﻿Non-reversed 
  Spectrum 
  Interferometry 
  . 
  155 
  

  

  lens 
  at 
  T' 
  the 
  phenomenon 
  could 
  be 
  traced 
  further 
  on 
  the 
  

   M 
  side. 
  

  

  To 
  secure 
  a 
  smaller 
  angle 
  of 
  incidence 
  and 
  reflection, 
  8/2, 
  

   at 
  M, 
  fig. 
  22, 
  the 
  combination 
  of 
  a 
  silvered 
  20° 
  prism, 
  P, 
  and 
  

   a 
  30° 
  prism, 
  P 
  ', 
  was 
  tested. 
  M 
  and 
  N 
  are 
  the 
  opaque 
  mirrors, 
  

   G 
  the 
  concave 
  grating 
  with 
  its 
  focus 
  at 
  T 
  for 
  inspection 
  by 
  a 
  

   strong 
  lens. 
  L 
  is 
  the 
  incident 
  beam 
  of 
  white 
  sunlight 
  from 
  

   the 
  collimator, 
  which 
  is 
  split 
  into 
  the 
  component 
  pencils 
  abed 
  

   and 
  a'Vc'd' 
  and 
  interfere 
  at 
  T. 
  The 
  results, 
  however, 
  were 
  

   about 
  the 
  same 
  as 
  above, 
  the 
  range 
  of 
  displacement 
  at 
  M 
  

   for 
  90° 
  of 
  rotation 
  of 
  fringes 
  being 
  about 
  '15 
  cm 
  . 
  As 
  a 
  and 
  b 
  

   make 
  angles, 
  <j> 
  and 
  (£', 
  with 
  the 
  line 
  of 
  symmetry 
  LL 
  ', 
  was 
  

   about 
  10°, 
  

  

  8= 
  <f>'— 
  cj>. 
  

  

  At 
  a 
  subsequent 
  opportunity 
  I 
  made 
  further 
  trials 
  with 
  the 
  

   paired 
  prisms 
  of 
  20° 
  and 
  30°, 
  but 
  failed 
  to 
  increase 
  the 
  fringes 
  

   above 
  about 
  D 
  X 
  DJ% 
  width. 
  Two 
  micrometers, 
  one 
  at 
  J^and 
  

   the 
  other 
  at 
  N, 
  were 
  installed, 
  and 
  moved 
  forward 
  in 
  alternate 
  

   steps, 
  within 
  a 
  range 
  of 
  over 
  2 
  cm 
  , 
  naturally 
  without 
  modify- 
  

   ing 
  the 
  fringes. 
  These 
  are 
  now 
  observed 
  on 
  both 
  sides 
  (xVand 
  

   M), 
  each 
  with 
  the 
  micrometer 
  which 
  is 
  manipulated. 
  One 
  

   may 
  note 
  in 
  passing 
  that 
  the 
  two 
  screws 
  are 
  being 
  incidentally 
  

   compared. 
  

  

  It 
  is 
  noteworthy 
  that 
  the 
  30° 
  prism 
  at 
  P' 
  is 
  no 
  marked 
  

   improvement 
  as 
  to 
  range 
  of 
  displacement 
  over 
  the 
  90° 
  prism 
  

   at 
  P\ 
  previously 
  used. 
  In 
  other 
  words, 
  the 
  effect 
  of 
  decreas- 
  

   ing 
  the 
  a*ngle 
  of 
  reflection, 
  8, 
  at 
  M 
  is, 
  unexpectedly, 
  of 
  small 
  

   importance 
  in 
  relation 
  to 
  the 
  range 
  of 
  displacement 
  at 
  M. 
  

   This 
  result 
  has 
  already 
  been 
  accentuated 
  in 
  other 
  ways, 
  above, 
  

   §13. 
  

  

  16. 
  Non-reversed 
  spectra. 
  Restricted 
  coincidence. 
  — 
  In 
  tig. 
  

   24, 
  the 
  white 
  ray 
  L 
  from 
  the 
  collimator 
  is 
  diffracted 
  by 
  the 
  

   grating 
  G 
  and 
  the 
  two 
  spectra 
  a 
  and 
  a\ 
  thereafter 
  reflected 
  by 
  

   the 
  parallel 
  opaque 
  mirrors 
  M 
  and 
  N, 
  to 
  be 
  again 
  diffracted 
  by 
  

   the 
  grating 
  G' 
  . 
  The 
  rays 
  are 
  observed 
  by 
  a 
  telescope 
  at 
  T. 
  

   If 
  the 
  gratings 
  G, 
  G' 
  have 
  nearly 
  the 
  same 
  constant, 
  it 
  is 
  obvi- 
  

   ous 
  that 
  the 
  field 
  of 
  the 
  telescope 
  will 
  show 
  a 
  sharp 
  white 
  image 
  

   of 
  the 
  slit, 
  for 
  each 
  mirror. 
  If 
  M 
  N 
  G 
  G' 
  are 
  adjusted 
  for 
  

   symmetry 
  by 
  aid 
  of 
  the 
  adjustment 
  screws 
  on 
  each 
  and 
  the 
  rul- 
  

   ings 
  are 
  parallel, 
  the 
  two 
  white 
  slit 
  images 
  will 
  coincide 
  hori- 
  

   zontally 
  and 
  vertically. 
  If 
  now 
  a 
  direct 
  vision 
  spectroscopic 
  

   prism, 
  or 
  a 
  direct 
  vision 
  prism-grating 
  G" 
  , 
  is 
  placed 
  in 
  front 
  of 
  

   the 
  telescope, 
  the 
  superposed 
  white 
  slit 
  images 
  will 
  be 
  drawn 
  

   out 
  into 
  overlapping 
  non-reversed 
  spectra, 
  which 
  will 
  usually 
  

   show 
  a 
  broad 
  strip 
  of 
  interference 
  fringes. 
  The 
  equation 
  is 
  

   n\=2e 
  cos 
  S/2=2<? 
  sin 
  6. 
  

  

  