﻿Rotation 
  in 
  Iron 
  Cathode 
  Films. 
  91 
  

  

  of 
  reflecting 
  power 
  with 
  increasing 
  wave-length, 
  while 
  films 
  

   22 
  and 
  24 
  show 
  a 
  somewhat 
  greater 
  decrease, 
  but 
  on 
  the 
  

   whole 
  the 
  reflecting 
  power 
  for 
  the 
  pure 
  films 
  is 
  also 
  relatively 
  

   non-selective. 
  

  

  Thickness 
  of 
  Films, 
  — 
  The 
  absorption 
  and 
  reflexion 
  data 
  

   would 
  afford 
  a 
  possible 
  means 
  for 
  determining 
  the 
  relative 
  

   thicknesses 
  of 
  the 
  various 
  films 
  ; 
  but, 
  as 
  Houllevigue 
  and 
  

   Passa 
  * 
  have 
  shown, 
  it 
  is 
  more 
  accurate 
  to 
  compute 
  the 
  thick- 
  

   nesses 
  from 
  the 
  rotation 
  produced, 
  since 
  this 
  is 
  proportional 
  

   to 
  the 
  thickness. 
  Accordingly 
  the 
  thicknesses 
  of 
  the 
  four 
  

   fihns 
  already 
  mentioned, 
  15, 
  17, 
  22, 
  and 
  24 
  have 
  been 
  com- 
  

   puted 
  in 
  this 
  way, 
  assuming 
  Houllevigue 
  and 
  Passa''s 
  data 
  and 
  

   iron 
  cathode 
  films, 
  as 
  approximately 
  b^ 
  /jl/uL, 
  30 
  fifju, 
  13 
  fifx, 
  for 
  

   8 
  /jl/jL 
  respectively 
  t. 
  Kundt^s 
  results 
  for 
  films 
  by 
  electrolysis 
  

   would 
  give 
  thicknesses 
  about 
  one 
  third 
  smaller. 
  

  

  Ve7\i/ 
  Thin 
  Films. 
  — 
  Tests 
  were 
  made 
  also 
  on 
  a 
  number 
  of 
  

   very 
  thin 
  films, 
  some 
  of 
  them 
  less 
  than 
  0'5 
  /ifM 
  in 
  thickness, 
  

   but 
  the 
  results 
  are 
  of 
  questionable 
  value, 
  particularly 
  those 
  

   for 
  rotation 
  on 
  reflexion. 
  For 
  in 
  such 
  thin 
  films 
  reflexion 
  

   would 
  take 
  place 
  not 
  only 
  at 
  the 
  first 
  or 
  air-film 
  surface, 
  

   but 
  also 
  at 
  the 
  film-glass 
  boundary 
  and 
  at 
  the 
  glass-air 
  or 
  

   rear 
  surface, 
  and, 
  because 
  of 
  the 
  small 
  loss 
  on 
  transmission, 
  

   these 
  three 
  surfaces 
  — 
  or 
  at 
  least 
  the 
  first 
  and 
  last 
  — 
  would 
  

   each 
  return 
  about 
  the 
  same 
  amount 
  of 
  radiation. 
  The 
  latter 
  

   part 
  was 
  generally 
  excluded 
  by 
  covering 
  the 
  rear 
  glass 
  

   surface 
  with 
  an 
  absorbing 
  mixture 
  of 
  the 
  same 
  optical 
  density- 
  

   as 
  the 
  glass, 
  i. 
  e. 
  Canada 
  balsam 
  and 
  lamp-black 
  ; 
  but 
  there 
  

   would 
  seem 
  to 
  be 
  no 
  way 
  of 
  separating 
  the 
  first 
  two 
  parts. 
  

   In 
  one 
  case 
  there 
  was 
  tried 
  the 
  experiment 
  of 
  depositing 
  a 
  

   thick 
  film 
  of 
  platinum, 
  and 
  on 
  this 
  a 
  thin 
  film 
  of 
  iron. 
  The 
  

   rotation 
  shown 
  on 
  reflexion 
  was 
  positive, 
  as 
  might 
  be 
  expected, 
  

   due 
  to 
  the 
  preponderance 
  of 
  the 
  rotation 
  of 
  the 
  doubly 
  trans- 
  

   mitted 
  part 
  of 
  the 
  beam. 
  The 
  only 
  striking 
  feature 
  of 
  the 
  

   experiment 
  was 
  the 
  enormously 
  varying 
  reflecting 
  power 
  

   shown 
  by 
  such 
  a 
  film, 
  increasing 
  from 
  almost 
  nothing 
  in 
  the 
  

   visible 
  to 
  about 
  50 
  per 
  cent, 
  at 
  X 
  = 
  2/jl. 
  

  

  In 
  general, 
  the 
  results 
  on 
  very 
  thin 
  films 
  indicated 
  a 
  some- 
  

   what 
  larger 
  reflexion 
  rotation 
  than 
  the 
  transmission. 
  The 
  

   latter 
  rotation 
  sometimes 
  measured 
  not 
  more 
  than 
  '003°, 
  with 
  

   the 
  reflexion 
  rotation 
  perhaps 
  twice 
  as 
  large 
  and 
  frequently 
  

   appearing 
  positive 
  in 
  the 
  early 
  infra-red. 
  

  

  Efect 
  of 
  Oxidation. 
  — 
  Considerable 
  data 
  on 
  this 
  point 
  have 
  

  

  * 
  Comptes 
  Rondus, 
  cxli. 
  p. 
  29 
  (1905). 
  

  

  t 
  This 
  scale 
  of 
  thickness 
  has 
  been 
  checked 
  to 
  within 
  a 
  few 
  per 
  cent, 
  

   by 
  subsequent 
  measurements, 
  by 
  interference 
  methods, 
  of 
  films 
  deposited 
  

   on 
  optical 
  glass. 
  See 
  "Addendum." 
  

  

  