PHASE-CONTRAST MICROSCOPY 



77 



tive Oi, Si is imaged at S[ in the objective focal plane F. The direct 

 light passes through the image S[ of S^. To dephase by 7r/2 the direct 

 vibrations in relation to the diffracted ones, all the light must penetrate 

 in the phase-plate at S[. The phase-plate, therefore, must overspread 

 the imaged Si of the diaphragm Di. If the phase-plate is ring-shaped, 

 the diaphragm Di aperture should be a ring which, when imaged 

 at S[ does not overlap the phase-plate. When the latter is shaped 

 as a small rectangular band, the aperture 5*1 of the diaphragm Z>i 

 should be a slit which, when imaged at S'l, is overspread by the phase- 

 plate. When a hnear-phase plate is used, objects shaped lengthwise, 

 extending at right angles to the phase-plate, are not perceived. Such 

 a strip affords a preferred direction of vision: merely rotating the 

 object shows up details that would remain invisible in the initial 

 position. Regardless of the phase-plate shape, it follows that segraga- 

 tion of the diffracted and direct light is unsatisfactory if the phase- 

 plate is too wide. In Fig. 2.4, the beam of diffracted hght overspreads 

 the objective and lies in the MM' area in the focal plane F. The 

 phase-plate area should be small in relation to that of the MM' 

 diameter. 



Figure 2.10 shows diagrammatically a phase-contrast microscope 

 with a ring-shaped phase-plate. Diaphragm Di, set in the focal plane 



Fig. 2.10. Phase-contrast microscope with annular phase plate. 



of the condenser C is provided with an annular aperture. The imaged 

 filament of the lamp So, adequately enlarged as to overspread the 

 whole ring sections, is cast on the diaphragm ring. The annular phase- 

 plate Q, located in the focal plane of objective Oi overspreads the 

 imaged ring D^. The shaded area in Fig. 2.10 does not show the whole 

 direct beam of light but only a source-generated beam of Hght. The 

 beams originated from other points of the lamp S^) filament follow 

 similar paths but at different slopes. 



