﻿160 
  C. 
  Barus 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  j[j 
  and 
  p', 
  the 
  appurtenances 
  being 
  shown 
  in 
  dotted 
  lines. 
  In 
  

   this 
  case 
  the 
  separated 
  rays, 
  a, 
  a\ 
  b, 
  b', 
  are 
  collected 
  at 
  e", 
  c 
  nf 
  , 
  

   to 
  be 
  joined 
  in 
  the 
  telescope 
  at 
  T. 
  The 
  internal 
  reflection 
  

   being 
  total, 
  I 
  made 
  use 
  of 
  it 
  for 
  the 
  following 
  experiments. 
  

   M 
  and 
  JV 
  and 
  P' 
  are 
  on 
  micrometers 
  with 
  the 
  screw 
  in 
  the 
  

   directions 
  normal 
  to 
  their 
  faces, 
  P, 
  31, 
  JV, 
  P' 
  must 
  all 
  be 
  

   adjustable. 
  After 
  preliminary 
  measurement 
  for 
  equal 
  dis- 
  

   tances, 
  the 
  fringes 
  were 
  found. 
  They 
  were 
  strong 
  but 
  tine, 
  

   beginning 
  with 
  vertical 
  hair 
  lines 
  and 
  gradually 
  rotating 
  as 
  they 
  

   grew 
  coarser 
  till 
  they 
  rather 
  abruptly 
  vanished. 
  The 
  displace- 
  

   ment 
  of 
  the 
  M 
  mirror 
  did 
  not 
  exceed 
  *06 
  cm 
  , 
  nor 
  the 
  rotation 
  

   30°. 
  The 
  spectra 
  being 
  non-reversed, 
  the 
  fringes 
  covered 
  the 
  

   whole 
  field. 
  

  

  One 
  would 
  naturally 
  suppose 
  that 
  the 
  abrupt 
  evanescence 
  of 
  

   fringes 
  was 
  due 
  to 
  the 
  escape 
  of 
  the 
  beam 
  of 
  the 
  edge 
  of 
  the 
  

   prism 
  P', 
  but 
  this 
  is 
  not 
  possible 
  as 
  the 
  mirror 
  J/was 
  traveling 
  

   toward 
  the 
  rear. 
  Furthermore 
  the 
  fore-and-aft 
  motion 
  of 
  the 
  

   prism 
  P' 
  over 
  several 
  millimeters 
  had 
  scarcely 
  any 
  effect 
  on 
  

   the 
  fringes. 
  This 
  is 
  unexpected 
  ; 
  for 
  the 
  rays, 
  c, 
  <?', 
  are 
  com- 
  

   pelled 
  to 
  approach 
  or 
  recede 
  from 
  each 
  other 
  by 
  this 
  motion. 
  

   Finally 
  the 
  sodium 
  doublets 
  may 
  be 
  moved 
  at 
  some 
  distance 
  

   (many 
  times 
  their 
  breadth) 
  apart, 
  without 
  destroying 
  the 
  

   fringes. 
  They 
  are 
  often 
  most 
  distinct 
  when 
  the 
  D 
  lines 
  are 
  

   not 
  superposed. 
  The 
  same 
  is 
  also 
  true 
  for 
  the 
  longitudinal 
  

   axes, 
  though 
  to 
  a 
  less 
  degree. 
  

  

  To 
  enlarge 
  the 
  fringes, 
  the 
  prism 
  P' 
  may 
  be 
  rotated 
  around 
  

   a 
  horizontal 
  axis 
  parallel 
  to 
  LT. 
  The 
  fringes 
  then 
  also 
  rotate, 
  

   but 
  the 
  increase 
  of 
  size 
  so 
  obtained 
  is 
  usually 
  not 
  striking. 
  

   Moreover 
  no 
  observable 
  effect 
  either 
  on 
  the 
  size 
  of 
  fringes 
  

   or 
  on 
  the 
  range 
  of 
  displacement 
  is 
  produced 
  by 
  inserting 
  

   compensators 
  in 
  one 
  beam 
  or 
  both. 
  A 
  great 
  variety 
  of 
  differ- 
  

   ent 
  adjustments 
  showed 
  a 
  range 
  of 
  displacement 
  at 
  M, 
  

   about 
  the 
  same 
  ('06 
  cm 
  ), 
  whether 
  the 
  patch 
  of 
  light 
  on 
  

   the 
  prism 
  was 
  wide 
  or 
  narrow. 
  The 
  range 
  of 
  fore 
  and 
  aft 
  

   motion 
  of 
  P' 
  within 
  which 
  fringes 
  are 
  visible 
  was 
  '52 
  cm 
  . 
  They 
  

   vanish 
  quite 
  abruptly 
  when 
  the 
  light 
  is 
  near 
  the 
  edge 
  of 
  the 
  

   prism, 
  although 
  both 
  spectra 
  are 
  still 
  strongly 
  visible. 
  When 
  

   the 
  light 
  is 
  nearer 
  the 
  base 
  of 
  the 
  prism, 
  they 
  vanish 
  more 
  

   gradually. 
  Definite 
  strips 
  of 
  white 
  light 
  on 
  both 
  sides 
  of 
  the 
  

   prism 
  therefore 
  cooperate 
  to 
  produce 
  the 
  fringes. 
  The 
  attempt 
  

   to 
  iind 
  a 
  systematic 
  method 
  for 
  enlarging 
  the 
  fringes 
  failed, 
  

   possibly 
  because 
  the 
  prism 
  angles 
  were 
  not 
  quite 
  identical. 
  

   The 
  striking 
  contrast 
  in 
  the 
  results 
  obtained 
  here 
  in 
  comparison 
  

   with 
  those 
  of 
  the 
  preceding 
  paragraph, 
  although 
  both 
  methods 
  

   are 
  essentially 
  the 
  same, 
  is 
  noteworthy. 
  

  

  It 
  is 
  for 
  this 
  reason 
  that 
  I 
  thought 
  it 
  desirable 
  to 
  test 
  the 
  

   method 
  in 
  fig. 
  26 
  which 
  accomplishes 
  with 
  a 
  prism, 
  what 
  was 
  

  

  