﻿246 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Absorption 
  and 
  

  

  line 
  in 
  the 
  fluorescent 
  spectrum 
  are 
  more 
  refrangible 
  than 
  

   any 
  which 
  appear 
  in 
  the 
  photographs. 
  

  

  The 
  difficulties 
  which 
  accompanied 
  the 
  solution 
  of 
  this- 
  

   problem 
  were 
  still 
  further 
  complicated 
  by 
  the 
  fact 
  that 
  even 
  

   with 
  the 
  aluminium 
  spark 
  placed 
  close 
  to 
  the 
  bulb, 
  the 
  line 
  

   was 
  often 
  found 
  to 
  be 
  absent 
  in 
  the 
  spectrum. 
  The 
  cause 
  of 
  

   this 
  was 
  eventually 
  found. 
  The 
  line 
  only 
  appears 
  when 
  the 
  

   vapour 
  is 
  quite 
  rare, 
  i. 
  e. 
  when 
  the 
  temperature 
  of 
  the 
  bulb 
  

   is 
  comparatively 
  low. 
  The 
  best 
  density 
  is 
  that 
  at 
  which 
  

   visible 
  fluorescence 
  first 
  appears 
  and 
  is 
  quite 
  faint. 
  The 
  

   line 
  then 
  comes 
  out 
  strong. 
  If 
  the 
  temperature 
  is 
  raised 
  a 
  

   trifle 
  and 
  the 
  density 
  of 
  the 
  vapour 
  increased, 
  the 
  visible- 
  

   fluorescence 
  becomes 
  very 
  bright, 
  but 
  no 
  trace 
  of 
  the 
  line 
  

   2536 
  appears 
  in 
  the 
  spectrum. 
  This 
  circumstance 
  gave 
  a 
  

   great 
  deal 
  of 
  trouble 
  in 
  the 
  labour 
  of 
  securing 
  the 
  line 
  with 
  

   the 
  monochromatic 
  aluminium 
  radiation 
  (shortest 
  wave- 
  

   length) 
  as 
  the 
  vapour 
  density 
  was 
  at 
  first 
  arranged 
  so 
  as 
  to 
  

   give 
  the 
  brightest 
  fluorescence. 
  In 
  fig. 
  2 
  are 
  reproduced 
  

   four 
  spectra 
  taken 
  under 
  identical 
  conditions 
  as 
  to 
  time 
  of 
  

   exposure, 
  position 
  of 
  the 
  cadmium 
  spark, 
  &c. 
  The 
  upper 
  

   was 
  taken 
  with 
  highly 
  attenuated 
  vapour, 
  and 
  while 
  showing 
  

   the 
  bright 
  line 
  distinctly 
  (see 
  arrow) 
  exhibits 
  only 
  a 
  very 
  

   faint 
  trace 
  of 
  the 
  continuous 
  spectrum 
  fluorescence. 
  In 
  the 
  

   third 
  spectrum 
  the 
  line 
  is 
  barely 
  visible, 
  while 
  the 
  fluores- 
  

   cence 
  is 
  much 
  brighter. 
  In 
  this 
  case 
  the 
  vapour 
  was 
  denser. 
  

   In 
  the 
  fourth 
  the 
  bright 
  line 
  has 
  disappeared, 
  and 
  the 
  

   continuous 
  spectrum 
  is 
  at 
  its 
  maximum 
  brilliancy, 
  .s^^' 
  The 
  

   fluorescent 
  line 
  is 
  double 
  like 
  the 
  absorption-line, 
  2536 
  '7 
  

   strong, 
  2539*3 
  faint. 
  Both 
  appear 
  in 
  fig. 
  2. 
  

  

  Effect 
  of 
  Temperature 
  upon 
  the 
  Fluorescence. 
  

  

  If 
  a 
  very 
  small 
  drop 
  of 
  mercury 
  is 
  sealed 
  up 
  in 
  an 
  

   exhausted 
  bulb 
  it 
  can 
  be 
  completely 
  vaporized. 
  The 
  

   temperature 
  can 
  now 
  be 
  raised 
  to 
  a 
  bright 
  red 
  heat 
  without 
  

   danger 
  of 
  bursting 
  the 
  bulb. 
  It 
  was 
  found 
  that 
  as 
  the 
  

   temperature 
  was 
  raised 
  the 
  fluorescence 
  diminished 
  gradually 
  

   in 
  intensity 
  and 
  finally 
  disappeared 
  entirely. 
  On 
  removing 
  

   the 
  flame 
  the 
  fluorescence 
  presently 
  reappeared 
  again. 
  No 
  

   effect 
  of 
  temperature 
  on 
  the 
  appearance 
  of 
  the 
  absorption- 
  

   bands 
  could 
  be 
  detected. 
  

  

  If 
  a 
  quartz 
  flask 
  with 
  a 
  long 
  neck 
  is 
  filled 
  to 
  a 
  depth 
  of 
  

   several 
  millimetres 
  with 
  mercury, 
  and 
  the 
  flame 
  of 
  a 
  Bunsen 
  

   burner 
  is 
  made 
  to 
  play 
  around 
  the 
  bulb 
  heating 
  it 
  red 
  hot, 
  

   fluorescence 
  can 
  be 
  detected 
  only 
  close 
  to 
  the 
  surface 
  o£ 
  the 
  

  

  