﻿Rotation 
  in 
  Iron 
  CatJiode 
  Films. 
  87 
  

  

  It 
  will 
  be 
  seen 
  from 
  the 
  above 
  table 
  that 
  the 
  actual 
  and 
  

   computed 
  rotations 
  usually 
  differ 
  by 
  1 
  per 
  cent, 
  or 
  less. 
  This, 
  

   then, 
  may 
  be 
  taken 
  as 
  about 
  the 
  order 
  of 
  the 
  outstanding 
  

   probable 
  error 
  o£ 
  rotation 
  measurements 
  in 
  this 
  work. 
  It 
  is 
  

   about 
  as 
  great 
  an 
  accuracy 
  as 
  may 
  be 
  reasonably 
  expected 
  in 
  

   almostany 
  spectro-bolometric 
  or 
  radiation 
  work, 
  and 
  is 
  ample 
  

   for 
  the 
  present 
  investigation. 
  

  

  In 
  the 
  matter 
  of 
  certainty 
  of 
  wave-length, 
  as 
  great 
  accuracy 
  

   cannot 
  be 
  claimed 
  as 
  might 
  be 
  expected 
  in 
  radiation 
  problems 
  

   where 
  a 
  spectrum 
  of 
  large 
  dispersion 
  and 
  a 
  very 
  narrow 
  

   bolometer-strip 
  can 
  be 
  used. 
  The 
  spectrometer 
  was 
  calibrated 
  

   in 
  the 
  present 
  case 
  by 
  double 
  dispersion 
  tests, 
  using 
  a 
  rock- 
  

   salt 
  prism 
  on 
  the 
  second 
  spectrometer 
  to 
  fix 
  the 
  wave-length 
  

   scale. 
  Each 
  of 
  the 
  two 
  prisms 
  was 
  tested 
  several 
  times 
  in 
  

   this 
  way. 
  For 
  the 
  25" 
  prism 
  the 
  estimated 
  probable 
  error 
  

   rises 
  from 
  a 
  few 
  thousandths 
  of 
  a 
  micron 
  in 
  the 
  visible 
  and 
  

   early 
  infra-red 
  to 
  about 
  '01 
  yu, 
  at 
  \=1'1 
  fj,, 
  and 
  is 
  perhaps 
  

   three 
  or 
  four 
  times 
  as 
  much 
  at 
  2'3 
  /n. 
  For 
  the 
  45° 
  prism 
  the 
  

   probable 
  error 
  hardly 
  exceeds 
  '02 
  fx 
  anywhere. 
  It 
  must 
  be 
  

   remembered, 
  however, 
  that 
  the 
  bolometer-strip 
  covers 
  a 
  

   much 
  greater 
  width 
  of 
  spectrum 
  than 
  this 
  ; 
  hence 
  the 
  deter- 
  

   mination 
  of 
  a 
  rotation 
  dispersion 
  curve 
  which 
  had 
  sharp 
  bends 
  

   or 
  dips 
  corresponding 
  to 
  sharp 
  absorption-bands 
  would 
  be 
  

   practically 
  impossible. 
  

  

  Part 
  II. 
  — 
  Kotation 
  Dispersion 
  Curves 
  for 
  the 
  

   Faraday 
  and 
  Kerr 
  Effects. 
  

  

  Deposition 
  of 
  Films. 
  — 
  The 
  films 
  were 
  deposited 
  cathodically 
  

   on 
  pieces 
  of 
  microscope 
  cover-glass 
  in 
  an 
  atmosphere 
  of 
  

   hydrogen, 
  an 
  induction-coil 
  being 
  used 
  as 
  a 
  source 
  of 
  current 
  

   and 
  a 
  small 
  spark-gap 
  introduced 
  in 
  the 
  secondary 
  circuit 
  to 
  

   make 
  it 
  practically 
  one 
  directional. 
  A 
  flat 
  coil 
  of 
  pure 
  iron 
  

   wire, 
  making 
  a 
  disk 
  of 
  2*5 
  cms. 
  diam., 
  served 
  as 
  a 
  cathode, 
  and 
  

   the 
  best 
  results 
  were 
  obtained 
  with 
  a 
  vacuum 
  of 
  a 
  fraction 
  of 
  

   a 
  millimetre, 
  and 
  with 
  the 
  pieces 
  of 
  cover-glass 
  just 
  outside 
  

   the 
  cathode 
  dark 
  space 
  and 
  about 
  1 
  cm. 
  fiom 
  the 
  cathode. 
  It 
  

   was 
  possible 
  to 
  use 
  thin 
  microscope-cover 
  glass 
  as 
  a 
  backing 
  

   for 
  the 
  films, 
  in 
  spite 
  of 
  its 
  poor 
  optical 
  surfaces 
  ; 
  for 
  it 
  will 
  

   be 
  noted 
  that 
  in 
  all 
  cases 
  the 
  film 
  was 
  placed 
  where 
  the 
  beam 
  

   of 
  light 
  converged 
  to 
  a 
  focus. 
  Hence 
  the 
  loss 
  of 
  light 
  

   occasioned 
  by 
  the 
  departure 
  from 
  planeness 
  of 
  the 
  surfaces 
  

   was 
  immaterial, 
  while 
  the 
  advantage 
  of 
  having 
  only 
  a 
  small 
  

   thickness 
  of 
  glass 
  is 
  evident. 
  

  

  Considerable 
  care 
  was 
  required 
  to 
  secure 
  films 
  which 
  were 
  

   sufficiently 
  uniform 
  over 
  the 
  required 
  area 
  of 
  4 
  x 
  15 
  mms., 
  

   and 
  free 
  from 
  oxidation. 
  With 
  the 
  first 
  induction-coil 
  used 
  — 
  

  

  