PHASE VERTICAL ILLUMINATION MICROSCOPE 147 



Benford and Seidenberg (1950) introduced a projector system, anal- 

 ogous to the ordinary eyepiece of a microscope i)ut better corrected 

 for spherical and chromatic aberration, and mounted the diffraction 

 plate at the conjugate aperture in the projector system. If the di- 

 aphragm is located at the aperture of the condenser system, an image 

 of the diaphragm is formed at the exit pupil of the ocular of the micro- 

 scope as well as at the back aperture of the objective. It has been 

 shown (Osterberg, 1948a) that any one of the conjugate pupils which 

 follows the entrance pupil of a phase microscope can serve as the location 

 of the diffraction plate. 



Whatever optical system is adopted, the surface of the specimen must 

 be perpendicular to the optical axis of the microscope so that the image 

 of the diaphragm is centered upon this optical axis when the condenser 

 diaphragm has been centered. If the surface of the specimen diffuses 

 the incident light completely, no image of diaphragm D is formed after 

 the light passes through the objective the second time, and phase con- 

 trast cannot be obtained. 



The phase vertical illuminator increases the contrast in the image of 

 surface details which consist of elevations and depressions and which 

 therefore give rise to small differences in optical path between rays re- 

 flected from neighboring areas at different heights on the surface of the 

 specimen. If a well-polished, homogeneous specimen is being observed 

 with the phase vertical illuminator and the diffraction plate is of the 

 type TA— 0.25X, a small elevation appears brighter than the surround- 

 ing area, and a depression, scratch, or hollow appears darker than the 

 surrounding area, provided that the depression is not too deep. If the 

 diffraction plate is of the type TA+0.25X, then a depression appears 

 bright, and an elevation appears dark. When an image is to be observed 

 in dark contrast, care must be taken that the optical path difference 

 between the detail of interest and the surround is not too great. It 

 should be noted that, since light is incident on the specimen and then 

 reflected, the ray of light which is incident on the lower area travels 

 through twice the optical path difference between the lower and higher 

 regions before it re-enters the objective after reflection. Neighboring 

 areas that introduce small differences in phase at reflection, areas that 

 produce small optical path differences as a result of penetration and 

 subsequent reflection of the incident light, and neighboring areas that 

 show small differences in reflectivity or in absorption either with or 

 without a small change in the relative phase also appear as areas of 

 different brightness in the phase vertical-illumination microscope if the 

 diffraction plate is suitably chosen. 



In 1949 Cooke, Troughton and Simms, Ltd., began to supply phase 



