A HIGHLY FLEXIBLE POLANRET SYSTEM 157 



deviated or the deviated bundles of rays. Fixed phase differences may 

 be inckided by adding the correct thickness of a dielectric material over 

 the required area. Polanret systems of this type are both relatively 

 simple in principle and relatively easy to construct with resultant good 

 optical performance. A fixed phase difference may also be obtained by 

 placing a birefringent plate between the zonal polarizers and the 

 analyzer. The fast or slow direction of vibration of the electric vector 

 in the birefringent plate should be parallel either to the direction of 

 vibration of the electric vector which emerges from the conjugate area 

 or to the direction of vibration of the electric vector which emerges from 

 the complementary area. 



In order to obtain variable phase difference, a direct procedure, for 

 example, is to replace the plate with fixed birefringence by a variable 

 birefringent plate such as the Babinet-Soleil compensator or by a plate 

 the birefringence of which is varied electro-optically. Although in- 

 corporating elements with variable birefringence into a polanret system 

 is sound in principle, such components form a cumbersome optical unit, 

 and several objections arise. In these more direct methods of attack, 

 the amplitude ratio is altered preferably by rotating a polarizer which 

 is located either below or above the object specimen according as it is 

 desired that the specimen shall be illuminated by polarized or un- 

 polarized light. 



9.2. A highly flexible polanret system 



The polanret system illustrated in Fig. III. 14 is capable of varying the 

 amplitude ratio and the phase difference continuously and independently 

 throughout the complete ranges of values that are of interest in phase 

 microscopy. This method has been described by Osterberg (19-17). 

 The phase variation is obtained by an indirect method which includes 

 simple optical components and which w^as discussed in an earlier paper 

 (Osterberg, 1946). 



The conventional diffraction plate is replaced by the micoid disk. 

 This disk together with the rotatable analyzer and polarizer constitute 

 the essential elements of the polanret system. The polarizer may be 

 located anywhere between the micoid disk and the source of light. The 

 analyzer may be placed anywhere between the micoid disk and the eye 

 or viewing device. Since lenses produce elliptical polarization, the 

 analyzer and the polarizer preferably are located adjacent to the micoid 

 disk, but in the usual phase microscope these locations are awkward for 

 mechanical reasons. The micoid disk is composed of two zonal polarizers 

 which transmit electrical vibrations along the directions Ei and E2 over 

 the conjugate and complementary areas, respectively, and of a quarter- 



