﻿158 
  C. 
  Barns 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  Rotation 
  of 
  the 
  compensator 
  in 
  the 
  first 
  place 
  moves 
  the 
  

   fringes 
  as 
  in 
  interferometry, 
  as 
  does 
  also 
  the 
  normal 
  micromet- 
  

   ric 
  displacement 
  of 
  M. 
  If 
  this 
  motion 
  requires 
  readjustment 
  

   of 
  M 
  the 
  range 
  of 
  displacement 
  is 
  curtailed 
  and 
  the 
  corre- 
  

   sponding 
  change 
  of 
  phase 
  appears. 
  In 
  the 
  second 
  place 
  the 
  

   compensator 
  on 
  rotation 
  traces 
  the 
  contours 
  of 
  the 
  curves 
  by 
  

   successively 
  accentuating 
  vaguer 
  parts, 
  as 
  will 
  presently 
  be 
  

   explained. 
  

  

  The 
  most 
  remarkable 
  results 
  occurred 
  on 
  widening 
  the 
  slit. 
  

   Supposing 
  that 
  large 
  strands 
  were 
  visible 
  in 
  case 
  of 
  the 
  fine 
  

   slit, 
  and 
  that 
  this 
  was 
  gradually 
  widened 
  until 
  the 
  slit 
  width 
  

   was 
  half 
  a 
  millimeter 
  or 
  more 
  ; 
  the 
  strands 
  were 
  found 
  to 
  have 
  

   coalesced 
  in 
  a 
  way 
  which 
  defies 
  description. 
  In 
  their 
  place 
  

   appeared 
  a 
  wide 
  vertical 
  strip 
  of 
  equidistant 
  parallel 
  crescents. 
  

   The 
  Fraunhofer 
  lines 
  had 
  long 
  vanished 
  and 
  the 
  appearance 
  of 
  

   the 
  spectrum 
  was 
  whitish 
  and 
  intense. 
  The 
  fringes 
  in 
  ques- 
  

   tion 
  may 
  thus 
  be 
  termed 
  achromatic. 
  The 
  strips 
  appear 
  quite 
  

   regular 
  through 
  the 
  breadth 
  of 
  the 
  spectrum 
  and 
  its 
  width 
  may 
  

   be 
  one-third 
  of 
  the 
  length 
  of 
  the 
  spectrum. 
  The 
  fringes 
  move 
  

   with 
  the 
  normal 
  displacement 
  of 
  31 
  (interferometry) 
  and 
  the 
  

   range 
  is 
  large 
  ('5 
  cm 
  without 
  adjustment) 
  provided 
  M 
  does 
  not 
  

   require 
  readjustment 
  by 
  rotation. 
  Simultaneously 
  the 
  strip 
  is 
  

   displaced 
  longitudinally 
  in 
  the 
  spectrum 
  in 
  the 
  usual 
  way. 
  

  

  On 
  closing 
  the 
  slit 
  the 
  ellipses 
  break 
  up 
  into 
  sharp 
  strands 
  

   again 
  without 
  offering 
  a 
  systematic 
  clue 
  as 
  to 
  the 
  manner 
  in 
  

   which 
  this 
  is 
  done. 
  The 
  strands 
  usually 
  trend 
  more 
  or 
  less 
  

   vertically 
  with 
  two 
  sharp 
  strong 
  groups, 
  flanked 
  by 
  one 
  or 
  

   more 
  weak 
  groups 
  on 
  each 
  side. 
  

  

  On 
  removing 
  the 
  condenser, 
  these 
  crescents 
  became 
  more 
  

   slender 
  but 
  much 
  sharper, 
  so 
  that 
  in 
  spite 
  of 
  the 
  diminished 
  

   light 
  they 
  could 
  be 
  well 
  seen. 
  They 
  were 
  then 
  found 
  to 
  be 
  

   like 
  the 
  approximately 
  confocal 
  ellipses 
  of 
  displacement 
  inter- 
  

   ferometry, 
  though 
  not 
  subject 
  to 
  the 
  same 
  laws. 
  They 
  

   embraced 
  over 
  one-third 
  of 
  the 
  visibly 
  overlapping 
  (green 
  

   yellow 
  through 
  red) 
  spectra, 
  terminating 
  in 
  very 
  fine 
  hair-lines 
  

   on 
  one 
  side 
  but 
  coarse 
  lines 
  on 
  the 
  other. 
  On 
  opening 
  the 
  slit 
  

   from 
  a 
  breadth 
  of 
  cm 
  to 
  about 
  *l 
  mm 
  the 
  evolution 
  was 
  curious. 
  

   With 
  a 
  very 
  fine 
  slit 
  a 
  relatively 
  narrow 
  strip 
  of 
  strong 
  slant- 
  

   ing 
  lines 
  was 
  seen 
  in 
  the 
  yellow. 
  As 
  the 
  slit 
  widened 
  they 
  

   developed 
  curvature, 
  adding 
  the 
  more 
  slender 
  complements 
  of 
  

   the 
  ellipses 
  on 
  the 
  red 
  side, 
  until 
  this 
  part 
  of 
  the 
  spectrum 
  was 
  

   filled 
  with 
  confocal 
  half 
  ellipses 
  having 
  a 
  transverse 
  major 
  axis. 
  

   The 
  range 
  of 
  displacement 
  of 
  3fis 
  practically 
  indefinite, 
  depend- 
  

   ing 
  simply 
  on 
  the 
  degree 
  to 
  which 
  the 
  spectra 
  overlap. 
  Three 
  

   or 
  four 
  centimeters 
  were 
  tried. 
  Both 
  sides 
  of 
  the 
  ellipses 
  may 
  

   be 
  traversed 
  by 
  rotating 
  the 
  plate 
  compensator, 
  which 
  suc- 
  

   cessively 
  accentuates 
  (in 
  a 
  transverse 
  strip) 
  a 
  definite 
  part 
  of 
  

  

  