﻿Non-reversed 
  Spectrum 
  Intcrferometry. 
  147 
  

  

  hair 
  line 
  fringes 
  came 
  from 
  rays 
  about 
  5 
  ,nm 
  behind 
  the 
  edge 
  of 
  

   the 
  prism 
  P 
  '. 
  Hence 
  the 
  pencils 
  were 
  here 
  about 
  l*2 
  cm 
  apart 
  

   when 
  they 
  entered 
  the 
  telescope. 
  The 
  largest 
  and 
  last 
  of 
  the 
  

   fringes 
  come 
  from 
  close 
  to 
  the 
  edge 
  of 
  P 
  '. 
  The 
  experiment 
  

   was 
  varied 
  as 
  follows 
  : 
  supposing 
  both 
  screens 
  s 
  and 
  s' 
  placed 
  

   as 
  far 
  to 
  the 
  rear 
  as 
  the 
  visibility 
  of 
  fringes 
  permits, 
  let 
  the 
  

   former, 
  s, 
  be 
  slowly 
  pushed 
  forward. 
  The 
  fringes 
  then 
  con- 
  

   tract 
  from 
  the 
  very 
  broad 
  set, 
  fig. 
  17, 
  case 
  1, 
  to 
  the 
  strong 
  and 
  

   narrow 
  set, 
  2 
  (which 
  is 
  a 
  mere 
  line 
  for 
  a 
  full 
  wave-front), 
  and 
  

   then 
  expand 
  again 
  to 
  3. 
  If 
  now 
  s 
  is 
  left 
  in 
  place 
  and 
  s' 
  moved 
  

   forward, 
  slowly 
  in 
  the 
  same 
  way, 
  the 
  identical 
  contraction 
  and 
  

   expansion, 
  1, 
  2, 
  3, 
  is 
  reproduced. 
  The 
  screen 
  s' 
  may 
  then 
  be 
  

   left 
  in 
  place 
  and 
  s 
  in 
  turn 
  slowly 
  moved 
  forward 
  with 
  the 
  

   same 
  results, 
  etc. 
  (there 
  may 
  be 
  six 
  alternations), 
  until 
  finally 
  

   the 
  effective 
  parts 
  of 
  the 
  pencils 
  b 
  and 
  V 
  are 
  beyond 
  the 
  edge 
  

   of 
  the 
  prism 
  P' 
  . 
  In 
  case 
  2, 
  the 
  two 
  slits 
  s 
  and 
  §! 
  are 
  obviously 
  

   symmetrical 
  to 
  the 
  interfering 
  rays, 
  whereas 
  in 
  cases 
  1 
  and 
  3 
  

   the 
  diagonally 
  opposite 
  edges 
  of 
  the 
  slits, 
  s 
  and 
  s 
  f 
  , 
  limit 
  the 
  

   efficient 
  pencils 
  rigorously 
  to 
  a 
  sheet. 
  

  

  A 
  similar 
  result 
  (passage 
  of 
  case 
  2 
  into 
  3, 
  fig. 
  17) 
  may 
  be 
  

   produced 
  by 
  moving 
  P 
  forward, 
  the 
  case 
  3 
  appearing 
  just 
  

   before 
  the 
  pencils 
  b 
  V 
  leave 
  the 
  edge 
  of 
  P'. 
  Again, 
  when 
  M 
  

   is 
  moved 
  rearward, 
  when 
  both 
  b 
  and 
  V 
  are 
  near 
  the 
  edge 
  of 
  

   P', 
  the 
  cases 
  2, 
  3 
  are 
  obtained. 
  In 
  general 
  the 
  width 
  of 
  the 
  

   diffraction 
  pattern 
  increases 
  without 
  changing 
  the 
  size 
  of 
  

   fringes, 
  as 
  the 
  width 
  of 
  the 
  available 
  wave-front 
  decreases. 
  

  

  12. 
  Displacement 
  parallel 
  to 
  rays.- 
  — 
  It 
  now 
  becomes 
  of 
  im- 
  

   portance 
  to 
  test 
  the 
  range 
  of 
  displacement 
  as 
  modified 
  by 
  the 
  

   angle 
  of 
  reflection, 
  increasing 
  from 
  5 
  = 
  0. 
  It 
  is 
  therefore 
  

   desirable 
  to 
  make 
  a 
  few 
  direct 
  measurements. 
  The 
  angle 
  6 
  at 
  

   P, 
  fig. 
  15, 
  was 
  found 
  to 
  be 
  about 
  49° 
  45', 
  so 
  that 
  the 
  total 
  

   angle 
  at 
  M 
  is 
  8 
  = 
  40° 
  15'. 
  M 
  and 
  A^are 
  both 
  on 
  micrometers, 
  

   with 
  the 
  screws 
  normal 
  to 
  their 
  faces. 
  P' 
  is 
  on 
  a 
  micrometer 
  

   with 
  its 
  screw 
  parallel 
  to 
  bb\ 
  so 
  that 
  this 
  prism 
  is 
  shifted 
  right 
  

   and 
  left. 
  The 
  range 
  of 
  displacement 
  was 
  found 
  at 
  

  

  M, 
  about 
  -04 
  cm 
  ; 
  x 
  — 
  2 
  X 
  '04 
  X 
  "939 
  = 
  -076 
  cm 
  , 
  

   P', 
  about 
  y 
  = 
  07 
  cm 
  ; 
  2y 
  = 
  -14u 
  cm 
  , 
  

  

  where 
  x 
  — 
  2e 
  cos 
  (90 
  — 
  d)/2 
  and 
  2y 
  are 
  the 
  corresponding 
  path 
  

   differences 
  between 
  the 
  inception 
  and 
  evanescence 
  of 
  fringes. 
  

   With 
  a 
  very 
  fine 
  slit, 
  2z/ 
  was 
  possibly 
  smaller 
  (see 
  fig. 
  18). 
  

  

  The 
  question 
  at 
  issue 
  is 
  thus 
  in 
  the 
  first 
  place, 
  how 
  the 
  value 
  

   of 
  2y 
  compares 
  with 
  x 
  ; 
  for 
  in 
  the 
  former 
  case 
  the 
  angle 
  8 
  is 
  

   effectively 
  zero. 
  In 
  other 
  words, 
  when 
  M 
  is 
  displaced 
  from 
  

   M 
  to 
  M' 
  over 
  a 
  distance 
  6, 
  the 
  pencil 
  b, 
  fig. 
  18, 
  changes 
  to 
  5„ 
  

   and 
  is 
  soon 
  lost 
  at 
  the 
  edge 
  of 
  P' 
  ; 
  whereas, 
  when 
  P 
  is 
  dis- 
  

   placed 
  in 
  the 
  direction 
  bb\ 
  over 
  a 
  distance 
  y, 
  the 
  rays 
  b 
  and 
  b' 
  

  

  