Osterberg, H. and Richards, O. W.
Phase microscopy.  Transactions of the American Microscopical Society 65: 99-131 (1946).

The difficulties of seeing specimens whose structure varies in optical path2 but not in light transmission present an old problem in microscopy. When the optical path differences between neighboring portions of the structure are large so that the gradients in the optical path are high, several methods. e.g, oblique and dark field illumination become effective. The recent method of phase microscopy is advantageous for viewing structures which have small differences in optical path. At the other extreme the microscopic field may show small differences in light transmission only which can be amplified in the image by the method of phase microscopy. In many cases unwanted details can be suppressed and others emphasized. These details may involve amplitude (transmission) or phase (optical path) differences or both. The method is effective over a wide range in both phase and transmission differences. Corresponding to a given phase and transmission difference in the specimen suitable phase and absorption distributions are introduced elsewhere in the optical system in order to alter the contrast in the image. This method of varying contrast in the image by utilizing absorption and optical path differences in the optical system is called phase microscopy.