﻿The 
  Reflexion 
  of 
  the 
  X-rays, 
  211 
  

  

  confirmed 
  his 
  theory 
  by 
  throwing 
  a 
  beam 
  of 
  X-rays 
  on 
  to 
  a 
  

   cleavage 
  surface 
  of 
  mica, 
  and 
  observing 
  a 
  strong 
  image 
  in 
  

   the 
  position 
  which 
  an 
  optically 
  reflected 
  beam 
  would 
  occupy. 
  

  

  So 
  far, 
  the 
  subject 
  had 
  been 
  developed 
  entirely 
  by 
  

   photography. 
  The 
  first 
  object 
  of 
  our 
  investigation 
  was 
  to 
  

   test 
  whether 
  the 
  " 
  reflected 
  " 
  radiation, 
  which 
  was 
  so 
  clearly 
  

   shown 
  to 
  have 
  an 
  extended 
  wave-front, 
  possessed 
  those 
  

   properties 
  which 
  have 
  led 
  Prof. 
  W. 
  H. 
  Bragg 
  to 
  suggest 
  

   that 
  X-rays 
  are 
  corpuscular. 
  It 
  was 
  still 
  possible 
  that 
  the 
  

   X-rays 
  were 
  really 
  a 
  mixture 
  of 
  two 
  different 
  constituents. 
  

  

  At 
  first 
  we 
  followed 
  the 
  experimental 
  arrangement 
  of 
  

   Fried 
  rich 
  and 
  Knipping. 
  When, 
  however, 
  W. 
  L. 
  Bragg 
  

   discovered 
  that 
  a 
  greater 
  effect 
  was 
  obtained 
  by 
  " 
  reflexion 
  " 
  

   from 
  mien, 
  his 
  method 
  was 
  adopted, 
  and 
  was 
  found 
  to 
  be 
  

   much 
  more 
  convenient. 
  Later 
  W. 
  H. 
  Bragg 
  * 
  announced 
  

   in 
  ' 
  Nature 
  ' 
  that 
  the 
  reflected 
  radiation 
  ionized 
  air, 
  and 
  we 
  f 
  

   were 
  able 
  to 
  add 
  that 
  it 
  had 
  all 
  the 
  principal 
  properties 
  

   which 
  characterize 
  ordinary 
  X-rays. 
  We 
  then 
  tried 
  to 
  

   determine 
  how 
  the 
  intensity 
  of 
  reflexion 
  depends 
  on 
  the 
  

   angle 
  of 
  incidence 
  on 
  the 
  surface 
  of 
  a 
  crystal. 
  We 
  first 
  

   measured 
  the 
  reflexion 
  from 
  selenite, 
  and 
  found 
  that 
  as 
  the 
  

   angle 
  of 
  incidence 
  was 
  varied 
  from 
  88° 
  to 
  55° 
  the 
  reflexion 
  

   fell 
  to 
  a 
  thousandth 
  of 
  its 
  initial 
  value. 
  Our 
  results 
  showed 
  

   certain 
  singularities, 
  which 
  we 
  hoped 
  to 
  explain 
  by 
  studying 
  

   other 
  crystals. 
  At 
  this 
  stage, 
  however, 
  Prof. 
  Bragg 
  very 
  

   kindly 
  communicated 
  to 
  us 
  results 
  obtained 
  with 
  other 
  

   crystals, 
  which 
  he 
  has 
  since 
  published 
  in 
  conjunction 
  with 
  

   Mr. 
  W. 
  L. 
  Bragg 
  {. 
  They 
  found 
  the 
  intensity 
  of 
  reflexion 
  

   to 
  be 
  abnormally 
  large 
  at 
  certain 
  angles. 
  They 
  attributed 
  

   this 
  selective 
  reflexion 
  to 
  the 
  presence 
  in 
  the 
  radiation 
  of 
  

   monochromatic 
  waves. 
  These 
  waves 
  were 
  thought 
  to 
  be 
  

   characteristic 
  radiation 
  from 
  the 
  platinum 
  target 
  in 
  the 
  

   X-ray 
  tube. 
  When 
  the 
  phase 
  difference 
  between 
  the 
  waves 
  

   " 
  reflected 
  " 
  from 
  successive 
  layers 
  of 
  atoms 
  is 
  27T, 
  the 
  

   waves 
  reinforce 
  one 
  another 
  in 
  the 
  reflected 
  beam. 
  In 
  

   the 
  light 
  of 
  this 
  communication 
  we 
  re-examined 
  the 
  reflexion 
  

   from 
  selenite. 
  We 
  found 
  that 
  this 
  was 
  the 
  cause 
  of 
  the 
  

   singularities 
  which 
  we 
  had 
  noticed. 
  By 
  using 
  very 
  fine 
  slits 
  

   we 
  had 
  missed 
  the 
  position 
  at 
  which 
  the 
  principal 
  selective 
  

   reflexion 
  occurs. 
  Our 
  later 
  work, 
  in 
  which 
  selenite. 
  rocksalt, 
  

   an 
  1 
  potassium 
  ferrocyanide 
  have 
  been 
  used 
  as 
  reflectors, 
  fully 
  

   confirm- 
  the 
  conclusions 
  of 
  Prof, 
  and 
  W. 
  L. 
  Bragg. 
  At 
  the 
  

  

  

  * 
  W. 
  H. 
  Bragg, 
  'Nature,' 
  Jan. 
  23 
  (1918). 
  

  

  t 
  Moseley 
  and 
  Darwin, 
  'Nature.' 
  Jan. 
  :'>0 
  (1013) 
  

  

  \ 
  Proc. 
  Koy. 
  Soc. 
  A, 
  vol. 
  lxxxviii. 
  p. 
  128. 
  

  

  r 
  2 
  

  

  