﻿324 
  Dr. 
  T. 
  M. 
  Lowry 
  on 
  a 
  Method 
  of 
  

  

  of 
  relatively 
  short 
  wave-length, 
  in 
  order 
  that 
  the 
  actual 
  

   readings 
  may 
  be 
  large 
  *, 
  without 
  incurring 
  the 
  loss 
  of 
  optical 
  

   intensity 
  and 
  the 
  fatigue 
  which 
  result 
  from 
  the 
  use 
  of 
  blue 
  

   light. 
  The 
  intense 
  green 
  mercury 
  line, 
  which 
  can 
  be 
  read 
  

   with 
  a 
  considerably 
  smaller 
  half-shadow 
  angle, 
  and 
  gives 
  

   readings 
  about 
  15 
  per 
  cent, 
  larger 
  than 
  the 
  sodium 
  doublet, 
  

   is 
  therefore 
  much 
  superior 
  to 
  the 
  traditional 
  standard, 
  apart 
  

   altogether 
  from 
  the 
  question 
  of 
  spectral 
  purity. 
  In 
  the 
  latter 
  

   respect 
  the 
  contrast 
  is 
  extreme 
  ; 
  in 
  the 
  case 
  of 
  quartz 
  I 
  have 
  

   been 
  able, 
  without 
  any 
  noticeable 
  loss 
  of 
  accuracy, 
  to 
  secure 
  

   readings 
  showing 
  a 
  total 
  rotation 
  of 
  50 
  right 
  angles 
  for 
  the 
  

   mercury 
  green 
  line, 
  two 
  independent 
  series 
  of 
  determinations 
  

   giving 
  average 
  values 
  4487*78° 
  and 
  4487*79°; 
  sodium 
  under 
  

   similar 
  conditions 
  gave 
  no 
  extinction 
  at 
  all. 
  The 
  green 
  

   mercury 
  line 
  promises, 
  indeed, 
  wholly 
  to 
  replace 
  the 
  sodium 
  

   doublet 
  as 
  a 
  chief 
  standard 
  in 
  polarimetric 
  work, 
  and 
  it 
  is 
  

   highly 
  desirable 
  that 
  it 
  should 
  acquire 
  as 
  quickly 
  as 
  possible 
  

   a 
  like 
  predominance 
  in 
  the 
  measurement 
  of 
  refractive 
  indices, 
  

   and 
  in 
  all 
  other 
  optical 
  determinations. 
  

  

  The 
  violet 
  mercury 
  line 
  has 
  proved 
  indispensable 
  in 
  the 
  

   measurement 
  of 
  rotatory 
  dispersion 
  on 
  account 
  of 
  its 
  

   extraordinary 
  brilliancy. 
  In 
  spite 
  of 
  the 
  low 
  sensitiveness 
  

   of 
  the 
  eye 
  for 
  light 
  of 
  such 
  small 
  wave-length 
  it 
  has 
  been 
  

   found 
  possible 
  to 
  read 
  this 
  line 
  with 
  a 
  half-shadow 
  angle 
  

   of 
  only 
  6°, 
  and 
  to 
  secure 
  series 
  of 
  readings 
  (each 
  an 
  

   average 
  of 
  10 
  settings) 
  which 
  only 
  differed 
  from 
  one 
  

   another 
  by 
  a 
  hundredth 
  of 
  a 
  degree. 
  The 
  violet 
  line 
  is 
  less 
  

   pure 
  than 
  the 
  green, 
  as 
  it 
  is 
  accompanied 
  by 
  two 
  satellites 
  

   of 
  smaller 
  wave-length, 
  but 
  these 
  are 
  so 
  weak 
  that 
  they 
  

   cannot 
  be 
  seen 
  at 
  all 
  in 
  the 
  polarimeter, 
  and 
  cannot, 
  therefore, 
  

   produce 
  any 
  appreciable 
  disturbance 
  in 
  the 
  readings. 
  The 
  

   yellow 
  doublet 
  is 
  made 
  up 
  of 
  two 
  lines 
  separated 
  by 
  about 
  

   three 
  times 
  as 
  great 
  an 
  interval 
  as 
  in 
  the 
  case 
  of 
  sodium 
  ; 
  

   for 
  small 
  rotations 
  they 
  may 
  be 
  read 
  as 
  one 
  line, 
  but 
  I 
  have 
  

   also 
  been 
  able^ 
  by 
  using 
  a 
  narrow 
  slit, 
  to 
  read 
  them 
  separately; 
  

   they 
  are, 
  however, 
  altogether 
  unsuited 
  for 
  general 
  use. 
  

  

  For 
  refractometer 
  work 
  the 
  mercury 
  lines 
  are 
  at 
  least 
  as 
  

   easily 
  available 
  as 
  those 
  of 
  hydrogen 
  ; 
  a 
  warmed 
  vacuum- 
  

   tube 
  containing 
  a 
  drop 
  of 
  mercury 
  gives 
  the 
  lines 
  with 
  

   greater 
  brilliance 
  than 
  those 
  of 
  hydrogen, 
  and 
  it 
  is 
  therefore 
  

   not 
  unreasonable 
  to 
  suggest 
  that 
  — 
  as 
  a 
  minimum 
  concession 
  

   to 
  the 
  correlation 
  of 
  optical 
  measurements 
  of 
  various 
  kinds 
  — 
  

   the 
  use 
  of 
  H/3 
  4861 
  and 
  Hy 
  4341 
  shall 
  be 
  abandoned 
  in 
  favour 
  

   of 
  Ilg5461 
  and 
  Hg4359 
  in 
  future 
  refractometric 
  work, 
  and 
  

  

  * 
  As 
  a 
  rule 
  the 
  specific 
  rotation 
  is 
  doubled 
  on 
  passing 
  from 
  yellow 
  to 
  

   violet. 
  

  

  