﻿914 
  Dr. 
  J. 
  P. 
  V. 
  Madsen 
  on 
  the 
  

  

  the 
  case 
  of 
  all 
  three, 
  and 
  the 
  difference 
  in 
  magnitude 
  may 
  

   possibly 
  be 
  explained 
  by 
  the 
  difference 
  in 
  distribution 
  of 
  the 
  

   fields 
  of 
  the 
  rays 
  concerned. 
  

  

  Curves 
  similar 
  to 
  C, 
  figs. 
  3 
  and 
  4, 
  have 
  been 
  obtained 
  for 
  

   Ag 
  and 
  paper 
  ; 
  they 
  show 
  the 
  same 
  general 
  characteristics. 
  

   It 
  is 
  remarkable, 
  however, 
  that 
  the 
  maximum 
  value 
  of 
  the 
  

   curve 
  C 
  is 
  very 
  nearly 
  the 
  same 
  for 
  all 
  the 
  substances 
  tested. 
  

  

  In 
  a 
  recent 
  paper 
  by 
  McClelland 
  * 
  an 
  account 
  is 
  given 
  of 
  

   the 
  distribution 
  of 
  the 
  returned 
  ^ 
  radiation 
  from 
  plates 
  of 
  

   diff'erent 
  substances 
  when 
  the 
  incident 
  beam 
  of 
  radiation 
  

   is 
  inclined 
  to 
  the 
  plate. 
  The 
  results 
  seem 
  capable 
  of 
  expla- 
  

   nation, 
  in 
  view 
  of 
  the 
  effects 
  which 
  have 
  just 
  been 
  described, 
  

   upon 
  a 
  theory 
  of 
  scattering 
  without 
  the 
  need 
  of 
  introducing 
  

   the 
  idea 
  of 
  a 
  true 
  secondary 
  radiation 
  proceeding 
  from 
  the 
  

   atoms 
  affected 
  by 
  the 
  incident 
  ^ 
  rays. 
  

  

  The 
  general 
  effect 
  observed 
  by 
  McClelland 
  is 
  that 
  the 
  

   distribution 
  of 
  the 
  returned 
  radiation 
  is 
  more 
  uniform 
  for 
  

   Pb 
  than 
  for 
  Al. 
  This 
  is 
  to 
  be 
  expected 
  in 
  view 
  of 
  the 
  nature 
  

   of 
  distribution 
  of 
  the 
  scattered 
  rays 
  from 
  thin 
  films 
  of 
  such 
  

   substances 
  as 
  Au 
  and 
  Al, 
  which 
  has 
  been 
  described 
  in 
  the 
  

   present 
  paper. 
  

  

  From 
  the 
  results 
  shown 
  in 
  figs. 
  3 
  and 
  4 
  it 
  is 
  at 
  once 
  seen 
  

   that 
  the 
  effects 
  of 
  scattering 
  may 
  considerably 
  modify 
  the 
  

   results 
  obtained 
  in 
  the 
  usual 
  form 
  of 
  absorption 
  experiment 
  

   with 
  /3 
  rays. 
  The 
  shape 
  of 
  the 
  ionization-chamber 
  and 
  the 
  

   positions 
  of 
  the 
  screen 
  and 
  active 
  material 
  relatively 
  to 
  the 
  

   chamber 
  and 
  to 
  each 
  other 
  may 
  produce 
  considerable 
  modifi- 
  

   cations 
  in 
  the 
  results. 
  

  

  Again, 
  in 
  studying 
  the 
  absorption 
  of 
  /3 
  rays 
  it 
  would 
  seem 
  

   necessary 
  to 
  deal 
  with 
  very 
  thin 
  screens 
  as 
  is 
  necessary 
  in 
  

   observing 
  the 
  effects 
  of 
  scatterinp^ 
  ; 
  for 
  thicker 
  screens 
  the 
  

   results 
  are 
  likely 
  to 
  become 
  considerably 
  complicated. 
  

  

  It 
  would 
  seem 
  almost 
  better 
  to 
  replace 
  the 
  name 
  of 
  

   " 
  absorption 
  coefficient," 
  as 
  it 
  is 
  usually 
  employed, 
  by 
  that 
  of 
  

   " 
  transmission 
  coefficient," 
  reserving 
  the 
  former 
  as 
  a 
  measure 
  

   or 
  effects 
  which, 
  as 
  has 
  been 
  explained, 
  can 
  probably 
  be 
  

   obtained 
  only 
  from 
  a 
  study 
  of 
  very 
  thin 
  screens. 
  

  

  If 
  the 
  interpretation 
  of 
  the 
  foregoing 
  experiments 
  be 
  correct 
  

   it 
  seems 
  that 
  the 
  /3 
  particle 
  in 
  traversing 
  a 
  thick 
  screen 
  may 
  

   suffer 
  many 
  collisions 
  and 
  deflexions. 
  

  

  Now 
  it 
  has 
  been 
  shown 
  by 
  Allen 
  (Phys. 
  Review, 
  Aug. 
  

   1906) 
  that 
  the 
  secondary 
  or 
  reflected 
  radiation 
  consists 
  of 
  

   electrons 
  moving 
  on 
  the 
  whole 
  with 
  a 
  somewhat 
  slower 
  speed 
  

   than 
  the 
  original 
  radiation. 
  

  

  As 
  the 
  experiments 
  described 
  in 
  the 
  present 
  paper 
  indicate 
  

   that 
  in 
  some 
  cases 
  these 
  reflected 
  electrons 
  have 
  suffered 
  

   * 
  Proc. 
  Roy. 
  Soc, 
  Series 
  A, 
  vol. 
  Ixxx. 
  

  

  