﻿producing 
  an 
  intense 
  Cadmium 
  Spectrum. 
  323 
  

  

  impurities 
  ^' 
  of 
  the 
  mercury 
  green 
  and 
  sodium 
  yellow 
  lines 
  

   are 
  given 
  by 
  the 
  ratios 
  y^^^^;^, 
  2000 
  ' 
  these 
  figures 
  serve 
  to 
  

   show 
  that 
  the 
  change 
  of 
  principal 
  standard 
  now 
  proposed 
  on 
  

   the 
  basis 
  of 
  practical 
  polarimetric 
  work 
  is 
  fully 
  justified 
  by 
  

   minute 
  spectroscopic 
  tests 
  on 
  the 
  lines 
  themselves. 
  

  

  Mercury, 
  

  

  The 
  use 
  of 
  the 
  enclosed 
  mercury 
  arc 
  as 
  a 
  source 
  of 
  light 
  in 
  

   spectroscopy 
  dates 
  back 
  to 
  18G0 
  (J. 
  H. 
  Gladstone, 
  " 
  On 
  the 
  

   Electric 
  Light 
  of 
  Mercury/^ 
  Phil. 
  Mag. 
  [4] 
  xx. 
  pp. 
  249-253), 
  

   but 
  its 
  use 
  in 
  polarimetric 
  measurements 
  was 
  apparently 
  intro- 
  

   duced 
  by 
  Disch 
  (Ann. 
  Phys. 
  (4) 
  xii. 
  p. 
  1155) 
  in 
  1903, 
  who 
  

   made 
  use 
  of 
  the 
  Arons 
  lamp. 
  The 
  Bastian 
  mercury 
  lamp, 
  

   which 
  has 
  been 
  in 
  use 
  in 
  my 
  own 
  laboratory 
  since 
  1906, 
  and 
  at 
  

   the 
  Central 
  Technical 
  College 
  since 
  1907, 
  has 
  the 
  advantage 
  

   of 
  being 
  a 
  commercial 
  article 
  of 
  much 
  lower 
  cost; 
  it 
  is 
  con- 
  

   structed 
  w^ith 
  a 
  suitable 
  resistance 
  in 
  the 
  holder, 
  so 
  that 
  it 
  can 
  

   be 
  plugged 
  into 
  the 
  ordinary 
  lighting 
  circuit 
  without 
  using 
  a 
  

   resistance-frame 
  or 
  any 
  special 
  leads. 
  This 
  lamp 
  is 
  unfortu- 
  

   nately 
  no 
  longer 
  on 
  the 
  market, 
  though 
  it 
  is 
  still 
  constructed 
  

   to 
  order 
  by 
  the 
  Brush 
  Electrical 
  Engineering 
  Company 
  ; 
  but 
  

   silica 
  lamps 
  of 
  moderate 
  price 
  are 
  promised 
  which 
  may 
  

   prove 
  to 
  be 
  as 
  economical 
  in 
  working 
  as, 
  and 
  even 
  more 
  

   efficient 
  in 
  illumination 
  than, 
  the 
  earlier 
  glass 
  lamps. 
  

  

  Of 
  the 
  six 
  chief 
  mercury 
  lines, 
  

  

  5769-45 
  J 
  I^^l^^^ 
  doublet, 
  

   5460*97 
  a 
  splendid 
  green 
  line, 
  

   4358*58 
  a 
  strong 
  violet 
  line, 
  

  

  404r-78 
  ^ 
  ^* 
  ^^^ 
  extreme 
  limit 
  of 
  the 
  visible 
  spectrum, 
  

  

  two, 
  the 
  green 
  and 
  the 
  violet, 
  have 
  already 
  proved 
  to 
  be 
  of 
  

   the 
  utmost 
  value 
  in 
  polarimetry, 
  and 
  are 
  likely 
  in 
  the 
  future 
  

   to 
  prove 
  of 
  equal 
  value 
  in 
  the 
  measurement 
  of 
  refraction 
  and 
  

   dispersion. 
  

  

  Their 
  use 
  in 
  polarimetry 
  has 
  been 
  due 
  to 
  the 
  following- 
  

   considerations. 
  For 
  accurate 
  measurements 
  of 
  the 
  specific 
  

   rotatory 
  power 
  of 
  a 
  substance, 
  and 
  to 
  any 
  even 
  larger 
  extent 
  

   for 
  tracing 
  the 
  course 
  of 
  chemical 
  changes 
  (isomeric 
  change, 
  

   sugar-hydrolysis, 
  (fcc.) 
  by 
  polarimetric 
  observations, 
  it 
  is 
  

   essential 
  to 
  use 
  an 
  intense 
  source 
  of 
  light 
  in 
  association 
  with 
  

   a 
  very 
  small 
  half-shadow 
  angle, 
  since 
  only 
  thus 
  can 
  a 
  maximum 
  

   of 
  sensitiveness 
  be 
  secured 
  : 
  it 
  is 
  also 
  desirable 
  to 
  use 
  a 
  light 
  

  

  