over the entire field of view; the deviated fight tends to focus at the 

 plane of the eyepiece lens (Figure 11-12). The image of the object is 

 formed as a result of interference between the deviated and undeviated 

 light coming from the object. The image of its surroundings is formed 

 by the undeviated light transmitted by this component only. Separation 

 of the deviated and undeviated light occurs in the optical microscope 

 at the rear focal plane of the objective. 



Specimen 



Figure 11-11. Diagram Showing the Diffraction of Light on Passing 

 Through and Around the Edges of an Inhomogeneous Object. Light deviated 

 from its original path by the object is spread over the entire surface of the 

 objective lens, while the light transmitted undeviated by the object passes 

 through the more central part of this same lens. 



The optical system of the phase-contrast microscope differs from that 

 of the ordinary optical microscope only in the addition of ( 1 ) a substage 

 annular diaphragm to illuminate the object with a narrow cone of light, 

 and (2) a diffraction plate mounted in the objective (Figure 11-13). 

 The diffraction plate is located at the rear focal plane of the objective 

 where the deviated and undeviated light coming from the object is sepa- 

 rated. The relative phase of the deviated and undeviated light is changed 

 by introducing a layer of phase-retarding material on that part of the 

 diffraction plate covered by either one of these rays. As already pointed 

 out, the deviated light originating at the object is retarded or altered in 

 phase (about V4 wavelength) with respect to the light transmitted un- 

 deviated by the object. By further retarding this deviated light by another 

 V4 wavelength at the diffraction plate, it wiU have a total retardation 

 of Vi wavelength and will be completely out of phase with that of the 

 undeviated light (Figure 11-13). The deviated and undeviated light 



SURVEY OF CYTOLOGICAL TECHNIQUES / 229 



