PHASE-CONTRAST MICROSCOPY 87 



when using variable-characteristics phase-plates. In powerful objec- 

 tives, reaching the focal plane, lying in the objective itself, is hardly 

 feasible. Designing a phase-plate, varied in the objective, is no easy 

 matter whereas the converse is true if the focal plane is imaged outside 

 the objective in an accessible area. Zernike was the first to use this 

 image transfer in the first phase-contrast microscope he designed. 



Instead of placing the phase-plate next after the objective, it oc- 

 curred that it might be placed ahead of it. In Frangon's half-sphere, 

 the specimen is at P (Fig. 2.23) and the detail A close to the centre 



p 



Fig. 2.23. Fran^on's half-sphere. 



of the glass half-sphere MNA'N'M' which is a metalUzed reflecting 

 layer at NA'N'. A small quantity of metal was removed at A'. The 

 half-sphere was split in two at NN'. On the upper-half surface NN' 

 a thin semi-reflecting layer was deposited. The annular phase-plate is 

 at Q and a small opaque screen at R. The half- spheres were then 

 cemented. 



The condenser (not shown in Fig. 2.23) has, in its focal plane, 

 an annular diaphragm on which the source is imaged as usual. After 

 passing through the condenser, the specimen being reflected on 

 the spherical mirror NA'N', the annular diaphragm is imaged on the 

 phase-ring Q. A ray of light, diffracted by A, travels along the path 

 ABCBA' and the specimen is imaged at v4'. This image is of the same 

 size as the specimen's and stigmatic because A lies in the centre of the 

 sphere NA'N'. After travelling along an identical path, the direct 

 light passing through the phase-plate Q ends at A' . The image A' is 

 therefore a phase-contrast image, observable in an ordinary microscope. 

 The object of the opaque screen R is to stop the rays of light from 

 entering the objective after straying from the path shown. Provided 

 the device be soundly designed, highly absorbing phase-plates may be 

 used as the direct fight is stopped before penetrating the objective 

 and it does not originate any stray light. 



7 



