l.'iC) 



MICROSCOPIC techniquj:.s 



ation has a wavelength 200-600 X.U. shorter than that for the 

 absorption edge of the element whose secondary radiation is desired. 

 Accordingly, it is preferable to use K-radiation from copper ( atomic 

 number, 29) to excite K-radiation from iron (26). If there are no 

 suitable lines for the secondary radiation the continuous radiation 

 from an element of high atomic number such as tungsten (74» or 

 platinum (78) is used. 



Spectrograph for Microanalysis. For wavelengths shorter 

 than 2.5 A it is not necessary to enclose the spectrograph in a vaccum, 

 since the absorption due to air becomes appreciable only for wave- 

 lengths greater than 2.5 A. The spectrograph for microanalysis is 



Fig. 25. Microcuvette. The hatched area is the volume employed. 



From Engstrom (1946) 



used with these shorter wavelengths; the diagram of the instrument 

 is shown in Figure 24. Either photographic or ionization measure- 

 ments may be made of the radiation intensity with this apparatus. 

 The roentgen tube (.4) delivers a stream of radiation through the 

 spectrograph slit (B) to the crystal (C) which is mounted in the 

 holder (D). The monochromatic beam produced passes both above 

 and through the microcuvette (E) and onto the photographic film 

 in the holder (F) . The cuvette can be moved relative to the film by 

 the micrometer screw (G). An ionization chamber (H) may be 

 substituted for the photographic film; the chamber is directly con- 

 nected to an electrometer by its central electrode. (An Edelmann 

 string electrometer was employed.) A Bakelite bar (7) operates a 

 cog by which the cuvette may be moved into or out of the path of 



