OPTICAL FUNDAMENTALS 11 



equal. Generally, the deviated light, D, is much weaker than the direct 

 light, S, and since we cannot increase the intensity of *S' without also 

 increasing D proportionately we have available only the alternative of 

 decreasing the intensity of the direct light, *S'. This decrease is accom- 

 plished by placing some absorbing material on the diffraction plate in 

 the path of the light S. In many specimens suitable visibility of the 

 desired details requires control of the contrast in the image in order to 

 avoid too strong or too weak a contrast. This requirement has led to 

 the use of a series of diffraction plates having different absorption and 

 retardation characteristics or to the changing of either the absorption 

 or retardation or both in the diffraction plate by methods discussed in 

 Chapters II and III. 



Although the foremost function of the microscope is to make small 

 particles visible, the phase microscope, particularly one in which the 

 absorption and retardation of the diffraction plate can be varied, is a 

 useful instrument for measuring properties of the specimen (Osterberg 

 and Pride, 1950). 



Since the specimen diffracts light by virtue of its light-absorbing 

 and/or light-retarding properties, phase microscopy is of value in the 

 e.xamination of specimens which, in addition to introducing small optical 

 path differences, also weakly absorb the light. Such specimens have a 

 combination of density and phase contrasts, designated in Chapter IV 

 as densiphase contrast. 



In the literature of Zernike and Kohler and Loos, the term "phasenkon- 

 trast" denotes the kind of microscopy discussed in this book. This 

 term, translated as "phase contrast," is much used. Since the above 

 authors refer to phase contrast, as distinguished from amplitude con- 

 trast, in the specimen, it seems likely that the designation "phase 

 contrast microscopy" really means "the microscopy of specimens pos- 

 sessing phase contrast." In England the earlier writers on this subject 

 refer to it as "phase difference microscopy," presumably because phase 

 differences exist in the light traversing various portions of the specimen. 

 In the United States the authors at the outset of their work applied the 

 simpler term "phase microscopy." They preferred to base the name 

 on the uniciue properties of the apparatus rather than on one of the 

 optical properties which a specimen may or may not possess. The 

 microscopic equipment introduces phase changes in the light passing 

 through the microscope, and preferably the observer should have some 

 choice in selecting these phase changes in order to present the greatest 

 variety of aspects of the many kinds of specimens (see Frontispiece). 

 But, regardless of whether the emphasis should be placed on the phase 

 contrast of the specimen or on the phase changes introduced by the 



