﻿618 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Absorption, 
  

  

  very 
  accurate, 
  as 
  no 
  precautions 
  were 
  taken 
  to 
  eliminate 
  the 
  

   reduction 
  in 
  the 
  intensity 
  of 
  the 
  light 
  due 
  to 
  the 
  reflexion 
  at 
  

   the 
  surface. 
  In 
  determining 
  curves 
  of 
  absorption 
  it 
  is 
  always 
  

   better 
  to 
  use 
  two 
  films 
  of 
  different 
  thickness. 
  The 
  errors 
  in 
  

   the 
  case 
  of 
  selenium 
  may 
  be 
  considerable 
  on 
  account 
  of 
  the 
  

   high 
  refractive 
  index, 
  and 
  I 
  expect 
  to 
  redetermine 
  the 
  curve 
  

   in 
  the 
  near 
  future. 
  In 
  the 
  present 
  preliminary 
  work 
  the 
  

   object 
  was 
  merely 
  to 
  determine 
  whether 
  there 
  was 
  a 
  return 
  

   to 
  partial 
  transparency 
  in 
  the 
  ultra-violet 
  region, 
  which 
  

   question 
  appears 
  to 
  be 
  answered 
  in 
  the 
  negative, 
  although 
  a 
  

   possible 
  turning-point 
  in 
  the 
  curve 
  may 
  be 
  masked 
  by 
  the 
  

   high 
  reflexion 
  coefficient. 
  It 
  appears 
  to 
  me 
  to 
  be 
  extremely 
  

   doubtful 
  that 
  there 
  is 
  but 
  a 
  single 
  absorption-band 
  in 
  the 
  

   present 
  case, 
  the 
  more 
  probable 
  condition 
  being 
  a 
  series 
  of 
  

   overlapping 
  bands. 
  We 
  are 
  in 
  fact 
  forced 
  to 
  this 
  conclusion 
  

   if 
  we 
  attempt 
  to 
  apply 
  the 
  dispersion 
  formula 
  to 
  the 
  results. 
  

   Though 
  we 
  are 
  unable 
  to 
  determine 
  experimentally 
  the 
  value 
  

   of 
  X 
  m 
  , 
  the 
  wave-length 
  corresponding 
  to 
  that 
  at 
  the 
  centre 
  of 
  

   the 
  absorption-band, 
  we 
  can 
  calculate 
  a 
  value 
  for 
  it 
  from 
  three 
  

   determinations 
  of 
  the 
  refractive 
  index 
  in 
  the 
  region 
  of 
  com- 
  

   parative 
  transparency. 
  Writing 
  the 
  dispersion 
  formula 
  in 
  

   the 
  form* 
  

  

  m 
  r 
  \ 
  2 
  

  

  we 
  can 
  calculate 
  \m 
  from 
  

  

  in 
  which 
  

  

  , 
  A 
  3 
  W-V)-A 
  2 
  2 
  lV-V)C 
  

   " 
  (A! 
  2 
  -V)-(V-V)C 
  ; 
  

  

  

  nf 
  — 
  n 
  x 
  

  

  nJ—n 
  } 
  2 
  

  

  where 
  n 
  u 
  n 
  2 
  , 
  and 
  n 
  3 
  are 
  the 
  values 
  of 
  the 
  refractive 
  index 
  for 
  

   wave-lengths 
  \ 
  1} 
  X 
  2 
  , 
  and 
  A3. 
  Applying 
  this 
  formula 
  to 
  the 
  

   values 
  determined 
  by 
  the 
  prism 
  method 
  we 
  find 
  Xm 
  to 
  be 
  

   0*00056, 
  which 
  we 
  may 
  assume 
  to 
  be 
  not 
  very 
  far 
  from 
  the 
  

   centre 
  of 
  the 
  first 
  band 
  in 
  the 
  series. 
  The 
  true 
  position 
  of 
  

   the 
  centre 
  of 
  the 
  first 
  member 
  of 
  the 
  series 
  cannot 
  be 
  exactly 
  

   determined, 
  owing 
  to 
  the 
  effects 
  of 
  the 
  free 
  periods 
  of 
  higher 
  

   frequency, 
  but 
  the 
  value 
  given 
  above 
  can 
  be 
  considered 
  a 
  fair 
  

   approximation. 
  Ketteler 
  has 
  given 
  a 
  method 
  for 
  decomposing 
  

   a 
  complex 
  absorption-band 
  into 
  its 
  constituents; 
  but 
  as 
  the 
  

   process 
  is 
  very 
  laborious 
  and 
  somewhat 
  arbitrary, 
  it 
  has 
  not 
  

  

  * 
  "Dispersion 
  of 
  Ultra- 
  Violet 
  Ravs," 
  F. 
  F. 
  Martens, 
  Annalen 
  dcr 
  

   Physik, 
  No. 
  11, 
  p. 
  612 
  (11)01). 
  

  

  