With the phase-contrast method scientists usually observe unstained objects or very thin cells. The procedure is really nothing more than the consideration of an interference image. That is, the light beam split (full Koehler illumination and phase contrast condenser required) with an annular aperture. So the light is partially passing through the medium and partially driven past it. Depending on the structure and thickness of the medium it produces a phase difference with the background light.

There is an excellent description of phase contrast microscopy on the website of the National High Magnetic Field Laboratory of the Florida State University:

http://micro.magnet.fsu.edu/primer/techniques/phasecontrast/phaseindex.html

You can try online working with a virtual phase-contrast microscope (Java applet) on the following link of the National High Magnetic Field Laboratory of the Florida State University:

Virtual Phase-contrast Microscope
http://micro.magnet.fsu.edu/primer/java/phasecontrast/phasemicroscope/index.html