DIFFRACTION PLATES FOR A = +x/4 AND g ^1 63 



comparison in Fig. 11.16. In A, Gp vanishes and the contrast K = — 1 at 

 the point /i = 0.2 for the curve 5 = 0. For the curve 5 = 30°, A' = -0.7 

 at h = 0.2. Provided that the h value is set at 0.2, variations of 20° in 5 

 from its preferred value 5 = will not alter the contrast seriously. 

 Provided that 5 = 0, /i may be permitted to vary by the amount ±0.04 

 about the point h = 0.2. Although the choice of h and 5 is not critical, 

 it must be made with care. The curve with 5 = 90° has been included 

 in order to illustrate that quarter-wave diffraction plates are not useful 

 with particles in this category. It is interesting to observe from the 

 curves drawn for 5 = and 30° that it is theoretically possible to reverse 

 contrast in the image by reducing the h value below h = 0.1. This 

 behavior is to be expected, for the particle must appear in bright contrast 

 in the ultimate Schlieren case, /i = 0. An inspection of B shows that 

 conditions for bright contrast are more flexible than those for dark 

 contrast. Thus, at h == 0.2, K is reduced only from K = 3.0 to 2.7 

 as 5 is reduced by 30° with respect to its best value of 180°. The 

 contrast K = qo at /i = and decreases with increasing h. 



The contrast K has been plotted in Fig. 11.17 as a function of g for a 

 diffraction plate for which h = 0.2 and 5 = 0. The contrast values —K 

 obtained with this diffraction plate are seen to exceed 0.8 for particles 

 having g values in the range 0.712 < g < 0.887. 



16. CHOICE OF DIFFRACTION PLATE WITH STAINED PARTICLES 

 WHOSE OPTICAL PATH DIFFERENCE IS +X/4 



The energy densities Gg and Gp are plotted in Fig. 11.18 as functions 

 of h at the indicated fixed values of 5 for the case A = +X/4 and g = 0.9. 

 Since g^ = 0.81, the particle transmits 0.81 times as much light as its 

 surround. Such particles are weakly stained and will show fair contrast 

 in the standard microscope. On the other hand, such particles should 

 appear to be black in the phase microscope with the choice of a diffrac- 

 tion plate for which h = 1.345 and 5 = —42°. The curves Gp with 

 5 = —27° and —57° practically coincide. Variations of ±15° in 5 from 

 the best value 5 = —42° do not produce serious deteriorations in the 

 contrast. 



Furthermore, h may be varied from 1.0 to 1.7 without seriously 

 disturbing the contrast. Contrast in the image of the particle will be 

 dark so long as /i > 0.7, but the h values must exceed unity in order to 

 produce excellent contrast with a particle for which A = +X/4 and 

 g = 0.9. Diffraction plates for which h > I are called* the B type and 

 differ from the A tj^pe in that the absorbing material is placed upon the 

 complementary area instead of upon the conjugate area. The useful- 



* See Section 2.5, Chapter III. 



