﻿MetJiod 
  of 
  producing 
  an 
  intense 
  Cadmium 
  Spectrum. 
  321 
  

  

  wave-length 
  is 
  transmitted 
  with 
  considerable 
  intensity 
  when 
  

   the 
  other 
  is 
  extinguished, 
  and 
  in 
  addition 
  there 
  is 
  always 
  some 
  

   uncertainty 
  as 
  to 
  the 
  " 
  optical 
  mass-centre 
  '■' 
  of 
  the 
  doublet_, 
  

   which 
  may 
  indeed 
  vary 
  in 
  different 
  types 
  of 
  sodium-lamp, 
  on 
  

   account 
  of 
  changes 
  in 
  the 
  relative 
  intensity 
  of 
  the 
  two 
  con- 
  

   stituents 
  *. 
  It 
  should 
  also 
  be 
  noted 
  that 
  the 
  sodium 
  flame 
  

   emits 
  a 
  considerable 
  amount 
  of 
  light 
  of 
  other 
  colours, 
  which 
  

   in 
  accurate 
  work, 
  or 
  in 
  reading 
  large 
  rotations, 
  must 
  be 
  

   removed 
  by 
  filtering 
  through 
  a 
  coloured 
  screen, 
  or, 
  better, 
  by 
  

   means 
  of 
  a 
  spectroscopic 
  eyepiece 
  (Perkin). 
  

  

  Hydrogen^ 
  

   The 
  hydrogen 
  lines, 
  

  

  n„ 
  (red), 
  w.-l. 
  = 
  6560-04, 
  

  

  H^ 
  (blue), 
  w.-l. 
  = 
  4861-49, 
  

   H,^ 
  (violet), 
  w.-l. 
  = 
  4340-66, 
  

  

  have 
  been 
  universally 
  employed 
  with 
  the 
  sodium 
  doublet 
  in 
  

   refractometric 
  work 
  when 
  dispersion- 
  values 
  were 
  required. 
  

   The 
  choice 
  has 
  been 
  wholly 
  one 
  of 
  convenience 
  and 
  has 
  no 
  

   other 
  merit 
  to 
  recommend 
  it. 
  The 
  vacuum-tube, 
  though 
  

   easily 
  fitted 
  up, 
  can 
  hardly 
  be 
  seriously 
  considered 
  as 
  a 
  source 
  

   of 
  light. 
  The 
  red 
  line 
  is 
  by 
  far 
  the 
  strongest, 
  and 
  has 
  been 
  

   used 
  with 
  advantage 
  to 
  produce 
  interference 
  - 
  fringes 
  in 
  

   measurements 
  of 
  length 
  f, 
  but 
  would 
  be 
  utterly 
  useless 
  for 
  

   polarimetric 
  work 
  in 
  which 
  the 
  source 
  of 
  light 
  must 
  be 
  viewed 
  

   through 
  a 
  Xicol's 
  prism 
  set 
  within 
  2° 
  or 
  3° 
  of 
  the 
  extinction 
  

   position. 
  The 
  violet 
  line 
  is 
  unpleasantly 
  weak 
  even 
  for 
  

   refractometric 
  measurements, 
  and 
  demands 
  the 
  use 
  of 
  the 
  

   full 
  power 
  of 
  a 
  six-inch 
  coil, 
  with 
  an 
  efficient 
  optical 
  

   condenser, 
  before 
  readings 
  can 
  be 
  made 
  with 
  any 
  degree 
  of 
  

   comfort. 
  The 
  hydrogen 
  spectrum 
  has 
  the 
  further 
  dis- 
  

   advantage 
  of 
  showing, 
  at 
  least 
  in 
  an 
  ordinary 
  vacuum-tube, 
  

   an 
  almost 
  continuous 
  back-ground 
  of 
  weak 
  lines. 
  Although, 
  

   therefore, 
  refractometers 
  are 
  regularly 
  sent 
  out 
  with 
  tables 
  

   for 
  the 
  sodium 
  and 
  the 
  three 
  hydrogen 
  lines 
  — 
  and 
  no 
  data 
  

   whatever 
  for 
  light 
  of 
  any 
  other 
  wave-length 
  — 
  it 
  is 
  evident 
  

   that 
  this 
  position 
  is 
  radically 
  unsound 
  and 
  cannot 
  be 
  perma- 
  

   nently 
  maintained. 
  

  

  * 
  Compare 
  Landolt, 
  Optische 
  Drehungsvermogen, 
  1898, 
  pp. 
  362 
  et 
  seq. 
  

  

  t 
  See, 
  for 
  instance, 
  Tutton's 
  measurements 
  of 
  the 
  coefficients 
  of 
  

   expansion 
  and 
  of 
  elasticity 
  of 
  crystal-plates 
  (Phil. 
  Trans. 
  1903, 
  a. 
  202, 
  

   p. 
  143). 
  Compare 
  also 
  Tutton, 
  Proc. 
  K. 
  S., 
  June 
  10, 
  1909. 
  

  

  