82 



PROGRESS IN MICROSCOPY 



Microscopes with variable phase-plates 



In Osterberg's polanret microscope, the phase-plate proper, denoted 

 by Q, consists of two polaroid elements the axes of which cross each 

 other (Fig. 2.17). The ring, shown in thick hnes in Fig. 2.17, is cut 



, nO 



( 



Fig. 2.17. Polanret microscope (after Osterberg). 



out from a polaroid sheet. Two other polaroid elements the axes of 

 which are paralleled and at right angles to the axis of the first polaroid 

 element of the ring are on each side of the ring. These various elements 

 form the phase-plate Q just ahead of which is the quarter-wave plate 

 the axes of which are 45° from the axes of the polaroid elements of 

 the plate Q. The polarizer P^ and the analyser P.^ complement the 

 device. Rotating the polarizer P^ varies the phase difference between 

 the direct and the diffracted vibrations. Rotating the analyser P., 

 alters the amplitude's ratio of such vibrations while their phase differ- 



2 



Fig. 2.18. The Kastlcr and Montarnal phase-plate. 



ence remains constant. The result is an absorbing phase-plate ad- 

 justable ad libitum. The Kastler and Montarnal phase-plate consists 

 of two half-wave plates the axes of which are 45° from each other 

 (Fig, 2.18), to which is added a quarter-wave plate one preferred 

 direction of which is parallel to the direct vibration. Both plates are 

 inserted between a pohvrizer and an analyser as shown in Fig. 2.17. 

 Assuming that the polarizer generates a vibration directed along the 



