140 INSTRUMENTATION 



were a little more apparent than when diffraction plates with a conjugate 

 annulus of greater outer diameter were substituted. 



When the conjugate area was formed slightly closer (N.A. 0.40-N.A. 

 0.30) to the optical axis of the objective, Fig. III. 7. 15 was obtained. In 

 the specimen being discussed, there was no noticeable difference in the 

 image whether the diffraction plate related to Fig. III. 7. 13 or to Fig. 

 III. 7. 15 was inserted in the objective except that the conjugate area 

 having the smaller mean diameter increased, by a small amount, the 

 visibility of the diffraction patterns around the smallest particles. , A 

 more critical object specimen with closely spaced detail, such as the 

 chromosome specimen shown in Fig. V.5, indicated that some loss of 

 resolution had resulted from changing the dimensions of the conjugate 

 area from N.A. 0.52-N.A. 0.36 to N.A. 0.40-N.A. 0.30. Figure III.7.17 

 shows the effect of decreasing the energy transmission of the annulus 

 with the dimensions N.A. 0.40-N.A. 0.30. The contrast became greater, 

 and the particles G and F appeared uniformly dark. There was no 

 reversal of contrast in the images of the K, L, and R groups. 



When the image produced with the addition of diffraction plate 

 0.13A-0.25X, N.A. 0.31-N.A. 0.17 (Fig. III. 7. 18), was compared 

 with the image seen with the aid of diffraction plate 0.15A — 0.25X, 

 N.A. 0.52-N.A. 0.36 (Fig. III.7.3), it was observed that definition ap- 

 peared better with the first diffraction plate because there was less haze 

 over the field of view. However, the limitation of the cone of illumina- 

 tion to N.A. 0.31-N.A. 0.17 resulted in a noticeable loss of resolution. 

 A greater number of diffraction rings were easily seen arovmd particles in 

 groups L, M, N, and R. The images of particles R resembled diffi-action 

 patterns consisting of a bright center and one dark outer ring. Narrow 

 dark bands were visible within the region occupied by the halo around 

 the image of particles as large. as those in group H. The extent of the 

 halo around the image of an oil pool in Fig. III. 7. 18 is most nearly like 

 that shown in Fig. III. 7.1. Particle E did not appear uniformly dark, 

 but it appeared more nearly so with diffraction plate 0.13A— 0.25X, 

 N.A.0.31-N.A. 0. 17, than with diffraction plate 0. 1 1 A -0.25X, N.A. 0.52- 

 N.A. 0.30. The images of particles F and G were again non-uniform 

 in brightness, as is seen in Fig. III. 7. 18. 



The last phase photomicrograph in this series, Fig. III. 7. 20, was taken 

 with diffraction plate 0.14A-0.25X, N.A. 0.26-N.A. 0.18, in the ob- 

 jective. The halo extended farther away from the boundary of the 

 image of an oil pool than in Fig. III. 7. 18 but not finite so far as in Fig. 

 III. 7. 5. Again, dark diffraction bands were clearly evident around the 

 images of the particles in //, M, N, P, and R and within the region oc- 

 cupied by the halo. There was some indication that the distribution 



