GENERAL CONSIDERATIONS OF DESIGN 85 



Sections II. 11 and 11.15 have shown that the diffraction plate for which 

 8 = ±180° or for which 5=0 can l)e expected to be more useful in 

 producing bright or dark contrast, respectively, than the diffraction plate 



for which 5 = ±90°. When A = and 5 = ±180°, Eq. II. 8.8 becomes 



G, = [A+ (1 -g)?. (1.8) 



When A = and 5 = 0, Va[. II.8.8 becomes 



G, = [h - (1 - g)f. ' (1.9) 



In each case 



Gs = Ir. 



Table III. 5 permits comparison of the contrast values K = (Gp — Gs)/Gs 

 which are computed with the help of Eqs. 1.7, 1.8, and 1.9 for values of 

 h in the range 0.02 ^ h ^ 1.0 and for a particle for which A = and 

 g = 0.9. Such a particle transmits 0.9" = 0.81 times as much light as 

 its surround. The diffraction plates for which 5 = ±90° produce only 

 bright contrast, but as l)right-contrast plates they do not compare 

 favorably with the diffraction plates for which 5 = ±180°. The dark 

 contrast produced by the diffraction plate for which 5 = and for which 

 0.1 ^ h ^ 1.0 does not deteriorate so rapidly with increasing value of h 

 as does the bright contrast produced by the diffraction plate for which 

 5 = ±180°. Table III. 5 shows that a value less than 0.5 must be 

 assigned to h if good contrast, either bright or dark, is to be obtained. 

 The ordinary oljjective [h = 1, 5 = 0) produces dark contrast K = 

 —0.19. Even when the value of h is as high as 0.5 on a diffraction plate 

 for which 5 = 0, the resulting dark contrast K = —0.36 certainly shows 

 an improvement o\'er the contrast produced by an ordinary ol)jective. 



Although the simple theory states that the diffraction plates charac- 

 terized by 5 = ±90° are not generally useful when the object specimen 

 is slightly absorbing and has an optical path equal to that of the sur- 



