138 INSTRUMENTATION 



always being brighter than the areas near the boundaries. (Unless 

 explicit reference is made to inclusions, the variation in contrast caused 

 by their presence is omitted from the general description of the appear- 

 ance of the images of the larger oil pools.) The brightness at the center 

 of the images of .4, B, and C seemed to be approximately equal to the 

 brightness of the image of the surround beyond the regions occupied by 

 the halos. However, as the conjugate area became narrower, the dark 

 region near the edge of the image of the particle extended farther toward 

 the center. The manner in which the V-shaped lower edge of B darkened 

 as the diffraction plate was changed is of interest. As the darker region 

 extended farther toward the center of B, the darkening spread across the 

 entire bottom corner, and in Fig. III. 7. 5 the darker border in the image 

 does not follow the contour of B as closely as it does in the other two 

 phase photomicrographs. This is an example of the fact that neighbor- 

 ing discontinuities in the optical path of the object specimen can interact 

 to affect the amount of contrast present in an image formed by the phase 

 microscope. Therefore caution must be used in trying to interpret 

 amount of contrast in terms of relative optical path differences in the 

 object specimen. 



Particle E appeared clearly non-uniformly dark across its width when 

 the diffraction plate with the widest conjugate area was in the objective. 

 The diffraction plate having the conjugate annulus of intermediate 

 width caused E to appear uniformly dark, and the narrowest conjugate 

 annulus also made E appear uniformly dark. Particles G and F ap- 

 peared uniformly dark only when the diffraction plate with the narrowest 

 conjugate annulus was used. The particles in H, M, and N (inclusions 

 excepted) appeared uniformly black with all three diffraction plates. 

 The narrowest conjugate area produced partial reversal of contrast in the 

 images of the small areas in P, R, and L that have been referred to as 

 inclusions; otherwise particles in these groups appeared uniformly dark. 



Figures III. 7. 9, III. 7. 10, and III. 7. 11 show the appearance of the 

 oil pools when the dimensions of the conjugate area were given by 

 N.A. 0.52-N.A. 0.36, as in Fig. III. 7. 3, but the energy transmission of the 

 conjugate area was varied. As the transmission decreased, the halo 

 around each image became more visible although it did not appear 

 to change appreciably in area. Also, the area over which darkening 

 occurred did not seem to be appreciably affected by the transmission of 

 the conjugate area. The effect which this variation in the energy trans- 

 mission had upon the small areas considered to be inclusions has already 

 been discussed. Figure III. 7. 12 was obtained when the absorption 

 in the conjugate annulus having the same dimensions as those indicated 

 below Fig. III. 7. 5 was increased. There were no apparent changes in 



