﻿Non-reversed 
  Spectrum 
  Interferomelry. 
  

  

  153 
  

  

  value 
  of 
  5. 
  The 
  range 
  of 
  measurement 
  is 
  small 
  (if 
  M 
  only 
  

   moves) 
  not 
  exceeding 
  1*5 
  millimeters 
  for 
  a 
  moderately 
  strong 
  

   telescope. 
  Usually 
  but 
  one-half 
  of 
  this 
  displacement 
  is 
  avail- 
  

   able, 
  as 
  the 
  fringes 
  increase 
  in 
  size 
  (with 
  rotation) 
  from 
  tine 
  

   vertical 
  hair 
  lines 
  to 
  a 
  nearly 
  horizontal 
  maximum, 
  and 
  then 
  

   abruptly 
  vanish. 
  This 
  is 
  only 
  one-half 
  of 
  the 
  complete 
  cycle. 
  

   If 
  we 
  regard 
  the 
  component 
  beams, 
  a 
  o 
  c 
  and 
  a' 
  V 
  <?', 
  as 
  being 
  

   of 
  the 
  width 
  of 
  the 
  pencil 
  diffracted 
  by 
  the 
  slit 
  of 
  the 
  collima- 
  

  

  Figs. 
  20-23. 
  

  

  a 
  

  

  ^Mk 
  

  

  &' 
  v 
  e 
  

  

  /< 
  

  

  n 
  

  

  tor, 
  it 
  is 
  clear 
  that 
  the 
  maximum 
  size 
  of 
  fringes 
  will 
  occur, 
  

   when 
  g 
  and 
  e/ 
  are 
  as 
  near 
  together 
  as 
  possible 
  ; 
  furthermore, 
  

   that 
  as 
  M 
  moves 
  toward 
  P\ 
  c 
  continually 
  approaches 
  c 
  ; 
  , 
  until 
  

   o 
  drops 
  off 
  (as 
  it 
  were) 
  from 
  the 
  right 
  angled 
  edge 
  of 
  the 
  

   prism 
  P' 
  . 
  To 
  get 
  the 
  best 
  conditions, 
  i. 
  e., 
  the 
  largest 
  fringes, 
  

   c 
  must 
  therefore 
  also 
  be 
  moved 
  up 
  to 
  the 
  edge 
  of 
  P 
  and 
  very 
  

   sharp 
  angled 
  prisms 
  be 
  used 
  at 
  both 
  P 
  and 
  P 
  f 
  . 
  The 
  largest 
  

   fringes 
  (lines 
  about 
  10 
  times 
  the 
  D^D^ 
  distance) 
  obtained 
  with 
  

   the 
  right 
  angled 
  prism 
  were 
  often 
  not 
  very 
  strong, 
  though 
  

   otherwise 
  satisfactory. 
  Much 
  of 
  the 
  light 
  of 
  both 
  spectra 
  does 
  

   not 
  therefore 
  interfere, 
  being 
  different 
  in 
  origin. 
  

  

  Results 
  very 
  similar 
  to 
  the 
  present 
  were 
  described 
  long 
  ago* 
  

   and 
  found 
  with 
  two 
  identical 
  half 
  gratings, 
  coplanar 
  and 
  

  

  *Phil. 
  Mag., 
  xxii, 
  pp. 
  118-129, 
  1911 
  ; 
  Carnegie 
  Publ. 
  No. 
  149, 
  chap. 
  vi. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XLIII, 
  No. 
  254.— 
  February, 
  1917. 
  

   11 
  

  

  