PHASE-CONTRAST MICROSCOPY 



73 



If rp is small, ip is virtually = 90^" and the most efficient phase- 

 plate is a quarter-wave one. Figure 2.3 shows the relevant result. 

 Phase-plates exhibiting variable characteristics aie, hence, valuable for 



N = IO 



ir,2,5 asrs 



l7P-/]i>\ (.7/8) 



90° 



^ 



Fig. 2.6. Contrast image y in relation to the phase difference (p produced by the object. 



they allow one to go fully into the specimen details. However, micro- 

 scopes suited to phase-plates of this type are usually elaborate in- 

 struments requiring much skill. 



4. IMAGE STRUCTURE IN PHASE-CON IRAST 



Referring to Fig. 2.7, the diagrammatic object shown is the de- 

 phasing object. The latter may be, e.g. a parallel-sided glass plate 

 with a rectangular band A. Positive phase-contrast observation shows 

 the area A in black against a white ground but the image structure 



A 



_„ n , , 



Fig. 2.7. Diagrammatic dephasing object. 



varies as the width of the object A. Assuming the latter to be minute, 

 the fight it diffracts overspreads along the plane F (Fig. 2.4); the small 

 portion of diffracted fight passing through phase-plate Q can be dis- 

 regarded. Under such conditions, the image structure is that shown 

 in Fig. 2.8(a). The imaged object as seen is correct: there is a smaU 

 dark band on a light ground the outlines of which are rounded off 

 by diffraction. As the band-width A increases, overspreading of the 



