﻿836 
  Prof. 
  Skinner 
  and 
  Mr. 
  Tool 
  on 
  the 
  Optical 
  

  

  magnetic 
  effects 
  and 
  also 
  in 
  their 
  extinction. 
  Compared 
  with 
  

   the 
  corresponding 
  ones 
  of 
  iron, 
  cathodic 
  films 
  of 
  cobalt 
  are 
  

   considerably 
  more 
  transparent 
  ; 
  though 
  such 
  is 
  not 
  the 
  case 
  

   with 
  electrolytic 
  films. 
  

  

  Nickel, 
  — 
  With 
  the 
  glow 
  current, 
  only 
  films 
  giving 
  metallic 
  

   reflexion 
  conld 
  be 
  produced 
  with 
  nickel, 
  though 
  these 
  varied 
  

   considerably 
  in 
  hardness. 
  The 
  densest 
  revealed 
  only 
  a 
  very 
  

   faint 
  activity 
  in 
  the 
  magnetic 
  field. 
  As 
  stated 
  above, 
  films 
  

   ol 
  nickel 
  of 
  relatively 
  pronounced 
  activity 
  were, 
  however, 
  

   obtained 
  by 
  electrolytic 
  deposition, 
  the 
  same 
  as 
  with 
  iron 
  and 
  

   cobalt. 
  

  

  2. 
  Thickness 
  of 
  Films* 
  

  

  This 
  was 
  measured 
  by 
  a 
  slight 
  but 
  very 
  convenient 
  modifi- 
  

   cation 
  of 
  the 
  customary 
  interference 
  method. 
  The 
  film 
  was 
  

   cleared 
  from 
  the 
  glass 
  to 
  an 
  abrupt 
  edge 
  at 
  the 
  point 
  to 
  be 
  

   measured, 
  I 
  hrer 
  this 
  was 
  laid 
  a 
  cover-glass, 
  which 
  produced 
  

   under 
  perpendicular 
  reflexion 
  of 
  monochromatic 
  light 
  two 
  

   adjacent 
  sets 
  of 
  interference- 
  bands 
  — 
  one 
  arising 
  from 
  the 
  

   interference 
  of 
  lighl 
  reflected 
  from 
  the 
  upper 
  and 
  lower 
  

   boundaries 
  of 
  the 
  air-space 
  between 
  the 
  two 
  plates 
  of 
  glass; 
  

   the 
  other 
  from 
  that 
  contained 
  between 
  the 
  upper 
  plate 
  and 
  

   the 
  film. 
  Obviously 
  the 
  thickness 
  of 
  the 
  film 
  is 
  one-half 
  the 
  

   relative 
  retardation 
  of 
  the 
  two 
  sets 
  of 
  bands, 
  provided 
  no 
  

   phase 
  difference 
  arises 
  at 
  the 
  surfaces. 
  That 
  no 
  measurable 
  

   surface 
  effect 
  of 
  this 
  kind 
  was 
  present 
  was 
  shown 
  by 
  the 
  fact 
  

   that 
  films 
  known 
  from 
  their 
  magnetic 
  rotation 
  to 
  be 
  too 
  

   thin 
  to 
  produce, 
  by 
  their 
  thickness 
  alone, 
  a 
  measurable 
  dis- 
  

   placement 
  of 
  the 
  bands, 
  gave 
  coincidence 
  of 
  the 
  two 
  sets. 
  

  

  A 
  micrometer 
  microscope 
  was 
  used 
  for 
  measuring 
  the 
  

   fractional 
  part 
  of 
  a 
  band 
  displacement. 
  

  

  The 
  modification 
  referred 
  to 
  consisted 
  in 
  the 
  use 
  of 
  sunlight 
  

   passed 
  through 
  a 
  Fuess 
  spectral 
  instrument 
  as 
  a 
  source 
  of 
  

   monochromatic 
  light, 
  variable 
  in 
  colour 
  and 
  intensity. 
  By 
  

   varying 
  the 
  width 
  of 
  the 
  collimator-slit 
  of 
  the 
  instrument, 
  

   that 
  intensity 
  producing 
  the 
  clearest 
  bands 
  — 
  a 
  very 
  important 
  

   factor 
  — 
  was 
  easily 
  obtained. 
  By 
  changing 
  the 
  colour, 
  the 
  

   direction 
  of 
  motion 
  of 
  the 
  bands 
  revealed 
  immediately 
  the 
  

   direction 
  of 
  the 
  relative 
  shift 
  ; 
  and 
  by 
  noting 
  the 
  successive 
  

   wave-lengths 
  that 
  gave 
  coincidence, 
  the 
  whole 
  number 
  of 
  

   wave-lengths 
  of 
  each 
  colour 
  was 
  obtained 
  readily 
  by 
  

   inspection, 
  each 
  setting 
  at 
  the 
  same 
  time 
  contributing 
  a 
  

   measured 
  value 
  to 
  the 
  desired 
  retardation. 
  Finally, 
  in 
  those 
  

   cases 
  where 
  the 
  film 
  was 
  too 
  thin 
  to 
  give 
  a 
  coincidence 
  of 
  the 
  

   bands 
  with 
  any 
  colour, 
  the 
  greatest 
  accuracy 
  was 
  attained 
  by 
  

   using 
  the 
  shortest 
  waves 
  consistent 
  with 
  clear 
  vision. 
  It 
  

  

  