162 INSTRUMENTATION 



5 = X 4 was introduced at the conjugate and complementary areas by 

 means of an auxiliary diffraction plate. Phase differences of iXM were 

 thus obtained between the undeviated and deviated waves according as 

 the analyzer was rotated in the neutral or anti-neutral quadrant. It is 

 especially noteworthy that Taylor's objective was only 4 mm in focal 

 length and produced excellent definition and contrast as demonstrated 

 by the reproductions of his photomicrographs of the epithelial cell. 



Kastler and Montarnal (1948) have described a scheme which uses 

 half-wave retarding plates as zonal polarizers. The micoid disk, in 

 Fig. III. 14 was constructed of two half-wave plates. One plate covered 

 the conjugate area. The second plate covered the complementary area. 

 The optical axis in the conjugate area was fixed at 45° with respect to the 

 optical axis in the complementary area. If, in such a system, the trans- 

 mission direction of the polarizer is parallel to the optical axis in the 

 conjugate area, the electric vector of the light that emerges from the 

 conjugate area vibrates along the direction of transmission of the 

 polarizer whereas the electric vector that emerges from the comple- 

 mentary area vibrates at right angles to the direction of transmission of 

 the polarizer. Therefore the amplitude ratio can be varied by rotating 

 the analyzer in either the neutral or the anti-neutral quadrant. Variable 

 amplitude ratios with a fixed phase difference 8 can be obtained by 

 depositing dielectric material upon the conjugate or complementary 

 area or by choosing two half-wave plates with different optical path. 

 If the half-wave plates are sheets of cellophane, it is possible to obtain 

 half- wave plates of different optical thickness. As an alternative, 

 the half-wave plates may be made of different crystalline materials, and 

 the required phase difference may be obtained by a suitable choice of 

 cement for retaining the half-wave plates between protecting plates of 

 glass. 



A simple method of adapting the half-wave plate has been described 

 by Locquin (1948). Locquin omitted the half-wave plate over the 

 complementary area and suggested that the half-wave plate covering 

 the conjugate area be cemented between glass plates with a cement which 

 produces the wanted phase difference. If, in such a scheme, the trans- 

 mission direction of the polarizer is set at 45° with respect to the optical 

 axis of the half-wave plate, the electric vector that emerges from the 

 complementary area vibrates along the direction of transmission of the 

 polarizer whereas the electric vector that emerges from the conjugate 

 area vibrates at right angles to the direction of transmission of the 

 polarizer. The amplitude ratio is varied by rotating the analyzer in the 

 neutral or anti-neutral cjuadrant. Locquin emphasized that an advan- 

 tage gained by his modification of the half -wave plate arrangement is that 



