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Microscopy /29 : 4 



case of the phase-contrast microscope, discussed in the next section, 

 which is somewhat less limited in its application than the dark-field 

 microscope. 



4. Phase-Contrast Microscopy 



The phase-contrast microscope is another optical variation of the basic 

 bright-field microscope, its purpose being to increase the contrast of 

 almost transparent specimens. In the phase-contrast microscope, the 



Condenser 

 Diaphragm 



Filament 



Lamp 

 Condenser 



Diffraction 

 f| Object p^te Image 



Deviated Beam Plane ■ > 



Undeviated Beam 



Substage 

 Condenser 



Objective 

 Lens 



Eyepiece 



Figure 5. The phase-contrast microscope. Figures 5, 6, 7, 

 and 8 are all after H. Osterberg, in Physical Techniques in 

 Biological Research, Vol. 1, Optical Techniques, G. Oster and 

 A. W. Pollister, eds. (New York: Academic Press, Inc., 1955). 



light beam incident on the specimen is of such a shape that, in the absence 

 of a specimen, it would pass through the objective and all be concen- 

 trated in a ring in the focal plane of the objective lens. This result is 

 very similar to the diffraction pattern introduced in Section 2. If part 

 of the specimen has an index of refraction slightly different from the 

 surrounding medium, the light through it will pass through a different 

 region in the focal plane of the objective. This is illustrated in Figure 5, 

 where the light is called undeviated if it is transmitted as it would be in 

 the absence of a specimen, or deviated if its direction is altered by the 

 specimen. 



Because the deviated and undeviated beams are separate in the focal 

 plane of the objective, it is possible to alter one and not the other at this 

 point. Perhaps the simplest alternative to consider is to absorb the 

 undeviated beam partially and change the phase of the deviated beam. 

 This is accomplished by the diffraction plate illustrated in Figure 6. 



Just as the different diffraction orders of the grating were combined 

 in Section 2 to form the final image, so in the phase-contrast microscope 

 the deviated and undeviated beams combine at the image plane of the 



