﻿and 
  the 
  Echelon 
  Grating. 
  763 
  

  

  arc 
  spectrum 
  under 
  the 
  same 
  conditions, 
  and 
  the 
  third 
  (No. 
  1) 
  

   the 
  iron 
  arc 
  without 
  the 
  retarding 
  plate. 
  These 
  spectra 
  are 
  

   reproduced 
  as 
  negatives 
  on 
  PI. 
  XXIII. 
  fig. 
  1. 
  At 
  some 
  points 
  

   in 
  the 
  spectra 
  the 
  iron 
  lines 
  appear 
  the 
  same, 
  in 
  other 
  points 
  

   they 
  appear 
  doubled. 
  These 
  latter 
  points 
  are 
  indicated 
  by- 
  

   arrows. 
  

  

  We 
  thus 
  see 
  that 
  the 
  Talbot 
  bands 
  can 
  be 
  fully 
  accounted 
  

   for 
  without 
  in 
  any 
  way 
  making 
  use 
  o£ 
  the 
  idea 
  utilized 
  by 
  

   Schuster, 
  that 
  a 
  stream 
  of 
  waves 
  which 
  is 
  already 
  behind 
  

   another 
  stream 
  cannot 
  interfere 
  with 
  the 
  latter, 
  if 
  it 
  is 
  put 
  

   still 
  further 
  behind 
  by 
  retardation. 
  The 
  advanced 
  portion 
  of 
  

   the 
  train 
  must 
  be 
  retarded 
  if 
  interference 
  is 
  to 
  take 
  place. 
  

   While 
  this 
  explanation 
  is 
  not 
  open 
  to 
  criticism, 
  if 
  it 
  is 
  true 
  

   that 
  the 
  grating 
  constructs 
  the 
  monochromatic 
  elements 
  of 
  

   the 
  spectrum 
  from 
  the 
  assumed 
  irregular 
  pulses 
  which 
  are 
  

   supposed 
  to 
  constitute 
  white 
  light, 
  it 
  appears 
  to 
  me 
  that 
  we 
  

   can 
  account 
  for 
  the 
  fringes 
  without 
  making 
  this 
  supposition. 
  

   In 
  other 
  words, 
  Talbot^s 
  fringes 
  would 
  be 
  formed 
  even 
  if 
  

   the 
  two 
  streams 
  could 
  interfere 
  regardless 
  of 
  which 
  one 
  was 
  

   retarded, 
  and 
  this 
  being 
  the 
  case 
  it 
  does 
  not 
  seem 
  as 
  if 
  

   Schuster's 
  explanation 
  can 
  be 
  accepted 
  as 
  the 
  correct 
  one. 
  

  

  We 
  can, 
  in 
  fact, 
  form 
  Talbot's 
  fringes 
  under 
  conditions 
  

   which 
  preclude 
  the 
  factor 
  considered 
  by 
  Schuster, 
  and 
  we 
  

   find 
  that 
  they 
  fail 
  to 
  appear 
  when 
  the 
  retarding 
  plate 
  is 
  on 
  

   the 
  wrong 
  side, 
  though 
  interference 
  is 
  still 
  taking 
  place. 
  

  

  We 
  find 
  that 
  if 
  we 
  take 
  highly 
  homogeneous 
  light 
  to 
  start 
  

   with, 
  such 
  as 
  the 
  iron 
  spectrum, 
  the 
  lines 
  are 
  doubled 
  as 
  

   before 
  even 
  when 
  the 
  plate 
  is 
  introduced 
  from 
  the 
  wrong 
  

   side. 
  This 
  is 
  what 
  we 
  should 
  expect, 
  for 
  we 
  now 
  have 
  much 
  

   longer 
  trains 
  of 
  consecutive 
  disturbances, 
  and 
  one 
  train 
  can 
  

   interfere 
  with 
  the 
  other 
  regardless 
  of 
  whether 
  it 
  is 
  advanced 
  

   or 
  retarded. 
  The 
  same 
  will 
  hold 
  true 
  if 
  we 
  take 
  a 
  band 
  

   spectrum 
  of 
  very 
  narrow 
  and 
  close 
  lines 
  which 
  appears 
  con- 
  

   tinuous 
  in 
  our 
  spectroscope. 
  Talbot's 
  bands 
  can 
  be 
  seen 
  in 
  

   the 
  nitrogen 
  band 
  spectrum 
  only 
  when 
  the 
  plate 
  is 
  intro- 
  

   duced 
  from 
  the 
  proper 
  side, 
  though 
  we 
  must 
  consider 
  

   interference 
  as 
  still 
  taking 
  place 
  when 
  the 
  plate 
  is 
  put 
  on 
  

   the 
  wrong 
  side. 
  

  

  Supplementary 
  Note, 
  

  

  The 
  interference 
  in 
  this 
  case 
  is 
  not 
  to 
  be 
  considered 
  as 
  

   introducing 
  dark 
  regions 
  in 
  the 
  spectrum, 
  but 
  as 
  redistri- 
  

   buting 
  the 
  wave-lengths 
  ; 
  in 
  other 
  words, 
  decreasing 
  its 
  

   purity. 
  

  

  Let 
  us 
  calculate 
  what 
  happens 
  to 
  the 
  continuous 
  spectrum 
  

   for 
  the 
  two 
  cases 
  involving 
  the 
  correct 
  and 
  incorrect 
  placing 
  

  

  3 
  E 
  2 
  

  

  