78 



PROGRESS IN MICROSCOPY 



Fixed-characteristics phase-plate microscopes 



In Zernike's original microscope the phase-plate was of collodion, 

 immersed in Canada balsam and cemented between the two components 

 of one of the objective doublets (Fig. 2.11). Let u^ be the collodion- 

 plate index and /Zg that of the Canada balsam (dark area in Fig. 2.11), 



(2) (I) 



Fig. 2.11. Annular phase plate between two lenses of a microscope objective. 



the thicknesses of the collodion plate and of the balsam are the same 

 and equate e. To show the quarter-wave lag taking place between 

 the ray 1 traversing the phase-plate and the ray 2, passing next to it 

 in the balsam, all that is needed is that the difference between indices u^ 

 and 772 be given by the expression: 



{n^—m^e = A/4. 



(2.14) 



In modern objectives, phase-plates are generally obtained by evapo- 

 ration of thin layers in vacuo. One or several of such layers, such as a, 

 are deposited on the surface of the objective-lens and evaporated 



Fig. 2.12. Phase plate deposited on the surface of a lens by evaporation /// vacuo. 



in vacuo (Fig. 2.12). Proper adjustment of the layer thickness achieves 

 either a lag or lead equal to 2/4 between the traversing ray 1 and the 

 ray 2 passing next to it. Such chemicals as magnesium or cryolite 

 are used in making transparent phase-plates and produce the required 

 phase-displacement of 7r/2. Absorption is achieved by evaporating 



