96 INSTRUMENTATION 



X2, but it is not necessarily exactly achromatized for all wavelengths 

 between Xi and X2. If the diffraction plate is to be exactly achromatized 

 at A'' wavelengths, then a set of A^ simultaneous linear equations of the 

 form of Eqs. 2.8 and 2.9 must be satisfied. 



Dark contrast and achromatism are also obtained at wavelengths 

 Xi and X2 if, instead of conditions 2.8 and 2.9, the equations (rii — n2)t = 

 — (Xi/4 + ^-Xi), and (% — n^)t = — (X2/4 + A:X2), in which k is an 

 integer, either positive or negative, are satisfied. Equations 2.8 and 

 2.9 are the special cases in which A; = 0. Similarly the conditions for 

 bright contrast and achromatism at wavelengths Xi and X2 are also 

 satisfied if (rii — 112)1 = (Xi/4 + AXj) and (W3 — n^)t = (X2/4 + AX2), 

 in which k is again an integer, either positive or negative. However, 

 Eq. 2.12 follows unchanged, regardless of how high a multiple of J^ 

 wavelength is chosen for the optical path difference between the con- 

 jugate and the complementary area. At any wavelength, the difference 

 between the indices of refraction of the two substances forming the 

 conjugate and complementary areas is proportional to that wavelength, 

 provided that the conjugate and complementary areas are of ecjual 

 physical thickness. 



2.4. Principles of color phase contrast 



Color phase contrast was first discussed by Zernike in 1948 (National 

 Academy of Sciences,* Spring Meeting, 1948, and Symposium on Elec- 

 tron and Light Microscopy, June 1948). Ho\Vever, the published 

 abstracts and reports of these meetings contain no mention of color 

 phase contrast. Since that time Saylor, Brice, and Zernike (1949 and 

 1950) have described additional investigations of the technics available 

 for designing color phase plates by controlling or selecting the dispersions 

 of the substances forming the conjugate and complementary areas. 

 Zernike also considered the use of controlled dispersions to make achro- 

 matic diffraction plates. Some discussion of the achromatic diffraction 

 plate is contained in the publication by Saylor et al. (1950). Grigg 

 (1950) substituted colored filters for the ordinary diaphragm of the 

 substage condenser in order to achieve color phase contrast with a phase 

 objective which contained a neutral diffraction plate of standard design. 

 Two filters of contrasting colors were so assembled that undeviated light 

 of one color passed through that area of the diffraction plate which 

 normally forms the conjugate area, and undeviated light of the second 

 color passed through that area which has been designated as the com- 

 plementary area of the standard diffraction plate. 



The color phase microscope forms a colored image of a transparent 

 specimen which differs in optical path from its surround. In the ideal 



