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  XIV. 
  The 
  Reflexion 
  of 
  the 
  X-rays. 
  By 
  H. 
  G. 
  J. 
  Moseley, 
  

   M.A., 
  John 
  Harling 
  Fellow, 
  and 
  0. 
  Gr. 
  Darwin, 
  M.A., 
  

   Lecturer 
  in 
  Mathematical 
  Physics 
  in 
  the 
  University 
  of 
  

   Manchester 
  *. 
  

   IT^HE 
  discovery 
  in 
  July 
  1912 
  by 
  Friedrich 
  and 
  Knipping 
  f 
  

   JL 
  that 
  the 
  X-rays 
  can 
  show 
  interference 
  has 
  opened 
  up 
  

   a 
  new 
  region 
  to 
  research. 
  It 
  had 
  been 
  suggested 
  to 
  them 
  

   by 
  Laue 
  that 
  the 
  ordered 
  arrangement 
  o£ 
  the 
  atoms 
  in 
  a 
  

   crystal 
  would 
  do 
  the 
  same 
  for 
  X-rays 
  that 
  a 
  diffraction- 
  

   grating 
  does 
  for 
  light. 
  They 
  therefore 
  sent 
  a 
  fine 
  beam 
  of 
  

   X-rays 
  through 
  a 
  thin 
  crystal 
  of 
  zinc- 
  blende 
  on 
  to 
  a 
  photo- 
  

   graphic 
  plate. 
  After 
  prolonged 
  exposure 
  to 
  the 
  radiation 
  

   the 
  plate 
  showed 
  an 
  elaborate 
  system 
  of 
  spots 
  surrounding 
  

   the 
  central 
  image. 
  These 
  spots 
  were 
  caused 
  by 
  fine 
  beams 
  

   of 
  X-rays, 
  which 
  came 
  from 
  the 
  crystal. 
  Explanations 
  of 
  

   this 
  phenomenon 
  have 
  been 
  offered 
  by 
  Laue 
  | 
  and 
  by 
  

   W. 
  L. 
  Bragg 
  %. 
  Laue 
  assumes 
  the 
  radiation 
  to 
  consist 
  of 
  a 
  

   finite 
  number 
  of 
  monochromatic 
  waves, 
  while 
  Bragg 
  believes 
  

   it 
  to 
  be 
  similiar 
  in 
  constitution 
  to 
  white 
  light. 
  The 
  

   explanation 
  of 
  the 
  latter 
  accounts 
  perfectly 
  for 
  the 
  positions 
  

   •of 
  the 
  spots 
  in 
  question. 
  The 
  present 
  paper 
  shows 
  that 
  a 
  

   large 
  proportion 
  of 
  the 
  radiation 
  from 
  an 
  X-ray 
  tube 
  hns 
  no 
  

   definite 
  wave-length, 
  but 
  that 
  five 
  types 
  of 
  monochromatic 
  

   waves 
  are 
  also 
  present 
  in 
  the 
  radiation 
  from 
  a 
  tube 
  with 
  a 
  

   platinum 
  target. 
  The 
  following 
  is 
  a 
  brief 
  statement 
  of 
  

   W. 
  L. 
  Bragg's 
  theory. 
  

  

  It 
  is 
  known 
  that 
  every 
  atom 
  in 
  a 
  crystal 
  scatters 
  a 
  small 
  

   fraction 
  of 
  the 
  incident 
  radiation. 
  Since 
  the 
  atoms 
  are 
  

   regularly 
  arranged 
  these 
  scattered 
  disturbances 
  have 
  definite 
  

   time 
  relations. 
  In 
  certain 
  directions 
  a 
  number 
  of 
  scattered 
  

   disturbances 
  reinforce 
  one 
  another, 
  and 
  so 
  give 
  rise 
  to 
  a 
  new 
  

   disturbance 
  of 
  considerable 
  strength. 
  The 
  co-operating 
  dis- 
  

   turbances 
  come 
  from 
  atoms 
  which 
  lie 
  in 
  a 
  plane. 
  This 
  plane, 
  

   the 
  incident 
  beam, 
  and 
  the 
  direction 
  of 
  the 
  new 
  beam 
  have 
  

   the 
  same 
  geometrical 
  relation 
  as 
  have 
  a 
  mirror, 
  an 
  incident 
  

   ray, 
  and 
  the 
  reflected 
  ray. 
  Wherever 
  a 
  plane 
  containing 
  

   many 
  atoms 
  can 
  be 
  drawn 
  in 
  the 
  crystal 
  a 
  ray 
  will 
  be 
  found 
  

   in 
  the 
  direction 
  which 
  is 
  the 
  reflexion 
  in 
  this 
  plane 
  of 
  the 
  

   incident 
  beam. 
  The 
  position 
  of 
  every 
  spot 
  in 
  Friedrich 
  and 
  

   Knipping's 
  photographs 
  can 
  be 
  found 
  from 
  this 
  principle. 
  

   A 
  cleavage-plane 
  is 
  exceptionally 
  rich 
  in 
  atoms, 
  and 
  Bragg 
  § 
  

  

  * 
  Communicated 
  by 
  Prof. 
  E. 
  Paitherford, 
  F.R.S. 
  

  

  + 
  Laue, 
  Friedrich 
  and 
  Knipping, 
  Kgl. 
  Bay. 
  Akad. 
  p. 
  303 
  (1912). 
  

  

  \ 
  W. 
  L. 
  Bragg, 
  Proc. 
  Camb. 
  Phil. 
  Soc. 
  xvii. 
  p. 
  43. 
  

  

  § 
  W. 
  L. 
  Bragg, 
  'Nature,' 
  vol. 
  xc. 
  p. 
  410, 
  Dec. 
  12 
  (1912). 
  

  

  