﻿Non-reversed 
  Spectrum 
  Interferometry 
  . 
  151 
  

  

  The 
  surprising 
  success 
  obtained 
  with 
  the 
  film 
  grating 
  at 
  short 
  

   distances 
  induced 
  me 
  to 
  test 
  similar 
  methods 
  at 
  long 
  distances. 
  

   Figure 
  23 
  is 
  an 
  apparatus 
  of 
  this 
  kind, 
  in 
  which 
  L 
  is 
  the 
  white 
  

   beam 
  incident 
  from 
  a 
  collimator, 
  G 
  and 
  G' 
  are 
  the 
  transmit- 
  

   ting 
  gratings, 
  M, 
  JV, 
  m, 
  w, 
  pairs 
  of 
  opaque 
  mirrors, 
  T 
  the 
  tele- 
  

   scope. 
  The 
  undeviated 
  raj, 
  d, 
  is 
  screened 
  off. 
  The 
  component 
  

   paths 
  a-\-b 
  + 
  c, 
  a' 
  + 
  b' 
  + 
  c' 
  were 
  each 
  about 
  4 
  meters 
  long. 
  The 
  

   method 
  of 
  adjustment 
  again 
  consists 
  in 
  bringing 
  the 
  shadow 
  of 
  

   the 
  thin 
  wire 
  across 
  the 
  slit, 
  into 
  the 
  same 
  position 
  of 
  the 
  spectra 
  

   seen 
  in 
  the 
  telescope 
  when 
  the 
  spectra 
  coincide. 
  For 
  this 
  pur- 
  

   pose 
  the 
  adjustment 
  screws 
  for 
  horizontal 
  and 
  vertical 
  axes 
  on 
  

   M, 
  JV, 
  ?n, 
  n, 
  must 
  be 
  actuated 
  together. 
  To 
  facilitate 
  this 
  

   tiresome 
  work, 
  with 
  the 
  observer 
  at 
  T 
  7 
  , 
  long 
  levers 
  brought 
  

   from 
  m 
  and 
  n, 
  with 
  their 
  ends 
  near 
  his 
  hands, 
  as 
  well 
  as 
  a 
  

   lever 
  from 
  G' 
  (fore 
  and 
  aft 
  motion) 
  were 
  useful. 
  Since 
  the 
  

   adjustment 
  screws 
  at 
  JS^and 
  JV 
  are 
  already 
  within 
  reach, 
  it 
  is 
  

   thus 
  easy 
  to 
  bring 
  any 
  Fraunhofer 
  line 
  to 
  the 
  middle 
  of 
  the 
  

   field 
  and 
  to 
  make 
  these 
  fields 
  overlap, 
  with 
  the 
  guide 
  wire 
  cen- 
  

   tral 
  in 
  both. 
  

  

  The 
  fringes 
  were 
  found 
  after 
  some 
  searching 
  and 
  seemed 
  to 
  

   be 
  of 
  D 
  1 
  X> 
  2 
  breadth, 
  a 
  strip 
  of 
  oblique 
  lines 
  of 
  the 
  usual 
  char- 
  

   acter. 
  They 
  were 
  not 
  brilliant 
  and 
  hard 
  to 
  recover 
  when 
  lost. 
  

   The 
  Fraunhofer 
  lines 
  were 
  still 
  disagreeably 
  blurred. 
  

  

  On 
  exchanging 
  the 
  gratings 
  (weaker 
  ruled 
  glass 
  grating 
  at 
  

   G 
  and 
  film 
  at 
  G'), 
  though 
  the 
  dispersion 
  was 
  smaller, 
  the 
  bril- 
  

   liancy 
  of 
  spectra 
  was 
  greatly 
  improved. 
  On 
  cutting 
  down 
  the 
  

   incident 
  beam 
  at 
  the 
  collimator 
  and 
  near 
  G, 
  to 
  a 
  breadth 
  of 
  not 
  

   more 
  than 
  *5 
  cm 
  , 
  the 
  fringes 
  were 
  acceptable 
  and 
  capable 
  of 
  

   high 
  magnification. 
  They 
  remained 
  visible 
  for 
  a 
  displacement 
  

   of 
  5 
  millimeters 
  at 
  the 
  micrometer 
  at 
  M. 
  With 
  fore 
  and 
  aft 
  

   motion 
  of 
  G', 
  the 
  fringes 
  rotated 
  as 
  usual 
  from 
  fine 
  vertical 
  

   hair 
  lines, 
  through 
  the 
  horizontal 
  (probably 
  arrow-shaped 
  

   forms 
  of 
  maximum 
  size,) 
  back 
  again 
  to 
  hair 
  lines. 
  Here 
  the 
  

   excursion 
  of 
  G' 
  was 
  about 
  l*5 
  em 
  . 
  On 
  tilting 
  the 
  grating 
  G' 
  

   in 
  its 
  own 
  plane 
  and 
  readjusting 
  M, 
  the 
  rotation 
  is 
  through 
  

   the 
  vertical 
  maximum 
  (the 
  linear 
  phenomenon). 
  

  

  The 
  film 
  grating 
  may 
  be 
  used 
  by 
  reflection, 
  on 
  adapting 
  the 
  

   method 
  fig. 
  14, 
  for 
  this 
  purpose, 
  with 
  a 
  ruled 
  grating 
  or 
  

   prism 
  at 
  P 
  and 
  the 
  film 
  grating 
  (with 
  its 
  ruled 
  toward 
  P) 
  at 
  

   G. 
  If 
  a 
  ruled 
  grating 
  is 
  put 
  at 
  P, 
  the 
  spectra 
  and 
  fringes 
  are 
  

   good 
  ; 
  but 
  naturally 
  there 
  is 
  deficient 
  illumination. 
  Neverthe- 
  

   less 
  a 
  strong 
  telescope 
  may 
  be 
  used 
  and 
  a 
  range 
  of 
  displace- 
  

   ment 
  of 
  4 
  mm 
  , 
  at 
  3f, 
  is 
  available. 
  This 
  may 
  be 
  increased 
  

   indefinitely 
  by 
  using 
  a 
  micrometer 
  at 
  M 
  and 
  JV 
  alternately. 
  

   The 
  chief 
  difficulty 
  was 
  the 
  (incidentally) 
  unequal 
  brightness 
  

   of 
  spectra. 
  

  

  Again, 
  the 
  method 
  of 
  fig. 
  15, 
  apart 
  from 
  the 
  drawbacks 
  to 
  

  

  