84 



INSTRUMENTATION 



Table III. 3 



The contrast values K as functions of h and A when 

 ^ A ^ 90° in the case g = 1 and 5 = +90° 



bright contrast, K = 3, has already been exceeded when A = 45° = X/8 

 for all listed values of h. Optimum contrast is not reached, however, for 

 h values in the range 0.5 S h ^ 0.632 when A is less than 30°. If as a 

 criterion of good bright contrast it is refjuired that K ^ 2, i.e., it is re- 

 quired that the light intensity in the image of the particle shall be at 

 least three times the light intensity in the image of the surround, there 

 will exist for every value of /i > a value of A = A3 such that the dif- 

 fraction plate can be expected to produce good bright contrast for all 

 values of A in the interval A3 ^ A ^ 90°. These values of A3 have been 

 determined from Eqs. 1.6 and are listed as a function of h in Table III. 4. 

 Tables III. 3 and III. 4 indicate that good bright contrast may be ex- 

 pected with a 0.25A+X/4 diffraction plate (see Fig. III. 3a) in observing 

 particles for which A falls in the range 21° < A ^ 90°. For smaller 

 values of A the simple theory indicates that h must be less than 0.5 in 

 order to obtain sufficiently good bright contrast. It is possible that the 

 phase microscopist will prefer a general-purpose bright-contrast plate 

 with a more highly absorbing conjugate area than would be selected for 

 a general-purpose dark-contrast plate. 



Object specimens which are absorbing and which have a refractive 

 index equal to the refractive index of the surround (g ^ I, A = 0) 

 present a different set of requirements for a useful diffraction plate. If 

 the values A = and hi = I are substituted into Ecj. II. 8. 8, then when 

 either 8 = +90° or 8 = -90° is also substituted into Eq. II. 8.8 the 

 expression for Gp becomes 



G^= (g- 1)^ + Al (1.7) 



