VARYING THE CONJUGATE AREA 143 



aperture and given inekitletl difference in numerical aperture produced 

 a halo that extended farther into the image of the surround, but the 

 aberrations of the objective and the presence of pronounced diffraction 

 patterns complicated the observations. The fact that the halo exists 

 as an artifact does not limit the usefulness of phase microscopy. Care 

 must be taken in trying to interpret amount of contrast in terms of 

 optical path differences between the structural details within a specimen. 



It has been explained that spherical aberration can affect the per- 

 formance of a diffraction plate because the aberration also produces 

 relative changes in phase over the wave front as it passes through the 

 objective. The phase changes introduced by spherical aberration are 

 not abrupt changes. However, if a condenser diaphragm with an 

 annular opening is placed at the first focal plane of the substage con- 

 denser, it may be possible to introduce a net change in phase between 

 the deviated and the undeviated light by sufficiently overcorrecting or 

 undercorrecting the objecti\'e. Some degree of phase contrast should 

 be observed in such a case although the definition may be less good than 

 if a step-type diffraction plate is used in a well-corrected objective. 



The photomicrographs reproduced in Fig. II 1.8 show the result of 

 undercorrecting or overcorrecting the objective used to obtain Fig. III. 7. 

 The amount of spherical aberration introduced by altering the separation 

 between two lens components in the objective is most easily described 

 in terms of the equivalent effect that can be produced by leaving the 

 objective adjusted according to design for a standard cover-glass thick- 

 ness but by changing the thickness of the cover glass. The condenser 

 diaphragm contained an annular opening described by N.A. 0.52- 

 N.A. 0.3G. The specimen consisted of the same group of oil pools 

 photographed in Fig. III. 7. 



Fig. III. 8. Photomicrogniphs illustrating changes produced in the image of the 

 oil pool specimen by altering the spherical aberration of the ol)jective. The cone of 

 illumination was defined byN. A. 0.52-N.A. 0.3(5. The undercorrection or overcorrec- 

 tion is expressed in terms of a corresponding deviation fromstandard cover-glass thick- 

 ness. (1) Undercorrection equivalent to a decrease of 0.03 mm in cover-glass thick- 

 ness. (2) Undercorrection equivalent to a decrease of 0.07 mm in cover-glass 

 thickness. (3) Undercorrection equivalent to a decrease of 0.03 mm in cover-glass 

 thickness superimi)osed on diffraction plate 0.15A — 0.25X. (4) Undercorrection 

 equivalent to a decrease of 0.07 mm in cover-glass thickness superimposed on dif- 

 fraction plate 0.15A— 0.25X. (5) Overcorrection equivalent to an increase of 0.05 mm 

 in cover-glass thickness. (6) Overcorrection equivalent to an increase of 0.08 mm 

 in cover-glass thickness. (7) Overcorrection equivalent to an increase of 0.05 mm in 

 cover-glass thickness superimposed on diffraction plate 0.15A — 0.25X. (8) Over- 

 correction equivalent to an increase of 0.08 mm in cover-glass thickness superimposed 

 on diffraction plate 0.15A-0.25X. 450 X. 



