﻿Phenomena 
  with 
  Gamma 
  Rays. 
  191 
  

  

  plate 
  tended 
  to 
  render 
  the 
  presence 
  of 
  possible 
  "spots" 
  

   indeterminable. 
  (2) 
  It' 
  the 
  direct 
  j3 
  rays 
  were 
  eliminated, 
  

   an 
  exposure 
  of 
  over 
  an 
  hour 
  was 
  necessary 
  in 
  order 
  to 
  

   produce 
  the 
  faintest 
  impression 
  on 
  a 
  photographic 
  plate 
  with 
  

   a 
  beam 
  of 
  y 
  rays 
  three 
  millimetres 
  in 
  diameter, 
  the 
  source 
  

   being 
  so 
  arranged 
  that 
  the 
  beam 
  was 
  produced 
  by 
  about 
  

   fifteen 
  milligrams 
  of 
  radium 
  bromide. 
  This 
  meant 
  that 
  an 
  

   exposure 
  of 
  many 
  weeks 
  would 
  be 
  required 
  in 
  order 
  to 
  

   obtain 
  the 
  desired 
  " 
  spots. 
  1 
  ' 
  and 
  in 
  acting 
  over 
  such 
  an 
  

   extended 
  period 
  the 
  first 
  difficulty 
  tended 
  to 
  become 
  in- 
  

   superable. 
  

  

  W. 
  L. 
  Bragg* 
  has 
  shown 
  that 
  the 
  spots 
  in 
  the 
  inter- 
  

   ference 
  patterns 
  obtained 
  by 
  Laue 
  can 
  be 
  treated 
  as 
  though 
  

   they 
  were 
  formed 
  by 
  reflexion 
  of 
  the 
  incident 
  beam 
  at 
  

   planes 
  which 
  are 
  packed 
  with 
  atoms 
  (or 
  molecules) 
  deviating 
  

   from 
  the 
  planes 
  by 
  distances 
  that 
  are 
  small 
  compared 
  with 
  

   atomic 
  dimensions. 
  He 
  found 
  that 
  the 
  reflexion 
  of 
  "Rontgen 
  

   rays 
  from 
  crystals 
  containing 
  very 
  marked 
  cleavage-planes 
  

   illustrated 
  the 
  phenomenon 
  much 
  more 
  readily. 
  It 
  required 
  

   only 
  a 
  short 
  exposure 
  in 
  order 
  to 
  get 
  the 
  reflected 
  spot, 
  

   while 
  in 
  the 
  case 
  of 
  the 
  complete 
  pattern, 
  exposures 
  of 
  many 
  

   hours 
  were 
  necessary. 
  It 
  was 
  at 
  first 
  assumed 
  that 
  if 
  the 
  

   phenomenon 
  existed 
  for 
  y 
  rays, 
  it 
  would 
  likewise 
  be 
  easier 
  

   to 
  detect 
  their 
  reflexion 
  from 
  the 
  main 
  cleavage-planes. 
  

   Experiments, 
  however, 
  showed 
  that 
  this 
  was 
  not 
  the 
  case, 
  

   but 
  actual 
  results 
  were 
  obtained 
  which 
  suggested 
  an 
  in- 
  

   tcresting 
  explanation 
  upon 
  comparison 
  with 
  X-ray 
  photo- 
  

   graphs 
  taken 
  under 
  similar 
  conditions. 
  

  

  Fig. 
  1. 
  

   A 
  

  

  1 
  tA 
  diagram 
  of 
  the 
  experimental 
  arrangement 
  is 
  shown 
  in 
  

   fig. 
  1. 
  The 
  " 
  source" 
  is 
  contained 
  in 
  a 
  lead 
  cylinder 
  B, 
  and 
  

  

  • 
  W. 
  L. 
  Bragg, 
  Camb. 
  Phil. 
  Soc. 
  vol. 
  xvii. 
  p. 
  43 
  (1913), 
  and 
  .Science 
  

   Progress, 
  vol. 
  vii. 
  No. 
  27, 
  p. 
  372 
  (1918). 
  

  

  