﻿154 
  C. 
  Bar 
  us 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  parallel 
  as 
  to 
  rulings, 
  etc., 
  when 
  one 
  grating 
  was 
  displaced 
  

   normally 
  to 
  its 
  plane 
  relative 
  to 
  the 
  other. 
  The 
  edges 
  of 
  the 
  

   two 
  gratings 
  must 
  be 
  close 
  together 
  ; 
  but 
  even 
  then 
  the 
  fringes 
  

   remain 
  small 
  and 
  the 
  available 
  paths 
  also. 
  Strong 
  large 
  fringes, 
  

   but 
  with 
  small 
  paths, 
  were 
  obtained 
  by 
  the 
  later 
  method* 
  of 
  

   two 
  identical 
  transmitting 
  gratings, 
  superposed. 
  

  

  If 
  the 
  prism 
  P' 
  is 
  right 
  angled 
  (a 
  special 
  case 
  of 
  fig. 
  19), 
  it 
  

   may 
  be 
  rotated 
  as 
  in 
  H^. 
  21, 
  so 
  that 
  the 
  rays 
  c 
  and 
  c' 
  pass 
  off 
  

   towards 
  the 
  observer. 
  They 
  are 
  then 
  to 
  be 
  regarded 
  through 
  

   an 
  Ives 
  prism 
  grating 
  G 
  and 
  a 
  telescope 
  at 
  T. 
  This 
  method 
  

   admits 
  of 
  much 
  easier 
  adjustment. 
  With 
  the 
  component 
  

   beams 
  a 
  b, 
  a' 
  b\ 
  coplaner, 
  horizontal 
  and 
  of 
  about 
  equal 
  length 
  

   in 
  the 
  absence 
  of 
  the 
  prism 
  P 
  ', 
  the 
  latter 
  is 
  now 
  inserted 
  with 
  

   its 
  edge 
  vertical 
  (rotation) 
  and 
  the 
  white 
  slit 
  images 
  in 
  T 
  

   (without 
  G) 
  superposed, 
  horizontally 
  and 
  vertically. 
  G 
  is 
  then 
  

   added 
  and 
  the 
  micrometer 
  at 
  M 
  or 
  JV 
  manipulated 
  till 
  the 
  

   fringes 
  appear. 
  As 
  above, 
  they 
  are 
  largest 
  when 
  c 
  and 
  e' 
  are 
  

   as 
  nearly 
  as 
  possible 
  coincident 
  and 
  vanish 
  as 
  horizontal 
  fringes 
  

   at 
  the 
  maximum. 
  

  

  The 
  case 
  of 
  fig. 
  20 
  was 
  subsequently 
  again 
  tried 
  on 
  the 
  large 
  

   interferometer, 
  the 
  distance 
  P 
  to 
  M-JV 
  being 
  about 
  2 
  meters. 
  

   G, 
  in 
  these 
  experiments, 
  was 
  a 
  concave 
  grating 
  and 
  T 
  & 
  strong 
  

   lens 
  near 
  the 
  principal 
  focus 
  of 
  G. 
  The 
  adjustment 
  for 
  long 
  

   distances 
  is 
  not 
  easy. 
  The 
  equilateral 
  triangle 
  of 
  rays, 
  a, 
  a 
  ; 
  , 
  

   b', 
  b, 
  should 
  be 
  first 
  carefully 
  levelled, 
  the 
  edges 
  of 
  P 
  and 
  P' 
  

   being 
  on 
  the 
  median 
  line. 
  With 
  G 
  placed 
  at 
  the 
  proper 
  dis- 
  

   tance, 
  the 
  two 
  spectra 
  seen 
  at 
  Twill 
  usually 
  be 
  quite 
  distinct 
  

   in 
  the 
  field. 
  They 
  should 
  show 
  the 
  shadow 
  of 
  the 
  black 
  line 
  

   across 
  the 
  slit, 
  at 
  the 
  same 
  level 
  in 
  the 
  spectra. 
  The 
  longi- 
  

   tudinal 
  axis 
  of 
  the 
  spectra 
  may 
  then 
  be 
  made 
  collinear 
  by 
  

   slightly 
  tilting 
  the 
  edge 
  of 
  P' 
  to 
  the 
  vertical, 
  on 
  a 
  horizontal 
  

   axis, 
  with 
  the 
  adjusting 
  screws. 
  M 
  and 
  JV 
  are 
  then 
  rotated 
  

   on 
  a 
  vertical 
  axis 
  till 
  the 
  D 
  lines 
  coincide. 
  Small 
  changes 
  

   may 
  be 
  completed 
  at 
  M 
  and 
  JV. 
  The 
  fringes 
  when 
  found 
  are 
  

   usually 
  strongly, 
  but 
  very 
  fine, 
  less 
  than 
  the 
  P 
  X 
  D^ 
  distance 
  in 
  

   width. 
  I 
  have 
  been 
  able 
  to 
  increase 
  them 
  to 
  a 
  width 
  of 
  2/^Z^, 
  

   but 
  they 
  are 
  then 
  faint. 
  The 
  two 
  illuminated 
  strips 
  on 
  the 
  

   grating 
  may 
  even 
  be 
  an 
  inch 
  apart; 
  but 
  the 
  fringes 
  are 
  as 
  usual 
  

   larger, 
  when 
  this 
  distance 
  is 
  the 
  smallest 
  attainable 
  (virtual 
  

   coincidence). 
  The 
  grating 
  may 
  be 
  moved 
  fore 
  and 
  aft 
  without 
  

   effect. 
  As 
  JV 
  is 
  displaced 
  on 
  its 
  micrometer, 
  the 
  interferences 
  

   are 
  first 
  seen 
  as 
  vertical 
  h 
  airlike 
  striatums, 
  which 
  gradually 
  en- 
  

   large, 
  rotate 
  and 
  vanish 
  just 
  before 
  reaching 
  the 
  horizontal 
  and 
  

   at 
  maximum 
  size. 
  The 
  range 
  of 
  displacement 
  did 
  not 
  exceed 
  

   *15 
  cm 
  for 
  this 
  rotation 
  of 
  90°. 
  Since 
  JV 
  and 
  T 
  are 
  close 
  

   together, 
  the 
  manipulation 
  is 
  convenient 
  here, 
  but 
  with 
  another 
  

  

  * 
  Physical 
  Keview, 
  vii, 
  1916, 
  p. 
  587, 
  1916 
  ; 
  Science, 
  xlii, 
  p. 
  841, 
  1915. 
  

  

  