﻿producing 
  an 
  intense 
  Cadmium 
  Spectrum, 
  327 
  

  

  it 
  would 
  be 
  a 
  better 
  chief 
  standard 
  than 
  mercury 
  green, 
  

   as 
  it 
  is 
  considerably 
  brighter 
  and 
  gives 
  readings 
  about 
  

   15 
  per 
  cent, 
  higher, 
  but 
  in 
  view 
  of 
  the 
  greater 
  trouble 
  

   involved 
  in 
  producing 
  the 
  light, 
  and 
  persuading 
  it 
  to 
  burn 
  

   steadily, 
  it 
  is 
  better 
  to 
  use 
  it 
  as 
  a 
  secondary 
  line 
  for 
  the 
  

   study 
  of 
  rotatory 
  dispersion. 
  The 
  red 
  and 
  blue 
  lines 
  are 
  also 
  

   very 
  bright 
  and 
  can 
  be 
  read 
  quite 
  easily. 
  The 
  dark 
  blue 
  

   line 
  is 
  of 
  much 
  less 
  intensity, 
  and 
  is 
  not 
  likely 
  to 
  be 
  widely 
  

   used, 
  as 
  it 
  is 
  difficult 
  to 
  read, 
  and 
  does 
  not 
  differ 
  sufficiently 
  

   in 
  wave-length 
  from 
  the 
  light 
  blue 
  line 
  to 
  justify 
  the 
  extra 
  

   trouble 
  involved 
  ; 
  this 
  observation 
  applies, 
  however, 
  only 
  to 
  

   the 
  existing 
  arrangements_, 
  as 
  it 
  is 
  quite 
  possible 
  that 
  when 
  a 
  

   more 
  powerful 
  source 
  of 
  steady 
  light 
  is 
  available 
  the 
  dark 
  

   blue 
  line 
  may 
  prove 
  to 
  be 
  of 
  considerable 
  value 
  in 
  shortening 
  

   the 
  gap 
  between 
  Cd 
  4800 
  and 
  Hg 
  4359. 
  

  

  Unlike 
  copper 
  the 
  silver 
  spectrum 
  does 
  not 
  clash 
  at 
  all 
  

   with 
  that 
  of 
  cadmium 
  ; 
  the 
  brilliant 
  silver 
  green 
  lines 
  

  

  rAar.aa 
  J" 
  doublet 
  (compare 
  sodium) 
  

  

  5209-25 
  

   are 
  separated 
  from 
  the 
  cadmium 
  green, 
  Cd 
  5086, 
  by 
  an 
  

   interval 
  nearly 
  as 
  great 
  as 
  that 
  which 
  separates 
  the 
  two 
  

   cadmium 
  blues, 
  and 
  the 
  only 
  other 
  line 
  that 
  shows 
  at 
  all 
  

   stronglv 
  in 
  the 
  spectrum 
  is 
  a 
  line 
  in 
  the 
  far-violet, 
  perhaps 
  

   Ag4055. 
  

  

  In 
  conclusion 
  : 
  It 
  is 
  suggested 
  that 
  the 
  mercury 
  line 
  

   Hg 
  5461 
  should 
  be 
  used 
  as 
  chief 
  standard 
  in 
  optical 
  w^ork 
  of 
  

   all 
  kinds, 
  and 
  that 
  dispersion 
  should 
  be 
  measured 
  from 
  this 
  

   line 
  to 
  Hg4359 
  instead 
  of 
  from 
  Ha 
  6561 
  or 
  H/3 
  4861 
  to 
  

   Hy4341. 
  As 
  secondary 
  standards 
  are 
  suggested 
  the 
  flame 
  

   spectra 
  Li 
  6708 
  and 
  Na 
  5893, 
  purified 
  spectroscopically, 
  

   together 
  with 
  the 
  three 
  cadmium 
  lines 
  Cd 
  6438, 
  Cd 
  5086, 
  

   Cd 
  4800, 
  giving 
  a 
  well-distributed 
  series 
  of 
  seven 
  wave- 
  

   lengths. 
  

  

  6708, 
  6438, 
  5893, 
  5461, 
  5086, 
  4800, 
  4359. 
  

  

  Red 
  red 
  yellow 
  green 
  green 
  blue 
  violet. 
  

  

  130 
  Horseferry 
  Road, 
  Westminster, 
  S.W. 
  

  

  ISote 
  added 
  July 
  1909. 
  — 
  I 
  am 
  glad 
  to 
  find 
  that 
  the 
  desirability 
  of 
  a 
  

   change 
  in 
  the 
  standard 
  wave-lengths 
  for 
  use 
  in 
  refractometry 
  is 
  appa- 
  

   rently 
  recognized 
  by 
  other 
  workers 
  : 
  in 
  particular, 
  Dorn 
  & 
  Lohmann 
  in 
  

   their 
  measurements 
  of 
  the 
  refractive 
  indices 
  of 
  liquid 
  crystals 
  {A7i7i. 
  

   Physik, 
  June 
  10th, 
  1909, 
  [4] 
  xxix. 
  pp. 
  o35-o65) 
  have 
  used 
  the 
  series 
  

   Li 
  6708, 
  Xa 
  5893, 
  Hg 
  5461, 
  Hg 
  4359, 
  which 
  is 
  identical 
  with 
  a 
  series 
  

   that 
  I 
  am 
  using 
  for 
  the 
  measurement 
  of 
  the 
  refractive 
  dispersions 
  

   of 
  the 
  alcohols 
  and 
  acids 
  of 
  the 
  aliphatic 
  series, 
  and 
  differs 
  from 
  the 
  

   series 
  of 
  seven 
  lines 
  used 
  in 
  the 
  measurement 
  of 
  rotatory 
  dispersion 
  only 
  

   in 
  the 
  omission 
  of 
  the 
  three 
  cadmium 
  line?. 
  

  

  Z 
  2 
  

  

  