VARYING THE CONJUGATE AREA 133 



Figure III. 7.3 was obtained with diffraction plate 0.15A— 0.25X, 

 N.A. 0.52-N.A. 0.36, which produced a high-contrast image of the oil 

 pools. With this plate particles K and the other inclusions appeared 

 darker than the edges of the larger particles. When the energy trans- 

 mission of the conjugate area of the diffraction plate was reduced to 6%, 

 partial reversal of contrast in the image of particles K was observed, 

 and the images of some of the small areas in groups P, R, and L showed 

 reversal of contrast (Fig. III.7.1 1 ). There was no indication of reversal 

 of contrast in the images of the edges of the oil pools. When the 

 conjugate area of the diffraction plate contained no absorbing material, 

 the inclusions again appeared dark and were more easily detected 

 (Fig. III. 7.9) than with the diffraction plate described by 0.15A-0.25X. 

 However, particles K and the other small inclusions were even more 

 clearly visible with the diffraction plate designated by 0.32A — 0.25X. 

 N.A. 0.52-N.A. 0.36 (Fig. III.7.10). 



The simple theory would indicate that, since these inclusions appear 

 darker than the edges of the oil pools with the diffraction plates having 

 the higher transmissions in the conjugate area (in particular, even with 

 the non-absorbing diffraction plate), the optical path through the in- 

 clusions is greater than the average thickness, in terms of optical path, of 

 the edges of the larger oil pools. However, the simple theory does not 

 exclude the possibility that the inclusions were also partially absorbing. 

 When the bright-contrast diffraction plate 0.19A+0.25X, N.A. 0.52- 

 N.A. 0.36, was inserted in the objective, the images of the oil pools 

 and the inclusions were bright (Fig. III. 7. 8), and the areas which formerly 

 were darkest (except when contrast reversal occurred with the 

 0.06A— 0.25X plate) now appeared brightest. According to the simple 

 theory, this does not contradict the evidence that the inclusions had a 

 greater optical path than did the edges of the larger oil pools, but again 

 it also allows the possibility that some absorbing material is present in 

 these small brightest areas. Thus the qualitative information obtained 

 with this series of diffraction plates having a 3^-wavelength optical 

 path step is not sufficient to determine conclusively whether the in- 

 clusions represent regions of greater optical path only or whether partial 

 absorption is present. 



As stated previously', the series of observations represented by the 

 photomicrographs in Fig. 1 1 1. 7 was made chiefly to study the dependence 

 of both the halo and the darkening of the images of the particles on the 

 dimensions and energy transmission of the conjugate area. Because 

 the halo is more pronounced when the amount of contrast is high, 

 diffraction plates with relatively high absorption in the conjugate 

 area were chosen for most of the observations. In any single phase 



