554 



BELL SYSTEM TECHNICAL JOURNAL 



values which satisfy the Bragg formula sin 6 = n\l2d, where X repre- 

 sents wave-length, d the distance between atom planes parallel to the 

 surface, and n an integer. We expect this to occur with reflection of 

 electrons, if the de Broglie waves are scattered by underlying atom 

 layers as well as by the outermost. I will show later that the reflection 

 portrayed in Fig. 1 is, indeed, selective. But with low-speed electrons, 

 there are confusing complications which we can avoid by dealing with 

 electrons of considerably higher speed; I will, therefore, begin by dis- 

 playing the selective reflection of electrons accelerated through thou- 

 sands rather than tens of volts only. 



I' 



ST 



ORDER 



l2ND l3RD 



4TH 



# 



4* 



A 6 



DEGREES 



10 



Fig. 2— Photographic record of selective reflection of 53-kv. electrons by a crystal 



of iron — (100) face. 



In Fig. 2 we have a photographic record of the reflection of a beam of 

 53 kilovolt electrons from a crystal of iron. The experimental ar- 

 rangement is essentially the same as that indicated in Fig. 1, though 

 devices rather more elaborate are required for producing electron beams 

 of such speeds. A photographic plate set at right angles to the direc- 

 tion of the primary beam and 30 or 40 cms. beyond the crystal replaces 

 the exploring electrode. Each strip in Fig. 2 is the record of reflection 

 at a particular angle of glancing. After each exposure the cr^-stal is 

 turned through the position of grazing (zero glancing angle), the pri- 

 mary beam falls directly upon the plate, and its direction is thus 

 recorded. These fiducial marks appear in a column on the left. The 



