PHASE VERTICAL-ILLUMINATION MICROSCOPE 145 



5 between the conjugate and complementary areas and that the increased 

 contrast of the images of the inclusions was due to a decrease in |5|. 

 The two stages of undercorrection affected the appearance of the bright- 

 contrast images produced with plate 0. 19A +0.25X in a similar way except 

 that the contrast of the inclusions was not so greatly enhanced as the 

 objective was undercorrected. 



The diffraction plate was removed from the objective and the ob- 

 jecti^'e was overcorrected first by an amount equivalent to an increase of 

 0.05 mm in the thickness of the cover glass (as measured from the thick- 

 ness of the standard cover glass for which the objective was initially 

 adjusted to obtain Fig. III. 7) and then by an amount equivalent to an 

 increase of 0.08 mm in thickness from that of the standard cover glass. 

 Figures IILS.S and III. 8. 6 were obtained. The oil pools then ap- 

 peared in dark contrast and the definition was good. It is interesting 

 to compare Fig. III. 7. 9. When the diffraction plate designated by 

 0.15A-0.25X, N.A. 0.52-N.A. 0.3G, was placed in the objective at these 

 two stages of overcorrection, the images seen in Figs. III. 8. 7 and III. 8. 8 

 were formed. The superposition of the first step in overcorrection 

 caused the particles A' to appear in bright contrast and out of focus 

 when the edges of the oil pools appeared sharpest, although all the images 

 appeared blurred. Superposition of the next step in overcorrection 

 caused a blue haze to obscure much of the detail. The overcorrection 

 produced similar deterioration of the bright-contrast images formed 

 when the diffraction plate 0.19A+0.25X was placed in the objective at 

 the two stages of overcorrection. This illustrates the fact that the 

 choice of the proper cover-glass thickness as well as the avoidance of a 

 thick layer of mounting material between the specimen and the cover 

 glass can be important for good image formation with a dry phase ob- 

 jective of relatively high numerical aperture. It also follows that the 

 correct tube length is then of consequence. 



6. THE PHASE VERTICAL-ILLUMINATION MICROSCOPE 



The method of phase microscopy has been adapted to vertical-illumi- 

 nation or incident-light microscopes for observing specularly reflecting 

 specimens (Jupnik, Osterberg, and Pride, 1946, 1948; Cuckow, 1947, 

 1949; Taylor, 1949; Benford and Seidenberg, 1950). Figure III. 9 

 illustrates the optical system used by Jupnik et al. (1948) in their 

 experimental test of the applicability of the method. A diaphragm, D, 

 having an annular opening served as the entrance pupil of an optical 

 system consisting of a field lens, the microscope objective, and the re- 

 flecting surface of the specimen. The light source was imaged on di- 

 aphragm D by the condenser lens. The field lens and the objective 



