﻿S30 
  Sir 
  J. 
  J. 
  Thomson 
  on 
  

  

  over 
  the 
  hole, 
  it 
  cannot 
  be 
  due 
  to 
  any 
  ordinary 
  form 
  of 
  

   Rontgen 
  radiation. 
  

  

  The 
  conductivity 
  can 
  easily 
  be 
  exphiined 
  if 
  we 
  suppose 
  that 
  

   there 
  are 
  neutral 
  doublets 
  in 
  the 
  ionized 
  gas 
  in 
  the 
  discharge- 
  

   tube, 
  and 
  that 
  since 
  these 
  are 
  not 
  deflected 
  by 
  either 
  electric 
  

   or 
  magnetic 
  forces 
  they 
  can 
  pass 
  through 
  the 
  hole 
  though 
  the 
  

   ions 
  are 
  stopped. 
  Then 
  by 
  collision 
  with 
  the 
  molecules 
  of 
  

   the 
  gas 
  in 
  the 
  Faraday 
  cylinder, 
  they 
  break 
  up 
  into 
  ions 
  and 
  

   thus 
  make 
  the 
  gas 
  a 
  conductor. 
  

  

  The 
  velocity 
  of 
  the 
  neutral 
  doublets 
  in 
  the 
  discharge 
  pro- 
  

   duced 
  by 
  an 
  induction-coil 
  is 
  approximately 
  the 
  same 
  as 
  that 
  

   of 
  the 
  positively 
  charged 
  particles 
  which 
  accompany 
  them. 
  

   This 
  follows 
  from 
  the 
  fact 
  that 
  the 
  deflexion 
  of 
  the 
  spot 
  h 
  

   (fig. 
  6) 
  is 
  the 
  same 
  as 
  that 
  of 
  spot 
  a, 
  though 
  the 
  former 
  spot 
  is 
  

   produced 
  by 
  positively 
  charged 
  particles 
  which 
  have 
  been 
  

   detached 
  from 
  the 
  neutral 
  doublets 
  after 
  passing 
  the 
  first 
  

   magnetic 
  field, 
  and 
  the 
  second 
  by 
  positively 
  electrified 
  

   particles 
  which 
  were 
  free 
  before 
  the 
  first 
  field 
  was 
  reached. 
  

  

  The 
  doublets 
  seem 
  to 
  travel 
  in 
  directions 
  approximately 
  

   normal 
  to 
  the 
  surface 
  of 
  the 
  cathode. 
  Experiments 
  described 
  

   in 
  my 
  paper 
  on 
  '' 
  Positive 
  Rays 
  " 
  (Phil. 
  Mag. 
  Oct. 
  1908) 
  

   show 
  that 
  the 
  retrograde 
  rays 
  travel 
  in 
  this 
  direction, 
  and 
  it 
  

   is 
  well 
  known 
  that 
  Canalstrahlen 
  are 
  not 
  obtained 
  unless 
  the 
  

   aperture 
  in 
  the 
  cathode 
  is 
  approximately 
  normal 
  to 
  the 
  face 
  

   of 
  the 
  cathode 
  facing 
  the 
  anode. 
  The 
  direction 
  of 
  the 
  

   doublet 
  is 
  in 
  short 
  approximately 
  that 
  of 
  the 
  lines 
  of 
  force 
  

   close 
  to 
  the 
  cathode; 
  they 
  may 
  move 
  in 
  either 
  direction 
  along 
  

   the 
  lines. 
  

  

  The 
  doublets 
  seem 
  to 
  start 
  from 
  beyond 
  the 
  dark 
  space 
  

   next 
  the 
  cathode, 
  for 
  if 
  a 
  vertical 
  obstacle 
  such 
  as 
  a 
  thin 
  

   glass 
  rod 
  or 
  a 
  metal 
  wire 
  be 
  placed 
  inside 
  the 
  discharge-tube 
  

   in 
  such 
  a 
  position 
  that 
  its 
  projection 
  on 
  the 
  plane 
  of 
  the 
  

   cathode 
  passes 
  across 
  the 
  hole 
  in 
  the 
  cathode 
  through 
  which 
  

   the 
  Canalstrahlen 
  pass, 
  then 
  when 
  the 
  pressure 
  is 
  so 
  low 
  that 
  

   the 
  dark 
  space 
  extends 
  beyond 
  the 
  obstacle, 
  the 
  dark 
  shadow 
  

   of 
  the 
  obstacle 
  is 
  apparent 
  on 
  the 
  willemite 
  screen, 
  stretching 
  

   across 
  the 
  phosphorescent 
  patch 
  even 
  when 
  this 
  is 
  spread 
  

   out 
  by 
  the 
  electric 
  and 
  magnetic 
  fields. 
  If 
  the 
  pressure 
  is 
  

   increased 
  so 
  that 
  the 
  obstacle 
  is 
  outside 
  the 
  dark 
  space 
  and 
  

   in 
  the 
  negative 
  glow, 
  no 
  shadow 
  can 
  be 
  perceived. 
  

  

  In 
  this 
  form 
  of 
  the 
  experiment 
  the 
  shadows 
  cast 
  by 
  an 
  

   obstacle 
  are 
  much 
  more 
  easily 
  perceived 
  and 
  much 
  more 
  

   definite 
  than 
  in 
  the 
  original 
  lorm 
  in 
  which 
  it 
  was 
  tried 
  by 
  

   Schuster 
  and 
  Wehnelt. 
  I 
  have 
  got 
  quite 
  sharp 
  shadows 
  with 
  

   a 
  fine 
  wire 
  placed 
  several 
  centimetres 
  away 
  from 
  the 
  cathode. 
  

  

  Some 
  very 
  interesting 
  questions 
  arise 
  in 
  connexion 
  with 
  

  

  