PHASE-CONTRAST MICROSCOPY 



79 



above the previous layer a very thin metal layer that absorbs the hght 

 virtually without affecting phase displacement. Evaporation of the 

 thin layers constituting the annular phase-plate is carried out on one 

 of the lenses closest to the objective's focal plane. Figure 2.13 shows 



Phase 

 plate 



^^ 



Fig. 2.13. Phase contrast immersion objective. 



an immersion objective in which the phase-plate was evaporated on 

 one of the components of a doublet prior to cementing. The ob- 

 jective's focal plane hes in this area. 



In absorbent phase-plates made by means of thin metal layers, 

 a not inconsiderable portion of the direct light is reflected by the 

 phase-plate and becomes stray hght. In Reichert's "anoptral" device, 



FiG. 2.14. Reichert's anoptral device: (1) and (4) no reflecting thin film; (2) dielectric 



thin film; (3) absorbing metallic thin film. 



there is no stray light by reflection and the phase-plate may exhibit 

 substantial absorption (Fig. 2.14). The ''anoptral" phase-plate is 

 featured by Number TV = 15. This phase-plate divides the incident 

 intensity of the direct light by 15 and brings about a njl phase difference. 



Adjustable annular-diaphragm microscopes 



The characteristics of the foiegoing phase-plates are permanent. 

 If two objectives of identical geometrical characteristics are used 

 seriatim, the annular diaphragm remains the same in both cases provided 



