SYSTEMS OF VARIABLE PHASE MICROSCOPY 155 



undeviated rays also forms a second image of the condenser diaphragm 

 at the back focal plane of the microscope objective. 



Either method of producing the image of the condenser diaphragm 

 introduces some aberration in the rays passing through CD, but, since 

 in practice the angle a. is small, this aberration should not be trouble- 

 some. The presence of spherical aberration in the microscope objective 

 will affect the quality of the final, magnified phase contrast image in 

 the same way as if the diffraction plate were contained in the objective. 

 If the microscope objective has been corrected for a cover glass 0.17 mm 

 thick, then it is necessary to place a cover glass 0.17 mm thick, or its 

 equivalent, over the image at 0' . Fran^on found that a more practical 

 solution was to form the image oi }i mm above the aperture CD 

 by displacing the interface MGN slightly toward the top of the hemi- 

 sphere and then to cement over CD a planoconcave lens }4 mm thick 

 (0.33 mm + 0.17 mm) and having the same radius of curvature as the 

 hemisphere. 



The device described by Fran^-on can be expected to have several 

 advantages. The image of the condenser diaphragm will be formed with 

 less curvature of field in the plane of the diffraction plate within the 

 hemisphere than within a microscope objective of short focal length. 

 It will be easier to avoid parallax between the image of the opening in 

 the condenser diaphragm and the conjugate area of the diffraction plate. 

 The relatively intense undeviated beam that is present with most trans- 

 parent specimens is weakened within the hemisphere, and there should 

 be less loss of contrast produced by the scattering of light from the direct 

 beam within the microscope. Fran^-on's device is an example of the 

 application of the multipupil principle to facilitate the practice of phase 

 microscopy. 



9. SYSTEMS OF VARIABLE PHASE MICROSCOPY 



Throughout this section and its subdivisions the term phase difference 

 will refer to the optical path or phase difference 5 between the conjugate 

 and complementary area of the diffraction plate or its equivalent, and 

 the term amplitude ratio will refer to the ratio h of the amplitude trans- 

 mission of the conjugate area to the amplitude transmission of the 

 complementary area of the diffraction plate or its equivalent. The 

 purpose of variable phase microscopy is to obtain continuous variation 

 in the phase difference or in the amplitude ratio or in both. Although 

 limited ranges of variation are useful, greatest flexibility is achieved 

 when the phase difference is continuously variable from zero to 1 wave- 

 length and when the amplitude ratio can be varied continuously from 

 zero to infinity. When such a range of variation is available, it becomes 



