﻿Rotation 
  in 
  Iron 
  Catliode 
  Films, 
  85 
  

  

  Table 
  I. 
  

  

  Yalues 
  o£ 
  Sensibility 
  Reduction 
  Factors. 
  

  

  A. 
  B. 
  C. 
  

  

  (1) 
  Values 
  griven 
  by 
  step 
  by 
  step 
  method 
  . 
  . 
  2*95 
  9'45 
  27'2o 
  

  

  (2) 
  Measured 
  with 
  rotating- 
  sector 
  2-90 
  9-20 
  2670 
  

  

  (3) 
  Computed 
  from 
  rotations 
  of 
  the 
  polarizer 
  2*92 
  9'15 
  26*45 
  

  

  Accepted 
  values 
  2-90 
  9-20 
  26-60 
  

  

  J), 
  wliich 
  was 
  rarely 
  used, 
  was 
  found 
  by 
  comparison 
  with 
  C 
  to 
  be 
  104*5. 
  

  

  This 
  table 
  is 
  o£ 
  interest 
  in 
  that 
  it 
  gives 
  an 
  idea 
  o£ 
  the 
  

   maximum 
  accuracy 
  which 
  could 
  be 
  expected 
  of 
  this 
  way 
  of 
  

   measuring 
  magnetic 
  rotations, 
  or 
  rather, 
  of 
  the 
  largest 
  error 
  

   which 
  is 
  likely 
  to 
  enter 
  from 
  this 
  source. 
  It 
  is 
  true 
  that 
  sets 
  

   (1) 
  and 
  (2) 
  disagree 
  by 
  2 
  per 
  cent., 
  and 
  (1) 
  and 
  (3) 
  by 
  3 
  per 
  

   cent. 
  ; 
  but, 
  as 
  stated 
  above, 
  method 
  (1) 
  is 
  not 
  as 
  free 
  from 
  

   objection 
  as 
  the 
  other 
  two, 
  whose 
  results 
  differ 
  by 
  less 
  than 
  

   1 
  per 
  cent., 
  and 
  this 
  is 
  estimated 
  as 
  about 
  the 
  probable 
  error. 
  

  

  Correction 
  for 
  Errors 
  due 
  to 
  Low 
  Dispersion. 
  — 
  This 
  was 
  

   determined, 
  as 
  previously 
  indicated, 
  by 
  comparison 
  of 
  the 
  

   two 
  curves 
  made 
  with 
  the 
  same 
  rotating 
  substance 
  under 
  the 
  

   two 
  different 
  conditions 
  of 
  dispersion. 
  As 
  might 
  be 
  antici- 
  

   pated, 
  the 
  error 
  in 
  determining 
  any 
  particular 
  rotation 
  is 
  

   dependent 
  not 
  only 
  on 
  the 
  wave-length, 
  that 
  is, 
  on 
  the 
  

   position 
  of 
  the 
  point 
  on 
  the 
  curve, 
  but 
  also 
  on 
  the 
  actual 
  

   form 
  of 
  the 
  curve. 
  Furthermore, 
  the 
  largest 
  error 
  is 
  to 
  be 
  

   expected 
  when 
  we 
  have 
  the 
  combination 
  of 
  an 
  energy 
  curve 
  

   rapidly 
  increasing 
  — 
  as 
  the 
  Nernst-glower 
  energy 
  curve 
  in 
  the 
  

   early 
  infra-red 
  — 
  and 
  a 
  rotation 
  dispersion 
  curve 
  rapidly 
  

   falling, 
  as 
  is 
  the 
  case 
  for 
  glass 
  and 
  similar 
  substances. 
  

  

  This 
  is 
  well 
  shown 
  bv 
  the 
  rotation 
  dispersion 
  curves 
  of 
  

   fluorite 
  in 
  PI. 
  I. 
  fig. 
  3. 
  The 
  rotations 
  on 
  the 
  upper 
  curve 
  are 
  

   30 
  per 
  cent, 
  higher 
  at 
  'GoyLt, 
  and 
  the 
  difference 
  gradually 
  

   lessens 
  until 
  they 
  are 
  equal 
  at 
  2*0 
  yu,. 
  An 
  extrapolation 
  to 
  

   give 
  the 
  true 
  fonn 
  of 
  the 
  curve 
  as 
  determined 
  with 
  a 
  spectrum 
  

   of 
  great 
  dispersion 
  would 
  raise 
  the 
  upper 
  curve 
  by 
  only 
  about 
  

   1 
  per 
  cent. 
  

  

  That 
  the 
  correction 
  is 
  much 
  smaller, 
  if 
  not 
  negligible, 
  when 
  

   the 
  rotation 
  increases 
  with 
  wave-length 
  in 
  the 
  early 
  infra- 
  

   red 
  is 
  indicated 
  in 
  PI. 
  I. 
  fig. 
  4. 
  The 
  curve 
  is 
  for 
  rotation 
  on 
  

   reflexion 
  from 
  a 
  polished 
  steel 
  mirror 
  in 
  a 
  field 
  of 
  5700 
  units. 
  

   It 
  is 
  essentially 
  identical, 
  though 
  plotted 
  somewhat 
  differently, 
  

   with 
  the 
  curve 
  as 
  measured 
  for 
  the 
  same 
  mirror 
  with 
  the 
  

   earlier 
  form 
  of 
  apparatus 
  *. 
  The 
  corrections 
  for 
  a 
  third 
  type 
  

   of 
  curve 
  — 
  a 
  horizontal 
  straight 
  line 
  — 
  could 
  be 
  deduced 
  at 
  

   once 
  from 
  the 
  figures 
  in 
  Table 
  II., 
  and 
  are 
  negligible. 
  With 
  

  

  * 
  Phil. 
  Mag. 
  he. 
  cit. 
  p. 
  65. 
  

  

  