﻿612 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Absorption, 
  

  

  displaced 
  fringe 
  can 
  be 
  identified 
  with 
  the 
  undisplaced. 
  In 
  

   working 
  with 
  films 
  in 
  the 
  red 
  and 
  orange, 
  where 
  the 
  dis- 
  

   placement 
  is 
  equal 
  to 
  the 
  width 
  of 
  several 
  fringes, 
  the 
  only 
  

   method 
  of 
  identifying 
  the 
  fringes 
  is 
  to 
  make 
  visual 
  observa- 
  

   tions 
  with 
  white 
  light 
  at 
  the 
  centre 
  of 
  the 
  system. 
  With 
  

   the 
  wedge-shaped 
  film 
  there 
  is 
  no 
  difficulty 
  in 
  telling 
  which 
  

   fringes 
  belong 
  together, 
  for 
  the 
  same 
  fringe 
  can 
  be 
  traced 
  

   clear 
  across 
  the 
  plate, 
  and 
  the 
  total 
  displacement 
  determined. 
  

   The 
  chance 
  of 
  making 
  an 
  error 
  of 
  an 
  entire 
  fringe-width 
  in 
  

   estimating 
  the 
  displacement 
  by 
  visual 
  observations 
  with 
  white 
  

   light 
  is 
  very 
  great, 
  owing 
  to 
  the 
  great 
  dissimilarity 
  in 
  the 
  

   appearance 
  of 
  the 
  chromatic 
  fringes 
  seen 
  through 
  clear 
  glass 
  

   and 
  selenium. 
  

  

  The 
  photographs 
  of 
  the 
  curved 
  fringes 
  were 
  measured 
  by 
  

   placing 
  them 
  in 
  contact 
  with 
  a 
  glass 
  plate 
  ruled 
  with 
  parallel 
  

   diamond 
  scratches, 
  one 
  of 
  the 
  scratches 
  being 
  placed 
  in 
  

   coincidence 
  with 
  the 
  line 
  marking 
  the 
  thin 
  edge 
  of 
  the 
  film. 
  

   The 
  displacement 
  was 
  measured 
  as 
  before 
  with 
  a 
  filar 
  micro- 
  

   meter, 
  setting 
  the 
  cross-hair 
  first 
  on 
  the 
  undisplaced 
  fringe, 
  

   and 
  then 
  on 
  the 
  point 
  at 
  which 
  the 
  corresponding 
  fringe 
  

   intersected 
  the 
  scratch 
  nearest 
  the 
  end 
  of 
  the 
  oblique 
  system. 
  

  

  The 
  values 
  for 
  the 
  refractive 
  indices 
  obtained 
  in 
  the 
  red 
  by 
  

   means 
  of 
  the 
  prisms 
  were 
  taken 
  as 
  a 
  basis 
  for 
  the 
  calculation 
  

   of 
  the 
  indices 
  in 
  the 
  rest 
  of 
  the 
  spectrum 
  from 
  the 
  interfero- 
  

   meter 
  measurements. 
  If 
  we 
  find 
  the 
  displacement 
  for 
  wave- 
  

   length 
  \ 
  r 
  to 
  be 
  n 
  r 
  (n 
  being 
  measured 
  in 
  terms 
  of 
  fringe- 
  

   width), 
  and 
  know 
  the 
  ref. 
  index 
  fju 
  r 
  for 
  this 
  wave-length, 
  we 
  can 
  

   determine 
  the 
  refractive 
  index 
  fi 
  g 
  for 
  wave-length 
  X 
  g 
  (in 
  the 
  

   green) 
  by 
  measuring 
  the 
  displacement 
  n 
  g 
  for 
  the 
  same 
  thick- 
  

   ness, 
  from 
  the 
  formula 
  

  

  ll 
  r 
  /\ 
  r 
  

  

  Determinations 
  were 
  made 
  in 
  this 
  way 
  down 
  to 
  wave-length 
  

   •0004, 
  beyond 
  which 
  point 
  it 
  was 
  impossible 
  to 
  go 
  owing 
  to 
  

   the 
  powerful 
  absorption, 
  which 
  made 
  the 
  fringes 
  too 
  faint 
  to 
  

   measure. 
  It 
  was 
  only 
  by 
  employing 
  exceedingly 
  thin 
  films 
  

   and 
  making 
  the 
  measurements 
  with 
  the 
  greatest 
  care 
  that 
  

   consistent 
  results 
  were 
  obtained 
  in 
  the 
  violet. 
  The 
  reason 
  

   for 
  this 
  will 
  be 
  apparent 
  when 
  the 
  subject 
  of 
  absorption 
  is 
  

   taken 
  up, 
  for 
  a 
  film 
  which 
  in 
  red 
  light 
  is 
  so 
  transparent 
  that 
  

   its 
  presence 
  cannot 
  be 
  detected, 
  in 
  the 
  violet 
  and 
  ultra- 
  

   violet 
  absorbs 
  nearly 
  as 
  strongly 
  as 
  a 
  metal 
  film 
  of 
  the 
  same 
  

   thickness. 
  

  

  The 
  dispersion-curve 
  obtained 
  by 
  means 
  of 
  the 
  inter- 
  

   ferometer 
  is 
  given 
  on 
  fig. 
  4, 
  points 
  determined 
  with 
  uniform 
  

  

  