﻿C. 
  Barus 
  — 
  Non-reversed 
  Spectrum 
  Interferometry 
  . 
  145 
  

  

  Art. 
  XV. 
  — 
  Methods 
  in 
  Reversed 
  and 
  N 
  on-reversed 
  Spectrum 
  

   Interferometry 
  (continued) 
  ; 
  by 
  Carl 
  Barus.* 
  

  

  11. 
  Prism 
  methods 
  without 
  grating. 
  — 
  A 
  more 
  interesting 
  

   method, 
  in 
  some 
  respects, 
  in 
  which 
  the 
  grating 
  is 
  entirely 
  dis- 
  

   pensed 
  with, 
  is 
  shown 
  in 
  fig. 
  15. 
  L 
  is 
  the 
  beam 
  of 
  white 
  light 
  

   from 
  a 
  collimator, 
  P 
  a 
  refracting 
  prism 
  (here 
  with 
  a 
  60° 
  prism 
  

   angle), 
  M 
  and 
  N 
  the 
  opaque 
  mirrors 
  with 
  either 
  or 
  both 
  on 
  a 
  

   micrometer, 
  P' 
  a 
  silvered 
  reflecting 
  prism 
  (here 
  right 
  angled). 
  

   The 
  telescope 
  is 
  at 
  T 
  and 
  should 
  have 
  high 
  magnification. 
  

   The 
  rays 
  L 
  are 
  refracted 
  into 
  abc 
  and 
  a'b'c 
  and 
  the 
  two 
  

   spectra 
  observed 
  by 
  the 
  telescope 
  at 
  T. 
  Each 
  of 
  the 
  prisms 
  

   should 
  be, 
  on 
  three 
  adjustment 
  screws, 
  as 
  well 
  as 
  the 
  mirrors. 
  

   P 
  must 
  be 
  revolvable 
  slightly 
  around 
  a 
  vertical 
  axis 
  and 
  capa- 
  

   ble 
  of 
  fore 
  and 
  aft 
  motion. 
  P' 
  is 
  preferably 
  a 
  large 
  prism 
  

   placed 
  on 
  a 
  tablet. 
  The 
  rays 
  b 
  and 
  V 
  are 
  made 
  collinear 
  

   before 
  P' 
  is 
  inserted 
  and 
  both 
  the 
  rays 
  c 
  and 
  c' 
  must 
  come 
  

   from 
  near 
  its 
  edge. 
  

  

  The 
  fringes 
  are 
  strong 
  and 
  large 
  and 
  lie 
  within 
  a 
  relatively 
  

   remarkably 
  wide 
  transverse 
  strip. 
  This 
  may 
  be 
  ten 
  or 
  twenty 
  

   times 
  as 
  wide 
  as 
  the 
  P.D^ 
  doublets, 
  which 
  in 
  view 
  of 
  the 
  

   small 
  dispersion 
  are 
  hardly 
  separated. 
  For 
  the 
  same 
  reason, 
  

   moreover, 
  the 
  range 
  of 
  displacement 
  of 
  M 
  within 
  which 
  

   fringes 
  are 
  visible, 
  rarely 
  attains 
  half 
  a 
  millimeter. 
  Within 
  

   this 
  the 
  fringes 
  grow 
  from 
  the 
  fine 
  hair 
  lines, 
  usually 
  oblique, 
  

   to 
  their 
  maximum 
  coarseness. 
  Apart 
  from 
  the 
  §mall 
  range 
  of 
  

   displacement, 
  these 
  fringes 
  are 
  available 
  for 
  measurement. 
  If 
  

   both 
  mirrors 
  M 
  and 
  JY 
  are 
  on 
  micrometers, 
  they 
  may 
  be 
  

   brought 
  forward 
  or 
  the 
  reverse, 
  alternately, 
  and 
  the 
  range 
  in- 
  

   creased 
  5 
  or 
  10 
  times. 
  

  

  To 
  change 
  the 
  form 
  of 
  the 
  fringes, 
  the 
  first 
  prism, 
  JP, 
  may 
  

   be 
  tilted 
  slightly 
  on 
  an 
  axis 
  parallel 
  to 
  Ll\ 
  fig. 
  15. 
  The 
  

   fringes 
  then 
  pass 
  through 
  a 
  maximum 
  in 
  the 
  vertical 
  direction 
  

   (linear 
  phenomenon). 
  Fore 
  and 
  aft 
  motion 
  of 
  P 
  rotates 
  the 
  

   fringes 
  partially 
  toward 
  the 
  horizontal 
  ; 
  but, 
  as 
  a 
  rale, 
  the 
  

   component 
  beams 
  b 
  and 
  V 
  pass 
  beyond 
  the 
  edge 
  of 
  P' 
  and 
  the 
  

   fringes 
  vanish. 
  Just 
  before 
  this 
  (the 
  spectra 
  separating), 
  the 
  

   strip 
  within 
  which 
  the 
  fringes 
  lie, 
  widens 
  enormously. 
  In 
  

   other 
  words, 
  the 
  breadth 
  of 
  the 
  phenomenon 
  depends 
  on 
  dif- 
  

   fraction, 
  not 
  on 
  dispersion, 
  so 
  that 
  even 
  though 
  the 
  prism 
  P 
  

   scarcely 
  separates 
  the 
  D 
  lines, 
  the 
  striated 
  strip 
  has 
  about 
  the 
  

   same 
  width 
  as 
  when 
  it 
  is 
  produced 
  by 
  highly 
  resolving 
  grat- 
  

   ings. 
  

  

  * 
  Abridged 
  from 
  a 
  Report 
  to 
  trie 
  Carnegie 
  Institution 
  of 
  Washington, 
  

   D. 
  C. 
  See 
  this 
  Journal, 
  pp. 
  402-420, 
  November, 
  1916. 
  

  

  