﻿and 
  Fluorescence 
  of 
  Mercury 
  Vapour, 
  245 
  

  

  to 
  the 
  continuous 
  spectrum, 
  a 
  bright 
  line 
  (indicated 
  by 
  an 
  

   arrow 
  on 
  fig. 
  2) 
  which 
  is 
  not 
  present 
  in 
  the 
  spark 
  spectrum, 
  

   and 
  which 
  coincides 
  in 
  position 
  with 
  the 
  sharp 
  absorption- 
  

   line 
  2536-7, 
  shown 
  by 
  vapour 
  of 
  small 
  density. 
  It 
  was 
  at 
  

   first 
  thought 
  that 
  this 
  line 
  was 
  excited 
  by 
  the 
  bright 
  

   cadmium 
  line 
  which 
  falls 
  within 
  the 
  region 
  of 
  the 
  expanded 
  

   part 
  of 
  the 
  absorption-band. 
  The 
  zinc 
  spark 
  had 
  a 
  bright 
  

   line 
  which 
  lies 
  even 
  nearer 
  the 
  mercury 
  line, 
  and 
  it 
  was 
  

   expected 
  that 
  the 
  emission 
  line 
  would 
  be 
  stronger 
  in 
  the 
  

   case 
  of 
  zinc 
  excitation. 
  To 
  my 
  surprise 
  it 
  was 
  impossible 
  to 
  

   obtain 
  any 
  trace 
  of 
  the 
  line 
  using 
  the 
  zinc 
  spark 
  as 
  a 
  

   stimulus. 
  The 
  fluorescence 
  spectrum 
  was 
  then 
  photographed 
  

   when 
  excited 
  by 
  the 
  cadmium 
  lines 
  in 
  succession 
  hut 
  in 
  no 
  

   case 
  was 
  am/ 
  trace 
  of 
  the 
  line 
  found 
  in 
  the 
  spectrum. 
  This 
  

   seemed 
  quite 
  bafiling, 
  and 
  much 
  time 
  was 
  spent 
  in 
  repeating 
  

   the 
  work 
  using 
  longer 
  exposures. 
  The 
  results 
  were 
  all 
  

   negative 
  however. 
  It 
  was 
  then 
  found 
  that 
  when 
  the 
  image 
  

   of 
  the 
  spark 
  was 
  thrown 
  upon 
  the 
  bulb 
  with 
  a 
  quartz 
  lens 
  and 
  

   the 
  bright 
  spot 
  of 
  fluorescent 
  light 
  photographed 
  with 
  the 
  

   spectrograph 
  the 
  line 
  was 
  absent 
  in 
  the 
  spectrum. 
  The 
  

   aluminium 
  spark 
  was 
  found 
  to 
  be 
  even 
  more 
  efficient 
  than 
  

   the 
  cadmium 
  in 
  bringing 
  out 
  the 
  line. 
  (See 
  fig. 
  6. 
  The 
  

   aluminium 
  spark 
  spectrum 
  is 
  recorded 
  below 
  for 
  comparison. 
  

   Fluorescent 
  line 
  2536 
  marked 
  with 
  arrow.) 
  

  

  This 
  at 
  once 
  made 
  me 
  suspicious 
  that 
  the 
  bright 
  line 
  

   might 
  be 
  excited 
  by 
  the 
  very 
  short 
  waves, 
  which 
  penetrate 
  

   quartz 
  ^yith 
  difficulty, 
  and 
  I 
  arranged 
  my 
  auxiliary 
  quartz 
  

   spectrograph 
  to 
  deliver 
  the 
  light 
  of 
  the 
  last 
  aluminium 
  line 
  

   which 
  can 
  be 
  observed 
  with 
  ordinary 
  apparatus. 
  This 
  light 
  

   excited 
  a 
  feeble 
  fluorescence, 
  and 
  by 
  giving 
  a 
  very 
  long 
  

   exposure 
  I 
  was 
  able 
  to 
  secure 
  its 
  spectrum. 
  The 
  line 
  

   appeared 
  in 
  his 
  photograph, 
  faint 
  but 
  unmistakable. 
  I 
  then 
  

   focussed 
  the 
  spark 
  upon 
  the 
  bulb 
  with 
  the 
  quartz 
  lens, 
  

   placing 
  the 
  lens 
  much 
  nearer 
  the 
  bulb 
  than 
  before, 
  after 
  

   having 
  made 
  a 
  rough 
  calculation 
  of 
  its 
  focal 
  length 
  for 
  these 
  

   very 
  short 
  waves. 
  Again 
  the 
  line 
  appeared. 
  As 
  a 
  final 
  

   test 
  I 
  placed 
  the 
  spark 
  close 
  to 
  the 
  bulb 
  and 
  took 
  a 
  series 
  of 
  

   spectra, 
  interposing 
  in 
  succession 
  quartz 
  plates 
  of 
  increasing 
  

   thickness. 
  It 
  was 
  found 
  that 
  a 
  plate 
  8 
  mm. 
  in 
  thickness 
  

   reduced 
  the 
  intensity 
  of 
  the 
  fluorescent 
  line 
  fully 
  one 
  half, 
  

   while 
  a 
  plate 
  18 
  mm. 
  caused 
  its 
  disappearance 
  from 
  the 
  

   spectrum 
  entirely. 
  

  

  The 
  mystery 
  was 
  thus 
  completely 
  solved. 
  The 
  cadmium 
  

   spectrum 
  has 
  lines 
  of 
  much 
  shorter 
  wave-length 
  than 
  any 
  

   shown 
  by 
  zinc. 
  The 
  cadmium 
  lines 
  which 
  excite 
  the 
  2536 
  

  

  PUl. 
  ^Mag. 
  S. 
  6. 
  Vol. 
  18. 
  No. 
  104. 
  Awj, 
  1909. 
  S 
  

  

  