﻿132 
  ProL 
  R. 
  W. 
  Wood 
  on 
  the 
  Anomalous 
  

  

  of 
  the 
  vapour 
  was 
  essentially 
  identical 
  with 
  that 
  used 
  by 
  

   Becquerel. 
  The 
  light 
  of 
  an 
  arc 
  lamp 
  was 
  focussed 
  on 
  the 
  

   horizontal 
  slit 
  of 
  a 
  collimator, 
  after 
  traversing 
  which 
  the 
  

   parallel 
  rays 
  passed 
  lengthwise 
  through 
  the 
  dispersion-tube. 
  

   A 
  second 
  lens 
  brought 
  them 
  to 
  a 
  focus 
  on 
  the 
  slit 
  of 
  a 
  

   spectroscope, 
  when 
  the 
  dispersion 
  was 
  to 
  be 
  studied 
  b}^ 
  the 
  

   method 
  of 
  crossed 
  prisms_, 
  or 
  in 
  the 
  focus 
  of 
  an 
  eyepiece 
  

   when 
  the 
  anomalous 
  spectrum 
  was 
  to 
  be 
  viewed 
  subjectively 
  

  

  (%• 
  *). 
  

  

  Fig. 
  2. 
  

  

  Screen. 
  ' 
  

  

  # 
  

  

  The 
  first 
  experiments 
  were 
  made 
  by 
  the 
  method 
  of 
  crossed 
  

   prisms, 
  the 
  spectrometer 
  being 
  furnished 
  with 
  a 
  "Rowland 
  

   plane 
  grating, 
  which 
  showed 
  the 
  sodium 
  lines 
  widely 
  

   separated. 
  It 
  was 
  at 
  once 
  apparent 
  that 
  far 
  better 
  results 
  

   could 
  be 
  obtained 
  with 
  the 
  dispersion-tubes 
  than 
  had 
  ever 
  

   been 
  observed 
  with 
  prismatic 
  flames. 
  The 
  curved 
  branches 
  

   of 
  the 
  diffraction-spectrum 
  on 
  each 
  side 
  of 
  the 
  D 
  lines 
  were 
  

   perfectly 
  sharp 
  and 
  steady, 
  and 
  the 
  dispersion 
  could 
  be 
  

   traced 
  to 
  a 
  considerable 
  distance 
  up 
  and 
  down 
  the 
  spectrum. 
  

   On 
  the 
  slit 
  of 
  the 
  spectrometer 
  appeared, 
  instead 
  of 
  the 
  

   white 
  image 
  of 
  the 
  horizontal 
  slit, 
  a 
  most 
  beautiful 
  anomalous 
  

   spectrum, 
  of 
  great 
  brilliancy 
  and 
  purity. 
  The 
  spectrometer 
  

   was 
  at 
  once 
  removed 
  and 
  an 
  eyepiece 
  put 
  in 
  its 
  place, 
  when 
  

   a 
  most 
  superb 
  spectrum 
  revealed 
  itself. 
  The 
  general 
  ap- 
  

   pearance 
  is 
  shown 
  in 
  the 
  coloured 
  plate 
  accompanying 
  

   this 
  paper, 
  though 
  it 
  is 
  quite 
  impossible 
  to 
  represent 
  by 
  

   means 
  of 
  pigments 
  the 
  sparkling 
  brilliancy 
  of 
  the 
  colours. 
  

  

  Before 
  discussing 
  this 
  spectrum 
  in 
  detail, 
  it 
  will 
  be 
  better 
  

   to 
  take 
  up 
  the 
  results 
  of 
  the 
  experiments 
  made 
  by 
  the 
  method 
  

   of 
  crossed 
  prisms. 
  On 
  first 
  heating 
  the 
  tube, 
  the 
  curvature 
  

   of 
  the 
  spectrum 
  between 
  the 
  D 
  lines 
  as 
  well 
  as 
  on 
  each 
  side 
  

   is 
  observed, 
  the 
  appearance 
  being 
  identical 
  with 
  that 
  figured 
  

   by 
  Becquerel, 
  but 
  in 
  a 
  few 
  seconds 
  the 
  vapour 
  becomes 
  so 
  

   dense 
  that 
  total 
  absorption 
  of 
  all 
  the 
  light 
  between 
  the 
  

   lines 
  occurs. 
  Julius 
  expresses 
  the 
  opinion 
  that 
  this 
  dis- 
  

   appearance 
  of 
  the 
  light 
  between 
  the 
  lines 
  is 
  only 
  a 
  result 
  of 
  

   the 
  strong 
  dispersion 
  ; 
  that 
  is, 
  it 
  is 
  net 
  absorbed 
  but 
  turned 
  

   off 
  to 
  one 
  side 
  so 
  that 
  it 
  does 
  not 
  enter 
  the 
  instrument. 
  It 
  

   appears 
  to 
  me, 
  however, 
  that 
  this 
  is 
  not 
  the 
  case, 
  for 
  I 
  have 
  

   Observed 
  the 
  same 
  effect 
  under 
  conditions 
  where 
  lateral 
  

   deflexion 
  seems 
  quite 
  out 
  of 
  the 
  question. 
  As 
  I 
  shall 
  show 
  

  

  