﻿94 
  Erof. 
  L. 
  R. 
  Ingersoll 
  on 
  Magnetic 
  

  

  results 
  for 
  partially 
  oxidized 
  films 
  show 
  a 
  considerable 
  

   difference 
  between 
  the 
  two 
  curves. 
  Kundt 
  explained 
  the 
  

   varying 
  effects 
  obtained 
  when 
  the 
  steel 
  mirror 
  for 
  which 
  he 
  

   was 
  measuring 
  the 
  Kerr 
  effect 
  w^as 
  slightly 
  oxidized, 
  by 
  the 
  

   mutually 
  interfering 
  action 
  of 
  the 
  two 
  beams^ 
  one 
  reflected 
  

   from 
  the 
  air-oxide 
  surface, 
  and 
  the 
  other 
  reflected 
  and 
  rotated 
  

   by 
  the 
  oxide-metal 
  surface. 
  In 
  the 
  present 
  case, 
  however, 
  

   w^e 
  are 
  dealing 
  with 
  a 
  homogeneous 
  layer, 
  not 
  a 
  film 
  of 
  oxide 
  

   on 
  the 
  metal, 
  and 
  the 
  discrepancies 
  cannot 
  be 
  as 
  easily 
  

   ■explained 
  away. 
  

  

  The 
  resemblance 
  between 
  the 
  reflexion 
  rotation 
  curve 
  for 
  

   film 
  29 
  of 
  PL 
  I. 
  fig. 
  13 
  and 
  that 
  obtained 
  with 
  a 
  steel 
  mirror, 
  

   PI. 
  I. 
  fig. 
  4, 
  would 
  indicate 
  that 
  the 
  latter, 
  though 
  polished 
  

   brightly, 
  still 
  had 
  more 
  or 
  less 
  oxide 
  on 
  its 
  surface, 
  perhaps 
  

   in 
  the 
  minute 
  scratches 
  left 
  by 
  polishing. 
  In 
  the 
  previous 
  

   ^vork 
  on 
  the 
  Kerr 
  effect 
  (Fhil. 
  Mag. 
  l. 
  c.) 
  the 
  rotation 
  dis- 
  

   persion 
  curves 
  for 
  polished 
  surfaces 
  of 
  nickel 
  and 
  cobalt 
  

   ^showed 
  this 
  decrease 
  of 
  rotation 
  with 
  wave-length 
  after 
  about 
  

   lyLfc, 
  as 
  well 
  as 
  steel. 
  It 
  remains 
  to 
  determine 
  if 
  cathodic 
  

   ■surfaces 
  of 
  these 
  metals 
  show 
  a 
  similar 
  effect. 
  If, 
  as 
  shown 
  

   above, 
  the 
  Kerr 
  rotation 
  takes 
  place 
  in 
  an 
  extremely 
  thin 
  

   surface 
  film, 
  it 
  is 
  not 
  remarkable 
  if 
  polished 
  surfaces 
  fail 
  

   to 
  give 
  results 
  identical 
  with 
  those 
  from 
  the 
  more 
  perfect 
  

   cathodic 
  surfaces. 
  

  

  Some 
  such 
  connexion 
  as 
  is 
  seen 
  to 
  exist 
  between 
  rotatory 
  

   power 
  and 
  absorbing 
  or 
  reflecting 
  power 
  might 
  reasonably 
  be 
  

   •expected. 
  Thus 
  in 
  film 
  29 
  a 
  rotatory 
  power 
  decreasing 
  with 
  

   w^ave-length 
  is 
  accompanied 
  by 
  an 
  increasing 
  reflecting 
  power. 
  

   The 
  readiest 
  explanation 
  of 
  this 
  — 
  overlooking 
  for 
  the 
  moment 
  

   the 
  somewhat 
  different 
  state 
  of 
  the 
  iron 
  — 
  would 
  be 
  that 
  the 
  

   Kerr 
  effect 
  is 
  a 
  function 
  of 
  the 
  penetration 
  of 
  the 
  wave 
  on 
  

   reflexion, 
  and 
  this 
  may 
  on 
  the 
  whole 
  be 
  less 
  for 
  a 
  greater 
  

   reflecting 
  power. 
  Similarly 
  for 
  the 
  pure 
  films, 
  the 
  flat 
  part 
  

   of 
  the 
  rotation 
  dispersion 
  curve 
  accompanies 
  a 
  non-selective 
  

   absorption 
  or 
  reflexion. 
  

  

  But 
  this 
  does 
  not 
  account 
  in 
  either 
  case 
  for 
  the 
  rapid 
  

   change 
  of 
  rotatory 
  power 
  with 
  wave-length 
  in 
  the 
  visible 
  and 
  

   early 
  infra-red 
  spectrum. 
  This 
  accords 
  with 
  the 
  conclusions 
  

   reached 
  by 
  the 
  writer 
  from 
  a 
  study 
  of 
  the 
  Kerr 
  rotation 
  

   =curves 
  for 
  mirrors 
  of 
  steel, 
  cobalt, 
  and 
  nickel, 
  that 
  this 
  

   spectral 
  region, 
  — 
  viz. 
  from 
  perhaps 
  \ 
  = 
  '4:jjb 
  to 
  l/ii 
  — 
  has 
  much 
  

   the 
  same 
  optical 
  significance 
  in 
  the 
  case 
  of 
  metals 
  that 
  a 
  

   region 
  of 
  resonance 
  absorption 
  has 
  for 
  a 
  dielectric. 
  As 
  is 
  

   well 
  known, 
  the 
  refractive 
  indices 
  of 
  metals 
  increase 
  with 
  

   wave-length 
  in 
  the 
  visible 
  spectrum. 
  It 
  remains 
  to 
  investi- 
  

   gate 
  their 
  optical 
  constants 
  for 
  the 
  remainder 
  of 
  this 
  region 
  

  

  