MAKING A DIFFRACTION PLATE 89 



and that (ris — ^2)^3 either is neghgible or is apphed later as a small 

 correction. A series of equations with the form of Eq. 2.7 formulate 

 the problem if the dependence of the refractive index on wavelength is 

 relevant, and any such considerations must be superimposed upon the 

 general discussion included in Section 2.1. The phase accessories now 

 supplied as standard equipment by various manufacturers produce a 

 neutral, i.e., a black, gray, and white image of most transparent speci- 

 mens when white light is incident on the specimen. If the conjugate 

 area of the diffraction plate is colored, the image of the material sur- 

 rounding a specimen (the field of view) will appear similarly colored. 

 The color of the image of a particle will be affected by color in either the 

 conjugate or the complementary area. Wavelengths transmitted by 

 the complementary area but not by the conjugate area will superimpose 

 a colored image like that produced by darkfield illumination with these 

 wavelengths. The introduction of color in either the conjugate or the 

 complementary area or in both is not a satisfactory method for produc- 

 ing color phase contrast. Color phase contrast is a means of causing the 

 detail in a specimen to appear colored whereas the background in the 

 image remains more or less neutral. To convert an ordinary objective 

 to a phase objective, one or more of the existing lens surfaces may be 

 altered to produce the conjugate and the complementary areas, or an 

 additional unit may be mounted in the objective. For example, the 

 additional unit may consist of one or more plane parallel glass plates 

 which have been coated or otherwise treated to form the diffraction 

 plate. The existing optical design of an objective system may not 

 always permit the insertion of an additional glass plate at a suitable 

 plane in the objective. 



2.1. Making a diffraction plate 



It is mentioned in Section 3.2 of Chapter II that an etching process 

 was- used to make some of the first diffraction plates. These plates did 

 not include an additional absorbing layer on either the conjugate or 

 the complementary area. A possible procedure is to etch a glass surface 

 in order to form a trough having the shape of the conjugate area and then 

 to cement a second glass surface over the etched surface. This second 

 glass surface should introduce no additional change in optical path across 

 the diffraction plate. The cement which fills the trough forms the con- 

 jugate area. The complementary area consists of a layer of glass equal 

 in thickness to the depth of the trough. If the index of refraction of the 

 cement is greater than that of the etched glass element, then the optical 

 path through the conjugate area is greater than the optical path through 

 the complementary area. If the index of refraction of the cement is 



