90 INSTRUMENTATION 



less than that of the etched glass element, then the optical path through 

 the conjugate area is less than the optical path through the comple- 

 mentary area. If a lens surface is etched, its refractive index has been 

 predetermined by the optical design, and either the depth of the etch 

 or the refractive index of the cement is varied to produce the specified 

 optical path difference. Ob^iously, it is possible to begin by etching 

 the trough in the shape of the complementary area and again to achieve 

 the recjuired optical path difference by using a cement with a suitable 

 index of refraction. It would also be possible to add an absorbing coat- 

 ing over the specified area of the etched surface or over the second glass 

 surface. If the absorbing film of the correct size and shape is applied 

 over the second glass surface, this second element must be centered 

 with respect to the etched surface so that the discontinuity in amplitude 

 transmission occurs at the same place as the discontinuity in optical 

 path. At present other methods of forming the optical path step are 

 preferred. 



The technics developed for e\'aporating thin films in a high \'acuum 

 offer a very useful method for making difiraction plates. Optical path 

 differences may be produced by evaporating a dielectric material to the 

 required thickness, and absorption may be introduced by evaporating 

 a metallic film. However, the absorbing film need not be a metallic one. 

 In general, a neutral conjugate area is preferred. For example, mag- 

 nesium fluoride and Inconel form a suitable combination of materials. 

 It is possible to proceed by evaporating soft films which are then partly 

 rubbed off to leave a coating having the required shape and area, or it is 

 possible to use masks during the evaporation so that durable films of 

 only the required pattern are deposited. A second surface may or may 

 not be cemented over the coated surface. For the purposes of this 

 discussion it will be assumed that the optical path is produced by a 

 transparent dielectric film and that the absorption is produced by a film 

 so thin that the optical path through it is negligible compared with the 

 final optical path difference required. Many metallic films, if properly 

 formed, have a thickness equi\'alent to an optical path of less than X/20 

 even at energy transmissions of only a few per cent. 



If soft dielectric and absorl)ing films are successively deposited on the 

 same surface and if a single ruling procedure is used to remove the coat- 

 ing from the complementary area, then the coating that remains on the 

 conjugate area forms an absorbing step which has a greater optical path 

 than the adjacent air path through the complementary area. This by 

 itself would make a bright-contrast plate for particles having an optical 

 path greater than that of the surround. If a second element is cemented 

 over the coated surface, then a cement with a higher or lower index of 



