﻿Phenomena 
  with 
  Gamma 
  Rays. 
  193 
  

  

  3'5 
  mm.) 
  at 
  almost 
  grazing 
  incidence, 
  the 
  photographic 
  plate 
  

   was 
  only 
  2 
  cm. 
  from 
  the 
  mica, 
  and 
  the 
  hole 
  in 
  C 
  was 
  8 
  mm. 
  

   in 
  diameter. 
  With 
  this 
  aperture 
  much 
  more 
  radioactive 
  

   material 
  lay 
  in 
  the 
  path 
  of 
  the 
  beam, 
  and 
  a 
  faint 
  impression 
  

   from 
  a 
  direct 
  beam 
  could 
  be 
  obtained 
  with 
  between 
  five 
  and 
  

   ten 
  minutes 
  exposure. 
  An 
  exposure 
  of 
  five 
  days 
  was 
  taken. 
  

  

  Three 
  other 
  photographs 
  were 
  taken 
  with 
  the 
  mica 
  deviat- 
  

   ing 
  from 
  grazing 
  incidence 
  bv 
  about 
  five 
  degrees, 
  but 
  no 
  

   effects 
  were 
  obtained 
  at 
  this 
  angle. 
  

  

  As 
  the 
  j3 
  radiation 
  could 
  not 
  be 
  entirely 
  prevented 
  from 
  

   reaching 
  the 
  plate, 
  two 
  photographs 
  were 
  taken 
  in 
  which 
  the 
  

   f3 
  rays 
  had 
  only 
  to 
  traverse 
  one 
  millimetre 
  of 
  glass 
  in 
  the 
  

   path 
  of 
  the 
  beam. 
  Xo 
  intensification 
  of 
  the 
  "spots" 
  was 
  

   obtained, 
  and 
  it 
  was 
  assumed 
  that 
  the 
  7 
  rays 
  were 
  responsible 
  

   for 
  the 
  markings 
  in 
  figs. 
  2, 
  3, 
  and 
  4. 
  

  

  In 
  fig. 
  5 
  the 
  result 
  was 
  produced 
  by 
  hard 
  X-rays 
  under 
  

   the 
  same 
  general 
  experimental 
  conditions 
  as 
  in 
  fig. 
  3, 
  the 
  

   mica, 
  however, 
  deviating 
  slightly 
  from 
  grazing 
  incidence. 
  

   Mr. 
  W. 
  L. 
  Bragg 
  kindly 
  showed 
  the 
  writer 
  several 
  in- 
  

   teresting 
  photographs 
  which 
  also 
  illustrated 
  the 
  same 
  type 
  

   of 
  arrangement, 
  in 
  certain 
  cases 
  the 
  reflexions 
  from 
  the 
  main 
  

   cleavage-planes 
  being 
  less 
  intense 
  than 
  from 
  other 
  planes 
  at 
  

   greatly 
  inclined 
  angles. 
  

  

  The 
  intensity 
  of 
  the 
  "spots" 
  apparently 
  depends 
  on 
  the 
  

   number 
  of 
  molecules 
  in 
  a 
  reflecting 
  plane, 
  but 
  more 
  

   especially 
  on 
  the 
  distance 
  between 
  the 
  planes 
  considered 
  and 
  

   on 
  the 
  wave-length 
  of 
  the 
  incident 
  radiation. 
  In 
  the 
  case 
  of 
  

   the 
  soft 
  X-rays 
  used 
  for 
  the 
  reflexion 
  photographs 
  obtained 
  

   by 
  W. 
  L. 
  Bragg 
  with 
  mica, 
  the 
  distance 
  between 
  the 
  main 
  

   cleavage-planes 
  apparently 
  bears 
  such 
  a 
  proportion 
  to 
  the 
  

   wave-length 
  of 
  the 
  rays 
  that 
  the 
  main 
  reflected 
  spot 
  is 
  of 
  

   much 
  greater 
  intensity 
  than 
  any 
  of 
  those 
  reflected 
  from 
  

   other 
  planes. 
  If, 
  however, 
  very 
  hard 
  X-rays 
  are 
  used, 
  other 
  

   spots 
  spring 
  into 
  much 
  greater 
  prominence 
  than 
  the 
  one 
  

   reflected 
  from 
  the 
  cleavage-planes. 
  In 
  this 
  case 
  the 
  chief 
  

   reflecting 
  planes 
  are 
  closer 
  together 
  but 
  must 
  each 
  have 
  

   fewer 
  molecules, 
  hence 
  it 
  appears 
  that 
  the 
  distance 
  between 
  

   the 
  planes 
  must 
  be 
  the 
  more 
  important 
  factor 
  in 
  the 
  deter- 
  

   mination 
  of 
  the 
  intensity 
  of 
  reflexion 
  for 
  a 
  given 
  wave- 
  

   length. 
  

  

  An 
  examination 
  of 
  the 
  photographs 
  shows 
  that 
  the 
  effects 
  

   with 
  7 
  rays 
  are 
  obtained 
  in 
  one 
  of 
  the 
  directions 
  in 
  which 
  

   the 
  more 
  intense 
  spots 
  were 
  obtained 
  with 
  hard 
  X-rays. 
  The 
  

   distances 
  between 
  the 
  markings 
  in 
  the 
  different 
  cases 
  are 
  

   approximately 
  proportional 
  to 
  the 
  distance 
  of 
  the 
  plate 
  from 
  

   the 
  mica. 
  

  

  J'lul. 
  Mag. 
  S. 
  G. 
  Vol. 
  20. 
  No. 
  151. 
  July 
  1913. 
  

  

  