﻿150 
  C. 
  Barns 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  the 
  spectra 
  c 
  and 
  c' 
  are 
  reversed, 
  or 
  that 
  the 
  colors 
  of 
  like 
  

   origin 
  and 
  wave-length 
  are 
  successively 
  farther 
  apart. 
  When 
  

   the 
  screens 
  are 
  alternately 
  moved 
  therefore, 
  the 
  same 
  phenome- 
  

   non 
  is 
  in 
  turn 
  produced 
  in 
  slightly 
  different 
  colors. 
  But 
  as 
  ah 
  

   continually 
  increases 
  whereas 
  the 
  efficient 
  breadth 
  of 
  the 
  strips 
  

   does 
  not, 
  the 
  fringes 
  soon 
  pass 
  beyond 
  appreciable 
  smallness. 
  

  

  When 
  as 
  in 
  the 
  earlier 
  methods 
  but 
  a 
  simple 
  grating 
  is 
  used 
  

   with 
  two 
  successive 
  diffractions 
  through 
  it, 
  the 
  patches 
  a 
  and 
  b 
  

   are 
  obviously 
  in 
  the 
  same 
  level 
  when 
  the 
  longitudinal 
  axes 
  of 
  

   spectra 
  coincide. 
  Hence 
  the 
  fringes 
  are 
  essentially 
  vertical. 
  

  

  In 
  the 
  experiment 
  with 
  screens, 
  s, 
  s 
  ; 
  . 
  fig. 
  15, 
  it 
  is 
  obvious 
  

   that 
  path 
  difference 
  remains 
  constant. 
  The 
  distance 
  from 
  the 
  

   same 
  wave-front 
  in 
  the 
  pencils 
  b 
  and 
  b 
  '. 
  fig. 
  18, 
  to 
  the 
  prin- 
  

   cipal 
  plane 
  j)p, 
  is 
  always 
  the 
  same; 
  but 
  pencils 
  different 
  in 
  

   lateral 
  position 
  are 
  successively 
  selected. 
  On 
  the 
  other 
  hand, 
  

   when 
  the 
  prism 
  P' 
  is 
  moved 
  in 
  the 
  direction 
  y, 
  parallel 
  to 
  bb', 
  

   path 
  difference 
  only 
  is 
  introduced, 
  while 
  the 
  pencils 
  selected 
  

   remain 
  the 
  same. 
  Supposing 
  the 
  ordinary 
  conditions 
  of 
  visi- 
  

   bility 
  (magnification, 
  etc.) 
  to 
  remain 
  unaltered, 
  the 
  wave- 
  

   fronts 
  are, 
  as 
  it 
  were, 
  explored 
  in 
  depth 
  as 
  to 
  their 
  uniformity 
  ; 
  

   i. 
  e. 
  the 
  distance 
  is 
  apparently 
  recorded, 
  throughout 
  which 
  a 
  

   wave 
  train 
  consists 
  of 
  identical 
  wave 
  elements. 
  Effectively, 
  

   however, 
  the 
  rapidity 
  with 
  which 
  fringes 
  decrease 
  in 
  size 
  

   beyond 
  visibility 
  is 
  directly 
  in 
  question. 
  Finally 
  when 
  the 
  

   opaque 
  mirror 
  M 
  (or 
  JV) 
  is 
  moved 
  from 
  If 
  to 
  M\ 
  both 
  effects 
  

   occur 
  together. 
  Path 
  difference 
  x 
  — 
  ^e 
  cos 
  6/2 
  is 
  introduced 
  

   and 
  the 
  pencil 
  is 
  displaced 
  from 
  b 
  to 
  b' 
  ' 
  . 
  

  

  14. 
  Film 
  Grating. 
  — 
  The 
  method 
  of 
  two 
  gratings 
  was 
  now 
  

   again 
  resorted 
  to, 
  except 
  that 
  the 
  first 
  at 
  G, 
  fig. 
  3, 
  was 
  a 
  

   film 
  grating. 
  This 
  attempt 
  failed 
  in 
  my 
  earlier 
  work, 
  when 
  

   but 
  a 
  single 
  Him 
  grating 
  was 
  used 
  for 
  the 
  two 
  diffractions, 
  

   because 
  of 
  insufficient 
  light. 
  In 
  the 
  present 
  case, 
  two 
  gratings 
  

   (G 
  f 
  being 
  reflecting) 
  are 
  employed, 
  and 
  the 
  method 
  succeeded 
  

   at 
  once. 
  The 
  first 
  grating 
  constant 
  was 
  D 
  = 
  10" 
  6 
  Xl67 
  cm 
  ; 
  

   observations 
  were 
  therefore 
  necessarily 
  made 
  in 
  the 
  second 
  

   order 
  of 
  G\ 
  so 
  that 
  the 
  spectra 
  are 
  not 
  as 
  intense 
  as 
  with 
  

   prisms. 
  But 
  the 
  fringes 
  are 
  perfect 
  and 
  may 
  be 
  made 
  as 
  large 
  

   as 
  desirable, 
  with 
  but 
  two 
  in 
  the 
  breadth 
  of 
  the 
  spectrum, 
  for 
  

   instance. 
  The 
  range 
  of 
  displacement 
  was 
  found 
  to 
  be 
  about 
  6 
  

   millimeters 
  under 
  the 
  best 
  conditions 
  (arrows). 
  If 
  both 
  J^and 
  

   JY 
  are 
  successively 
  displaced 
  in 
  the 
  same 
  direction, 
  the 
  total 
  

   displacement 
  available 
  between 
  the 
  hairlike 
  fringes 
  at 
  the 
  

   extremes 
  is 
  about 
  l*5 
  cm 
  for 
  each 
  mirror. 
  

  

  At 
  these 
  extremes 
  the 
  two 
  patches 
  of 
  light 
  on 
  the 
  grating 
  G 
  r 
  

   may 
  have 
  been 
  separated 
  by 
  several 
  millimeters. 
  The 
  nature 
  

   of 
  the 
  transformation 
  from 
  arrows 
  to 
  the 
  oblique 
  striations 
  

   would 
  be 
  well 
  reproduced, 
  if 
  equidistant 
  vertical 
  wedges 
  were 
  

   moved 
  from 
  right 
  to 
  left, 
  or 
  the 
  reverse, 
  behind 
  a 
  vertical 
  slit. 
  

  

  