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to the framework of the apparatus. The left-hand or collector enclo- 

 sure contains the second reflector and the collector, and is supported 

 from the frame of the apparatus through bearings by which it can be 

 rotated about a horizontal axis. Communication between the en- 

 closures is through the right-hand bearing which is hollow. The 

 sections of the enclosures at right angles to the plane of the drawing 

 are square. 



The electron gun is similar in construction to the one described in an 

 earlier paper ^ to which we refer for the details. The apertures are 

 circular and those which define the beam are 2 mm. in diameter. 



Fig. 1 — Cross-section of the experimental apparatus — 0.8 actual size. 



The reflectors were cut from a single crystal of nickel formed by the 

 slow freezing of pure nickel in vacuum. Their faces, which were 

 polished to fairly good optical flats and then lightly etched by acid, 

 are approximately 4X4 mm. in extent. The normals to these faces 

 diverge from one of the {111} -directions of the crystal structure by only 

 about 10 minutes of arc. 



The reflectors are so mounted that for each of them the incident 

 beam lies in what we have designated as a {111} -azimuth of the crystal 

 structure, as illustrated in the schematic diagram, Fig. 2. This 

 adjustment may be unimportant, but was made because it has not yet 

 been established that the selectivity of reflection is independent of the 

 azimuth of the incident beam. The { 111 } -azimuth was chosen rather 



3 C. J. Davisson and L. H. Germer, Phys. Rev., 30, 705 (1927). 



