28 AN ELEMENTARY THEORY OF PHASE MICROSCOPY 



wavelength with respect to the optical path of the complementary area 

 of the diffraction plate. 



The early phase microscopes were constructed with diffraction plates 

 whose conjugate area consisted of a suitably shaped groove etched to a 

 sufficient depth in a glass plate to reduce the optical path of the groove 

 by }/i wavelength with respect to the unetched poi'tion of the plate. 

 By etching the groove more deeply and by filling it with a clear cement 

 of higher index than that of the glass plate, the optical path of the groove 

 could be increased by }/i wavelength with respect to the unetched portion 

 of the plate. In accordance with the conclusions stated above, a particle 

 whose optical path was slightly greater than that of the surround ap- 

 peared brighter or darker than the surround according as the etched 

 groove was filled or unfilled. Although it is possible to find particles 

 that appear in excellent bright or dark contrast when the conjugate and 

 complementary areas of the diffraction plate are treated by etching or 

 by deposition of refracting materials so that they differ in optical path by 

 }/i wavelength, it was soon discovered that the contrast in the image of 

 most particles can be improved greatly by adding an absorbing material 

 to the conjugate area. 



The reason for the above-mentioned improvement in contrast can be 

 understood from Figs. II. 6 and II. 7. It has been noted that the deviated 

 D wave vanishes with the optical path difference A and has an amplitude 

 that is small compared with the amplitude of the undeviated *S wave 

 when the optical path difference A between the particle and surround is 

 small and when the particle and surround are equally transparent. 

 Under these circumstances the wave that is deviated from its course by 

 diffraction at the object is much weaker, as is to be suspected, than the 

 wave that is undeviated by diffraction at the object. With reference 

 first to Fig. II. 7, suppose that the amplitude of the undeviated wave is 

 reduced by passage through an absorbing material placed upon the 

 conjugate area to the amplitude of the deviated wave. The amplitudes 

 of the D and *S waves of Fig. II. 7 are now to be drawn alike, as in Fig. 

 II. 8. These two waves are, therefore, equal and opposed and so an- 

 nihilate one another by destructive interference; thus their resultant is 

 zero. Because the particle is illuminated by the vanishing resultant 

 wave whereas the surround is illuminated by the S wave of reduced 

 amplitude, the particle should appear black against a surround of 

 reduced brightness. Provided that the original source of illumination is 

 strong enough, the particle should appear in greatly improved dark 

 contrast. This phenomenon, which is predictable from the simplified 

 theory, is in qualitative agreement with experiment. 



With reference to Fig. II. 6, suppose again that the amplitude of the 



