PRINCIPLES OF COLOR PHASE CONTRAST 99 



is hardly distinguishable from its surround if a red filter is inserted in 

 front of the light source. If a reversal color diffraction plate is mounted 

 in the objective, then with white light illumination the particle appears 

 green. If a green filter is introduced the image of the particle is brighter 

 than that of the surround, but if the filter is red the particle appears 

 black or darker than the surround. 



The reversal color diffraction plate is more efficient than the simple 

 color diffraction plate. If white light illuminates the specimen the color 

 of the image of a particle and of the halo may be considered to be com- 

 plementary in the ideal case and the image of the background is gray. 

 The publication by Saylor et al. (1950) is mainly concerned with what has 

 here been called reversal color contrast. 



Suppose that the design of a simple color diffraction plate is being 

 considered and that the undeviated light of a green wavelength Xi is 

 to be retarded by an amount Xi/4 relative to the deviated light but that 

 the undeviated and the deviated light of the red wavelength Xo undergo 

 no relative phase change on passing through the diffraction plate. The 

 conditions to be satisfied l)y the refractive indices of the materials 

 constituting the diffraction plate are 



(m - 7i2)t = —' (2.13) 



4 



(ng - n^)t = 0, (2.14) 



in which ni = refractive index of conjugate area at Xi; 

 ng = refractive index of conjugate area at X2; 

 712 — refractive index of complementary area at Xi ; 

 714 = refractive index of complementary area at X2 ; 

 t = thickness of the conjugate and the complementary area. 



Since t cannot be equal to zero, then, from Eq. 2.14, W3 = 714. Again, let 



Ma = rii - di; (2.15) 



714 = no — c?2- (2.16) 



If Eqs. 2.15 and 2.16 are substituted into Eq. 2.13, it follows that 



idi-d2)=j^' (2.17) 



Equation 2.17 is intuitively obvious. The refractive indices of the 

 materials forming the conjugate and complementary areas must be equal 

 at X2, and the dispersion of the material on the conjugate area is greater 

 in such a way that the optical path step between the conjugate and com- 

 plementary areas becomes Xi/4 at Xi. Similarly, if it is preferred that 



