﻿156 
  

  

  C. 
  Barm 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  This 
  equation 
  is 
  not 
  obvious, 
  as 
  for 
  constant 
  X, 
  the 
  distance 
  

   being 
  G 
  and 
  G' 
  measured 
  along 
  a 
  given 
  raj 
  (prolonged) 
  for 
  

   any 
  position 
  of 
  21 
  or 
  A 
  T 
  is 
  also 
  constant. 
  The 
  equation 
  may 
  

   be 
  corroborated 
  by 
  drawing 
  the 
  diffracted 
  wave-front 
  at 
  G' 
  for 
  

   M 
  and 
  M\ 
  which 
  cuts 
  off 
  a 
  length 
  2e 
  sin 
  6 
  from 
  d" 
  '. 
  

  

  Since 
  sin 
  d=\/D, 
  if 
  D 
  is 
  the 
  grating 
  space, 
  the 
  last 
  equation 
  

   becomes 
  n 
  — 
  2e/JD 
  or 
  per 
  fringe 
  

  

  Se=D/2 
  

  

  a 
  remarkable 
  result, 
  showing 
  that 
  the 
  displacement 
  of 
  the 
  mir- 
  

   ror 
  21 
  per 
  fringe 
  is 
  independent 
  of 
  wave 
  length 
  and 
  equal 
  to 
  

  

  dh 
  \q! 
  

  

  

  J- 
  !' 
  

  

  3 
  

  

  half 
  the 
  grating 
  space. 
  An 
  interferometer 
  independent 
  of 
  X 
  

   and 
  available 
  throughout 
  relatively 
  enormous 
  ranges 
  of 
  dis- 
  

   placement 
  is 
  thus 
  at 
  hand. 
  It 
  may 
  be 
  shown 
  that 
  it 
  is 
  also 
  

   independent 
  of 
  the 
  angle 
  of 
  incidence 
  at 
  G. 
  

  

  To 
  change 
  the 
  size 
  of 
  fringes 
  it 
  is 
  necessary 
  to 
  rotate 
  the 
  

   grating 
  G' 
  (relatively 
  to 
  G) 
  on 
  a 
  horizontal 
  axis 
  normal 
  to 
  

   itself. 
  They 
  then 
  both 
  rotate 
  and 
  grow 
  larger, 
  attaining 
  the 
  

   maximum 
  of 
  size 
  when 
  the 
  fringes 
  are 
  vertical. 
  Fringes 
  quite 
  

   large 
  and 
  black 
  may 
  be 
  obtained 
  in 
  this 
  way. 
  

  

  To 
  show 
  the 
  close 
  relation 
  of 
  the 
  present 
  experiments 
  with 
  

   one 
  reflection, 
  to 
  the 
  earlier 
  work 
  with 
  crossed 
  rays 
  and 
  two 
  

   reflections, 
  experiments 
  may 
  be 
  made 
  with 
  homogeneous 
  light. 
  

   Accordingly 
  the 
  sodium 
  arc 
  with 
  a 
  wide 
  slit 
  was 
  installed. 
  

   Strands 
  of 
  fringes 
  with 
  nodules 
  were 
  obtained 
  as 
  before. 
  

   These 
  rotated 
  in 
  marked 
  degree 
  (180°) 
  from 
  vertical 
  hair 
  lines, 
  

   through 
  coarse 
  vertical 
  strands 
  with 
  horizontal 
  nodules, 
  back 
  

   to 
  vertical 
  hair 
  lines 
  again, 
  as 
  either 
  2f, 
  or 
  G, 
  were 
  suitably 
  

   displaced 
  normally 
  to 
  their 
  planes. 
  To 
  shift 
  the 
  fringes 
  of 
  any 
  

  

  