CONTINUOUSLY VARIABLE AMPLITUDE RATIOS 163 



auxiliary surfaces in the path of deviated rays are thus avoided. It is 

 pointed out, however, that in the usual microscope a large number of 

 surfaces lie in the path of the undeviated and deviated rays. The 

 optical problem of polanret microscopy consists primarily of selecting 

 and locating polarizing, analyzing, and retarding elements so that they 

 produce the least possible injury to definition. A second important 

 problem is to achieve polanret microscopy in spite of the elliptical 

 polarization introduced by the lenses. An important advantage of 

 Locquin's system is that the polarizer need not be rotated. Conse- 

 quently, the polarizer can be cemented into the micoid disk which con- 

 tains the half-wave plate. The light incident upon the half-wave 

 plate is then linearly polarized in spite of the elliptical polarization 

 introduced by the lenses. The polarizer may be included in the micoid 

 disk also in the arrangements proposed by Taylor and by Kastler and 

 Montarnal. 



It has been pointed out with respect to the method by Osterberg 

 that the quarter-wave plate in the micoid disk may be omitted together 

 with the polarizer when only variations in the amplitude ratio are 

 required. So doing results in the advantage that the zonal polarizers, 

 if made of polarizing film, polarize the conjugate and complementary 

 areas linearly or almost linearly for a relatively wide variation of the 

 angle of incidence of the rays upon the micoid disk and for wide varia- 

 tions in the wavelength of the incident light. These are important 

 properties of polarizing film which do not apply to the birefringent 

 properties of retarding plates or to the rotatory properties of a quartz 

 plate. 



In another class of polanret systems for obtaining variable amplitude 

 ratios with a fixed phase difference or with highly restricted variations 

 in the phase difference, both the conjugate and the complementary area 

 are not polarized linearly. A unique system of this class has been 

 described briefly by Hartley (1947). In Hartley's method the micoid 

 disk in Fig. III. 14 consists of two quarter-wave birefringent elements. 

 One of these covers the conjugate area; the other covers the complemen- 

 tary area. The direction of vibration of the fast ray in the conjugate 

 area is fixed at 45° with respect to the direction of vibration of the fast 

 ray in the complementary area. As in Fig. III. 14, a substage polarizer 

 and an analyzer complete the system. Suppose that the transmission 

 direction of the polarizer is made parallel to the direction of vibration 

 of the fast ray in the conjugate area. Then linearly polarized light 

 emerges from the conjugate area whereas circularly polarized light 

 emerges from the complementary area. Rotation of the analyzer varies 

 the transmitted component of the undeviated light only such that the 



