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BELL SYSTEM TECHNICAL JOURNAL 



direction of incidence (the angles of the equations). Now and then 

 the desired coincidence occurs. 



This is Bragg's method. Instead of the fluorescent screen there is 

 usually a photographic film bent to follow an arc, or an ionization- 

 chamber swinging over an arc, of the surface of the imaginary bulb. 



(II) If the incident beam is monochromatic or nearly so, and its 

 wave-length can be varied continuously, we can keep the crystal 

 motionless and the direction of incidence constant, and yet count on 

 the coincidence turning up occasionally (Figs. 13, 15 and 16). 



Fig. 7 — Illustrating how a crystal (T) is mounted so that it may be oriented in 

 various ways relatively to the oncoming beam of electron-waves (emerging from the 

 electron-gun on the left) while a collector (C) is moved around to catch the diffraction 

 beams. (Davisson and Germer.) 



This is the method used with electron-waves by Davisson and 

 Germer, the method whereby it was first shown that free negative 

 electricity possesses some of the qualities of waves. To waves of this 

 kind it is especially adapted, as their wave-lengths can easily be 

 varied by varying the voltage-rise which endows the electrons with 

 their speed. Instead of a fluorescent screen a Faraday chamber is 

 used which swings in an arc over the surface of the imaginary bulb. 



(Ill) If the incident beam is a mixture of wave-lengths covering a 

 very wide range, we can keep the crystal motionless and the direction 

 of incidence constant, and yet count on the coincidence turning up 

 for some of the wave-lengths (Figs. 8, 9, 10, 11 and 12). 



This is Laue's method, the first to be applied to the analysis of 

 crystal structure, and the one whereby it was first proved that X-rays 

 are waves — as people said at the time, which was 1912. Nowadays 



