86 



PROGRESS IN MICROSCOPY 



the diffracted light is almost wholly reflected by the metal layer. 

 Suitable determination of the metal-layer thickness and taking the 

 incidence / in consideration, the 7r/2 phase difference is obtained. 

 Owing to the reflection on the glass at ah, the direct light is polarized: 

 if / =^ 57° (Brewster's incidence), polarization is complete. The diffrac- 

 ted vibrations reflected on the metal are virtually unpolarized. Then 

 a polarizer is set at Pj ahead of the phase-plate Q and an analyser P.. 

 next after. Both direct and diffracted vibrations are polarized identi- 

 cally and may interfere. Pi is set so that the incident vibration is 45° 

 from the phase-plate plane of incidence. Rotating the analyser P^ 

 brings about a variable-absorption phase-plate with either positive 

 or negative phase-contrast. 



In B. O. Payne's device (Fig. 2.22) the prism R, next after the 

 objective O^, reflects the light on to the concave spherical mirror P' 



PV.W.V.V,>A>MA/^.W/// /77777^ 



Fig. 2.22. B. O. Payne's device (Cooke, Troughton and Simms). 



The radius of curvature of the latter is such that it images the focal 

 plane of the objective O, at Q, following reflection on R. The phase- 

 plate is at Q and the specimen imaged by the objective is close to 

 the mirror P' . The optical system L gives at P" a new image, per- 

 ceived by the eyepiece O2. Such devices are particularly convenient 



