﻿214 
  Messrs. 
  H. 
  G. 
  J, 
  Moseley 
  and 
  C. 
  G. 
  Darwin 
  on 
  

  

  the 
  required 
  angle. 
  In 
  practice 
  the 
  glass 
  was 
  never 
  fixed 
  

   on 
  the 
  crystal 
  at 
  exactly 
  the 
  correct 
  angle, 
  and 
  the 
  error 
  had 
  

   to 
  be 
  determined 
  for 
  each 
  crystal 
  by 
  optical 
  methods. 
  At 
  a 
  

   later 
  stage, 
  after 
  the 
  fact 
  that 
  the 
  X-rays 
  obey 
  the 
  ordinary 
  

   laws 
  of 
  reflexion 
  had 
  repeatedly 
  been 
  verified, 
  it 
  was 
  found 
  

   easier 
  to 
  calculate 
  the 
  error 
  by 
  setting 
  crystal 
  and 
  detector 
  

   at 
  the 
  estimated 
  position 
  of 
  reflexion 
  and 
  then 
  turning 
  both 
  

   together 
  until 
  the 
  reflected 
  beam 
  entered 
  the 
  detector. 
  

  

  The 
  Path 
  of 
  the 
  Rays. 
  — 
  The 
  detector 
  turned 
  on 
  the 
  

   telescope 
  arm 
  of 
  the 
  spectrometer, 
  and 
  was 
  always 
  20 
  cm. 
  

   from 
  the 
  crystal. 
  The 
  distance 
  to 
  the 
  crystal 
  from 
  the 
  

   target 
  was 
  40 
  cm., 
  and 
  so 
  the 
  rays 
  always 
  passed 
  through 
  

   60 
  cm. 
  of 
  air. 
  A 
  large 
  fraction 
  of 
  the 
  softest 
  constituents 
  

   of 
  the 
  beam 
  was 
  therefore 
  absorbed 
  before 
  reaching 
  the 
  

   detector. 
  Much 
  would 
  be 
  gained 
  by 
  removing 
  the 
  air 
  from 
  

   the 
  path 
  of 
  the 
  rays, 
  but 
  this 
  refinement 
  would 
  have 
  been 
  

   inconvenient, 
  because 
  the 
  arrangement 
  of 
  slits 
  and 
  crystal 
  

   had 
  constantly 
  to 
  be 
  altered. 
  There 
  were 
  two 
  reasons 
  for 
  

   putting 
  the 
  detector 
  so 
  far 
  from 
  the 
  crystal. 
  In 
  the 
  first 
  

   place 
  the 
  effect 
  of 
  the 
  general 
  scattered 
  radiation, 
  which 
  

   comes 
  from 
  the 
  crystal 
  in 
  all 
  directions, 
  was 
  thereby 
  

   diminished. 
  In 
  the 
  second 
  place 
  the 
  direction 
  of 
  the 
  

   reflected 
  beam 
  could 
  be 
  determined 
  with 
  increased 
  accuracy* 
  

   The 
  distance 
  between 
  P 
  and 
  Q 
  had 
  to 
  be 
  made 
  large 
  to 
  

   ensure 
  the 
  rays 
  being 
  sufficiently 
  parallel. 
  Distance, 
  

   combined 
  with 
  the 
  use 
  of 
  two 
  fine 
  slits, 
  is 
  the 
  only 
  possible 
  

   way 
  of 
  collimating 
  a 
  beam 
  of 
  X-rays. 
  

  

  The 
  Detection 
  of 
  the 
  Rays. 
  — 
  The 
  fine 
  slits, 
  the 
  distance 
  

   from 
  the 
  target, 
  and 
  the 
  small 
  reflexion 
  coefficient 
  of 
  the 
  

   radiation 
  together 
  reduced 
  the 
  intensity 
  of 
  the 
  beam 
  which 
  

   reached 
  the 
  detector 
  to 
  a 
  minute 
  fraction 
  of 
  the 
  total 
  

   radiation. 
  It 
  was 
  therefore 
  necessary 
  to 
  measure 
  this 
  

   intensity 
  by 
  a 
  method 
  which 
  should 
  be 
  both 
  sensitive 
  and 
  

   fairly 
  accurate 
  over 
  a 
  very 
  wide 
  range. 
  A 
  method 
  was 
  also 
  

   needed 
  which 
  would 
  discriminate 
  against 
  the 
  natural 
  ioniza- 
  

   tion, 
  which 
  was 
  very 
  troublesome. 
  The 
  following 
  plan 
  

   proved 
  to 
  be 
  quite 
  satisfactory. 
  The 
  reflected 
  beam 
  struck 
  

   a 
  metal 
  plate 
  at 
  almost 
  glancing 
  incidence. 
  Laub 
  * 
  has 
  

   shown 
  that 
  under 
  these 
  conditions 
  the 
  emission 
  of 
  corpus- 
  

   cular 
  radiation 
  by 
  X-rays 
  is 
  very 
  greatly 
  increased. 
  This 
  

   effect 
  was 
  found 
  to 
  occur 
  equally 
  for 
  the 
  reflected 
  radiation. 
  

   The 
  metal 
  plate 
  was 
  surrounded 
  by 
  helium 
  at 
  nearly 
  

   atmospheric 
  pressure. 
  The 
  ionization 
  of 
  the 
  helium 
  by 
  the 
  

   corpuscular 
  radiation 
  was 
  greatly 
  increased 
  by 
  using 
  

   the 
  principle 
  of 
  ionization 
  by 
  collision. 
  An 
  electric 
  field 
  was 
  

  

  * 
  Ann. 
  d. 
  Physik, 
  xxvi. 
  p. 
  712 
  (1908). 
  

  

  