NOMENCLATURE FOR DIFFRACTION PLATES 105 



of the complementary area, and 5, the optical path difference between 

 the conjugate and complementary areas of the diffraction plate. It is 

 relevant to point out any dependence of the value of h on wavelength 

 such that colored images analogous to those observed with darkfield 

 illumination are introduced and to indicate the dependence of 5 on 

 wavelength if variations in dispersions produce color phase contrast. 

 A 'complete description of a diffraction plate should also contain the 

 values of the numerical aperture corresponding to the boundaries of the 

 conjugate area (p2 — Pi, Fig. 11.20). In particular, a statement con- 

 cerning the effect produced in an image by a given, definite change in the 

 value of T is quantitatively meaningful only if the size and location of 



abed 







Fig. III.3. Notation devised by Bennett et al. to describe neutral diffraction plates. 



the conjugate area are also known. The notation adopted in this book 

 to describe diffraction plates that form neutral images with white light 

 was devised by Bennett et al. (1946). This nomenclature does not 

 include a designation of pi and po, but it is useful in a general discussion 

 of phase microscopy despite this limitation. This system of notation 

 is explained as follows: 



The letter A is used to identify all plates that reduce the ratio of the 

 energy transmission of the conjugate area to the energy transmission of 

 the complementary area, and the letter B identifies all plates that 

 increase this ratio. The ^'alue of the energy transmission ratio is 

 designated by T and precedes the letter A or B. T is less than or equal 

 to 1 for A-type diffraction plates and greater than or equal to 1 for B-type 

 plates. If the diffraction plate increases the optical path of the un- 

 deviated light relative to that of the deviated light, the letter A or B is 

 followed by a + sign. If the optical path of the undeviated light is 

 decreased relative to that of the deviated light, the letter A or B is 

 followed b}^ a — sign. The magnitude of the optical path difference, 

 expressed in fractions 5 of the wavelength X of the Hg line for which 

 X = 5461 A, is written after the + or — sign. The application of this 

 notation to diffraction plates with an annular conjugate area is illus- 

 trated in Fig. III. 3. The four ambiguities which may arise are avoided 

 by the inclusion of the inequalities contained in the more explicit 



