INSTRUMENTATION 



Table IIL5 



The contrast values X as a function of h and 5 in 

 the case g = 0.9 and A = 



round, this prediction no longer holds when the optical path of a slightly 

 absorbing particle is not negligible with respect to the optical path of 

 the surround. The comparative usefulness of the general-purpose plate 

 can be estimated from Eq. II. 8. 8 by substituting into this eciuation the 

 pertinent values of g, A, h, and 5. For example, Table III.l states that 

 K = -0.944 when g = I, A ^ A„,i„ = 26.57°, h = 0.5, and 8 = -90°. 

 If Eq. II.8.8 is solved with h = 0.5 and 5 = -90° when g = 0.9 and A is 

 again 26.57° and if K is computed as before, then K = —0.81. This 

 is still good dark contrast. An example that concerns itself with bright 

 contrast may be obtained by substituting g = 0.9, A = 30°, h = 0.5, 

 and 6 = 90° into Eq. II. 8. 8 and then computing the value of K. K is 

 then 2.80. This is good bright contrast. From Table III. 3 it is seen 

 that, if 5 = 90°, h = 0.5, g = 1, and A = 30°, then K = 3.07. No 

 comprehensive consideration, even from the point of view of the simple 

 theory, can be included here for the class of particles A < 0, </ 5^ 1. 



It is implicit in the simple theory that Lummer's theorem holds 

 and that there is no overlap of the deviated and undeviated light in 

 the conjugate area. No dependence on the shape and dimensions of 

 the object specimen is included. However, it is not to be expected 

 that the more general theory of phase microscopy will nullify the quali- 

 tative aspects of the simple theory. Sections 14 and 15 of Chapter VII 

 show that there can exist within the geometrical image of the particle an 

 extended area, called the A region, for which the laws predicted by the 

 general theory reduce to the laws of the simple theory. In practice 

 the preferred value of h depends strongly on the size of the conjugate 

 area. Users of the phase microscope have found a very wide field of 

 applicability for bright- and dark-contrast diffraction plates for which 

 5, the optical path difference between the conjugate and complementary 

 areas, is either +90° or —90° but for which h, the amplitude trans- 

 mission ratio of these areas, may be less than or greater than unity. The 

 range 0.3 < h < 0.7 is the most useful. It is obvious that the manu- 

 facturer of phase microscopes must depend on observations of many 

 microscopists in order to determine what compromise results in the best 



