﻿and 
  the 
  Echelon 
  Grating. 
  765 
  

  

  even 
  i£ 
  we 
  assume 
  interference 
  between 
  the 
  retarded 
  and 
  

   unretarded 
  portions 
  o£ 
  the 
  wave-train. 
  The 
  question 
  now 
  

   suggests 
  itself 
  " 
  Does 
  interference 
  take 
  place, 
  in 
  the 
  manner 
  

   supposed, 
  in 
  the 
  case 
  of 
  the 
  continuous 
  spectrum 
  formed 
  

   from 
  white 
  light 
  ?" 
  I 
  have 
  attempted 
  to 
  test 
  this 
  matter 
  in 
  

   the 
  following 
  way 
  : 
  — 
  The 
  Talbot 
  fringes 
  were 
  f 
  ocussed 
  upon 
  

   the 
  slit 
  of 
  a 
  second 
  spectroscope. 
  If 
  the 
  slit 
  fell 
  upon 
  a 
  

   bright 
  band, 
  the 
  band 
  appeared 
  much 
  drawn 
  out, 
  owing 
  to 
  

   the 
  presence 
  in 
  it 
  of 
  a 
  number 
  of 
  different 
  wave-lengths. 
  If 
  

   it 
  fell 
  upon 
  a 
  dark 
  band 
  the 
  image 
  in 
  the 
  eyepiece 
  appeared 
  

   much 
  fainter. 
  

  

  If 
  the 
  slit 
  of 
  the 
  second 
  spectroscope 
  was 
  opened 
  wide 
  the 
  

   bands 
  appeared, 
  but 
  they 
  were 
  very 
  faint, 
  owing 
  to 
  the 
  

   widening 
  of 
  the 
  bright 
  bands 
  by 
  the 
  dispersion, 
  which 
  caused 
  

   them 
  to 
  overlap 
  the 
  dark 
  regions. 
  The 
  slit 
  was 
  now 
  narrowed 
  

   again, 
  and 
  the 
  retarding 
  plate 
  introduced 
  from 
  the 
  wrong- 
  

   side. 
  If 
  interference 
  took 
  place 
  in 
  the 
  manner 
  assumed 
  we 
  

   should 
  expect 
  three 
  or 
  more 
  wave-lengths 
  (lines 
  separated 
  

   by 
  dark 
  spaces) 
  to 
  appear 
  in 
  the 
  eyepiece. 
  In 
  other 
  words, 
  

   if 
  we 
  put 
  the 
  slit 
  at 
  the 
  point 
  A 
  in 
  the 
  spectrum 
  we 
  should 
  

   expect 
  A 
  to 
  appear 
  very 
  bright, 
  and 
  C 
  and 
  B 
  as 
  well, 
  with 
  

   about 
  half 
  the 
  intensity 
  of 
  A. 
  This 
  was 
  found 
  to 
  be 
  the 
  

   case. 
  Three 
  bright 
  lines, 
  separated 
  by 
  very 
  black 
  regions, 
  

   appeared 
  in 
  the 
  eyepiece. 
  

  

  I 
  next 
  removed 
  the 
  mica 
  plate 
  from 
  the 
  aperture. 
  In 
  the 
  

   eyepiece 
  there 
  now 
  appeared 
  in 
  monochromatic 
  light 
  the 
  

   diffraction 
  maxima 
  and 
  minima 
  of 
  the 
  1st 
  class, 
  a 
  central 
  one 
  

   very 
  bright 
  bordered 
  by 
  others 
  much 
  fainter. 
  On 
  introducing 
  

   the 
  mica 
  plate, 
  again 
  on 
  the 
  wrong 
  side, 
  the 
  central 
  maximum 
  

   became 
  furrowed 
  by 
  black 
  minima, 
  breaking 
  up 
  into 
  the 
  

   three 
  bright 
  lines 
  previously 
  alluded 
  to. 
  This 
  seems 
  to 
  show 
  

   that 
  interference 
  takes 
  place 
  even 
  when 
  we 
  retard 
  the 
  w^rong 
  

   half 
  of 
  the 
  train. 
  Very 
  likely 
  I 
  have 
  fallen 
  into 
  some 
  pit- 
  

   fall, 
  and 
  I 
  am 
  sure 
  that 
  I 
  hope 
  that 
  Prof. 
  Schuster 
  will 
  

   discover 
  it. 
  The 
  subject 
  seems 
  to 
  me 
  to 
  be 
  a 
  tricky 
  one 
  to 
  

   deal 
  with, 
  so 
  many 
  factors 
  have 
  to 
  be 
  taken 
  into 
  account, 
  

   such 
  as 
  the 
  decrease 
  of 
  purity 
  resulting 
  from 
  the 
  limited 
  

   aperture^ 
  the 
  presence 
  of 
  the 
  second 
  spectroscope, 
  &c. 
  It 
  is 
  

   probable 
  that 
  the 
  second 
  spectroscope 
  can 
  be 
  shown 
  to 
  be 
  

   responsible 
  for 
  the 
  interference, 
  as 
  when 
  showing 
  fringes 
  of 
  

   high 
  order 
  in 
  the 
  case 
  of 
  the 
  so-called 
  direct 
  interference 
  of 
  

   white 
  light. 
  There 
  seems 
  at 
  first 
  sight 
  to 
  be 
  no 
  escape 
  from 
  

   the 
  conclusion, 
  that 
  if 
  a 
  spectroscope 
  shows 
  three 
  bright 
  

   lines 
  when 
  placed 
  at 
  a 
  given 
  point 
  in 
  the 
  spectrum, 
  these 
  

   three 
  wave-lengths 
  must 
  be 
  present. 
  We 
  must 
  remember, 
  

   however, 
  that 
  spectroscopic 
  analysis 
  of 
  white 
  light, 
  interfering 
  

  

  