74 



PROGRESS IN MICROSCOPY 



light diffracted along the plane F recedes (Fig. 2.4). The portion of 

 diffracted light passing through the phase-plate can no longer be 

 disregarded as it brings about the perturbations shown in Fig. 2.8(b). 

 The image still conforms to the object and a dark band on light 

 ground is discernible but the image is edged by the two clearly visible 

 bright fringes m and m'. The halo that always surrounds phase con- 

 trast images is due to these fringes. Naturally, this halo is also present 



(a) 



(b) 



(c) 



Fig. 2.8. Image structure in phase contrast method (after H. WoUer). 



in Fig. 2.8(a) but much more extensive and not so visible. If the width 

 of the band A continues to increase, a time comes when the width 

 of the diffraction phenomenon it originates along the plane F (Fig. 2.4) 

 is approximately the same as that of phase-plate. Now the image 

 is completely perturbed, as shown in Fig. 2.8 (c.) No longer can it be 

 said that a dark band on hght ground is discerned as the intensity 

 within the geometrical image if the band has virtually the same mean 

 value as the remainder of the field. Figure 2.8(c) shows that only the 

 phase breaks of the object, e.g. the edges of band A, are perceivable. 

 Let us take as example an epithelial cell of the tongue : in a powerful 

 objective this is a rather large object the diffracted light of which 

 does not extend much beyond ihe phase-plate. The imaged cell 

 shows but little contrast with the remainder of the field. Only the 

 edges and inner details of the cell are visible. The edges are seen as 

 discontinuities denoted by m and m' in Fig. 2.8(c). All inner details 

 display images similar to those in Fig. 2.8(a). These results are shown 

 diagrammatically in Fig. 2.9. Figure 2.9(a) shows a cross-section of the 



