﻿and 
  Secondary 
  Gamma 
  Rays. 
  

  

  283 
  

  

  screens 
  were 
  also 
  used, 
  and 
  these 
  resembled 
  iron, 
  not 
  lead, 
  in 
  

   the 
  effects 
  produced. 
  It 
  is 
  then 
  easy 
  to 
  produce 
  a 
  change 
  

   with 
  less 
  than 
  a 
  centimetre 
  of 
  lead 
  which 
  cannot 
  be 
  brought 
  

   about 
  by 
  many 
  centimetres 
  o£ 
  iron. 
  This 
  peculiarity 
  was 
  

   first 
  noticed 
  by 
  Bragg 
  and 
  Madsen 
  *, 
  and 
  it 
  cannot 
  be 
  attri- 
  

   buted 
  to 
  selective 
  absorption 
  because 
  a 
  variation 
  in 
  the 
  cha- 
  

   racter 
  o£ 
  the 
  screens 
  causes 
  corresponding 
  modifications 
  in 
  

   the 
  radiations 
  from 
  other 
  pairs 
  of 
  plates^ 
  such 
  as 
  copper-carbon 
  

   or 
  copper-aluminium. 
  

  

  After 
  the 
  7 
  rays 
  have 
  passed 
  through 
  iron 
  screens 
  similar 
  

   changes 
  can 
  be 
  produced 
  by 
  a 
  moderate 
  thickness 
  of 
  lead. 
  

   Thus,— 
  

  

  Screens 
  

  

  Fe 
  2-1. 
  

  

  fFe21 
  

   [Pb 
  -16 
  

  

  Fe 
  2-1. 
  

  

  Pb 
  -48. 
  

  

  Fe 
  2-1 
  cm. 
  

   Pb 
  •64 
  cm. 
  

  

  Pb 
  

  

  100 
  

   91 
  

  

  100 
  

   93 
  

  

  100 
  

   99 
  

  

  100 
  

   100 
  

  

  Al 
  

  

  

  Iron, 
  however 
  thick, 
  appeared 
  incapable 
  of 
  producing 
  

   equivalent 
  results. 
  

  

  It 
  is 
  possible 
  that 
  the 
  true 
  explanation 
  is 
  a 
  little 
  complex. 
  

   When 
  primary 
  7 
  rays 
  pass 
  through 
  iron 
  they 
  give 
  rise 
  to 
  

   secondary 
  7 
  rays, 
  softer 
  than 
  the 
  primary 
  which 
  caused 
  them, 
  

   but 
  nevertheless 
  capable 
  of 
  penetrating 
  a 
  considerable 
  thick- 
  

   ness 
  of 
  iron. 
  The 
  same 
  is 
  true 
  of 
  zinc, 
  copper, 
  and 
  other 
  

   substances 
  of 
  a 
  like 
  order 
  of 
  atomic 
  weight. 
  When, 
  however, 
  

   primary 
  7 
  rays 
  pass 
  through 
  lead, 
  secondary 
  7 
  rays 
  are 
  pro- 
  

   duced 
  to 
  a 
  less 
  extent 
  than 
  with 
  iron, 
  or 
  they 
  are 
  so 
  rapidly 
  

   absorbed 
  that 
  they 
  are 
  more 
  difficult 
  to 
  detect. 
  Hence 
  thick 
  

   iron 
  screens 
  do 
  not 
  harden 
  the 
  rays, 
  but 
  lead 
  screens 
  do 
  so 
  

   rapidly. 
  

  

  When 
  the 
  U-shaped 
  curves 
  shown 
  in 
  the 
  preceding 
  figures 
  

   have 
  the 
  left 
  branch 
  high 
  compared 
  with 
  the 
  right, 
  it 
  seems 
  

   to 
  indicate 
  hard 
  rays 
  ; 
  when 
  low, 
  soft 
  rays. 
  In 
  order 
  to 
  put 
  

   this 
  to 
  the 
  test, 
  I 
  examined 
  the 
  7 
  rays 
  from 
  uranium 
  X, 
  and 
  

   also 
  the 
  secondary 
  7 
  rays 
  from 
  iron 
  produced 
  by 
  radium 
  C. 
  

   Both 
  types 
  of 
  radiation 
  are 
  known 
  to 
  be 
  softer 
  than 
  the 
  

   primary 
  7 
  rays 
  of 
  radium. 
  

  

  * 
  Phil. 
  Ma?. 
  Dec. 
  1908. 
  

  

  