﻿N 
  on-reversed 
  Spectrum 
  Interferometry 
  . 
  

  

  159 
  

  

  their 
  contours. 
  In 
  this 
  way 
  the 
  thick 
  apices 
  or 
  either 
  of 
  the 
  

   hairlike 
  lateral 
  ends 
  may 
  be 
  clearly 
  brought 
  out. 
  

  

  To 
  further 
  study 
  this 
  result 
  the 
  grating 
  G'* 
  was 
  successively 
  

   rotated 
  in 
  small 
  amounts 
  on 
  a 
  normal 
  axis 
  with 
  adjustment 
  at 
  M. 
  

   It 
  was 
  thus 
  possible 
  to 
  find 
  both 
  the 
  upper 
  ends 
  of 
  the 
  ellipses 
  

   and 
  their 
  lower 
  ends, 
  as 
  well 
  as 
  the 
  central 
  part. 
  The 
  con- 
  

   focal 
  ellipses 
  are 
  extremely 
  eccentric 
  with 
  very 
  turgid 
  apices 
  

   so 
  that 
  the 
  central 
  part 
  (if 
  in 
  the 
  spectrum) 
  consists 
  of 
  trans- 
  

  

  Fig. 
  25. 
  

  

  Fig. 
  20. 
  

  

  verse 
  straight 
  lines. 
  Motion 
  of 
  M 
  moves 
  the 
  fringes 
  to 
  and 
  

   from 
  the 
  center 
  where 
  they 
  originate 
  or 
  evanesce. 
  The 
  ellipses 
  

   shift 
  as 
  a 
  whole 
  with 
  M 
  without 
  changing 
  form 
  appreciably 
  

   throughout 
  the 
  spectrum, 
  but 
  they 
  move 
  very 
  slowly, 
  quite 
  

   differently 
  in 
  this 
  respect 
  from 
  the 
  round 
  ellipses 
  in 
  displace- 
  

   ment 
  interferometry 
  which 
  are 
  extremely 
  sensitive 
  to 
  displace- 
  

   ment 
  of 
  M. 
  In 
  the 
  present 
  work 
  it 
  may 
  take 
  5 
  or 
  10 
  crn 
  at 
  M 
  

   to 
  pass 
  the 
  ellipses 
  quite 
  through 
  the 
  spectrum. 
  They 
  are 
  

   strong 
  and 
  fine 
  in 
  spite 
  of 
  the 
  film 
  gratings 
  used. 
  

  

  18. 
  The 
  sam,e 
  continued. 
  Prismatic 
  Adjustment. 
  — 
  The 
  

   60° 
  prism 
  has 
  certain 
  advantages 
  in 
  experiments 
  like 
  the 
  

   present, 
  particularly 
  when 
  non-reversed 
  spectra 
  are 
  to 
  be 
  

   obtained. 
  Fig. 
  25 
  is 
  a 
  device 
  of 
  this 
  kind, 
  in 
  which 
  P 
  is 
  the 
  

   separating 
  prism 
  and 
  P' 
  the 
  collecting 
  prism, 
  the 
  beam 
  of 
  

   white 
  light 
  L 
  from 
  a 
  collimator 
  entering 
  the 
  flat 
  face 
  normally 
  

   on 
  the 
  front 
  side 
  and 
  issuing 
  normally 
  on 
  the 
  rear 
  side 
  at 
  

   c 
  and 
  c' 
  . 
  M 
  and 
  iV^are 
  opaque 
  mirrors 
  parallel 
  to 
  each 
  other, 
  

   G 
  a 
  direct 
  vision 
  prism-grating. 
  The 
  telescope 
  is 
  at 
  T. 
  The 
  

   reflection 
  may 
  be 
  either 
  internal, 
  as 
  in 
  the 
  strong 
  lines 
  of 
  

   fig. 
  25 
  ; 
  or 
  it 
  may 
  be 
  external 
  on 
  silvered 
  faces 
  of 
  the 
  prisms 
  

  

  * 
  When 
  nearly 
  centered 
  rotation 
  of 
  M 
  about 
  a 
  horizontal 
  axis 
  is 
  also 
  suf- 
  

   ficient 
  to 
  complete 
  the 
  centering 
  of 
  the 
  ellipses. 
  

  

  