﻿90 
  Prof. 
  L. 
  R. 
  Ingersoll 
  on 
  Magnetic 
  

  

  silver 
  and 
  one 
  iron. 
  Such 
  rotations 
  were 
  always 
  found 
  to 
  

   be 
  extremely 
  small. 
  One 
  has 
  been 
  plotted 
  in 
  PL 
  1. 
  fig. 
  6, 
  and 
  

   denoted 
  by 
  the 
  letter 
  S. 
  It 
  amounts 
  to 
  only 
  about 
  "002". 
  

  

  It 
  is 
  undoubtedly 
  true 
  that 
  the 
  results 
  as 
  given 
  for 
  the 
  

   thinner 
  films 
  are 
  somewhat 
  in 
  error 
  because 
  of 
  multiple 
  

   internal 
  reflexions, 
  but 
  it 
  would 
  be 
  hopeless 
  to 
  try 
  to 
  allow 
  

   for 
  these 
  until 
  we 
  can 
  separate 
  more 
  clearly 
  the 
  two 
  rotation 
  

   effects 
  and 
  perhaps 
  localize 
  the 
  reflexion 
  rotation. 
  This 
  

   error 
  is 
  not 
  considered 
  of 
  importance 
  except 
  in 
  the 
  case 
  of 
  

   the 
  very 
  thinnest 
  films. 
  It 
  is 
  also 
  a 
  fact, 
  from 
  the 
  work 
  of 
  

   j)revious 
  observers 
  *, 
  that 
  transmitted 
  light 
  from 
  a 
  magnetized 
  

   film 
  is 
  slightly 
  elliptically 
  polarized. 
  This 
  would 
  not 
  cause- 
  

   any 
  error 
  in 
  the 
  present 
  work 
  ; 
  the 
  rotation 
  measured 
  would 
  

   be 
  that 
  of 
  the 
  major 
  axis 
  of 
  the 
  resulting 
  ellipse. 
  

  

  Absorption 
  Measurements. 
  — 
  It 
  was 
  thought 
  to 
  be 
  of 
  interest 
  

   to 
  determine 
  if 
  the 
  absorbing 
  and 
  reflecting 
  powers 
  of 
  the 
  

   films 
  showed 
  a 
  variation 
  with 
  wave-length 
  corresponding 
  to 
  

   the 
  rotation. 
  Accordingly 
  measurements 
  were 
  made 
  of 
  the 
  

   transmission 
  of 
  each 
  film, 
  as 
  well 
  as 
  of 
  the 
  reflecting 
  power 
  

   (by 
  comparison 
  with 
  silver) 
  of 
  the 
  front 
  and 
  rear 
  surfaces.. 
  

   The 
  curves 
  for 
  several 
  films 
  are 
  shown 
  in 
  PI. 
  I. 
  fig. 
  10, 
  where 
  T, 
  

   the 
  transmission, 
  is 
  approximately 
  equal 
  to 
  e~°-*^ 
  a. 
  being 
  the 
  

   absorbing 
  power 
  and 
  t 
  the 
  thickness. 
  No 
  attempt 
  has 
  been 
  

   made 
  to 
  correct 
  for 
  the 
  complicated 
  system 
  of 
  internal 
  

   reflexions 
  : 
  this 
  is 
  in 
  itself 
  an 
  extremely 
  difficult 
  problem 
  for 
  

   films 
  as 
  thin 
  as 
  some 
  of 
  these. 
  The 
  values 
  of 
  T 
  have 
  been 
  

   obtained 
  by 
  merely 
  dividing 
  the 
  fraction 
  of 
  radiation 
  trans- 
  

   mitted 
  by 
  1 
  — 
  ft, 
  where 
  R 
  is 
  the 
  reflecting 
  power 
  of 
  the 
  

   air-film 
  surface. 
  While 
  not 
  possessing 
  the 
  accuracy 
  of 
  the 
  

   rotation 
  measurements 
  they 
  serve 
  very 
  well 
  to 
  indicate 
  the 
  

   selective 
  or 
  non-selective 
  character 
  of 
  the 
  absorption 
  or 
  

   reflexion, 
  which 
  is 
  the 
  only 
  part 
  in 
  which 
  we 
  are 
  particularly 
  

   interested. 
  

  

  It 
  will 
  be 
  noted 
  that 
  the 
  transmission 
  — 
  and 
  therefore 
  alsO' 
  

   absorption 
  — 
  for 
  the 
  pure 
  iron 
  films 
  15, 
  22, 
  and 
  24 
  is 
  almost 
  

   non-selective 
  or 
  independent 
  of 
  wave-length. 
  The 
  same 
  is 
  

   true 
  for 
  film 
  17 
  whose 
  curve 
  is 
  not 
  shown. 
  This 
  explains 
  

   the 
  greyish 
  colours 
  of 
  these 
  films, 
  and, 
  in 
  agreement 
  with 
  the 
  

   conclusions 
  of 
  other 
  observers 
  as 
  already 
  pointed 
  out, 
  is 
  

   accepted 
  as 
  evidence 
  of 
  their 
  freedom 
  from 
  oxidation. 
  

  

  The 
  curve 
  of 
  reflecting 
  power 
  with 
  wave-length 
  for 
  film 
  15 
  

   is 
  incidentally 
  shown 
  in 
  PL 
  J. 
  fig. 
  13. 
  It 
  shows 
  a 
  slight 
  increase 
  

   of 
  reflecting 
  power 
  in 
  the 
  infra-red, 
  as 
  is 
  the 
  case 
  with 
  pilre 
  

   iron 
  in 
  the 
  massj. 
  Film 
  17 
  shows 
  a 
  slightly 
  greater 
  increase- 
  

  

  * 
  See 
  Harris, 
  Phys. 
  Rev. 
  xxiv. 
  p. 
  337 
  (1907.) 
  

  

  t 
  Hagen 
  and 
  Rubens, 
  Ann. 
  d. 
  Fhys. 
  viii. 
  p. 
  1 
  (1902). 
  

  

  