﻿Iiutation 
  in 
  Iron 
  Cathode 
  Films. 
  77 
  

  

  investigate 
  the 
  effect 
  on 
  the 
  Kerr 
  rotation 
  o£ 
  overlavino- 
  the 
  

   surface 
  Avith 
  liquids 
  o£ 
  various 
  refractive 
  indices. 
  

  

  These 
  questions 
  are 
  discussed 
  in 
  Parts 
  II. 
  and 
  III. 
  of 
  the 
  

   present 
  paper. 
  Iron 
  has 
  been 
  chosen 
  as 
  a 
  working 
  substance 
  

   for 
  these 
  experiments 
  as 
  it 
  gives 
  the 
  largest, 
  and 
  hence 
  most 
  

   accurately 
  measurable 
  effects, 
  but 
  it 
  is 
  intended 
  in 
  the 
  near 
  

   future 
  to 
  study 
  part 
  or 
  all 
  of 
  these 
  effects 
  with 
  films 
  of 
  cobalt 
  

   and 
  nickel 
  as 
  well. 
  The 
  cathode 
  type 
  of 
  film 
  has 
  been 
  used 
  

   cbiefly 
  because 
  it 
  may 
  be 
  readily 
  produced 
  of 
  any 
  desired 
  

   thickness 
  and 
  without 
  a 
  preliminary 
  deposit 
  of 
  platinum, 
  as 
  

   would 
  be 
  required 
  for 
  electrolytic 
  films 
  and 
  which 
  would 
  be 
  

   very 
  objectionable 
  for 
  some 
  of 
  the 
  experiments. 
  It 
  is 
  true 
  

   that 
  cathode 
  fihns 
  differ 
  somewhat 
  in 
  optical 
  properties 
  from 
  

   the 
  massive 
  metal, 
  but 
  the 
  same 
  may 
  be 
  said 
  of 
  the 
  electrolytic 
  

   film 
  which 
  is 
  well 
  known 
  to 
  be 
  granular 
  in 
  structure. 
  In 
  

   some 
  cases 
  the 
  cathode 
  film 
  appears 
  to 
  resemble 
  fused 
  metal 
  

   more 
  nearly 
  than 
  the 
  electrolytic. 
  

  

  Part 
  I. 
  — 
  The 
  Bolometric 
  Method 
  and 
  Apparatus 
  

   FOR 
  Measuring 
  Magnetic 
  Eotatioxs. 
  

  

  This 
  method 
  with 
  the 
  necessary 
  apparatus 
  for 
  measuring 
  

   rotations 
  for 
  the 
  shorter 
  infra-red 
  wave-lengths 
  has 
  already 
  

   been 
  described 
  by 
  the 
  writer*, 
  both 
  in 
  its 
  original 
  form 
  and 
  

   after 
  important 
  modifications 
  had 
  been 
  introduced 
  ; 
  but 
  so 
  

   many 
  changes 
  have 
  been 
  made 
  since 
  then 
  that 
  a 
  brief 
  

   description 
  of 
  method 
  and 
  apparatus 
  seems 
  desirable 
  here. 
  

  

  The 
  essential 
  difference, 
  in 
  the 
  optical 
  system, 
  between 
  

   this 
  method 
  and 
  the 
  customary 
  process 
  of 
  measurement 
  

   applicable 
  to 
  the 
  visible 
  spectrum, 
  arises 
  from 
  the 
  fact 
  that 
  

   any 
  small 
  rotation 
  of 
  the 
  plane 
  of 
  polarization 
  occurring 
  

   between 
  crossed 
  nicols, 
  or 
  other 
  polarizing 
  agents, 
  causes 
  the 
  

   greatest 
  cliamie 
  of 
  intensity 
  when 
  the 
  latter 
  are 
  crossed 
  at 
  

   45°, 
  a 
  principle 
  recognized 
  and 
  used 
  over 
  half 
  a 
  century 
  ago 
  

   by 
  Provostaye 
  aud 
  Desainsf. 
  This, 
  then, 
  is 
  evidently 
  the 
  

   most 
  effective 
  arrangement 
  when 
  a 
  bolometer 
  or 
  other 
  

   radiation 
  measuring 
  instrument 
  is 
  to 
  be 
  used 
  instead 
  of 
  the 
  

   human 
  eye. 
  

  

  In 
  the 
  first 
  arrangement 
  adopted, 
  piles 
  of 
  thin 
  glass 
  plates 
  

   were 
  used 
  as 
  polarizer 
  and 
  analyser. 
  Light 
  from 
  a 
  Nernst 
  

   glower, 
  after 
  being 
  polarized 
  by 
  refiexion, 
  passed 
  through 
  

   the 
  magnet 
  and 
  the 
  rotating 
  substance 
  between 
  its 
  pole-pieces, 
  

   and 
  after 
  reflexion 
  from 
  the 
  analyser 
  fell 
  on 
  the 
  slit 
  of 
  a 
  

  

  * 
  Phil. 
  Mag. 
  [6] 
  Ixi. 
  p. 
  41 
  (1906) 
  ; 
  Phys. 
  Rev. 
  xxiii. 
  p. 
  489 
  (1906). 
  

   Hereafter 
  referred 
  to 
  as 
  Phil. 
  Mag, 
  loc. 
  cit. 
  aud 
  Phys. 
  Rev. 
  loc. 
  cit. 
  

   t 
  Ann. 
  Chim. 
  Phys. 
  [3] 
  xxvii. 
  p. 
  232 
  (1849). 
  

  

  