﻿92 
  Prof. 
  L. 
  E. 
  Ingersoll 
  on 
  Magnetic 
  

  

  been 
  collected, 
  but 
  the 
  results 
  on 
  different 
  films 
  are 
  somewhat 
  

   at 
  variance. 
  This 
  is 
  in 
  accordance 
  with 
  the 
  observations 
  of 
  

   Kundt 
  *, 
  who 
  found 
  very 
  complicated 
  effects 
  when 
  the 
  mirror 
  

   for 
  which 
  he 
  was 
  measuring 
  the 
  Kerr 
  rotation 
  was 
  oxidized. 
  

   In 
  general 
  it 
  was 
  found, 
  however, 
  that 
  films 
  of 
  a 
  reddish 
  

   or 
  brownish 
  colour 
  — 
  indicating 
  more 
  or 
  less 
  oxidation 
  — 
  gave 
  

   curves 
  similar 
  to 
  those 
  shown 
  in 
  PL 
  I. 
  figs. 
  11, 
  12, 
  and 
  13. 
  The 
  

   difference 
  between 
  these 
  and 
  the 
  curves 
  for 
  the 
  pure 
  iron 
  

   films 
  is 
  seen 
  in 
  the 
  rapid 
  decrease 
  of 
  rotation 
  with 
  wave-length 
  

   after 
  the 
  early 
  infra-red. 
  This 
  is 
  particularly 
  true 
  for 
  the 
  

   case 
  of 
  rotation 
  on 
  transmission, 
  and 
  was 
  very 
  marked 
  in 
  a 
  

   film 
  deposited 
  in 
  air 
  instead 
  of 
  hydrogen, 
  and 
  which 
  was 
  

   presumably 
  completely 
  oxidized. 
  Furthermore, 
  the 
  curves 
  

   for 
  rotation 
  on 
  transmission 
  and 
  reflexion 
  do 
  not 
  show 
  the 
  

   same 
  similarity 
  as 
  in 
  the 
  case 
  of 
  pure 
  iron, 
  and 
  indeed 
  in 
  

   two 
  of 
  the 
  cases 
  shown 
  the 
  reflexion 
  rotation 
  appears 
  positive 
  

   for 
  the 
  shorter 
  wave-lengths. 
  

  

  The 
  change 
  in 
  the 
  character 
  of 
  the 
  rotation 
  curves 
  is 
  seen 
  

   to 
  be 
  accompanied 
  by 
  a 
  marked 
  variation 
  of 
  the 
  transmission 
  

   with 
  wave-length, 
  as 
  shown 
  by 
  the 
  dotted 
  curves 
  of 
  PL 
  I. 
  

   fig. 
  10. 
  For 
  the 
  opaque 
  partially 
  oxidized 
  film 
  29 
  of 
  PL 
  I. 
  

   fig. 
  13 
  only 
  the 
  reflexion 
  rotation 
  and 
  reflecting 
  power 
  could 
  

   be 
  measu.red, 
  but 
  each 
  is 
  seen 
  to 
  change 
  markedly 
  with 
  

   change 
  of 
  wave-length. 
  As 
  to 
  the 
  effect 
  of 
  oxidation 
  on 
  the 
  

   magnitude 
  of 
  the 
  rotation, 
  films 
  7 
  and 
  14 
  show 
  smaller 
  rota- 
  

   tions 
  than 
  the 
  pure 
  iron 
  films, 
  although 
  film 
  14 
  has 
  about 
  

   the 
  same 
  absorption 
  as 
  fihn 
  22. 
  The 
  reflexion 
  rotation 
  from 
  

   film 
  29, 
  however, 
  is 
  slightly 
  greater 
  for 
  the 
  early 
  infra-red 
  

   than 
  that 
  from 
  film 
  15 
  or 
  that 
  due 
  to 
  a 
  steel 
  surface 
  as 
  shown 
  

   in 
  PL 
  I. 
  fig. 
  4. 
  

  

  Discussion 
  of 
  Results. 
  — 
  The 
  first 
  fact 
  worthy 
  of 
  note 
  which 
  

   may 
  be 
  deduced 
  irom 
  the 
  curves 
  is 
  that 
  the 
  magnetic 
  rotation, 
  

   either 
  on 
  transmission 
  or 
  reflexion, 
  of 
  iron, 
  instead 
  of 
  in- 
  

   creasing 
  indefinitely 
  with 
  wave-lengthy 
  as 
  might 
  be 
  concluded 
  

   from 
  observations 
  in 
  the 
  visible 
  spectrum, 
  reaches 
  a 
  maximum 
  

   value 
  shortly 
  outside 
  the 
  limit 
  of 
  this 
  spectrum. 
  This 
  

   maximum 
  rotation, 
  which 
  is 
  reached 
  for 
  pure 
  iron 
  at 
  about 
  

   X 
  = 
  l*5//', 
  is 
  seen 
  to 
  be 
  on 
  the 
  average 
  a 
  little 
  over 
  one 
  and 
  

   two-thirds 
  times 
  that 
  for 
  the 
  wave-length 
  of 
  sodium 
  light. 
  

  

  In 
  the 
  second 
  place 
  it 
  will 
  be 
  noted 
  that 
  while 
  the 
  Faraday 
  

   rotation 
  decreases 
  by 
  five 
  times 
  in 
  going 
  from 
  film 
  15 
  to 
  

   film 
  24, 
  the 
  Kerr 
  rotation 
  is 
  only 
  halved. 
  In 
  other 
  words 
  

   the 
  reflexion 
  rotation 
  increases 
  less 
  rapidly 
  than 
  the 
  thickness 
  

   of 
  the 
  film 
  and 
  soon 
  reaches 
  a 
  maximum 
  value. 
  That 
  this 
  

   has 
  been 
  very 
  nearly 
  reached 
  in 
  film 
  15 
  is 
  indicated 
  by 
  the 
  

  

  * 
  Wied. 
  Ann. 
  xxvii. 
  p. 
  199 
  (1886). 
  

  

  