156 INSTRUMENTATION 



possible to adjust the microscope for optimum bright or dark contrast 

 for a wide variety of object specimens. An important advantage of 

 the continuously variable phase microscope is that quantitative informa- 

 tion about the area and the optical properties of the particle can fre- 

 quently be deduced from the h and 5 setting required for producing 

 optimum bright or dark contrast (Osterberg and Pride, 1950). 



9.1. Polanret methods for phase microscopy 



One of the methods for obtaining variable phase microscopy consists 

 in replacing the diffraction plate by suitably chosen polarizing, analyz- 

 ing, and birefringent retarding elements. This general method can be 

 executed in a large number of ways. It is called, for brevity, the 

 polanret method or polanret microscopy. Polanret methods can be- 

 come simple both in principle and in practice w'hen it suffices to vary 

 either the phase difference only or the amplitude ratio only. However, 

 when it is required that both the phase difference and the amplitude 

 ratio be continuously and independently variable, a polanret system of 

 high optical quality can become difficult to achieve with existing ob- 

 jective designs and with materials now available. 



In one class of polanret systems the light passing through the con- 

 jugate area and that passing through the complementary area are 

 polarized with different directions of vibration, preferably at right angles. 

 This can be accomplished by replacing the diffraction plate by two 

 zonal polarizers of high optical quality. One of these covers the con- 

 jugate area and the other covers the complementary area. After pass- 

 mg through the zonal polarizers, the deviated and undeviated bundles of 

 rays remain separated physically because of their different directions 

 of polarization until components of the deviated and undeviated bundles 

 are subsequently brought into a common direction of vibration by pass- 

 ing the deviated and undeviated light through an analyzer. An im- 

 portant consequence of introducing zonal polarizers is that the phase 

 difference and the amplitude ratio can be altered by the interaction of 

 other polarizers and bii'efringent elements which do not have to be 

 located adjacent to the zonal polarizers, that is, near the conjugate and 

 complementary areas. 



If only the zonal polarizers and the analyzer are included in the optical 

 system, a continuously variable amplitude ratio is obtained. The 

 amplitude ratio is varied by rotating the analyzer relative to the zonal 

 polarizers, and the numerical value of the amplitude ratio is easily 

 calculated from the angle of rotation of the analyzer. The amplitude 

 ratio can assume practically any value from zero to infinity because 

 the analyzer may be set, in the limiting cases, to block either the un- 



