X-RAY MICROSCOPY 



electrons 



electron source 



Fig. 1. Principle of the projection microscope. 



Fig. 2. 1500 mesh per inch silver grid, demon- 

 strating the large depth of field. (Experimental 

 Delft microscope.) 



these elements are replaceable. Thus, the 

 projection microscope is a demountable sys- 

 tem continuously evacuated to maintain the 

 required vacumn. 



Properties. The advantage of using x- 

 rays for microscopy is discussed elsewhere in 

 this volume. This section will deal primarily 



with the specific aspects of the projection 

 microscope as compared with the widely 

 used contact method. (See B. L. Henke, p. 

 675.) These are: 



(1) The resolution is not limited by the 

 resolution of either the recording material 

 or the optical microscope. As the initial mag- 

 nification can be adapted to the recording 

 material, the choice of the film is more a 

 matter of working convenience. Usually such 

 a fihn is chosen to allow some 10 X optical 

 magnification. For printing at this magnifi- 

 cation a normal enlarger may be used. Le 

 Poole and Ong (9) show that the use of 

 ultra-fine film (of the Lippmann type) for 

 projection microscopy has some advantages. 



(2) The resolution is determined by the 

 source size and the diffraction phenomena 

 exclusively. In the region where diffraction 

 does not affect the resolution, the unsharp- 

 ness is the product of source size and mag- 

 nification. As a result, the depth of field is 

 very large. (For diffraction error, see B. L. 

 Henke, p. 677.) The magnification varies 

 with the source to specimen distance result- 

 ing in a perfect perspective of the specimen. 

 See figs. 2 and 3. This fact is particularly im- 

 portant for stereo microscopy. 



(3) The specimen and film are spatially 

 separated. This fact allows us to: (a) study 

 specimens at high temperature; (b) study 



Fig. 3. Plastic sponge, ca. 50X. (Experimental 

 Delft microscope.) 



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