130 INSTRUMENTATION 



definition of the edges of the oil pools as was feasible in the phase 

 contrast image with the diffraction plate having a conjugate area lim- 

 ited by N.A. 0.52-N.A. 0.36 in the objective. Inspection of the objec- 

 tive itself in an interferometer illuminated by a narrow band of wave- 

 lengths in the neighborhood of X = 5461 A showed the presence of one 

 narrow fringe, due to spherical aberration, around the extreme edge of 

 the aperture. The conjugate area of each diffraction plate lay well 

 within the uniform area seen in the interferometer. The specimen was 

 observed with white-light illumination, but the general results were still 

 valid if any one of the Kodak series of Wratten M filters for photomicrog- 

 raphy was placed in front of the lamp. The final magnification in the 

 photomicrographs is 450 X . 



The size of the cone of light illuminating the specimen has some effect 

 on the visibility of the specimen when no diffraction plate is present in 

 the objective. Therefore, photomicrographs were also taken with the 

 diffraction plate removed but with the condenser diaphragm remaining 

 in position. When a photomicrograph was taken with a diffraction 

 plate in the objective, the designation of the diffraction plate according 

 to the notation in Section 2.5 is printed below each picture, and the 

 range of numerical aperture included by the conjugate area of the 

 diffraction plate is also given. If only the range of numerical aperture 

 appears below a photomicrograph, this indicates that the diffraction 

 plate had been removed from the objective but that the diaphragm 

 corresponding to this plate remained at the first focal plane of the sub- 

 stage condenser. The photomicrographs obtained with such conical 

 illumination are shown alongside the corresponding phase photomicro- 

 graphs. For further comparison a photomicrograph, Fig. III. 7. 7, 

 was taken with central conical illumination when the aperture of the 

 condenser had been reduced to N.A. 0.36 and no diffraction plate was 

 mounted in the objective. Neutral Inconel filters of different trans- 

 mission were placed near the substage mirror and interchanged so that 

 the same exposure time for each photomicrograph would result in 

 negatives of closely the same background density. 



The sketch of Fig. III. 6 represents the system of particles as seen 

 with a low-contrast phase objective at greater magnification than was 

 produced by the objective of 4-mm focal length (see also Fig. III. 7. 22). 

 Letters are placed near the image of a particle or group of particles in 

 order to identify them. These particles fall into four groups of interest. 

 The following comments apply only to the dark phase contrast images. 

 Particles of the size and shape of A, B, C, and D never appear uniformly 

 dark regardless of the characteristics of the conjugate areas of the 

 diffraction plates. Whether particles such as E, F, and G appear 



