ROBERT BARER, M.C., M.A., M.B. University Demonstrator in Anatomy, Oxford University, England. Alan Johnston, Lawrence and Moseley Research Fellow of the Royal Society
ACKNOWLEDGMENTS The writer wishes to express his thanks to all those with whom he has had valuable discussions, particularly Mr. J. Dyson and Dr. J. Philpot. It is a special pleasure to thank Professor F. Zernike for reading the manuscript and making a number of valuable suggestions.
The assistance of the Royal Society, the Nuffield Foundation, and the Smith, Kline and French Foundation is gratefully acknowledged.
The value of a technique depends on two things: the amount of information it can provide and the number of workers to which it is generally available. By these criteria, phase-contrast microscopy must be regarded as probably the most valuable single method at the disposal of the cytologist interested in the study of living material. Other methods may in some cases provide more specific information of a physical or chemical character, but as a rule they require complex apparatus, and can be used only by a few specialist workers in a small number of laboratories. This does not apply to phase-contrast microscopy. The apparatus is basically simple and can be obtained at reasonable cost in fairly standardized form from numerous manufacturers in all parts of the world. It can safely be said that phase contrast is no longer to be regarded as an experimental and untested method but has now reached the stage of a routine tool, the usefulness of which will certainly not diminish with the passage of time. Nor, so far as can be seen at present, is it likely to be entirely displaced by another method. The fundamen-