VARYING THE CONJUGATE AREA 



137 



N.A. 0.79-N.A. 0.56 

 0.20A — 0.25\ 

 Fig. III. 7. Photomicrographs of the object specimen consisting of oil pools in a 

 surround of magnesium fluoride. The dark-contrast phase photomicrographs il- 

 lustrate changes which occurred in the image when the dimensions or the transmission 

 or both of the conjugate area were varied and 5 remained fixed at 5 = — X/4 (X = 

 5461 A). The ordinary photomicrographs show the effect on the image of changing 

 the dimensions of the cone of light incident on the specimen. Figures 7.1-7.21, 



inclusive: 450 X. Fig. 7.22: 1000 X. 



contrast photomicrograph it is evident that the brightness and the 

 extent of the halo vary with the particle size or the amount of detail 

 present. The halo is more pronounced around a large particle or in 

 the neighborhood of a cluster of particles very close together than it is 

 around a small, isolated particle. After an isolated particle has increased 

 to a certain size, the brightness and the extent of the halo appear to 

 remain constant, as does the extent of the darkest area within the image 

 of the particle. 



Figures III.7.1, III. 7. 3, and III. 7. 5 were obtained when the numerical 

 aperture corresponding to the outer diameter of the conjugate area 

 remained the same but the conjugate area was made narrower by 

 increasing the inner diameter. As the conjugate area became narrower, 

 the halo extended farther out into the image of the surround. Particles 

 A, B, C, and D did not appear uniformly dark with any of these three 

 diffraction plates, some part near the center of the images of the particles 



