﻿554 
  Dr. 
  C. 
  G. 
  Barkla 
  and 
  Mr. 
  C. 
  A. 
  Sadler 
  

  

  on 
  

  

  duction 
  o£ 
  a 
  greater 
  proportion 
  of 
  tertiary 
  rays 
  due 
  to 
  

   the 
  emergence 
  of 
  the 
  secondary 
  rays 
  from 
  greater 
  depths 
  

   of 
  the 
  radiating 
  substance, 
  or 
  by 
  a 
  combination 
  of 
  these. 
  

  

  Though 
  the 
  experiments, 
  the 
  results 
  of 
  which 
  are 
  stated 
  

   above, 
  were 
  not 
  performed 
  concurrently, 
  it 
  was 
  evident 
  that 
  

   the 
  variation 
  in 
  intensity 
  of 
  secondary 
  radiation 
  exhibiting 
  

   the 
  polarity 
  of 
  the 
  primary 
  beam 
  changed 
  from 
  about 
  20 
  per 
  

   cent, 
  to 
  only 
  about 
  6 
  or 
  7 
  per 
  cent, 
  as 
  the 
  X-ray 
  tube 
  became 
  

   " 
  harder/' 
  even 
  before 
  a 
  difference 
  between 
  the 
  penetrating- 
  

   powers 
  of 
  primary 
  and 
  secondary 
  beams 
  could 
  be 
  detected 
  by 
  

   direct 
  comparison. 
  It 
  was 
  not 
  evident 
  whether 
  this 
  change 
  

   was 
  actually 
  one 
  in 
  the 
  polarization 
  of 
  the 
  primary 
  beam 
  

   itself 
  or 
  in 
  the 
  efficiency 
  of 
  the 
  secondary 
  rays 
  in 
  exhibiting 
  

   a 
  polarization 
  of 
  constant 
  magnitude. 
  The 
  latter 
  appeared 
  

   the 
  more 
  probable 
  when 
  considered 
  in 
  conjunction 
  with 
  the 
  

   changes 
  that 
  had 
  been 
  found 
  in 
  the 
  ratio 
  of 
  intensities 
  of 
  

   secondary 
  rays 
  in 
  a 
  direction 
  almost 
  opposite 
  to 
  that 
  of 
  

   primary 
  propagation 
  and 
  one 
  at 
  right 
  angles. 
  Further 
  ex- 
  

   periments 
  were 
  therefore 
  made 
  to 
  determine 
  if 
  the 
  changes 
  

   were 
  all 
  attributable 
  to 
  the 
  same 
  cause. 
  It 
  was 
  found, 
  how- 
  

   ever, 
  that 
  although 
  increasing 
  the 
  hardness 
  of 
  a 
  given 
  X-ray 
  

   tube 
  produced 
  a 
  decrease 
  in 
  the 
  amount 
  of 
  polarization 
  

   detected, 
  the 
  more 
  penetrating 
  portion 
  of 
  a 
  primary 
  which 
  

   was 
  transmitted 
  through 
  a 
  sheet 
  of 
  aluminium 
  did 
  not 
  

   exhibit 
  less 
  polarity 
  but 
  slightly 
  more, 
  indicating 
  that 
  the 
  

   effect 
  was 
  due 
  not 
  merely 
  to 
  change 
  in 
  the 
  penetrating 
  power 
  

   of 
  the 
  radiation 
  but 
  to 
  some 
  change 
  in 
  the 
  polarity 
  of 
  the 
  

   primary 
  beam 
  itself. 
  This 
  was 
  supported 
  by 
  the 
  fact 
  that 
  

   the 
  secondary 
  radiation 
  did 
  not 
  become 
  appreciably 
  different, 
  

   in 
  penetrating 
  power 
  from 
  the 
  primary 
  producing 
  it, 
  — 
  indi- 
  

   cating 
  a 
  fairly 
  perfect 
  scattering. 
  Finally, 
  later 
  experiments 
  

   have 
  shown 
  that 
  for 
  a 
  primary 
  radiation 
  proceeding 
  from 
  a 
  

   tube 
  in 
  the 
  state 
  of 
  hardness 
  which 
  has 
  invariably 
  been 
  found 
  

   to 
  exhibit 
  a 
  minimum 
  of 
  polarity 
  in 
  the 
  primary, 
  the 
  ratio 
  of 
  

   intensities 
  of 
  secondary 
  rays 
  in 
  the 
  two 
  directions 
  indicated 
  

   has 
  been 
  such 
  as 
  would 
  be 
  given 
  by 
  an 
  almost 
  perfectly 
  

   scattered 
  radiation. 
  

  

  We 
  cannot 
  then 
  attribute 
  the 
  decrease 
  in 
  the 
  amount 
  of 
  

   polarization 
  of 
  a 
  primary 
  beam 
  exhibited 
  by 
  the 
  secondary 
  

   rays 
  to 
  the 
  scattered 
  rays 
  being 
  only 
  partially 
  a 
  scattered 
  

   radiation 
  or 
  to 
  imperfection 
  of 
  scattering, 
  but 
  it 
  is 
  almost 
  

   certainly 
  a 
  decrease 
  in 
  the 
  polarity 
  actually 
  existing 
  in 
  a 
  

   primary 
  beam 
  when 
  the 
  tube 
  becomes 
  harder. 
  

  

  These 
  results 
  are 
  possibly 
  due 
  to 
  the 
  more 
  swiftly 
  moving 
  

   cathode 
  particles 
  in 
  the 
  X-ray 
  tube 
  being 
  productive 
  of 
  

   more 
  secondary 
  cathode 
  rays 
  in 
  the 
  anti-cathode. 
  As 
  the 
  

  

  