﻿152 
  C. 
  Barus 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  which 
  that 
  method 
  is 
  incident, 
  succeeds 
  almost 
  perfectly, 
  both 
  

   in 
  the 
  first 
  and 
  second 
  order 
  spectra. 
  The 
  fringes 
  are 
  strong 
  

   and 
  clear. 
  An 
  Ives 
  grating 
  of 
  high 
  dispersion 
  (D 
  = 
  167 
  X 
  10~ 
  6 
  

   cm.) 
  was 
  tested. 
  

  

  A 
  prismatic 
  method 
  with 
  auxiliary 
  mirrors 
  to 
  accommodate 
  

   the 
  dispersion 
  of 
  the 
  grating 
  was 
  also 
  successfully 
  tried. 
  A 
  

   concave 
  reflecting 
  grating 
  may 
  be 
  replaced 
  by 
  a 
  film 
  grating 
  

   used 
  as 
  a 
  reflecting 
  grating, 
  with 
  entire 
  success. 
  The 
  ruled 
  

   side 
  of 
  the 
  film 
  should 
  be 
  free 
  (without 
  cover 
  glass), 
  but 
  the 
  

   reversed 
  side 
  cemented 
  on 
  plate 
  glass, 
  as 
  usual, 
  and 
  the 
  latter 
  

   placed 
  towards 
  the 
  telescope. 
  The 
  prism 
  P, 
  in 
  other 
  words, 
  

   admits 
  an 
  abundance 
  of 
  light, 
  so 
  that 
  even 
  the 
  loss 
  in 
  reflection 
  

   from 
  the 
  film 
  is 
  not 
  serious. 
  Sunlight 
  should 
  be 
  used 
  without 
  

   a 
  condensing 
  lens 
  ; 
  or 
  if 
  the 
  latter 
  is 
  added, 
  the 
  light 
  leaving 
  

   the 
  telescope 
  is 
  to 
  be 
  narrowed 
  laterally. 
  

  

  15. 
  Non-reversed 
  spectra. 
  — 
  The 
  prismatic 
  method 
  of 
  cleav- 
  

   ing 
  the 
  incident 
  beam 
  of 
  white 
  light 
  is 
  available 
  for 
  the 
  super- 
  

   position 
  of 
  non-reversed 
  spectra, 
  under 
  conditions 
  where 
  the 
  

   paths 
  of 
  the 
  component 
  rays 
  may 
  have 
  any 
  length 
  whatever. 
  

   It 
  is 
  thus 
  an 
  essential 
  extension 
  of 
  the 
  method, 
  fig. 
  19, 
  given 
  

   in 
  a 
  preceding 
  paper 
  (PP 
  f 
  prisms, 
  M,JV 
  mirrors, 
  Gp, 
  Ives 
  

   prism 
  grating, 
  T 
  telescope), 
  where 
  the 
  path 
  differences 
  were 
  

   essentially 
  small 
  and 
  the 
  spectra 
  reversed. 
  

  

  In 
  fig. 
  20, 
  P 
  is 
  the 
  first 
  prism 
  cleaving 
  the 
  white 
  beam, 
  Z, 
  

   diffracted 
  by 
  the 
  slit 
  of 
  the 
  collimator. 
  M 
  and 
  JV 
  are 
  the 
  

   opaque 
  mirrors, 
  the 
  former 
  on 
  a 
  micrometer. 
  lor 
  greater 
  

   ease 
  in 
  adjustment, 
  the 
  second 
  prism 
  P' 
  is 
  here 
  right 
  

   angled, 
  though 
  this 
  is 
  otherwise 
  inconvenient, 
  since 
  the 
  angle 
  

   8 
  = 
  90° 
  — 
  <£, 
  is 
  too 
  large. 
  The 
  rays 
  reflected 
  from 
  P 
  r 
  impinge 
  

   normally 
  on 
  the 
  reflecting 
  grating 
  G(D 
  — 
  200 
  X 
  10~ 
  6 
  ) 
  and 
  are 
  

   observed 
  by 
  a 
  telescope 
  at 
  T. 
  P,P\ 
  M 
  and 
  IV 
  are 
  all 
  provided 
  

   with 
  the 
  usual 
  three 
  adjustment 
  screws. 
  P' 
  must 
  be 
  capable 
  

   of 
  being 
  raised 
  and 
  lowered 
  and 
  moved 
  fore 
  and 
  aft. 
  The 
  

   field 
  is 
  brilliantly 
  illuminated. 
  When 
  the 
  path 
  difference 
  is 
  

   sufficiently 
  small, 
  the 
  fringes 
  appear 
  and 
  cover 
  the 
  whole 
  

   length 
  of 
  superposed 
  spectra, 
  strongly. 
  Tney 
  are 
  displaced, 
  

   with 
  rotation, 
  if 
  M\§ 
  moved 
  normally 
  to 
  itself. 
  

  

  As 
  first 
  obtained 
  the 
  fringes 
  were 
  too 
  close 
  packed 
  for 
  

   accurate 
  measurement. 
  But 
  experiments 
  on 
  the 
  displacement 
  

   of 
  the 
  mirror 
  J/", 
  for 
  successions 
  of 
  40 
  fringes 
  replacing 
  each 
  

   other 
  at 
  the 
  sodium 
  lines, 
  showed 
  a 
  mean 
  displacement 
  of 
  

   39 
  X 
  10" 
  6 
  cm. 
  per 
  fringe. 
  The 
  computed 
  value 
  would 
  be 
  

  

  — 
  36*4 
  X 
  10- 
  6 
  cm., 
  

  

  2 
  cos 
  8 
  /2 
  

  

  assuming 
  S 
  = 
  90° 
  — 
  <p. 
  The 
  difference 
  is 
  due 
  both 
  to 
  the 
  

   small 
  fringes 
  which 
  are 
  difficult 
  to 
  count 
  and 
  to 
  the 
  rough 
  

  

  