MAKING A DIFFRACTION PLATE 91 



refraction than that of the dielectric film will cause the conjugate area to 

 have, respectively, a smaller or greater optical path than the comple- 

 mentary area. To produce a given optical path difference, the thickness 

 to which the coating must be deposited depends on the refractive index 

 of both the dielectric and the cement, according to E(i. 2.7. A single 

 ruling process can also be followed if it is required that the complemen- 

 tary area absorb part of the deviated light. In this case the coating is 

 cleaned off the conjugate area, and again cements with the correct index 

 of refraction will produce the necessary optical path step. Another 

 variation is to deposit the absorbing film on one surface and the dielectric 

 on the other of two surfaces to be ruled and cemented together. This 

 procedure may be useful if it is preferred that the absorbing and di- 

 electric films be on adjacent areas of the diffraction plate. For example, 

 the dielectric material is removed from the conjugate area and the 

 absorbing material is removed from the complementary area. Then a 

 cement of higher index of refraction than the dielectric forms a conjugate 

 area which has a greater optical path than the complementary area. 

 Such a procedure requires more steps and makes the additional demand 

 that the two parts be centered with respect to each other. The final 

 diffraction plate should always be centered on the optical axis of the 

 objective. 



A very satisfactory way to shield part of the surface during the 

 evaporation is to hold rings or disks punched out of very thin iron or 

 steel sheet in contact with the surface being coated by means of a pair 

 of small, unlike magnetic pole pieces placed behind the second surface 

 of the plate or lens. If the films so formed are embedded in cement, the 

 considerations concerning optical path differences are the same as when 

 part of the coating is ruled off. If air rather than cement is adjacent 

 to the coating, then the dielectric film must be deposited on the con- 

 jugate area if a particle having a greater optical path than its surround 

 is to appear in bright contrast; but if this same particle is to appear in 

 dark contrast, the dielectric must be placed on the complementary area. 

 With thin steel masks it is possible to shield one area while coating the 

 adjacent region, and then to remove the first mask, cover the coated 

 part of the surface with a second mask, and deposit a film over the area 

 that had been protected during the first evaporation. When an annular 

 conjugate area is being produced, a non-magnetic cap fitting over the 

 outer portion of the surface being coated or a holder with the required 

 aperture is used in conjunction with the steel disk in order to shield both 

 parts of the complementary area. 



Several facts should be kept in mind if evaporated metal and dielec- 

 tric films are used. Equation 2.7 states that the greater the difference 



