Some extensions of the simple theory of phase-contrast microscopy are considered. It is emphasized that transparency, rather than thickness, is the limiting factor for the successful employment of the method. Certain transparent insect larvae (Chaoborus, Chironomus) can be observed in the living state by phase-contrast illumination.

The statement that the method is of no value for the examination of fixed and stained sections is based on consideration of an ideal physical case. In practice the method may be a valuable adjunct to routine examination of such material. Examples are given of the application of phase-contrast microscopy to normal and pathological stained sections.

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