VARYING THE CONJUGATE AREA 141 



of light in the halo around the images of the largest particles was being 

 affected by the superposition of darker diffraction bands. The images 

 of particles E, F, and G appeared uniformly dark, and the image of D 

 was nearly so. This last diffraction plate produced a greater degree of 

 uniformity in darkness across the image of particle A than did any of 

 the other diffraction plates. Figure III. 7. 22 is included to show the 

 particles at 1000 X. It was obtained with an oil immersion phase ob- 

 jective of 1.8-mm focal length. 



Figures III.7.U, III.7.16, III.7.19, and III.7.21 are non-phase photo- 

 micrographs showing the effect of changing the dimensions of the cone 

 of illumination to correspond to the last four changes in the size of the 

 conjugate area. The visibility of the oil pools was greater with the 

 opening given under Fig. III. 7. 14 than with that cited under Figs. 

 III.7.2 and III.7.4. Figure III. 7. 16 is similar to Fig. III. 7. 6. The 

 two smallest annular openings in the condenser diaphragm resulted in 

 an image similar to that formed with the central aperture N.A. 0.30, 

 Fig. III. 7. 7. More diffraction rings could be counted around the small- 

 est particles when the illuminating cones N.A. 0.31-N.A. 0.17 and 

 N.A. 0.26-N.A. 0.18 were used than when the illuminating cones had a 

 higher mean numerical aperture. 



The influence of the residual aberrations of the objective on either 

 the formation of the phase contrast image or on the appearance of the 

 halo as the difTraction plates were changed is not known. If an ob- 

 jective contained a diffraction plate which had an annulus of the size 

 N.A. 0.52-N.A. 0.36 and which produced medium or high dark contrast, 

 then the addition of a corrector plate to decrease the spherical aberra- 

 tion of the objective did not improve noticeably the definition of a phase 

 contrast image. However, if the diffraction plate produced low contrast, 

 then less residual haze was apparent over the field of view when the 

 corrector plate was inserted. 



The phase photomicrographs of Fig. III. 7 indicated the following 

 trends. If the outer numerical aperture of the conjugate area remains 

 constant, a narrower conjugate annulus forms an image in which the 

 halo extends farther into the image of the surround and in which a 

 wider area adjacent to the boundary (i.e., one that extends closer to 

 the center of the object specimen) of a large, homogeneous, uniformly 

 thick particle appears in either bright or dark contrast. If the mag- 

 nitude of the difference in numerical aperture included within the con- 

 jugate annulus is fixed and small, then a conjugate area of lower mean 

 numerical aperture can form an image in which a wider region adjacent 

 to the boundary of the particle appears in dark or bright contrast. There 

 was some evidence that the conjugate area of greater mean numerical 



