﻿838 
  Prof. 
  Skinner 
  and 
  Mr. 
  Tool 
  on 
  the 
  Optical 
  

  

  axis 
  in 
  the 
  plane 
  of 
  polarization. 
  This 
  upper 
  strip 
  of 
  mica 
  is 
  

   so 
  mounted 
  that 
  its 
  inclination 
  to 
  the 
  ray 
  can 
  be 
  varied 
  by 
  

   rotating 
  abont 
  a 
  vertical 
  axis, 
  and 
  thns 
  a 
  retardation 
  produced 
  

   in 
  the 
  upper 
  half 
  of 
  the 
  ray 
  equal 
  to 
  that 
  which 
  the 
  film 
  intro- 
  

   duces 
  in 
  the 
  lower 
  half. 
  P 
  is 
  a 
  compensating 
  plate 
  used 
  to 
  

   right 
  the 
  irregularity 
  of 
  form 
  in 
  the 
  hand- 
  arising 
  from 
  the 
  

   plate 
  on 
  which 
  the 
  film 
  is 
  deposited. 
  Beyond 
  C 
  is 
  placed 
  a 
  

   i'n- 
  L. 
  which 
  focusses 
  (through 
  the 
  plane 
  mirror 
  M) 
  the 
  

   film 
  F 
  on 
  the 
  collimator-slit 
  of 
  a 
  spectroscope. 
  Viewed 
  

   through 
  the 
  spectroscope, 
  the 
  field 
  exhibits 
  two 
  parallel 
  

   channeled 
  spectra 
  with 
  the 
  interference-bands 
  in 
  the 
  one 
  

   displaced 
  relative 
  to 
  those 
  in 
  the 
  other. 
  If 
  the 
  movable 
  

   compensator 
  strip 
  be 
  parallel 
  to 
  its 
  companion 
  strip, 
  the 
  

   observed 
  displacement 
  arises 
  from 
  the 
  optical 
  path 
  in 
  the 
  

   film 
  being 
  greater 
  than 
  that 
  in 
  the 
  same 
  thickness 
  of 
  air. 
  

   The 
  angular 
  displacement 
  of 
  the 
  collimator, 
  which 
  gives 
  

   coincidence 
  of 
  bands 
  in 
  any 
  pari 
  of 
  the 
  spectrum, 
  is 
  used 
  

   for 
  determining 
  tin- 
  difference. 
  As 
  a 
  convenient 
  formnla 
  

   for 
  calculating 
  the 
  relative 
  retardation 
  we 
  have 
  used 
  the 
  

   following, 
  for 
  which 
  we 
  arc 
  indebted 
  to 
  Professor 
  Tuckerman 
  

   of 
  this 
  laboratory 
  : 
  — 
  

  

  8 
  = 
  ifca 
  2 
  (l 
  + 
  5-3. 
  1C- 
  5 
  .* 
  2 
  ), 
  

  

  where 
  8 
  is 
  the 
  retardation 
  in 
  wave-lengths, 
  a 
  the 
  displacement 
  

   of 
  the 
  compensator 
  in 
  degrees, 
  and 
  /• 
  a 
  constant 
  depending 
  on 
  

  

  the 
  thickness 
  of 
  the 
  mica 
  and 
  its 
  refractive 
  index. 
  The 
  value 
  

   of/- 
  is 
  experimentally 
  determined 
  for 
  the 
  various 
  colours 
  by 
  

   observing 
  the 
  value 
  of 
  a 
  required 
  to 
  produce 
  one 
  or 
  more 
  

   wave-lengths' 
  retardation 
  with 
  the 
  film 
  removed. 
  The 
  ac- 
  

   curacy 
  (one-half 
  per 
  cent.) 
  of 
  this 
  formula 
  is 
  well 
  within 
  the 
  

   experimental 
  error-. 
  

  

  The 
  optical 
  thickness 
  of 
  the 
  film 
  is 
  obtained 
  for 
  each 
  colour 
  

   by 
  adding 
  the 
  corresponding 
  8 
  to 
  the 
  air 
  thickness 
  given 
  in 
  

   terms 
  of 
  the 
  wave-length 
  considered. 
  

  

  ■4. 
  Transmission 
  and 
  Reflexion, 
  

  

  For 
  measuring 
  the 
  ratio 
  of 
  the 
  intensity 
  of 
  the 
  transmitted 
  

   to 
  the 
  incident 
  light, 
  a 
  Brace 
  spectrophotometer 
  was 
  used 
  in 
  

   the 
  customary 
  way 
  : 
  a 
  rotating 
  sector 
  and 
  an 
  adjustable 
  slit 
  

   serving 
  to 
  reduce 
  the 
  intensity 
  of 
  the 
  unabsorbed 
  component 
  

   to 
  match 
  it 
  with 
  that 
  which 
  has 
  passed 
  through 
  (or 
  been 
  

   reflected 
  from) 
  the 
  film. 
  To 
  eliminate 
  the 
  effect 
  of 
  reflexion 
  

   in 
  the 
  transmission 
  measurements, 
  the 
  usual 
  method 
  of 
  

   measuring 
  the 
  transmission 
  of 
  two 
  different 
  thicknesses 
  

   was 
  followed. 
  For 
  reflexion 
  measurements, 
  an 
  attachment 
  

   described 
  by 
  Clark 
  {I. 
  c.) 
  was 
  used. 
  

  

  