VARYING THE CONJUGATE AREA 139 



the extent of either the halo or the darkening. A conjugate area hg.ving 

 this inchided range of numerical aperture was not, in general, a satis- 

 factory choice for this objective. A residual bluish haze, which could 

 not be lessened by a different adjustment in separation between the 

 lenses of the objective, made the definition seem poorer than when any 

 of the other diffraction plates in this series were in the objective. The 

 definition was made worse by placing a filter which transmitted blue 

 light in front of the lamp, was im])roved somewhat by substituting a 

 filter which transmitted green light, and was considerably improved by 

 illuminating the specimen with red light. 



If the corresponding non-phase photomicrographs in Figs. III. 7. 2, 

 III. 7. 4, and III. 7. 6 are compared, it is not possible to decide whether 

 the cone of light N.A. 0.52-N.A. 0.30 or the cone N.A. 0.52-N.A. 0.36 

 makes the specimen more visible. Those boundaries of the oil pools 

 which contain no inclusions are hardly visible. The narrowest illuminat- 

 ing cone, X.A. 0.52-N.A. 0.43, caused the oil pools to appear in greater 

 contrast than did the wider cones. When the phase condenser dia- 

 phragms were removed and the specimen was illuminated through a 

 central opening corresponding to N.A. 0.36, the visibility of the oil 

 pools and the inclusions was greatly increased (Fig. III. 7. 7) in com- 

 parison with the pre\'ious non-phase observations; however, a greater 

 number of diffraction rings were visible around the smallest particles. 



The rest of the phase photomicrographs included in Fig. III. 7 show 

 the change that occurred in the images of the oil pools when the outer 

 diameter of the conjugate annulus became smaller. The conjugate area 

 of the diffraction plate inserted in the objective to obtain Fig. III. 7. 13 

 was such that the outer diameter of the annulus coincided with the 

 inner diameter of the conjugate area of the diffraction plates which are 

 described under Figs. III. 7. 5 and III.7.12. The conjugate area related to 

 Fig. III. 7. 13 was slightly wider than the conjugate area related to Figs. 

 III. 7. 5 and III. 7. 12. The excessive haze which was apparent in the 

 images of Figs. III. 7. 5 and III.7.12 disappeared. Diffraction plate 

 0.27A-0.25X, N.A. 0.43-N.A. 0.33, produced a halo which was slightly 

 less in extent than the halo observed with diffraction plate 0.15A — 0.25X, 

 N.A. 0.52-N.A. 0.36. In Fig. III. 7. 13 particle E appears uniformly 

 dark. A small region near the middle of the image of F and that of G 

 was not so dark as the edges, but it was much darker than the image 

 of the surround. The decrease in darkness from the boundary to the 

 center of the images of particles F and G was less than that observed 

 with the diffraction plate related to Fig. III.7.3. In Fig. III. 7. 13 there 

 was no reversal of contrast in the image of particle groups K, L, M, N, 

 R, and P. However, the diffraction rings around particles such as R 



