100 INSTRUMENTATION 



the diffraction plate produce phase contrast at the wavelength X2 but 

 not at the wavelength Xj, then 



(ni - na) = 0, (2.18) 



and 



(4 -rfi) = ^- (2.19) 



No optical path difference exists between the conjugate and comple- 

 mentary area at Xi, but at X2 the greater dispersion of the complementary 

 area introdvices an optical path difference of X2/-4:. 



The wavelengths at which no phase contrast occurs form images of 

 equal or nearly equal brightness of both the particle and the surround. 

 In the examples related to Eqs. 2.17 and 2.19 the refractive index of the 

 substance selected for the conjugate area is greater than or equal to the 

 refractive index of the material deposited over the complementary area. 

 Then, if the optical path of the particle exceeds that of the surround, the 

 wavelengths at which phase contrast is produced form a brighter image 

 of the particle than of the siu'round. The phase contrast and non-phase 

 contrast images are superimposed so that the image of the particle is 

 brighter than the image of the surround and is colored. However, if a 

 dielectric coating described by Ui and di is applied over the comple- 

 mentary area and the coating described by n2 and ^2 is deposited over 

 the conjugate area, then the materials selected according to Eq. 2.17 

 produce dark contrast with the wavelength Xi and neighboring wave- 

 lengths. Therefore the color corresponding to these (green) wave- 

 lengths is subtracted from the ordinary white or gray image of the parti- 

 cle and the image has the color corresponding to the complementary 

 wavelengths. Similarly, materials can be selected according to Eq. 

 2.19 and deposited on the diffraction plate to produce dark contrast at 

 X2 and neighboring wavelengths so that the image of the particle is 

 composed only of the shorter wavelengths of the spectrum. In order 

 that a color diffraction plate of this type function most efficiently with 

 white light illumination, the conditions for achromatic phase contrast 

 should be satisfied for a band of wavelengths at a selected part of the 

 spectrum, and the transition to no phase contrast for a band of wave- 

 lengths in the other part of the spectrum should take place over a 

 relatively narrow band of wavelengths. 



Several procedures for combining various dispersion characteristics of 

 materials in order to make a reversal color diffraction plate have been 

 suggested by Saylor et al. (1950). If the materials forming the conju- 

 gate and complementary areas have normal dispersion curves, then a 

 diffraction plate can function satisfactorily as a reversal color plate, 



