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  XXXYIII. 
  A 
  Method 
  of 
  producing 
  an 
  intense 
  Cadmium 
  Spec^ 
  

   trum, 
  with 
  a 
  proposal 
  for 
  the 
  use 
  of 
  Mercury 
  and 
  Cadmium 
  

   as 
  Standards 
  in 
  Refractometry 
  . 
  By 
  T. 
  Martin 
  Lowry, 
  

  

  OF 
  the 
  different 
  line 
  spectra 
  that 
  are 
  available 
  for 
  spectro- 
  

   scopic 
  standards 
  — 
  hydrogen, 
  mercury, 
  cadmium, 
  &c. 
  — 
  

   the 
  simplest 
  and 
  purest 
  is 
  undoubtedly 
  the 
  cadmium 
  spectrum. 
  

   The 
  visible 
  spectrum 
  is 
  made 
  up 
  o£ 
  four 
  strong 
  lines 
  (red, 
  

   green, 
  blue, 
  and 
  dark 
  blue) 
  , 
  which 
  are 
  so 
  narrow 
  and 
  of 
  such 
  

   a 
  high 
  degree 
  of 
  purity 
  in 
  respect 
  of 
  the 
  absence 
  of 
  satellites 
  

   that 
  they 
  have 
  been 
  used 
  by 
  Michelson 
  to 
  produce 
  interference- 
  

   bands 
  of 
  an 
  order 
  of 
  retardation 
  that 
  has 
  apparently 
  never 
  

   been 
  reached 
  in 
  the 
  case 
  of 
  any 
  other 
  lines. 
  Michelson's 
  

   measurements 
  of 
  the 
  wave-lengths 
  of 
  the 
  three 
  chief 
  Cadmium 
  

   lines 
  : 
  — 
  

  

  Cdred 
  6438-4722 
  10 
  "^"^ 
  metre. 
  

  

  CM 
  green 
  5085*8240 
  

  

  Cdblue 
  4799-9107 
  

  

  have 
  indeed 
  formed 
  the 
  standards 
  from 
  which 
  all 
  other 
  wave- 
  

   lengths 
  have 
  been 
  deduced. 
  It 
  is 
  therefore 
  evident 
  that 
  the 
  

   cadmium 
  spectrum 
  is 
  destined 
  to 
  play 
  an 
  extremely 
  important 
  

   part 
  in 
  optical 
  determinations 
  of 
  all 
  kinds. 
  Unfortunately, 
  

   the 
  difficulty 
  of 
  producing 
  a 
  cadmium 
  lamp 
  which 
  shall 
  burn 
  

   steadily 
  and 
  give 
  out 
  light 
  of 
  high 
  intensity 
  has 
  been 
  so 
  great 
  

   that 
  the 
  four 
  cadmium 
  lines 
  have 
  been 
  used 
  only 
  very 
  

   occasionally 
  in 
  optical 
  experiments. 
  

  

  Sodium 
  » 
  

  

  The 
  standard 
  monochromatic 
  light 
  employed 
  almost 
  uni- 
  

   versally 
  for 
  refractometric 
  and 
  polarimetric 
  measurements 
  

   has 
  been 
  the 
  yellow 
  flame-spectrum 
  of 
  sodium, 
  which 
  has 
  the 
  

   advantage 
  of 
  being 
  produced 
  with 
  very 
  great 
  readiness, 
  but 
  

   with 
  all 
  the 
  drawbacks 
  inseparable 
  from 
  the 
  use 
  of 
  a 
  doublet, 
  

   instead 
  of 
  a 
  single 
  line, 
  as 
  a 
  standard. 
  Thus 
  in 
  determining 
  

   the 
  refractive 
  index 
  n^^ 
  of 
  a 
  liquid, 
  the 
  Pulfrich 
  refractometer 
  

   gives 
  readings 
  for 
  the 
  less 
  refrangible 
  constituent, 
  whilst 
  a 
  

   hollow 
  prism 
  mounted 
  on 
  a 
  spectroscope 
  gives 
  an 
  average 
  

   value 
  for 
  the 
  two 
  constituents, 
  unless 
  indeed 
  the 
  resolution 
  be 
  

   sufficient 
  to 
  read 
  them 
  separately. 
  In 
  polarimetric 
  work 
  the 
  

   double 
  character 
  of 
  the 
  sodium 
  line 
  renders 
  it 
  impossible 
  to 
  

   secure 
  a 
  proper 
  extinction 
  for 
  large 
  values 
  of 
  a^, 
  since 
  one 
  

  

  Communicated 
  bj 
  the 
  Physical 
  Society 
  : 
  read 
  June 
  25, 
  1909. 
  

  

  