﻿JS 
  r 
  on-reversed 
  Spectrum 
  Interferometry 
  . 
  161 
  

  

  done 
  in 
  my 
  original 
  experiments* 
  with 
  reversed 
  spectra, 
  by 
  the 
  

   aid 
  of 
  a 
  grating. 
  In 
  the 
  figure 
  the 
  incident 
  beam 
  of 
  white 
  

   light 
  L 
  from 
  a 
  collimator 
  strikes 
  the 
  60° 
  prism 
  at 
  its 
  edge, 
  and 
  

   is 
  then 
  refracted 
  into 
  the 
  paired 
  pencils 
  a, 
  a'. 
  Those 
  are 
  

   reflected 
  normally 
  by 
  the 
  opaque 
  mirrors 
  M 
  and 
  JV, 
  again 
  

   refracted 
  by 
  P 
  as 
  each 
  pencil 
  nearly 
  retraces 
  its 
  path. 
  The 
  

   return 
  beams 
  however 
  are 
  given 
  a 
  slightly 
  upward 
  trend, 
  so 
  as 
  

   to 
  impinge 
  on 
  the 
  opaque 
  mirror 
  m 
  (curved 
  or 
  plane). 
  The 
  

   rays 
  reflected 
  from 
  m, 
  in 
  such 
  a 
  way 
  as 
  to 
  avoid 
  the 
  prism 
  P, 
  

   may 
  be 
  reunited 
  in 
  the 
  focus 
  F 
  observed 
  by 
  the 
  lens 
  T, 
  or 
  (if 
  

   parallel) 
  collected 
  by 
  a 
  telescope 
  at 
  T. 
  In 
  view 
  of 
  the 
  prism, 
  

   the 
  spectra 
  are 
  small 
  and 
  reversed, 
  but 
  may 
  be 
  brought 
  to 
  

   overlap 
  at 
  the 
  red 
  ends 
  which 
  are 
  towards 
  each 
  other. 
  

  

  The 
  small 
  dispersion 
  makes 
  it 
  necessary 
  to 
  use 
  a 
  strong 
  tele- 
  

   scope 
  if 
  the 
  Fraunhofer 
  lines 
  are 
  to 
  be 
  visible 
  and 
  the 
  D 
  lines 
  

   separated. 
  When 
  the 
  adjustment 
  has 
  been 
  made 
  symmetrically, 
  

   a 
  strong 
  linear 
  phenomena 
  may 
  be 
  found 
  not 
  differing 
  in 
  

   appearance 
  from 
  the 
  results 
  obtained 
  when 
  a 
  grating 
  was 
  used 
  

   at 
  P, 
  fig. 
  26. 
  When 
  the 
  mirror 
  M 
  is 
  displaced, 
  however, 
  the 
  

   fringes 
  first 
  appear 
  in 
  the 
  form 
  of 
  multiple 
  vertical 
  hair 
  lines, 
  

   which 
  grow 
  coarser 
  until 
  but 
  a 
  single 
  dark 
  line 
  flanked 
  by 
  a 
  

   bright 
  line 
  is 
  visible. 
  With 
  further 
  displacement 
  the 
  phe- 
  

   nomenon 
  again 
  vanishes 
  in 
  passing 
  through 
  multiple 
  hair 
  lines. 
  

   An 
  important 
  result 
  is 
  the 
  small 
  range 
  of 
  displacement. 
  This 
  

   was 
  found 
  to 
  be, 
  between 
  appearance 
  and 
  evanescence 
  of 
  

   fringes, 
  about 
  

  

  8e 
  = 
  \12 
  cm., 
  

   thus 
  scarcely 
  larger 
  than 
  a 
  millimeter, 
  whereas 
  in 
  the 
  case 
  

   where 
  a 
  grating 
  (D 
  = 
  352 
  X 
  10~ 
  6 
  cm.) 
  was 
  used 
  in 
  place 
  of 
  P, 
  

   the 
  range 
  of 
  displacement 
  was 
  of 
  the 
  order 
  of 
  5 
  millimeters. 
  

  

  Suppose 
  that 
  for 
  low 
  dispersion, 
  the 
  fringes 
  may 
  be 
  regarded 
  

   as 
  extremely 
  eccentric 
  virtually 
  linear 
  ellipses, 
  the 
  lateral 
  dis- 
  

   tance 
  between 
  which 
  very 
  rapidly 
  diminishes, 
  so 
  that 
  {Be 
  = 
  *12) 
  

   but 
  

  

  1 
  -12 
  

  

  -7T- 
  « 
  TTT^i 
  = 
  2000 
  

  

  2 
  3 
  X 
  10- 
  3 
  

  

  can 
  be 
  seen 
  by 
  the 
  given 
  telescope. 
  These 
  lines 
  would 
  move 
  

   behind 
  the 
  strip 
  carrying 
  interference 
  fringes, 
  as 
  M 
  is 
  dis- 
  

   placed. 
  If 
  now 
  the 
  dispersion 
  is 
  much 
  increased, 
  say 
  from 
  

   d6/d\ 
  = 
  2 
  X 
  760 
  for 
  the 
  prisms 
  to 
  2 
  X 
  2880 
  for 
  the 
  grating, 
  

   the 
  ellipses 
  will 
  be 
  much 
  less 
  eccentric 
  as 
  a 
  whole 
  and 
  their 
  

   lines 
  would 
  have 
  grown 
  coarser, 
  so 
  that 
  many 
  more 
  would 
  be 
  

   visible 
  by 
  the 
  given 
  optical 
  system. 
  As 
  the 
  dispersion 
  is 
  

   increased 
  2880/760 
  = 
  3*8 
  times, 
  the 
  range 
  of 
  displacement 
  

  

  * 
  This 
  Journal, 
  xl, 
  p. 
  486, 
  1915. 
  

  

  