﻿Rotation 
  in 
  Iron 
  Cathode 
  Films. 
  99 
  

  

  n 
  may 
  be 
  taken 
  as 
  relative 
  to 
  that 
  of 
  the 
  surrounding 
  medium, 
  

   while 
  the 
  absorption 
  index 
  k 
  is 
  constant 
  — 
  we 
  reach 
  at 
  once 
  

   the 
  conclusion 
  from 
  the 
  above 
  equation 
  that 
  the 
  Kerr 
  rotation 
  

   should 
  increase 
  directly 
  as 
  the 
  refractive 
  index 
  of 
  the 
  over- 
  

   lying 
  medium. 
  

  

  That 
  this 
  is 
  at 
  least 
  the 
  general 
  trend 
  of 
  the 
  facts 
  is 
  sliown 
  

   by 
  Table 
  V. 
  The 
  agreement 
  is 
  by 
  no 
  means 
  good, 
  but, 
  for 
  

   the 
  experiments 
  with 
  cathode 
  films, 
  the 
  mean 
  values 
  of 
  the 
  

   ratios 
  — 
  if 
  we 
  allow 
  that 
  a 
  mean 
  has 
  any 
  significance 
  when 
  

   there 
  is 
  such 
  a 
  decided 
  " 
  drift 
  " 
  of 
  the 
  results 
  — 
  differ 
  

   generally 
  by 
  only 
  a 
  few 
  per 
  cent, 
  from 
  the 
  refractive 
  indices. 
  

   There 
  is 
  no 
  evident 
  explanation 
  of 
  the 
  rapid 
  decrease 
  of 
  this 
  

   ratio 
  with 
  increasing 
  wave-length 
  ; 
  it 
  is 
  much 
  greater 
  than 
  

   the 
  decrease 
  of 
  index 
  of 
  the 
  liquid 
  would 
  require 
  it 
  to 
  be. 
  

   The 
  results 
  with 
  steel 
  mirrors 
  are 
  seen 
  to 
  differ 
  widely 
  from 
  

   those 
  with 
  cathode 
  films, 
  particularly 
  in 
  case 
  (3) 
  with 
  Canada 
  

   balsam 
  as 
  the 
  liquid. 
  But 
  a 
  polished 
  metal 
  surface, 
  with 
  all 
  

   its 
  microscopic 
  scratches, 
  is 
  not 
  to 
  be 
  compared 
  for 
  ultimate 
  

   smoothness 
  of 
  structure 
  with 
  a 
  cathode 
  mirror 
  on 
  a 
  glass 
  

   surface. 
  It 
  may 
  be 
  indeed 
  that 
  the 
  viscous 
  Canada 
  balsam 
  

   did 
  not 
  penetrate 
  to 
  the 
  bottom 
  of 
  crevices 
  of 
  the 
  steel 
  

   surface. 
  

  

  Passins: 
  to 
  the 
  case 
  of 
  rotation 
  from 
  the 
  rear 
  surfaces 
  of 
  

   films 
  we 
  should 
  expect 
  to 
  find 
  such 
  rotations 
  larger 
  than 
  those 
  

   from 
  the 
  first 
  surfaces 
  by 
  a 
  fraction 
  equal 
  to 
  the 
  refractive 
  

   index 
  of 
  the 
  glass, 
  in 
  this 
  case 
  about 
  1*49. 
  But 
  if 
  the 
  curves 
  

   of 
  PL 
  I. 
  fig. 
  14 
  are 
  compared 
  with 
  the 
  corresponding 
  ones 
  of 
  

   PL 
  I. 
  figs. 
  6-8, 
  it 
  will 
  be 
  seen 
  that 
  the 
  rotations 
  have 
  increased 
  

   from 
  over 
  1'7 
  in 
  film 
  15 
  to 
  nearly 
  3 
  times 
  in 
  film 
  22. 
  Two 
  

   explanations 
  are 
  offered 
  for 
  this 
  : 
  that 
  the 
  state 
  of 
  aggre- 
  

   gation 
  and 
  physical 
  properties 
  of 
  the 
  metal 
  are 
  modihed 
  in 
  

   the 
  layer 
  next 
  the 
  glass 
  ; 
  or, 
  more 
  probably, 
  that 
  the 
  matter 
  

   is 
  complicated 
  by 
  internal 
  reflexions 
  in 
  the 
  case 
  of 
  the 
  

   thinner 
  films. 
  

  

  This 
  would 
  possibly 
  explain 
  these 
  results 
  and 
  at 
  the 
  same 
  

   time 
  not 
  throw 
  discredit 
  on 
  the 
  work 
  of 
  Part 
  II. 
  where 
  the 
  

   reflected 
  rotation 
  is 
  measured 
  from 
  the 
  thinner 
  films, 
  because 
  

   the 
  reflecting 
  power 
  of 
  a 
  plain 
  or 
  air-iron 
  surface 
  is 
  much 
  

   greater 
  than 
  that 
  of 
  the 
  glass-iron. 
  Hence, 
  since 
  by 
  Stokes' 
  

   law 
  these 
  reflecting 
  powers 
  also 
  hold 
  for 
  the 
  internal 
  

   reflexions, 
  the 
  amount 
  of 
  energy 
  returned 
  on 
  the 
  first 
  

   internal 
  reflexion 
  is 
  much 
  less 
  in 
  the 
  cases 
  of 
  Part 
  II. 
  than 
  in 
  

   the 
  present 
  experiments. 
  

  

  An 
  attempt 
  was 
  made 
  to 
  verify 
  qualitatively 
  these 
  con- 
  

   clusions 
  as 
  to 
  the 
  variation 
  of 
  the 
  Kerr 
  effect 
  with 
  the 
  

   overlving 
  medium, 
  by 
  means 
  of 
  visual 
  observations 
  with 
  a 
  

  

  H2 
  

  

  