﻿Rotation 
  in 
  Iron 
  Cathode 
  Films. 
  75 
  

  

  gives 
  rise 
  to 
  the 
  Kerr 
  rotation 
  ; 
  and 
  the 
  relation 
  between 
  the 
  

   Kerr 
  rotation 
  and 
  the 
  optical 
  density 
  o£ 
  the 
  surrounding- 
  

   medium. 
  These 
  questions 
  have 
  been 
  studied 
  for 
  the 
  simplest 
  

   typical 
  case, 
  — 
  that 
  of 
  iron 
  in 
  the 
  form 
  of 
  thin 
  films, 
  the 
  

   cathode 
  film 
  being 
  chosen 
  for 
  reasons 
  which 
  will 
  appear 
  

   later. 
  

  

  The 
  rotation 
  of 
  plane 
  polarized 
  light 
  on 
  transmission 
  

   through 
  thin 
  metallic 
  fihns 
  in 
  a 
  magnetic 
  field 
  has 
  been 
  

   studied 
  by 
  Kimdt*, 
  Dn 
  Boisf, 
  LobachJ, 
  Righi§, 
  Harris 
  ||, 
  

   Skinner 
  and 
  Tool 
  If, 
  and 
  others. 
  This 
  effect, 
  discovered 
  by 
  

   Kundt, 
  has 
  since 
  been 
  measured 
  for 
  light 
  of 
  various 
  wave- 
  

   lengths 
  by 
  other 
  investigators, 
  and 
  so-called 
  rotation 
  disper- 
  

   sion 
  curves 
  thus 
  obtained 
  for 
  films 
  of 
  iron, 
  nickel, 
  and 
  cobalt. 
  

   Recently 
  Harris 
  made 
  some 
  very 
  careful 
  determinations 
  of 
  

   such 
  curves 
  for 
  iron 
  films 
  deposited 
  cathodically 
  in 
  atmo- 
  

   spheres 
  of 
  hydrogen, 
  nitrogen, 
  and 
  oxygen, 
  wdiile 
  Skinner 
  

   and 
  Tool 
  find 
  that, 
  independent 
  of 
  changes 
  in 
  atmosphere, 
  

   two 
  different 
  sorts 
  of 
  films 
  with 
  different 
  rotatory 
  powers 
  

   may 
  be 
  produced 
  for 
  some 
  metals 
  by 
  the 
  cathode 
  discharge, 
  

   and 
  moreover 
  that 
  the 
  rotation 
  for 
  any 
  wave-length 
  bears 
  

   more 
  or 
  less 
  relation 
  to 
  the 
  absorption 
  for 
  that 
  wave-length. 
  

  

  An 
  effect 
  quite 
  similar 
  to 
  this 
  of 
  rotation 
  by 
  transmission 
  

   through 
  a 
  magnetized 
  film, 
  and 
  which, 
  doubtless, 
  owes 
  its 
  

   existence 
  to 
  quite 
  similar 
  if 
  not 
  identical 
  causes, 
  is 
  the 
  so- 
  

   called 
  Kerr 
  effect, 
  or 
  rotation 
  of 
  the 
  plane 
  of 
  polarization 
  

   U])on 
  normal 
  reflexion 
  from 
  a 
  polished 
  mirror 
  held 
  perpen- 
  

   dicular 
  to 
  a 
  strong 
  magnetic 
  field. 
  The 
  mirror 
  may 
  be 
  of 
  

   iron, 
  cobalt, 
  nickel, 
  or 
  magnetite, 
  and 
  indeed 
  in 
  the 
  earlier 
  

   experiments 
  was 
  formed 
  by 
  merely 
  polishing 
  one 
  end 
  of 
  a 
  

   pole-piece 
  of 
  the 
  magnet. 
  

  

  The 
  work 
  of 
  Kerr** 
  was 
  extended 
  by 
  Du 
  Boisft^ 
  who 
  

   determined 
  the 
  variation 
  of 
  this 
  effect 
  with 
  wave-length 
  for 
  

   mirrors 
  of 
  the 
  above-mentioned 
  substances. 
  Such 
  rotation 
  

   dispersion 
  curves 
  resemble 
  those 
  obtained 
  for 
  the 
  case 
  of 
  

   transmission 
  through 
  thin 
  films, 
  in 
  that 
  they 
  are 
  anomalous, 
  

   showing 
  in 
  general 
  an 
  increase 
  of 
  rotation 
  with 
  wave-length, 
  

   as 
  far 
  as 
  the 
  limits 
  of 
  the 
  visible 
  spectrum, 
  instead 
  of 
  a 
  

   marked 
  decrease, 
  as 
  is 
  the 
  case 
  for 
  practically 
  all 
  transparent 
  

  

  * 
  Wied. 
  An7i. 
  xxiii. 
  p. 
  228 
  (1884) 
  ; 
  xxvii. 
  p. 
  191 
  (188G). 
  

  

  t 
  Wied. 
  Ann. 
  xxxi. 
  p. 
  941 
  (1887). 
  

  

  1 
  Wied. 
  Ami. 
  xxxix. 
  p. 
  347 
  (1890). 
  

  

  § 
  Me7n. 
  R. 
  Accnd. 
  Se. 
  d. 
  Bologna, 
  1886, 
  p. 
  443. 
  

  

  Pbys. 
  Rev. 
  xxiv. 
  p. 
  337 
  (190 
  

  

  ^ 
  Paper 
  read 
  before 
  the 
  American 
  Phvsical 
  Society. 
  Chicago, 
  Dec. 
  

   1907. 
  

  

  ** 
  Phil. 
  Mapr. 
  [ol 
  iii. 
  p. 
  339 
  (1877). 
  

   ft 
  Vwed. 
  Ann. 
  xxxix. 
  p. 
  -26 
  (1890) 
  ; 
  Phil. 
  Mag. 
  [5] 
  xxix. 
  p. 
  253 
  (1890). 
  

  

  