﻿302 
  Scientific 
  Intelligence. 
  

  

  vanishing 
  of 
  the 
  last 
  trace 
  of 
  the 
  positive 
  column 
  or 
  anode 
  glow 
  

   was 
  found 
  to 
  be 
  remarkably 
  definite 
  (cathode 
  settings 
  reprodu- 
  

   cible 
  to 
  within 
  one 
  per 
  cent.). 
  — 
  Proc. 
  Roy. 
  Soc, 
  96A, 
  200, 
  1919. 
  

  

  h. 
  s. 
  u. 
  

  

  8. 
  The 
  Line 
  Spectrum 
  of 
  Sodium 
  as 
  Excited 
  by 
  Fluores- 
  

   cence. 
  — 
  This 
  was 
  the 
  subject 
  of 
  the 
  Bakerian 
  lecture 
  delivered 
  

   by 
  E. 
  J. 
  Strutt 
  in 
  June, 
  1919 
  and 
  quite 
  recently 
  published 
  in 
  

   the 
  "Proceedings" 
  (vide 
  infra). 
  The 
  paper 
  contains 
  a 
  very 
  

   clear 
  general 
  account 
  of 
  the 
  author's 
  earlier 
  work 
  in 
  this 
  field 
  

   as 
  well 
  as 
  a 
  more 
  detailed 
  discussion 
  of 
  some 
  new 
  experiments 
  

   and 
  results. 
  The 
  success 
  of 
  the 
  latest 
  work 
  was 
  due 
  primarily 
  

   to 
  the 
  improved 
  form 
  of 
  sodium 
  vacuum-arc 
  lamp 
  devised 
  and 
  

   perfected 
  by 
  Strutt. 
  Without 
  a 
  diagram 
  it 
  is 
  not 
  feasible 
  to 
  

   describe 
  the 
  lamp 
  in 
  this 
  place. 
  Suffice 
  it 
  to 
  say, 
  that 
  this 
  piece 
  

   of 
  apparatus 
  fulfilled 
  the 
  following 
  conditions: 
  (a) 
  it 
  radiated 
  

   very 
  fine 
  lines 
  with 
  extremely 
  narrow 
  reversals, 
  (b) 
  the 
  intensity 
  

   of 
  the 
  lines 
  was 
  unusually 
  great, 
  and 
  (c) 
  the 
  lamp 
  could 
  be 
  run 
  

   continuously 
  and 
  unattended 
  for 
  24 
  hours 
  or 
  more 
  before 
  pro- 
  

   hibitive 
  opacity 
  of 
  the 
  silica 
  walls 
  set 
  in. 
  

  

  Of 
  the 
  seven 
  reproductions 
  of 
  excellent 
  photographs 
  pre- 
  

   sented 
  in 
  the 
  plate, 
  the 
  first 
  illustrates 
  the 
  extreme 
  sensitiveness 
  

   of 
  the 
  D 
  lines 
  to 
  reversal. 
  The 
  interference 
  fringes 
  were 
  

   obtained 
  by 
  using 
  a 
  Fabry 
  and 
  Perot 
  etalon 
  of 
  5 
  mm. 
  thickness. 
  

   When, 
  by 
  the 
  aid 
  of 
  a 
  weak 
  electromagnet, 
  the 
  luminous 
  vapor 
  

   was 
  thrown 
  against 
  the 
  wall 
  of 
  the 
  lamp 
  nearest 
  to 
  the 
  inter- 
  

   ferometer 
  the 
  reversals 
  were 
  very 
  much 
  narrower 
  than 
  when 
  the 
  

   radiating 
  column 
  was 
  deflected 
  against 
  the 
  more 
  remote 
  wall 
  of 
  

   the 
  tube. 
  "With 
  the 
  sodium 
  lamp 
  as 
  described, 
  focussed 
  upon 
  

   the 
  wall 
  of 
  an 
  exhausted 
  bulb 
  containing 
  sodium 
  vapour, 
  very 
  

   brilliant 
  resonance 
  is 
  obtained, 
  bright 
  enough 
  to 
  be 
  readily 
  

   shown 
  in 
  the 
  largest 
  lecture 
  room. 
  This 
  brightness 
  makes 
  pos- 
  

   sible 
  various 
  experiments 
  which 
  would 
  be 
  very 
  difficult 
  to 
  carry 
  

   out 
  with 
  the 
  salted 
  flame 
  as 
  source. 
  ' 
  ' 
  

  

  An 
  estimate 
  of 
  the 
  breadth 
  of 
  the 
  resonance 
  D 
  lines 
  was 
  made 
  

   with 
  the 
  aid 
  of 
  a 
  Lummer 
  plate 
  and 
  accessory 
  apparatus. 
  On 
  

   the 
  assumption 
  of 
  a 
  probability 
  distribution 
  of 
  radiation 
  in 
  one 
  

   D 
  line, 
  the 
  true 
  breadth 
  of 
  the 
  line 
  was 
  calculated 
  as 
  0-017 
  

   Angstrom. 
  The 
  value 
  obtained 
  by 
  using 
  Fabry 
  and 
  Buisson's 
  

   formula, 
  — 
  which 
  refers 
  to 
  the 
  Doppler 
  effect, 
  — 
  was 
  0-020 
  Ang- 
  

   strom. 
  The 
  latter 
  datum 
  involves 
  the 
  hypotheses 
  that 
  the 
  radi- 
  

   ator 
  was 
  the 
  sodium 
  atom 
  and 
  that 
  the 
  temperature 
  of 
  the 
  vapor 
  

   was 
  250° 
  C. 
  Under 
  the 
  circumstances, 
  the 
  two 
  values 
  for 
  the 
  

   breadth 
  of 
  a 
  D 
  line 
  may 
  be 
  considered 
  as 
  quite 
  concordant. 
  

  

  The 
  favorable 
  conditions 
  produced 
  by 
  the 
  new 
  lamp 
  enabled 
  

   Strutt 
  to 
  succeed 
  in 
  an 
  experiment 
  which 
  he 
  had 
  been 
  unable 
  

   to 
  realize 
  on 
  previous 
  occasions. 
  It 
  was 
  the 
  excitation 
  of 
  the 
  

   second 
  doublet 
  of 
  the 
  principal 
  series 
  by 
  using 
  the 
  same 
  line 
  

   alone 
  as 
  the 
  stimulator. 
  Not 
  only 
  was 
  the 
  doublet 
  at 
  A3303 
  

  

  