﻿Fluorescence 
  f 
  and 
  Magnetic 
  Rotation 
  of 
  Sodium 
  Vapour. 
  533 
  

  

  in 
  the 
  visible 
  region 
  ? 
  Tiiis 
  question 
  I 
  can 
  answer 
  in 
  the 
  

   affirmative 
  for 
  the 
  first 
  one 
  (at 
  line 
  3303). 
  The 
  fluorescence 
  

   can 
  be 
  powerfully 
  excited 
  by 
  the 
  triplet 
  in 
  the 
  zinc 
  arc 
  at 
  

   Tvave-length 
  3300. 
  The 
  fluorescent 
  light 
  is 
  of 
  course 
  in- 
  

   visible, 
  which 
  makes 
  its 
  detection 
  difficult. 
  The 
  sodium 
  tube 
  

   was 
  closed 
  with 
  a 
  large 
  quartz 
  lens, 
  and 
  powerful 
  fluorescence 
  

   excited 
  with 
  a 
  Heraeus 
  zinc 
  arc 
  in 
  quartz. 
  The 
  image 
  of 
  

   ihe 
  green 
  fluorescent 
  spot 
  was 
  focussed 
  upon 
  the 
  slit 
  of 
  a 
  

   quartz 
  spectrograph 
  by 
  means 
  of 
  a 
  quartz 
  lens. 
  Under 
  these 
  

   conditions 
  ultra-violet 
  fluorescent 
  light, 
  if 
  present, 
  should 
  

   appear 
  in 
  the 
  spectrum. 
  Such 
  was 
  found 
  to 
  be 
  the 
  case, 
  as 
  

   is 
  shown 
  by 
  Pi. 
  XV. 
  fig. 
  1 
  ; 
  the 
  zinc 
  arc 
  spectrum 
  is 
  shown 
  

   above, 
  the 
  fluorescent 
  spectrum 
  below. 
  The 
  banded 
  fluores- 
  

   cent 
  spectrum 
  is 
  strongest 
  at 
  some 
  distance 
  above 
  the 
  zinc 
  

   lines 
  {}. 
  e. 
  on 
  the 
  side 
  of 
  longer 
  wave-lengths). 
  The 
  inter- 
  

   vention 
  of 
  a 
  glass 
  plate 
  between 
  the 
  arc 
  and 
  the 
  sodium 
  tube 
  

   caused 
  the 
  disappearance 
  of 
  the 
  ultra-violet 
  fluorescent 
  bands, 
  

   showing 
  that 
  they 
  were 
  due 
  to 
  the 
  absorption 
  of 
  ultra-violet 
  

   and 
  not 
  of 
  visible 
  light. 
  

  

  An 
  attempt 
  was 
  next 
  made 
  to 
  see 
  whether 
  any 
  mechanical 
  

   connexion 
  existed 
  between 
  the 
  mechanisms 
  which 
  produced, 
  

   the 
  D 
  lines 
  and 
  the 
  visible 
  channelled 
  spectra 
  (which 
  have 
  

   been 
  shown 
  to 
  be 
  connected) 
  and 
  the 
  ultra-violet 
  lines. 
  The 
  

   zinc 
  ultra-violet 
  triplet 
  was 
  isolated 
  by 
  means 
  of 
  a 
  quartz 
  

   spectrograph 
  and 
  focussed 
  on 
  the 
  aperture 
  of 
  the 
  sodium 
  

   ■chamber. 
  The 
  room 
  was 
  absolutely 
  dark, 
  and 
  all 
  extraneous 
  

   light 
  carefully 
  screened 
  off. 
  No 
  visible 
  fluorescence 
  was 
  

   observed. 
  The 
  ultra-violet 
  fluorescence 
  was, 
  however, 
  pho- 
  

   tographed 
  with 
  a 
  second 
  quartz 
  spectrograph. 
  In 
  this 
  case 
  

   one 
  is 
  working 
  wholly 
  in 
  the 
  dark, 
  and 
  the 
  difficulties 
  

   attending 
  the 
  adjustment 
  of 
  the 
  two 
  optical 
  trains 
  may 
  be 
  

   well 
  imagined. 
  

  

  This 
  experiment 
  indicates 
  absence 
  of 
  connexion 
  between 
  

   the 
  two 
  systems. 
  

  

  The 
  converse 
  experiment 
  was 
  next 
  tried. 
  Powerful 
  stimu- 
  

   lation 
  by 
  the 
  entire 
  visible 
  spectrum 
  failed 
  to 
  show 
  any 
  trace 
  

   of 
  the 
  ultra-violet 
  band 
  in 
  the 
  spectrum 
  of 
  the 
  fluorescent 
  

   light, 
  even 
  though 
  the 
  visible 
  region 
  was 
  overexposed 
  ten 
  

   fold. 
  This 
  again 
  indicates 
  absence 
  of 
  connexion. 
  

  

  It 
  looks 
  very 
  much 
  as 
  if 
  the 
  different 
  lines 
  of 
  the 
  principal 
  

   (Balmer) 
  series 
  and 
  their 
  accompanying 
  channelled 
  spectra 
  

   may 
  be 
  considered 
  as 
  produced 
  by 
  different 
  entities. 
  The 
  

   enormous 
  increase 
  in 
  vapour 
  density 
  necessary 
  to 
  bring 
  out 
  

   the 
  higher 
  members 
  of 
  the 
  series 
  may 
  perhaps 
  be 
  ascribed 
  to 
  

   the 
  possible 
  circumstance 
  that 
  the 
  entities 
  producing 
  them 
  

   are 
  present 
  in 
  smaller 
  numbers. 
  

  

  