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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  If 
  it 
  were 
  possible 
  to 
  photograph 
  instantaneously 
  the 
  photons 
  in 
  

   an 
  intense 
  beam 
  of 
  light, 
  we 
  might 
  expect 
  them 
  to 
  have 
  somewhat 
  

   the 
  appearance 
  of 
  Figure 
  7. 
  Where 
  the 
  electric 
  field 
  of 
  the 
  correspond- 
  

   ing 
  electromagnetic 
  wave 
  is 
  a 
  maximum, 
  there 
  will 
  be 
  a 
  maximum 
  

   density 
  of 
  distribution 
  of 
  the 
  photons. 
  There 
  is, 
  however, 
  this 
  

   defect 
  with 
  our 
  picture, 
  that 
  there 
  seems 
  to 
  be 
  no 
  possible 
  way 
  in 
  

   which 
  we 
  can 
  experimentally 
  locate 
  the 
  individual 
  photons 
  within 
  

   the 
  wave. 
  Our 
  picture 
  must 
  thus 
  be 
  considered 
  to 
  be 
  a 
  purely 
  imag- 
  

   inary 
  one. 
  It 
  will, 
  however, 
  serve 
  to 
  indicate 
  that 
  the 
  conceptions 
  

   of 
  waves 
  and 
  particles 
  are 
  not 
  irreconcilable. 
  

  

  Perhaps 
  enough 
  has 
  been 
  said 
  to 
  show 
  that 
  by 
  grasping 
  both 
  

   horns 
  it 
  has 
  been 
  found 
  possible 
  to 
  overcome 
  the 
  dilemma. 
  Though 
  

   no 
  simple 
  picture 
  has 
  been 
  invented 
  affording 
  a 
  mechanical 
  model 
  

   of 
  a 
  light 
  ray, 
  by 
  combining 
  the 
  notions 
  of 
  waves 
  and 
  particles 
  a 
  

  

  

  AAA 
  

  

  FiGUKE 
  7.— 
  Waves 
  of 
  photons. 
  The 
  curve 
  represents 
  a 
  continuous 
  electromagnetic 
  wave; 
  below 
  the 
  

   curve 
  the 
  wave 
  is 
  represented 
  as 
  successive 
  sheets 
  of 
  photons 
  

  

  logically 
  consistent 
  theory 
  has 
  been 
  devised 
  which 
  seems 
  essentially 
  

   capable 
  of 
  accounting 
  for 
  the 
  properties 
  of 
  light 
  as 
  we 
  know 
  them. 
  

   Radio 
  rays, 
  heat 
  rays, 
  visible 
  and 
  ultra-violet 
  light, 
  all 
  are 
  thus 
  

   different 
  varieties 
  of 
  light. 
  We 
  find 
  from 
  experiments 
  on 
  diffraction 
  

   and 
  interference 
  that 
  light 
  consists 
  of 
  waves. 
  The 
  photoelectric 
  

   effect 
  and 
  the 
  scattering 
  of 
  X 
  rays 
  give 
  equally 
  convincing 
  reasons 
  

   for 
  believing 
  that 
  light 
  consists 
  of 
  particles. 
  For 
  centuries 
  it 
  has 
  

   been 
  supposed 
  that 
  the 
  two 
  conceptions 
  are 
  contradictory. 
  Goaded 
  

   on, 
  however, 
  by 
  obstinate 
  experiments, 
  we 
  seem 
  to 
  have 
  found 
  a 
  

   way 
  out. 
  We 
  continue 
  to 
  think 
  of 
  light 
  propagated 
  as 
  electromag- 
  

   netic 
  waves; 
  but 
  whenever 
  the 
  light 
  does 
  something, 
  it 
  does 
  it 
  as 
  

   photons. 
  Light 
  is 
  thus 
  in 
  some 
  respects 
  similar 
  to 
  waves 
  and 
  in 
  

   others 
  to 
  particles, 
  but 
  can 
  not 
  be 
  identified 
  completely 
  with 
  either. 
  

  

  