Deviated light passing through the specimen is retarded by about a quarter of a wavelength and passes through the phase plate without going through the ring. When the beams are recombined further along the light path, the differences in the phase of the deviated and undeviated light beams become additive and subtractive. The resultant wave is sum of the two waves which have their crests and troughs opposite each other and is four times darker than the background. Therefore, the specimen appears four times darker than the background. The net result is that features of the object are either lighter or darker than the surrounding field.
Dark phase is the most common optical method used for viewing living cells.
All of the inverted tissue culture microscopes employ it for quick examination of cells in tissue culture. Tissue culture microscopes should be cleaned periodically and their phase rings adjusted in order to obtain the best possible images of cells. Diagrams redrawn from Gray, P. 1964. Handbook of Basic Microtechnique.