﻿216 
  Messrs. 
  H. 
  G. 
  J. 
  Moseley 
  and 
  C. 
  G. 
  Darwin 
  on 
  

  

  aud 
  the 
  gas 
  was 
  then 
  no 
  longer 
  sensitive 
  to 
  impurities. 
  

   After 
  some 
  months 
  this 
  trouble 
  ceased, 
  and 
  the 
  helium 
  could 
  

   be 
  used 
  almost 
  pure. 
  

  

  The 
  Measurements. 
  — 
  The 
  factor 
  by 
  which 
  the 
  ionization 
  

   current 
  between 
  c 
  and 
  d 
  was 
  increased 
  by 
  collision 
  could 
  be 
  

   adjusted 
  to 
  any 
  desired 
  value 
  by 
  altering 
  the 
  potential 
  of 
  c 
  by 
  

   a 
  few 
  volts 
  with 
  the 
  aid 
  of 
  a 
  potentiometer. 
  A 
  factor 
  of 
  50 
  

   was 
  most 
  frequently 
  used, 
  and 
  it 
  was 
  usually 
  not 
  convenient 
  

   to 
  work 
  with 
  a 
  factor 
  much 
  greater 
  than 
  100. 
  When 
  the 
  

   multiplication 
  was 
  too 
  great 
  the 
  readings 
  became 
  unsteady 
  

   and 
  the 
  natural 
  ionization 
  gave 
  trouble. 
  Tn 
  order 
  to 
  detect 
  

   variations 
  in 
  the 
  primary 
  X-rays 
  a 
  pair 
  of 
  parallel 
  plates 
  

   X 
  and 
  Y 
  (fig. 
  1) 
  were 
  placed 
  in 
  front 
  of 
  the 
  slit 
  P. 
  The 
  

   beam 
  of 
  rays 
  passed 
  between 
  them, 
  and 
  the 
  resulting 
  

   saturation 
  ionization 
  current 
  was 
  measured 
  by 
  a 
  second 
  

   electrometer 
  F, 
  with 
  which 
  was 
  connected 
  a 
  large 
  capacity. 
  

  

  General 
  Properties 
  of 
  the 
  Reflected 
  Radiation. 
  

  

  Explanations 
  of 
  the 
  " 
  reflexion 
  " 
  of 
  the 
  X-rays 
  are 
  based 
  

   on 
  the 
  assumption 
  that 
  the 
  reflected 
  beam 
  is 
  due 
  to 
  the 
  co- 
  

   operation 
  of 
  a 
  number 
  of 
  the 
  waves 
  of 
  scattered 
  radiation 
  

   which 
  come 
  from 
  the 
  atoms 
  traversed 
  by 
  the 
  primary 
  beam. 
  

   It 
  is 
  well 
  known 
  that 
  the 
  scattered 
  radiation 
  is 
  in 
  every 
  way 
  

   similar 
  to 
  the 
  rays 
  which 
  excite 
  it. 
  A 
  description 
  will 
  now 
  

   be 
  given 
  of 
  some 
  simple 
  experiments 
  which 
  show 
  that 
  this 
  

   is 
  equally 
  true 
  in 
  the 
  case 
  of 
  the 
  a 
  reflected 
  " 
  radiation. 
  

  

  (1) 
  Ionization 
  of 
  gases.- 
  — 
  A 
  fine 
  beam 
  of 
  radiation 
  reflected 
  

   from 
  mica 
  was 
  sent 
  straight 
  down 
  the 
  detector 
  and 
  did 
  not 
  

   touch 
  the 
  plates 
  c 
  and 
  d. 
  Considerable 
  ionization 
  was 
  pro- 
  

   duced 
  in 
  air, 
  and 
  slight 
  ionization 
  in 
  helium. 
  

  

  (2) 
  Corpuscular 
  radiation. 
  — 
  The 
  detector 
  was 
  moved 
  a 
  

   little 
  to 
  one 
  side. 
  The 
  beam 
  now 
  touched 
  c 
  or 
  d. 
  The 
  

   ionization 
  in 
  air 
  was 
  somewhat 
  increased 
  ; 
  the 
  ionization 
  in 
  

   helium 
  was 
  increased 
  some 
  ten 
  times. 
  As 
  the 
  angle 
  at 
  which 
  

   the 
  beam 
  hit 
  the 
  metal 
  plate 
  was 
  altered, 
  the 
  ionization 
  

   changed 
  rapidly, 
  being 
  largest 
  at 
  almost 
  glancing 
  incidence. 
  

  

  (3) 
  Ionization 
  at 
  different 
  pressures. 
  — 
  The 
  detector 
  was 
  set 
  

   slightly 
  aslant, 
  so 
  that 
  the 
  whole 
  reflected 
  beam 
  fell 
  on 
  c, 
  and 
  

   the 
  corpuscular 
  radiation 
  was 
  a 
  maximum. 
  The 
  ionization 
  

   was 
  measured 
  in 
  air 
  at 
  pressures 
  between 
  10 
  cm. 
  and 
  an 
  

   atmosphere. 
  The 
  crystal 
  was 
  then 
  removed 
  and 
  the 
  same 
  

   process 
  carried 
  out 
  with 
  the 
  primary 
  beam. 
  The 
  two 
  beams 
  

   happened 
  to 
  be 
  equally 
  penetrating 
  and 
  the 
  two 
  curves 
  were 
  

   indistinguishable. 
  In 
  both 
  cases 
  the 
  effect 
  of 
  the 
  corpuscular 
  

   radiation 
  and 
  its 
  absorption 
  by 
  the 
  air 
  at 
  the 
  higher 
  pressures 
  

  

  