PHASE-CONTRAST MICROSCOPY 83 



axis of the half-wave plate forming the ring shown in Fig. 2.8, the 

 direction of the vibration passing through this plate remains unaltered 

 but the diffracted vibration traversing the other half- wave plate rotates 

 through an angle of 90°. In the latter plate the direct vibration is 45° 

 from the half-wave plate it traverses. It is well known that a half-wave 

 plate against which a reciilinear vibration impinges suppHes a vibration 

 which emerges as a rectilinear and symmetrical in relation to the 

 preferred directions. After passing through the half-wave plate, the 

 direct and the diffracted vibrations are converted in two perpendicular 

 vibrations. The 7r/2 phase difference, required between the direct and 

 the diffracted wave, is provided by the quarter-wave plate one of the 

 preferred directions of which is parallel to the direct vibration. Merely 

 rotating the analyser alters at will the ratio of the direct and the 

 diffracted ampHtudes. The outcome is an absorption-adjustable 

 phase-plate. 



Hartley devised an arrangement comprising two quarter-wave 

 plates 45° from each other (Fig. 2.19). Now, if the incident vibration 

 is parallel to the axis of the quarter-wave plate forming the ring, the 

 direct light is linearly polarized and the diffracted light circularly. 



Fig. 2.19. The Hartley phase-plate. 



Rotating the analyser varies the direct light-intensity without affecting 

 that of the diffracted light. Phase difference is obtained by some means 

 or other, e.g. deposits of thin layers on the ring. 



The rotatory power of quartz has been used by Taylor to vary 

 phase-plate absorption. As in the foregoing methods, the incident 

 Hght is hnearly polarized; the direct Hght passes through the dextro- 

 rotatory quartz plate Q^ and the diffracted light through the two 

 levorotatory quartz plates Q2 (Fig. 2.20). These quartz-crystals make 

 up a parallel-sided plate placed, as usual, in the imaged plane of the 

 radiant. The direct and the diffracted vibrations rotate 45° in a different 

 direction and meet 90° from each other at the plate outlet. As in the 

 foregoing methods, rotating the analyser varies the direct and 

 the diffracted amplitudes. Next, the phase difference is obtained 



