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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  as 
  shown 
  by 
  the 
  lowering 
  in 
  pitch 
  of 
  the 
  X-ray 
  echo, 
  but 
  we 
  have 
  

   found 
  also 
  the 
  recoiling 
  electrons 
  from 
  which 
  the 
  photons 
  have 
  

   bounced. 
  

  

  In 
  order, 
  however, 
  to 
  satisfy 
  ourselves 
  by 
  a 
  crucial 
  test 
  whether 
  

   X 
  rays 
  act 
  like 
  particles, 
  an 
  experiment 
  was 
  devised 
  which 
  should 
  

   enable 
  us 
  to 
  follow 
  the 
  path 
  of 
  the 
  photon 
  after 
  it 
  has 
  been 
  deflected 
  

   by 
  an 
  electron. 
  In 
  Figure 
  6 
  we 
  see 
  at 
  the 
  left 
  what 
  we 
  may 
  call 
  the 
  

   X-ray 
  gun, 
  which 
  shoots 
  a 
  few 
  X 
  rays 
  through 
  a 
  cloud-expansion 
  

   chamber. 
  In 
  this 
  chamber 
  is 
  photographed 
  the 
  trail 
  of 
  every 
  electron 
  

   set 
  in 
  motion 
  by 
  the 
  X 
  rays. 
  So 
  feeble 
  a 
  beam 
  of 
  X 
  rays 
  is 
  used 
  that 
  

   on 
  the 
  average 
  only 
  one 
  or 
  two 
  recoil 
  electrons 
  will 
  appear 
  at 
  a 
  time. 
  

   Let 
  us 
  suppose, 
  as 
  in 
  the 
  figure, 
  that 
  the 
  electron 
  struck 
  by 
  the 
  

   X-ray 
  particle 
  recoils 
  downward. 
  This 
  must 
  mean 
  that 
  the 
  X-ray 
  

  

  X-RAYS 
  

  

  FiGUKE 
  6.— 
  Diagram 
  of 
  an 
  experiment 
  in 
  which 
  one 
  observes 
  both 
  the 
  recoiling 
  electron 
  and 
  the 
  

   direction 
  in 
  which 
  the 
  deflected 
  photon 
  proceeds 
  

  

  particle 
  has 
  been 
  deflected 
  upward 
  toward 
  A. 
  If 
  this 
  X 
  ray 
  should 
  

   strike 
  another 
  electron 
  before 
  it 
  leaves 
  the 
  chamber, 
  this 
  event 
  must 
  

   occur 
  at 
  some 
  point 
  along 
  the 
  line 
  OA. 
  It 
  can 
  not 
  occur 
  on 
  the 
  same 
  

   side 
  as 
  the 
  recoil 
  electron. 
  If, 
  however, 
  the 
  X 
  ray 
  is 
  a 
  wave, 
  spreading 
  

   in 
  all 
  directions, 
  there 
  is 
  no 
  more 
  reason 
  why 
  the 
  second 
  electron 
  

   associated 
  with 
  the 
  scattered 
  ray 
  should 
  appear 
  at 
  A 
  than 
  at 
  B. 
  A 
  

   series 
  of 
  photographs 
  which 
  shows 
  the 
  relation 
  between 
  the 
  direction 
  

   of 
  recoil 
  of 
  the 
  scattering 
  electron 
  R 
  and 
  the 
  location 
  of 
  the 
  second 
  

   electron 
  struck 
  by 
  the 
  scattered 
  X 
  ray, 
  thus 
  affords 
  a 
  crucial 
  test 
  

   between 
  the 
  conceptions 
  of 
  X 
  rays 
  as 
  spreading 
  waves 
  and 
  X 
  rays 
  as 
  

   particles. 
  

  

  From 
  a 
  large 
  number 
  of 
  photographs 
  taken 
  in 
  this 
  manner 
  it 
  has 
  

   become 
  evident 
  that 
  an 
  X 
  ray 
  is 
  scattered 
  in 
  a 
  definite 
  direction, 
  

  

  