﻿76 
  Prof. 
  L. 
  R. 
  IngersoU 
  on 
  Magnetic 
  

  

  substances. 
  Moreover, 
  the 
  rotation, 
  while 
  o£ 
  about 
  the 
  same 
  

   order 
  o£ 
  magnitude 
  in 
  the 
  two 
  cases, 
  is 
  opposite 
  in 
  sign, 
  a 
  

   point 
  not 
  satisfactorily, 
  or 
  at 
  least 
  simply, 
  explained 
  by 
  the 
  

   theory. 
  Complicated 
  phenomena, 
  which 
  have 
  been 
  studied 
  

   by 
  many 
  other 
  observers, 
  arise 
  for 
  cases 
  of 
  oblique 
  incidence 
  

   or 
  of 
  magnetization 
  parallel 
  to 
  the 
  surface, 
  but 
  in 
  the 
  present 
  

   work 
  we 
  shall 
  consider 
  only 
  the 
  more 
  common 
  case 
  of 
  

   nearly 
  normal 
  incidence, 
  and 
  of 
  magnetization 
  perpendicular 
  

   to 
  the 
  surface. 
  

  

  A 
  few 
  years 
  ago 
  the 
  writer* 
  developed 
  a 
  method 
  of 
  

   measuring 
  such 
  magnetic 
  rotation 
  for 
  wave-lengths 
  of 
  the 
  

   infra-red 
  spectrum, 
  and 
  supplemented 
  the 
  work 
  of 
  Du 
  Bois 
  

   by 
  extending 
  his 
  curves 
  to 
  about 
  X 
  = 
  3'5/a. 
  The 
  striking 
  

   thing 
  shown 
  by 
  these 
  curves 
  was 
  that 
  the 
  rotation 
  in 
  each 
  

   case 
  reached 
  a 
  maximum 
  for 
  wave-length 
  about 
  1/jl, 
  and 
  

   decreased 
  rapidly 
  for 
  longer 
  wave-lengths. 
  Moreover, 
  in 
  

   the 
  case 
  of 
  nickel 
  the 
  rotation 
  chano-ed 
  sign 
  for 
  wave 
  -leno-ths 
  

   longer 
  than 
  1*4//,, 
  while 
  for 
  magnetite 
  the 
  case 
  was 
  still 
  more 
  

   complicated. 
  

  

  It 
  was 
  desired 
  at 
  the 
  time 
  to 
  proceed 
  at 
  once 
  with 
  measure- 
  

   ments 
  on 
  thin 
  films 
  of 
  the 
  magnetic 
  metals, 
  but 
  the 
  determi- 
  

   nation 
  of 
  such 
  small 
  rotations 
  was 
  too 
  difficult 
  and 
  uncertain 
  

   a 
  process 
  with 
  the 
  apparatus 
  as 
  it 
  then 
  existed. 
  Accordingly, 
  

   the 
  method 
  of 
  measurement 
  was 
  considerably 
  modified, 
  and 
  

   thereby 
  greatly 
  improved, 
  and 
  the 
  apparatus 
  almost 
  entirely 
  

   rebuilt 
  on 
  a 
  more 
  extensive 
  scale^ 
  with 
  a 
  more 
  careful 
  working 
  

   out 
  of 
  details 
  ; 
  this 
  has 
  made 
  possible 
  the 
  handling 
  of 
  the 
  

   sort 
  of 
  problems 
  it 
  was 
  desired 
  to 
  take 
  up 
  in 
  this 
  investi- 
  

   gation. 
  

  

  Ami 
  of 
  the 
  present 
  work. 
  — 
  This 
  has 
  been 
  : 
  First, 
  to 
  measure 
  

   the 
  transmission, 
  or 
  Faraday, 
  rotation 
  for 
  iron 
  films 
  of 
  various 
  

   thickness 
  for 
  as 
  wide 
  a 
  range 
  of 
  wave-lengths 
  as 
  possible 
  

   (\ 
  = 
  '6 
  jjb 
  to 
  2*2 
  fj.), 
  thus 
  supplementing 
  the 
  work 
  of 
  Lobach, 
  

   Harris, 
  and 
  others 
  by 
  extending 
  their 
  curves 
  over 
  a 
  spectral 
  

   region 
  of 
  greater 
  extent. 
  Second, 
  to 
  determine 
  the 
  reflexion 
  

   rotation, 
  or 
  Kerr 
  effect, 
  for 
  the 
  same 
  films 
  and 
  in 
  the 
  same 
  

   magnetic 
  field, 
  with 
  the 
  view 
  of 
  correlating 
  as 
  closely 
  as 
  

   possible 
  these 
  two 
  rotation 
  effects 
  due 
  to 
  transmission 
  and 
  

   reflexion 
  respectively. 
  . 
  Third, 
  to 
  study 
  the 
  dependence 
  of 
  

   the 
  Kerr 
  rotation 
  on 
  the 
  thickness 
  of 
  the 
  film, 
  a 
  problem 
  

   suggested 
  by 
  Kundt, 
  and 
  left 
  by 
  him 
  after 
  some 
  preliminary 
  

   observations 
  with 
  the 
  hope 
  of 
  more 
  careful 
  treatment 
  later.. 
  

   Fourth, 
  to 
  study 
  briefly 
  the 
  effects 
  of 
  oxidation 
  on 
  the 
  forms 
  of 
  

   the 
  rotation 
  dispersion 
  curves, 
  and 
  the 
  general 
  relation 
  between 
  

   absorbing 
  or 
  reflecting 
  power 
  and 
  rotatory 
  power. 
  Fifth, 
  to 
  

  

  * 
  Phil. 
  Mag. 
  [6] 
  Ixi. 
  p. 
  41 
  (1906). 
  

  

  