﻿Canal 
  Rays 
  from 
  Hollow 
  Cathodes. 
  173 
  

  

  Experiment 
  4. 
  — 
  A 
  further 
  series 
  of 
  experiments 
  was 
  under- 
  

   taken 
  in 
  order 
  to 
  find 
  out 
  the 
  conditions 
  under 
  which 
  canal- 
  

   rays 
  occur 
  outside 
  the 
  cathode. 
  A 
  tube 
  shown 
  in 
  fig. 
  12 
  was 
  

   employed. 
  The 
  anode 
  is 
  a 
  disk 
  of 
  aluminium 
  about 
  1 
  cm. 
  

   in 
  diameter. 
  The 
  cathode 
  at 
  first 
  was 
  a 
  sphere 
  of 
  brass, 
  

   perforated 
  in 
  three 
  directions 
  perpendicular 
  to 
  one 
  another. 
  

   A 
  bar 
  of 
  brass 
  surrounded 
  by 
  a 
  glass 
  tube 
  was 
  screwed 
  into 
  

   one 
  of 
  the 
  six 
  openings, 
  the 
  diameter 
  of 
  the 
  opening 
  being 
  

  

  Fiff. 
  12. 
  

  

  about 
  3 
  mm. 
  The 
  distribution 
  of 
  the 
  first 
  three 
  layers 
  next 
  

   to 
  the 
  cathode 
  changes 
  in 
  a 
  very 
  interesting 
  way 
  during 
  

   the 
  evacuation, 
  as 
  will 
  be 
  seen 
  from 
  the 
  three 
  following 
  

   figures. 
  One 
  of 
  the 
  openings 
  is 
  directed 
  towards 
  the 
  anode. 
  

   The 
  Crookes's 
  dark 
  space 
  in 
  fig. 
  13 
  is 
  exceedingly 
  small 
  ; 
  the 
  

   metallic 
  sphere 
  is 
  enveloped 
  by 
  a 
  spherical 
  shell 
  of 
  red-violet 
  

   light. 
  This 
  is 
  the 
  negative 
  glow, 
  and 
  is 
  pierced 
  at 
  the 
  five 
  

   openings 
  by 
  a 
  red 
  beam 
  of 
  the 
  shape 
  of 
  a 
  flat 
  table-plate. 
  

   The 
  holes 
  are 
  filled 
  with 
  bright 
  violet 
  light; 
  a 
  column 
  of 
  

   faint 
  red 
  light 
  goes 
  up 
  to 
  the 
  anode 
  without 
  striations. 
  At 
  

   this 
  comparatively 
  high 
  pressure 
  the 
  surface 
  of 
  the 
  glass 
  bulb 
  

   already 
  begins 
  to 
  get 
  fluorescent. 
  As 
  the 
  vacuum 
  increases 
  the 
  

   light 
  flaming 
  out 
  of 
  the 
  openings 
  assumes 
  the 
  shape 
  of 
  an 
  open 
  

   umbrella, 
  Crookes's 
  dark 
  space 
  becomes 
  much 
  larger, 
  the 
  velvet 
  

   glow 
  covers 
  the 
  surface 
  of 
  the 
  sphere 
  (fig. 
  14) 
  . 
  At 
  the 
  following 
  

  

  