THE TECHNICS OF PHASE MICROSCOPY 



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effect in developing increases the contrast of the specimen edge and halo 

 with dark contrast which aids measurement, although the same numeri- 

 cal results are obtained with both contrasts. With some specimens the 

 same detail is shown with both contrasts and the choice becomes purely 

 a matter of personal preference. Other specimens reveal detail with 



Fig. IV. 1. Glass fragment mounted in balsam, 150 X. (Refractive indices 1.52 

 and 1.54.) A, brightfield. B, darktield. C, bright-contrast phase. D, dark- 

 contrast (B — ) phase. E, dark-contrast (A — ) phase. 



one contrast that is not seen equally well with the other contrasts, and 

 several diffraction plates may be required to fully comprehend the nature 

 of the specimen (Bennett et al., 1946; Oliver, 1948; Richards, 1947c). 

 Of the two types of dark-contrast diffraction plates the B— is often 

 better with slightly absorbing materials. Lower contrasts (20%) are 

 better with larger specimens and higher contrast (7%) for smaller 

 specimens. The quarter-wave retardation is frequently most useful, 

 although more or less retardation may be required. 



With the phase microscope the best focus is the exact focus, in con- 

 trast to the necessity in brightfield microscopy of defocusing to see 

 specimens of low contrast (Linfoot, 1945). Also, the phase microscope 



