144 INSTRUMExXTATION 



The result of undercorrecting the objective by an amount equivalent 

 to a decrease of 0.03 mm in the thickness of the cover glass is seen in 

 Fig. III. 8.1. Figure III. 8. 2 shows the appearance of the oil pools when 

 the objective was further undercorrected by an amount equivalent to 

 an additional decrease of 0.04 mm in the thickness of the cover glass. 

 The first adjustment indicated a tendency for bright contrast to occur 

 when the objective was undercorrected, and the second adjustment pro- 

 duced a bright-contrast image. Inspection of the objective in an inter- 

 ferometer at the second stage of undercorrection showed that the part 

 of the wave front which passed through the region corresponding to the 

 conjugate area of the objective was retarded with respect to the part 

 of the wave front passing through the center and outermost area of 

 the objective. 



Figures III. 8. 3 and III. 8. 4 were obtained by inserting diffraction 

 plate 0.15A-0.25X, N.A. 0.52-N.A. 0.36, in the objective at these two 

 stages of undercorrection. The first step in undercorrection caused no 

 visible deterioration in the definition of the edges of the oil pools (Fig. 

 III. 8. 3). However, when the second step in undercorrection was in- 

 troduced, the field was covered by some haze which was visible to the 

 eye but which is only slightly evident in the photomicrograph, Fig. 

 III. 8. 4, since the haze was superimposed on a high-contrast image. If 

 more complicated structures are imaged in low contrast, such haze can 

 obscure some of the detail. The contrast of the very small areas which 

 have been considered as inclusions increased relative to the contrast of 

 the edges of the oil pools as the two successive stages of undercorrection 

 were superimposed on the diffraction plate. In fact, the inclusions were 

 more distinct in the image corresponding to Fig. III. 8. 4 than they were 

 in any of the images corresponding to the phase photomicrographs in- 

 cluded in Fig. 1 1 1. 7. With an additional small amount of undercorrec- 

 tion when the diffraction plate was in the objective, the boundaries of 

 the oil pools appeared blurred because of the overlying haze although 

 the inclusions were still clearly evident. 



Since it is to be expected that the choice of compromise focus differs 

 as the spherical aberration of the objective is altered or as the objective 

 is used with or without a diffraction plate coating, it is difficult to predict 

 from the theoretical spherical aberration curves for the objective what 

 effective phase difference is introduced between the deviated and un- 

 deviated bundles of rays when an objective is either undercorrected or 

 overcorrected and then used either with or without a diffraction plate 

 coating. It cannot be stated with certainty here that superimposing 

 upon the diffraction plate the spherical aberration due to undercorrection 

 had the effect of decreasing the magnitude of the optical path difference 



