84 PROGRESS IN MICROSCOPY 



by some means or other, e.g. evaporation of thin layers on the 

 ring Q^. 



B. O. Payne reverted to the principle of half-wave plates in order 

 to sunder the direct and the diffracted light in two perpendicular 

 components. Behind the plate is a Bravais compensator whereby the 

 phase difference can be varied continuously. Absorption is varied by 

 rotating the analyser. 



Locquin's phase-plate consists of the half-wave Cellophane ring ah, 

 cemented to Canada balsam between the two elements of the ob- 

 jective's doublet (Fig. 2.11). The incident light is polarized 45^ from 



Fig. 2.20. The Taylor phase-plate. 



the Cellophane-ring axis. The direct light (1) after passing through the 

 ring vibrates 90" from the diffracted light (2) the direction of which re- 

 mains unaltered. Intensity of the direct and the diffracted light is varied 

 by rotating the analyser. The phase-difference is obtained by adjusting 

 the index differences of the Cellophane and the Canada balsam. 



In another layout, Locquin suggested making use of interference 

 filters (Varicolor-Wild). The phase-ring is made up of stacked, evapo- 

 rated-///-racwo thin layers. Such an arrangement is an interference 

 filter letting through but a narrow spectrum band (60 A). The layers 

 are so arranged that a ray passing through the ring is phase-displaced 

 by n\l in relation to a ray passing next to it. The interference filter 

 acts concurrently as a phase-plate. When such a plate is illuminated 

 with the radiation for which the interference filter is designed, the 

 outcome is a low-absorption phase-plate. A slight wave-length change 

 in the incident light increases filter absorption. The outcome is 

 a variable-absorption phase-plate with monochromatic illumination the 

 wave-length of which can be varied. 



Traiisferrecl-imagc phase-contrast microscopes 



Figure 2.21 shows the diagram of the Fran^on-Nomarski device. 

 The image S[ of the diaphragm S^, after traversing the condenser C 



