The following are the main improvements that we have made in the method of phase-contrast microscopy described by Kempson, Thomas, and Baker (1948):

1. No bull's-eye condenser is used. The illuminant is an electric bulb with a ‘porcelain-processed’, ‘flashed white’, or ‘opal’ surface.

2. No oiled paper is placed over the illuminating annulus.

3. The thickness of the deposit of magnesium fluoride on the phase-plate is controlled by observations on the interference colours given by surface reflections.

4. Positive (dark) phase-contrast is preferred for most purposes to negative (bright).

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