﻿534 
  Ultra-violet 
  Absorption 
  ^c. 
  of 
  Sodium 
  Vapour. 
  

  

  It 
  appears 
  to 
  me 
  that 
  there 
  are 
  two 
  hypotheses 
  which 
  we 
  

   may 
  make 
  : 
  First, 
  that 
  the 
  Bahner 
  lines 
  and 
  their 
  accom- 
  

   panying 
  spectra 
  are 
  caused 
  by 
  atoms 
  which 
  have 
  lost 
  one, 
  

   two, 
  three, 
  four, 
  &c. 
  electrons. 
  Secondly, 
  that 
  they 
  are 
  pro- 
  

   duced 
  by 
  aggregates 
  or 
  complexes 
  of 
  one, 
  two, 
  three, 
  or 
  more 
  

   atoms. 
  In 
  either 
  case 
  it 
  seems 
  probable 
  that 
  the 
  members 
  

   would 
  be 
  present 
  in 
  continuously 
  decreasing 
  numbers. 
  

  

  I 
  feel 
  inclined 
  to 
  favour 
  the 
  first 
  hypothesis, 
  for 
  the 
  reason 
  

   that 
  the 
  channelled 
  specirum 
  accompanying 
  the 
  D 
  lines 
  

   appears 
  to 
  be 
  the 
  most 
  complex, 
  the 
  nltra-violet 
  channelled 
  

   spectra 
  decreasing 
  in 
  complexity 
  as 
  we 
  pass 
  down 
  the 
  

   spectrum. 
  It 
  seems 
  probable 
  that 
  complexes 
  made 
  up 
  of 
  

   large 
  numbers 
  of 
  atoms 
  would 
  have 
  more 
  available 
  frequencies 
  

   than 
  the 
  single 
  atom, 
  whereas 
  the 
  loss 
  of 
  a 
  large 
  number 
  of 
  

   electrons 
  might 
  be 
  expected 
  to 
  decrease 
  the 
  number 
  of 
  dif- 
  

   ferent 
  frequencies. 
  This, 
  however, 
  is 
  a 
  point 
  which 
  can 
  be 
  

   handled 
  by 
  mathematical 
  physicists. 
  Of 
  course 
  I 
  have 
  not 
  

   brought 
  as 
  high 
  dispersion 
  to 
  bear 
  upon 
  the 
  ultra-violet 
  

   region 
  as 
  has 
  been 
  used 
  in 
  the 
  visible, 
  for 
  this 
  region 
  has 
  

   been 
  photographed 
  by 
  Mr. 
  Clinkscales, 
  one 
  of 
  my 
  students, 
  

   in 
  the 
  second 
  order 
  spectrum 
  of 
  the 
  21 
  foot 
  grating*. 
  This 
  

   is 
  owing 
  to 
  the 
  fact 
  that 
  it 
  is 
  difficult 
  to 
  get 
  a 
  sufficiently 
  

   brilliant 
  source 
  giving 
  a 
  continuous 
  spectrum 
  below 
  wave- 
  

   length 
  3300. 
  The 
  crater 
  of 
  the 
  arc 
  can 
  be 
  used 
  for 
  the 
  first 
  

   band, 
  but 
  we 
  have 
  to 
  rely 
  upon 
  the 
  spark 
  below 
  this 
  point. 
  

   The 
  crater 
  is 
  comparatively 
  weak, 
  even 
  in 
  the 
  region 
  of 
  

   wave-length 
  3300. 
  

  

  Mr. 
  Clinkscales's 
  photographs 
  of 
  the 
  visible 
  channelled 
  

   spectrum 
  show 
  that 
  there 
  are, 
  in 
  round 
  numbers, 
  about 
  6000 
  

   absorption-lines, 
  as 
  many 
  as 
  thirty 
  being 
  crowded 
  together 
  

   in 
  a 
  region 
  not 
  wider 
  than 
  the 
  distance 
  between 
  the 
  D 
  lines. 
  

  

  Magnetic 
  Rotation, 
  

  

  The 
  study 
  of 
  the 
  magneto-optical 
  properties 
  of 
  the 
  vapour 
  

   in 
  the 
  ultra-violet 
  is 
  attended 
  with 
  great 
  experimental 
  diffi- 
  

   culties. 
  The 
  only 
  positive 
  result 
  thus 
  far 
  obtained 
  is 
  that 
  

   the 
  magnetic 
  rotation 
  at 
  the 
  first 
  two 
  ultra-violet 
  members 
  

   of 
  the 
  Balmer 
  series 
  is 
  in 
  the 
  same 
  direction 
  as 
  at 
  the 
  D 
  lines. 
  

   The 
  method 
  employed 
  was 
  the 
  usual 
  one, 
  polarized 
  light 
  

   being 
  passed 
  in 
  succession 
  through 
  the 
  magnetized 
  vapour, 
  

   a 
  Fresnel 
  double 
  prism 
  of 
  right- 
  and 
  left-handed 
  quartz, 
  and 
  

   a 
  second 
  polarizing 
  prism, 
  and 
  the 
  image 
  of 
  the 
  horizontal 
  

   fringes 
  focussed 
  on 
  the 
  slit 
  of 
  the 
  quartz 
  spectrograph. 
  

  

  * 
  Wood, 
  " 
  Resonance 
  Spectra 
  of 
  Sodium 
  Vapour," 
  Phil. 
  Mag. 
  [6] 
  xv. 
  

   p. 
  581 
  (May 
  1908) 
  ; 
  Fhys. 
  Zeit 
  ix. 
  no. 
  14, 
  p. 
  450. 
  

  

  