﻿Non-reversed 
  Spectrum 
  luterferometry 
  . 
  157 
  

  

  form 
  into 
  the 
  middle 
  of 
  the 
  wide 
  slit 
  image, 
  a 
  glass 
  compen- 
  

   sator 
  in 
  either 
  o 
  or 
  b 
  f 
  may 
  be 
  resorted 
  to, 
  or 
  both 
  M 
  and 
  G' 
  

   may 
  be 
  displaced 
  together. 
  Again, 
  whereas 
  the 
  micrometric 
  

   displacement 
  of 
  M 
  produces 
  a 
  marked 
  displacement 
  of 
  fringes 
  

   within 
  the 
  strip 
  in 
  accordance 
  with 
  the 
  equation, 
  the 
  micromet- 
  

   ric 
  displacement 
  of 
  G' 
  leaves 
  the 
  fringes 
  stationary 
  within 
  the 
  

   strip. 
  

  

  Very 
  remarkable 
  results 
  were 
  obtained 
  with 
  compensators 
  

   of 
  glass 
  plate. 
  Placed 
  in 
  one 
  or 
  both 
  beams 
  and 
  rotated 
  

   aixmnd 
  a 
  vertical 
  axis, 
  they 
  rotate 
  the 
  fringes. 
  If 
  however 
  

   they 
  are 
  placed 
  nearly 
  normally 
  in 
  one 
  beam, 
  they 
  produce 
  no 
  

   effect 
  either 
  of 
  rotation 
  or 
  on 
  the 
  size 
  of 
  the 
  fringes 
  ; 
  but 
  the 
  

   grid 
  is 
  displaced 
  bodily 
  across 
  the 
  wide 
  yellow 
  slit 
  image. 
  

   Glass 
  plates 
  *2, 
  '5 
  cm 
  were 
  used. 
  It 
  is 
  not 
  until 
  the 
  thickness 
  of 
  

   plate 
  reaches 
  2 
  cm 
  that 
  appreciable 
  thinning 
  of 
  the 
  interference 
  

   fringes 
  occurs 
  when 
  the 
  plate 
  is 
  placed 
  in 
  one 
  beam. 
  

  

  In 
  case 
  of 
  homogeneous 
  light 
  the 
  Ives 
  prism 
  grating 
  G" 
  is 
  

   not 
  needed 
  and 
  much 
  more 
  light 
  is 
  available 
  if 
  the 
  telescope 
  is 
  

   used 
  directly. 
  The 
  strands 
  of 
  interferences 
  being 
  on 
  a 
  yellow 
  

   ground 
  are 
  not 
  very 
  strong, 
  Nevertheless 
  a 
  few 
  measurements 
  

   of 
  ranges 
  of 
  displacement 
  were 
  made 
  by 
  moving 
  both 
  ^(dis- 
  

   placement 
  e) 
  and 
  G' 
  (displacement 
  h), 
  alternately. 
  The 
  follow- 
  

   ing 
  values 
  of 
  e, 
  h, 
  and 
  h 
  tan 
  f 
  were 
  found, 
  the 
  film 
  gratings 
  

   having 
  nearly 
  the 
  same 
  constants 
  : 
  

  

  6 
  = 
  -5 
  cm 
  0=19° 
  37' 
  

  

  A 
  = 
  l-30 
  cm 
  6' 
  = 
  20° 
  40' 
  

  

  h 
  tan 
  (9='49 
  cm 
  

  

  e 
  and 
  h 
  tan 
  6' 
  coincide 
  as 
  closely 
  as 
  may 
  be 
  expected, 
  seeing 
  

   that 
  the 
  fringes 
  in 
  neither 
  case 
  can 
  be 
  quite 
  brought 
  to 
  vanish. 
  

  

  17. 
  The 
  same, 
  continued. 
  Duplicate 
  fringes. 
  Achromatic 
  

   fringes. 
  — 
  The 
  occurrence 
  of 
  strands 
  and 
  apparently 
  duplicated 
  

   fringes 
  has 
  already 
  been 
  suggested 
  in 
  the 
  preceding 
  paragraph. 
  

   In 
  further 
  experiments 
  definite 
  results 
  were 
  eventually 
  obtained, 
  

   with 
  sunlight. 
  These 
  occur 
  in 
  very 
  great 
  variety, 
  but 
  typical 
  

   phases 
  may 
  be 
  accentuated. 
  In 
  intermediate 
  cases 
  fine 
  large 
  

   strands 
  occur. 
  These 
  pass 
  into 
  each 
  other 
  continuously 
  ; 
  the 
  

   manner 
  does 
  not 
  admit 
  of 
  description. 
  They 
  are 
  seen 
  best 
  in 
  

   the 
  principal 
  focal 
  plane 
  and 
  both 
  sets 
  are 
  about 
  equally 
  strong. 
  

  

  To 
  obtain 
  these 
  fringes 
  the 
  adjustment 
  was 
  first 
  carefully 
  

   made 
  with 
  the 
  sodium 
  arc. 
  Thereupon 
  the 
  arc 
  was 
  replaced 
  

   by 
  concentrated 
  sunlight 
  and 
  fine 
  fringes 
  were 
  recognized 
  in 
  

   the 
  superposed 
  spectra 
  (longitudinal 
  and 
  transverse 
  axes 
  coin- 
  

   ciding). 
  These 
  fine 
  fringes 
  were 
  then 
  enlarged 
  both 
  by 
  rota- 
  

   ting 
  the 
  grating 
  G' 
  (fig. 
  24) 
  on 
  its 
  normal 
  axis 
  and 
  readjusting 
  

   M 
  in 
  each 
  case, 
  and 
  by 
  adding 
  trial 
  compensators 
  in 
  the 
  M 
  or 
  

   N 
  pencils. 
  A 
  glass 
  plate 
  3 
  millimeters 
  thick 
  gave 
  the 
  best 
  

   results. 
  

  

  