﻿532 
  Prof. 
  E. 
  W. 
  Wood 
  on 
  the 
  Ultra-violet 
  Absorption, 
  

  

  shown 
  in 
  PI. 
  XY. 
  fig. 
  3. 
  Spectrum 
  d 
  was 
  made 
  with 
  moderate 
  

   vapour 
  density. 
  The 
  position 
  o£ 
  the 
  D 
  lines 
  is 
  indicated, 
  

   though 
  in 
  the 
  present 
  case 
  they 
  have 
  widened 
  into 
  a 
  very 
  

   broad 
  band. 
  A 
  channelled 
  spectrum 
  appears 
  in 
  the 
  red^ 
  

   orange, 
  and 
  yellow, 
  and 
  another 
  in 
  the 
  green-blue, 
  the 
  yellow- 
  

   green 
  region 
  being 
  transmitted 
  freely. 
  The 
  red-orange 
  

   portion 
  spreads 
  over 
  and 
  below 
  the 
  D 
  lines, 
  as 
  the 
  vapour- 
  

   density 
  increases 
  ; 
  while 
  the 
  blue-green 
  one 
  pushes 
  up 
  in 
  the 
  

   other 
  direction, 
  tbe 
  tvio 
  eventually 
  meeting 
  at 
  wave-length 
  

   5500, 
  where 
  a 
  broad 
  hazy 
  band 
  appears, 
  which 
  is 
  just 
  

   appearing 
  in 
  spectrum 
  h, 
  and 
  is 
  A^ery 
  distinct 
  in 
  a. 
  This 
  

   point 
  appears 
  to 
  be 
  the 
  " 
  centre 
  of 
  gravity 
  " 
  (i£ 
  we 
  may 
  use 
  

   the 
  term) 
  between 
  the 
  two 
  channelled 
  spectra 
  which 
  border 
  

   the 
  D 
  lines. 
  With 
  a 
  spectroscope 
  o£ 
  low 
  dispersion 
  we 
  see 
  

   merely 
  a 
  close 
  double 
  green 
  line 
  at 
  the 
  point, 
  and 
  the 
  extreme 
  

   violet-blue, 
  everything 
  else 
  being 
  cut 
  off. 
  The 
  colour 
  of 
  the 
  

   transmitted 
  light 
  is 
  as 
  deep 
  as, 
  and 
  about 
  the 
  colour 
  of, 
  the 
  

   light 
  which 
  gets 
  through 
  the 
  densest 
  cobalt 
  glass. 
  

  

  As 
  I 
  have 
  alrendy 
  shown, 
  there 
  is 
  some 
  direct 
  connexion 
  

   between 
  the 
  mechanism 
  which 
  produces 
  this 
  channelled 
  

   spectrum 
  and 
  the 
  one 
  which 
  produces 
  the 
  I) 
  lines, 
  for 
  ab- 
  

   sorption 
  of 
  blue-green 
  light, 
  carefully 
  freed 
  from 
  yellow 
  by 
  

   prismatic 
  analysis 
  and 
  colour 
  screens, 
  causes 
  the 
  vapour 
  to 
  

   emit 
  yellow 
  light 
  of 
  the 
  wave-length 
  of 
  the 
  D 
  lines, 
  in 
  

   addition 
  to 
  the 
  more 
  powerful 
  emission 
  (line 
  spectrum) 
  in 
  

   the 
  blue-green 
  region. 
  

  

  I 
  find 
  now 
  that 
  channelled 
  spectra 
  accompany 
  the 
  other 
  

   lines 
  of 
  the 
  Balmer 
  series, 
  but 
  that 
  they 
  are 
  by 
  no 
  means 
  the 
  

   counterpart 
  of 
  the 
  one 
  around 
  the 
  D 
  lines. 
  They 
  are 
  not 
  

   made 
  up 
  of 
  tivo 
  portions, 
  with 
  a 
  comparatively 
  bright 
  region 
  

   between, 
  but 
  spread 
  out 
  in 
  both 
  directions, 
  both 
  above 
  and 
  

   below 
  the 
  lines 
  which 
  they 
  accompany. 
  I 
  have 
  found 
  traces 
  

   of 
  them 
  as 
  far 
  down 
  as 
  the 
  line 
  for 
  which 
  w 
  = 
  8. 
  They 
  can 
  

   be 
  well 
  seen 
  in 
  PI. 
  XVI. 
  fig. 
  3, 
  spectra 
  d, 
  e^ 
  and/, 
  while 
  

   fig. 
  4 
  shows 
  the 
  one 
  accompanying 
  the 
  first 
  ultra-violet 
  line 
  

   (X 
  = 
  3300) 
  on 
  a 
  larger 
  scale; 
  this 
  photograph 
  was 
  made 
  with 
  

   a 
  concave 
  grating, 
  " 
  h 
  " 
  with 
  rare, 
  " 
  a 
  '' 
  with 
  denser 
  vapour. 
  

   The 
  Balmer 
  line 
  3303 
  appears 
  to 
  widen 
  on 
  the 
  side 
  towards 
  

   the 
  visible 
  spectrum 
  (see 
  (a)), 
  though 
  this 
  may 
  be 
  due 
  to 
  the 
  

   presence 
  of 
  a 
  group 
  of 
  heavy 
  lines 
  in 
  the 
  channelled 
  spectrum. 
  

   An 
  enlargement 
  of 
  this 
  region 
  is 
  reproduced 
  on 
  PI. 
  XV. 
  fig. 
  2' 
  

   to 
  show 
  the 
  complexity 
  of 
  the 
  spectrum. 
  The 
  spectrum 
  

   resembles 
  the 
  one 
  in 
  the 
  blue-green 
  region 
  in 
  its 
  general 
  

   appearance. 
  

  

  The 
  question 
  now 
  suggests 
  itself: 
  — 
  Do 
  these 
  ultra-violet 
  

   channelled 
  spectra 
  exhibit 
  the 
  fluorescence 
  phenomena 
  shown 
  

  

  