﻿924 
  Prof. 
  Bragg 
  and 
  Mr. 
  Mad 
  sen 
  : 
  An 
  Experimental 
  

  

  surface. 
  It 
  will 
  be 
  largest 
  if 
  the 
  surface 
  is 
  actually 
  within 
  

   the 
  chamber 
  ; 
  and 
  this 
  is 
  effectively 
  the 
  case 
  in 
  these 
  present 
  

   experiments. 
  

  

  Again, 
  the 
  incidence 
  radiation 
  should 
  be 
  somewhat 
  less 
  

   than 
  /> 
  times 
  the 
  emergence 
  radiation 
  ; 
  and 
  the 
  words 
  

   italicized 
  should 
  apply 
  specially 
  to 
  the 
  case 
  of 
  the 
  lighter 
  

   atoms. 
  Referring 
  back 
  to 
  the 
  argument 
  of 
  our 
  previous 
  

   paper, 
  it 
  was 
  there 
  pointed 
  out 
  that 
  the 
  cathode 
  radiation, 
  

   which 
  originated 
  in 
  any 
  layer 
  el* 
  the 
  plate 
  and 
  was 
  turned 
  

   back, 
  was 
  scattered 
  and 
  softened 
  in 
  the 
  process. 
  h\ 
  deter- 
  

   minations 
  o£ 
  p 
  this 
  effect 
  causes 
  the 
  result 
  to 
  be 
  too 
  high, 
  

   because 
  slow 
  ,3 
  rays 
  produce 
  more 
  ions 
  per 
  cm. 
  than 
  fast 
  

   Ones. 
  At 
  the 
  same 
  time, 
  such 
  rays 
  contribute 
  less 
  to 
  the 
  

   incidence 
  radiation 
  than 
  they 
  would 
  have 
  done 
  it' 
  they 
  had 
  

   possessed 
  the 
  same 
  penetrative 
  powers 
  as 
  the 
  (3 
  rays 
  which 
  

   go 
  on 
  and 
  emerge 
  from 
  the 
  plate. 
  This 
  applies 
  particularly 
  to 
  

   the 
  lighter 
  atoms. 
  for 
  it 
  is 
  in 
  their 
  case 
  thai 
  this 
  scattering 
  and 
  

   softening 
  effect 
  is 
  so 
  pronounced. 
  ("Quality 
  o£ 
  Secondary 
  

   Radiation 
  due 
  to 
  S 
  Rays," 
  Trans. 
  Roy. 
  Soc. 
  of 
  S.A., 
  Oct. 
  1907,. 
  

   p. 
  300.) 
  This 
  seems 
  a 
  reasonable 
  explanation 
  of 
  the 
  very 
  

   large 
  differences 
  between 
  the 
  emergence 
  and 
  incidence 
  

   radiation- 
  of 
  the 
  Lighter 
  atom-. 
  

  

  It 
  i- 
  interesting 
  to 
  observe 
  that 
  the 
  figures 
  for 
  the 
  incidence 
  

   radiations 
  were 
  obtained 
  by 
  the 
  use 
  of 
  a 
  stream 
  of 
  y 
  rays 
  

   fairly 
  free 
  from 
  /Q 
  rays. 
  So 
  fur 
  as 
  we 
  can 
  discover, 
  this 
  

   condition 
  has 
  not 
  been 
  realized 
  previously. 
  AVhen 
  we 
  have 
  

   successfully 
  applied 
  the 
  corrections 
  described 
  above, 
  the 
  

   results 
  Bhould 
  be 
  of 
  considerable 
  interest. 
  

  

  Having 
  carried 
  thes9 
  experiments 
  sufficiently 
  far 
  to 
  show 
  

   clearly 
  the 
  want 
  of 
  symmetry 
  between 
  the 
  secondary 
  radia- 
  

   tion- 
  Kit 
  the 
  two 
  sides 
  of 
  the 
  plate, 
  we 
  put 
  them 
  aside 
  for 
  

   the 
  time, 
  in 
  order 
  to 
  compare 
  the 
  emergence 
  radiations 
  of 
  

   plates 
  of 
  different 
  substances. 
  At 
  the 
  beginning 
  of 
  this 
  

   paper 
  it 
  was 
  stated 
  that 
  emergence 
  radiations 
  would 
  be 
  the 
  

   same 
  for 
  all 
  substances, 
  if 
  we 
  could 
  arrange 
  to 
  have 
  (1) 
  homo- 
  

   geneity 
  of 
  the 
  7 
  rays 
  : 
  (2) 
  a 
  density 
  law 
  of 
  absorption 
  for 
  

   the 
  7 
  rays 
  ; 
  (3) 
  a 
  density 
  law 
  of 
  absorption 
  for 
  the 
  /3 
  rays. 
  

   All 
  these 
  conditions 
  cannot 
  be 
  realized 
  : 
  but 
  we 
  can 
  go 
  along 
  

   way 
  towards 
  satisfying 
  the 
  first 
  two. 
  Wigger 
  has 
  shown 
  that 
  

   rays 
  which 
  have 
  passed 
  through 
  2'8 
  cm. 
  of 
  lead 
  are 
  then 
  

   absorbed 
  by 
  different 
  substances 
  according 
  to 
  a 
  density 
  law 
  

   simply 
  : 
  they 
  do 
  not 
  recognize 
  atomic 
  groupings. 
  We 
  may 
  

   safely 
  assume 
  that 
  the 
  first 
  condition 
  is 
  then 
  effectively 
  

   realized 
  also. 
  We 
  therefore 
  placed 
  a 
  lead 
  plug 
  (1*61 
  cm. 
  in 
  

   thickness) 
  in 
  the 
  conical 
  opening 
  (see 
  fig. 
  1), 
  and 
  hoped 
  to 
  

   find 
  that 
  the 
  emergence 
  radiation, 
  which 
  we 
  then 
  proceeded 
  

   to 
  measure, 
  would 
  depend 
  only 
  on 
  the 
  absorption 
  of 
  the 
  & 
  

  

  