﻿Rotation 
  in 
  Iron 
  Cathode 
  Films. 
  95 
  

  

  and 
  as 
  far 
  as 
  possible 
  in 
  the 
  infra-red, 
  for 
  it 
  quite 
  possible 
  

   that 
  the 
  anomalous 
  character 
  of 
  the 
  dispersion 
  may 
  cease, 
  or 
  

   at 
  least 
  change 
  — 
  as 
  it 
  does 
  for 
  the 
  rotation 
  dispersion 
  — 
  within 
  

   the 
  region 
  in 
  which 
  it 
  is 
  possible 
  to 
  study 
  it. 
  Experiments 
  

   to 
  this 
  end 
  are 
  now 
  in 
  progress. 
  

  

  Part 
  III. 
  — 
  Dependence 
  of 
  the 
  Kerr 
  Rotation 
  on 
  the 
  

   Refractive 
  Index 
  of 
  the 
  Overlying 
  Medium. 
  

  

  In 
  connexion 
  with 
  the 
  foregoing 
  experiments 
  tests 
  were 
  

   made 
  for 
  several 
  films 
  of 
  the 
  rotation 
  from 
  the 
  rear, 
  or 
  glass- 
  

   metal, 
  surface, 
  and 
  the 
  unexpected 
  result 
  was 
  found 
  that 
  the 
  

   rotation 
  iu 
  this 
  case 
  was 
  more 
  than 
  half 
  again 
  as 
  large 
  as 
  that 
  

   for 
  the 
  ordinary 
  air-metal 
  surface. 
  Thinking 
  that 
  some 
  con- 
  

   nexion 
  might 
  be 
  established 
  between 
  the 
  reflexion 
  rotation 
  

   and 
  the 
  optical 
  density 
  of 
  the 
  overlying 
  medium, 
  experiments 
  

   were 
  tried 
  with 
  films 
  whose 
  surfaces 
  were 
  covered 
  with 
  thin 
  

   layers 
  of 
  various 
  liquids, 
  held 
  in 
  place 
  and 
  protected 
  by 
  a 
  slip 
  

   of 
  microscope 
  cover 
  glass. 
  The 
  rotations 
  measured 
  in 
  this 
  

   way 
  all 
  proved 
  larger 
  than 
  for 
  the 
  bare 
  surface 
  and 
  by 
  a 
  

   factor 
  not 
  far 
  ditf'ereut, 
  in 
  most 
  cases, 
  from 
  the 
  refractive 
  

   index 
  of 
  the 
  liquid. 
  

  

  It 
  can 
  be 
  readily 
  seen 
  that 
  the 
  rotation 
  as 
  measured 
  by 
  the 
  

   bolometric 
  method 
  for 
  such 
  cases 
  as 
  these, 
  will 
  not 
  represent, 
  

   without 
  suitable 
  corrections, 
  the 
  actual 
  rotation 
  which 
  takes 
  

   place 
  at 
  the 
  liquid 
  metal 
  surface. 
  For 
  in 
  the 
  first 
  place 
  a 
  

   part 
  of 
  the 
  energy 
  would 
  be 
  reflected 
  from 
  the 
  surface 
  of 
  the 
  

   glass 
  cover 
  slip 
  and 
  would 
  be 
  returned 
  without 
  undergoing 
  

   any 
  rotation. 
  A 
  smaller 
  part 
  would 
  likewise 
  be 
  reflected 
  

   from 
  the 
  glass-liquid 
  surface, 
  while 
  the 
  part 
  which 
  is 
  reflected 
  

   and 
  negatively 
  rotated 
  by 
  the 
  true 
  liquid-metal 
  surface 
  must 
  

   also 
  be 
  afl'ected 
  by 
  the 
  positive 
  rotation 
  due 
  to 
  the 
  double 
  

   passage 
  through 
  glass 
  and 
  liquid. 
  The 
  rotation 
  due 
  to 
  the 
  

   latter 
  need 
  not 
  be 
  considered 
  since 
  the 
  layer 
  of 
  liquid 
  is 
  so 
  

   thin, 
  but 
  that 
  due 
  to 
  the 
  former 
  must 
  be 
  corrected 
  for. 
  If 
  we 
  

   are 
  dealing 
  with 
  rotation 
  from 
  the 
  elass-metal 
  siuiace 
  of 
  the 
  

   film 
  the 
  corrections 
  are 
  of 
  the 
  same 
  sort, 
  but 
  slightly 
  simpler, 
  

   .since 
  the 
  liquid 
  is 
  lacking. 
  To 
  make 
  the 
  first 
  correction, 
  

   that 
  is, 
  for 
  the 
  reflected 
  energy 
  which 
  does 
  not 
  penetrate 
  to 
  

   the 
  metal 
  surface, 
  we 
  must 
  know 
  the 
  combined 
  reflecting 
  

   power 
  R, 
  of 
  the 
  air-glass, 
  glass-liquid, 
  and 
  liquid-metal 
  

   surfaces, 
  and 
  also 
  R' 
  ot 
  the 
  air-glass 
  and 
  glass-Kquid 
  surfaces 
  

   alone. 
  The 
  former 
  is 
  obtained 
  by 
  measurement, 
  the 
  latter 
  by 
  

   calculation 
  from 
  the 
  known 
  refractive 
  indices. 
  Then 
  of 
  every 
  

  

  * 
  PhiJ. 
  .Mac-. 
  loc. 
  cit. 
  p. 
  70. 
  

  

  