Phase (different interference) Contrast microscopy:

Living cells are mostly   transparent. For viewing under ordinary light microscope, therefore, live cells must be stained with vital stains, or these must be fixed and then stained with acid or basic dyes. Zernicke (1940) designed a phase contrast microscope for viewing unstained living cells. This microscope has the same resolving power as the ordinary light microscope. but takes advantage  of the fact that light rays pass through  various cells parts at different velocities due to varying thickness and densities  of  the parts. In other words, various cell parts have different refractive indices i.e. they refract or bend the light rays to different extents. Obviously, light waves emerging from different cells parts are out of phase with each other. A phase contrast microscope converts these differences in phase to differences in light intensity, producing an image with good contrast, much of what we know about cell division, cell under phase contrast microscopes.

Merton (1947) Ambrose, Dyson, Smith and others have contributed in designing interference microscopes based on principles similar to those of phase contrast microscopes, but with an added advantage of giving quantitative data of the measurement of dry weight of various cell parts.