﻿322 
  Dr.'T. 
  M. 
  Lowrv 
  on 
  a 
  MetJiod 
  of 
  

  

  Choice 
  of 
  Standards. 
  

  

  The 
  essential 
  properties 
  for 
  a 
  standard 
  source 
  of 
  light 
  are, 
  

  

  (1) 
  that 
  it 
  should 
  be 
  of 
  sufficient 
  intensity 
  to 
  be 
  used 
  for 
  all 
  

   the 
  various 
  types 
  of 
  optical 
  measurements, 
  so 
  that, 
  for 
  

   instance, 
  refractive 
  indices 
  and 
  optical 
  and 
  magnetic 
  rotatory 
  

   powers 
  may 
  be 
  determined 
  for 
  the 
  same 
  wave-lengths, 
  

  

  (2) 
  that 
  it 
  should 
  be 
  strictly 
  monochromatic 
  and 
  as 
  far 
  as 
  

   possible 
  free 
  from 
  satellites, 
  and 
  (3) 
  that 
  it 
  should 
  be 
  pro- 
  

   duced 
  with 
  sufficient 
  readiness 
  to 
  render 
  it 
  generally 
  avail- 
  

   able. 
  These 
  requirements, 
  as 
  has 
  been 
  shown, 
  are 
  only 
  

   partially 
  fulfilled 
  by 
  sodium 
  light 
  and 
  fail 
  completely 
  in 
  the 
  

   case 
  of 
  the 
  hydrogen 
  spectrum. 
  The 
  purpose 
  of 
  the 
  present 
  

   communication 
  is 
  to 
  suggest 
  that 
  the 
  spectra 
  of 
  mercury 
  and 
  

   of 
  cadmium 
  fulfil 
  most 
  of 
  the 
  essential 
  conditions 
  outlined 
  

   above, 
  and 
  to 
  describe 
  a 
  method 
  by 
  which 
  the 
  cadmium 
  

   spectrum 
  may 
  be 
  rendered 
  more 
  generally 
  available 
  for 
  

   spectroscopic 
  work. 
  

  

  The 
  suggestion 
  — 
  which 
  is 
  made 
  on 
  the 
  basis 
  of 
  practical 
  

   experience 
  in 
  the 
  actual 
  measurement 
  of 
  optical 
  and 
  magnetic 
  

   rotations 
  and 
  of 
  refractive 
  indices 
  for 
  a 
  large 
  range 
  of 
  wave- 
  

   lengths 
  (see 
  for 
  instance 
  Proc. 
  Roy. 
  Soc. 
  1908, 
  81. 
  p. 
  472) 
  

   — 
  that 
  sodium 
  should 
  give 
  place 
  to 
  mercury 
  and 
  cadmium 
  

   as 
  a 
  chief 
  standard 
  source 
  of 
  light, 
  is 
  fully 
  supported 
  by 
  

   the 
  theoretical 
  considerations 
  recently 
  advanced 
  by 
  Bates 
  

   (" 
  Spectrum 
  Lines 
  as 
  Light 
  Sources 
  in 
  Polariscopic 
  Measure- 
  

   ments,''' 
  Bureau 
  of 
  Standards, 
  Bulletin, 
  1906, 
  ii. 
  p. 
  239) 
  and 
  

   by 
  Nutting 
  (" 
  Polarimetric 
  Sensibility 
  and 
  Accuracy," 
  ibid. 
  

   p. 
  249, 
  " 
  Purity 
  and 
  intensity 
  of 
  Monochromatic 
  Light 
  

   Sources,^' 
  ibid. 
  p. 
  439). 
  The 
  former 
  author 
  has 
  worked 
  out 
  

   a 
  formula 
  showing 
  the 
  errors 
  due 
  to 
  the 
  use 
  of 
  a 
  doublet 
  in 
  

   polariraetry, 
  and 
  has 
  redetermined 
  the 
  ratio 
  of 
  the 
  sodium- 
  

   yellow 
  and 
  mercury-green 
  rotations 
  for 
  quartz 
  ; 
  the 
  latter 
  

   has 
  developed 
  formulse 
  in 
  reference 
  to 
  polarimetric 
  sensi- 
  

   bility, 
  and 
  spectral 
  purity. 
  The 
  two 
  points 
  in 
  these 
  papers 
  

   that 
  bear 
  directly 
  on 
  the 
  practical 
  problem 
  now 
  under 
  con- 
  

   sideration 
  are, 
  (1) 
  the 
  confirmation 
  by 
  Bates 
  of 
  the 
  purity 
  

   of 
  the 
  mercury 
  green 
  line, 
  which 
  gave 
  very 
  sharp 
  readings 
  

   in 
  the 
  case 
  of 
  a 
  quartz 
  plate 
  of 
  about 
  5^ 
  mm. 
  thickness 
  ; 
  this 
  

   point 
  is, 
  however, 
  seriously 
  discounted 
  by 
  the 
  fact 
  that 
  he 
  

   professes 
  to 
  read 
  the 
  sodium 
  doublet 
  to 
  0*0001°, 
  and 
  gives 
  

   the 
  ratio 
  of 
  sodium 
  to 
  mercury 
  to 
  six 
  significant 
  figures 
  

   (0-850944 
  : 
  1), 
  (2) 
  the 
  statement 
  by 
  Nutting 
  that 
  on 
  one 
  

   basis 
  of 
  reckoning 
  the 
  '^ 
  spectral 
  purities 
  " 
  of 
  cadmium 
  green, 
  

   mercury 
  green, 
  and 
  sodium 
  yellow, 
  are 
  represented 
  by 
  the 
  

   ratios 
  joo^? 
  io^j 
  ^^^ 
  7^? 
  whilst 
  on 
  another 
  basis 
  the 
  ^' 
  specific 
  

  

  