102 INSTRUMENTATION 



Let Xo fall within the yellow region of the spectrum. It is known that 

 if a phase microscope produces a dark-contrast image of a particle 

 having an optical path exceeding that of the surround it also forms a 

 bright-contrast image of a particle with an optical path less than that 

 of the surround. Therefore the diffraction plate in this example is 

 designed so that, in an approximate sense, the color of the image of a 

 particle with an optical path greater than that of the surround is red 

 and the color of the image of a particle is blue-green if the particle has an 

 optical path less than that of the surround. The halo around the blue- 

 green image is red, and the halo around the red image is blue-green. 

 The color of the image of the particle and that of the halo is of course 

 altered when there is any change in the composition of the bands of 

 wavelengths which interfere to form bright- or dark-contrast images. 

 If the two dielectric substances selected for a reversal color diffraction 

 plate are interchanged as the materials from which the conjugate and 

 complementary areas are made, then also the colors which designate 

 whether the particle has an optical path greater or less than that of the 

 surround are interchanged. 



The conditions which the reversal color diffraction plate in the example 

 must satisfy are 



(m - n2)t = - j; (2.20) 



{ns-n,)t = ^; (2.21) 



(ns - ne)t = 0. (2.22) 



In these last equations rii, n^, and n^ are the refractive indices of the 

 conjugate area for Xi, X2, and Xq, respectively, and n2, n4, and tiq are the 

 refractive indices of the complementary area for Xi, X2, and Xq, respec- 

 tively, t is again the thickness of the complementary and conjugate 

 areas, and Xi < Xq < X2. As before, let 



ni - ng = di; (2.23) 



^2 — W4 = d2. (2.24) 



If t is eliminated between Eqs. 2.20 and 2.21 then 



d,-d2=^ ^^-^tA' (ni - 712) < 0. (2.25) 



The condition 2.22, which is related to the value of Xq, has not been used 

 in obtaining Eq. 2.25. Equation 2.25 does not fix the value of Xq, the 



