MAKING A DIFFRACTION PLATE 93 



so that films of both the required thickness and the required tra.ns- 

 mission are spread. 



Plastic films are particularly of interest when some control of the 

 dispersion of the materials constituting the diffraction plates is necessary. 

 It will be seen later in this section that the problem of finding substances 

 with a given dispersion occurs when either achromatic or color diffraction 

 plates are made. Glasses and those substances which can be evaporated 

 to produce a usable film do not offer so much variety and flexibility as 

 do plastics and cements in changing the relative dispersions of the 

 materials which form the conjugate and complementary areas. For 

 example, if cellulose nitrate, cellulose acetate, or cellulose butyrate in 

 solution, or a mixtiu'e of these, is spread on glass to produce the plastic 

 film, plasticizers are available which can be added to the solution in 

 order to control the dispersion of the final film. Also, as an example, 

 the addition of chlorinated or brominated aromatic hydrocarbons such 

 as bromonaphthalene is a means of changing the dispersion of the 

 common cement, Canada balsam. 



It is known that a phase ol)jective which contains a diffraction plate 

 made with magnesium fluoride and Inconel adjacent to air as the second 

 dielectric material produces an image with good contrast, either bright 

 or dark, when white light is used to illuminate the specimen, although 

 the optical path step has been made equal to 3^ wavelength at X = 

 5461 A. Such a diffraction plate is not achromatic; i.e., the optical 

 path is not equal to 3^ wavelength for all wavelengths of visible light. 

 The dispersion of the common dielectric optical substances, including 

 magnesium fluoride, is such that the index of refraction for the blue 

 wavelengths of light is greater than that for the red wavelengths. 

 Therefore, if one of these substances is used adjacent to air in order to 

 produce an optical path step of X/4 at X = 5461 A, the dispersion is such 

 as to increase a deviation from 3^ wavelength at both the blue and the 

 red ends of the visible range. Some predictions made by the simple 

 theory about the deterioration in contrast due to the use of white light 

 rather than a monochromatic source were discussed in Section 1. If the 

 optical paths through the two areas of the general-purpose diffraction 

 plate are paths through magnesium fluoride, metal, and air, some im- 

 provement in image contrast occurs if a green filter, such as the Wratten 

 B No. 58, is placed in front of a heterochromatic light source. At least 

 in some instances this improvement may be due in part to the fact that 

 some of the adverse effects due to the aberrations of the objective have 

 been decreased when the filter is used. If the general-purpose plate 

 produces good contrast with white light, it will, in general, still produce 

 satisfactory contrast if any one of the Wratten M series of filters for 



