﻿Rotation 
  in 
  Iron 
  Cathode 
  Films. 
  89 
  

  

  being 
  in 
  line. 
  The 
  field 
  was 
  approximately 
  5700 
  c.G.s. 
  units 
  

   with 
  a 
  magnet 
  current 
  o£ 
  16 
  amps. 
  

  

  For 
  the 
  " 
  reflexion 
  " 
  curve 
  the 
  film 
  was 
  held 
  in 
  position 
  B 
  

   and 
  a 
  piece 
  of 
  plane 
  silvered 
  glass 
  placed 
  at 
  C. 
  The 
  beam 
  

   striking 
  the 
  silver 
  surface 
  would 
  be 
  reflected 
  to 
  the 
  film 
  and 
  

   from 
  this 
  through 
  the 
  other 
  pole-piece, 
  which 
  was 
  thrown 
  out 
  

   of 
  line 
  to 
  the 
  dotted 
  position 
  to 
  allow 
  the 
  passage 
  of 
  the 
  beam: 
  

   it 
  would 
  then 
  follow 
  practically 
  the 
  same 
  path 
  as 
  in 
  the 
  first 
  

   case. 
  The 
  reflexion 
  at 
  the 
  silver 
  surface 
  did 
  not 
  aff'ect 
  the 
  

   polarization 
  of 
  the 
  beam 
  and 
  the 
  departure 
  from 
  normal 
  

   incidence 
  on 
  the 
  fllm 
  — 
  about 
  8° 
  — 
  was 
  well 
  within 
  the 
  limits 
  

   of 
  15°, 
  for 
  which, 
  as 
  shown 
  bv 
  Eighi 
  *, 
  the 
  rotation 
  is 
  prac- 
  

   tically 
  the 
  same 
  as 
  for 
  normal 
  incidence. 
  This 
  arrangement, 
  

   then, 
  served 
  the 
  purpose 
  of 
  the 
  customary 
  optical 
  system 
  in 
  

   which 
  the 
  normally 
  reflected 
  beam 
  is 
  turned 
  aside 
  by 
  a 
  plane 
  

   glass 
  plate 
  at 
  45*^; 
  such 
  an 
  arrangement 
  would 
  be 
  of 
  difficult, 
  

   if 
  not 
  wholly 
  impossible, 
  application 
  in 
  the 
  present 
  problem. 
  

   With 
  a 
  magnet 
  current 
  of 
  13*7 
  amps., 
  the 
  field 
  in 
  this 
  position 
  

   was 
  identical 
  wdth 
  that 
  in 
  position 
  A. 
  

  

  Corrections. 
  — 
  Following 
  the 
  practice 
  of 
  Kundt, 
  the 
  rotations 
  

   oiven 
  by 
  the 
  curves 
  and 
  tables 
  of 
  the 
  present 
  paper 
  are 
  for 
  

   the 
  reversal 
  of 
  the 
  magnetic 
  field, 
  unless 
  otherwise 
  specified, 
  

   and 
  hence 
  are 
  really 
  doubled 
  values. 
  They 
  were 
  all 
  deter- 
  

   mined 
  with 
  the 
  use 
  of 
  the 
  25° 
  prism, 
  and 
  therefore, 
  as 
  has 
  

   been 
  noted 
  in 
  Part 
  I., 
  some 
  correction 
  may 
  be 
  necessary 
  to 
  

   the 
  measured 
  values. 
  But 
  it 
  will 
  be 
  observed 
  that 
  the 
  curves 
  

   may 
  be 
  considered 
  as 
  partly 
  straight 
  lines 
  and 
  partly 
  of 
  the 
  

   type 
  of 
  PI. 
  I. 
  fig. 
  tt 
  ; 
  and 
  in 
  either 
  case 
  the 
  corrections 
  are 
  

   negligible. 
  

  

  For 
  the 
  transmission 
  curves, 
  correction 
  has 
  been 
  made 
  for 
  

   the 
  rotation 
  due 
  to 
  the 
  glass 
  on 
  which 
  the 
  films 
  were 
  deposited. 
  

   This 
  was 
  small 
  — 
  as 
  the 
  glass 
  was 
  only 
  -18 
  to 
  '22 
  mm. 
  thick, 
  — 
  

   but 
  by 
  no 
  means 
  negligible, 
  for 
  in 
  some 
  cases 
  it 
  may 
  be 
  com- 
  

   parable 
  with 
  the 
  rotation 
  of 
  the 
  film 
  itself. 
  It 
  was 
  carefully 
  

   determined 
  for 
  selected 
  sample 
  pieces 
  (correction 
  being 
  made 
  

   for 
  the 
  use 
  of 
  25° 
  prism 
  as 
  indicated 
  in 
  PI. 
  I. 
  fig. 
  3), 
  and 
  

   in 
  each 
  case 
  the 
  correction 
  applied 
  was 
  proportional 
  to 
  tlie 
  

   thickness 
  of 
  the 
  paiticular 
  piece 
  of 
  glass 
  on 
  which 
  the 
  film 
  

   was 
  deposited. 
  In 
  PI. 
  I. 
  figs. 
  6-9 
  the 
  dotted 
  curves 
  give 
  

   the 
  rotations 
  as 
  measured 
  while 
  the 
  solid 
  line 
  curve 
  has 
  been 
  

   corrected 
  for 
  the 
  glass 
  rotation. 
  The 
  curve 
  for 
  the 
  glass 
  

   alone 
  is 
  shown 
  in 
  PL 
  I. 
  fig. 
  6. 
  

  

  To 
  be 
  certain 
  that 
  no 
  other 
  influence 
  than 
  the 
  magnetized 
  

   film 
  was 
  causing 
  rotation, 
  trials 
  were 
  occasionally 
  made 
  for 
  

   rotation 
  on 
  reflexion 
  from 
  two 
  silver 
  surfaces, 
  instead 
  of 
  one 
  

   * 
  Ann. 
  Chim. 
  Phys. 
  (6) 
  ix. 
  p. 
  132 
  (1686). 
  

  

  