﻿766 
  Talbofs 
  Fringes 
  and 
  the 
  Echelon 
  Grating. 
  

  

  with 
  large 
  path-difference, 
  might 
  be 
  erroneously 
  interpreted, 
  

   as 
  indicating 
  the 
  presence 
  of 
  long 
  trains 
  of 
  monochromatic 
  

   radiation 
  in 
  the 
  white 
  light, 
  whereas 
  the 
  thing 
  is 
  in 
  reality 
  

   produced 
  in 
  the 
  spectroscope. 
  

  

  This 
  paper 
  must 
  not 
  be 
  considered 
  in 
  any 
  way 
  as 
  an 
  attack 
  

   upon 
  the 
  impulse 
  theory 
  of 
  white 
  light. 
  

  

  The 
  only 
  point 
  which 
  I 
  wish 
  to 
  question 
  is 
  this 
  : 
  If 
  we 
  can 
  

   account 
  for 
  the 
  presence 
  and 
  absence 
  of 
  the 
  Talbot 
  bands 
  in 
  

   the 
  two 
  cases, 
  by 
  the 
  rearrangement 
  of 
  the 
  wave-lengths, 
  

   assuming 
  interference 
  as 
  taking 
  place 
  in 
  each 
  case, 
  are 
  we 
  

   justified 
  in 
  saying 
  that 
  their 
  absence 
  in 
  one 
  case 
  can 
  be 
  ex- 
  

   plained 
  by 
  the 
  inability 
  of 
  the 
  retarded 
  portion 
  to 
  interfere 
  

   with 
  the 
  unretarded 
  ? 
  Would 
  they 
  not 
  be 
  absent 
  just 
  the 
  

   same 
  even 
  if 
  interference 
  did 
  take 
  place 
  ? 
  I 
  am 
  not 
  to 
  be 
  

   understood 
  as 
  claiming 
  that 
  interference 
  does 
  take 
  place 
  in 
  

   the 
  case 
  when 
  we 
  start 
  with 
  white 
  light, 
  in 
  spite 
  of 
  the 
  

   experiment 
  which 
  appears 
  to 
  show 
  that 
  it 
  does, 
  for 
  I 
  cannot 
  

   imagine 
  how 
  a 
  wave-train, 
  already 
  behind 
  another 
  train, 
  

   could 
  be 
  made 
  to 
  interfere 
  with 
  it 
  by 
  further 
  retardation^ 
  

   unless 
  perhaps 
  we 
  explain 
  it 
  in 
  the 
  same 
  way 
  (by 
  resonance) 
  

   as 
  Schuster 
  explains 
  the 
  formation 
  of 
  interference 
  fringes 
  

   with 
  white 
  light 
  when 
  no 
  spectroscope 
  is 
  employed. 
  

  

  The 
  object 
  of 
  the 
  present 
  paper 
  has 
  been 
  merely 
  to 
  

   " 
  A^isualize 
  ^' 
  the 
  manner 
  in 
  which 
  the 
  fringes 
  are 
  formed. 
  

   I 
  have 
  attempted 
  to 
  carry 
  the 
  thing 
  a 
  point 
  further, 
  for 
  it 
  

   has 
  occurred 
  to 
  me 
  that 
  even 
  with 
  a 
  source 
  giving 
  what 
  

   appears 
  to 
  be 
  a 
  continuous 
  spectrum 
  in 
  a 
  spectroscope 
  of 
  

   low 
  power 
  (e. 
  g. 
  the 
  band 
  spectrum 
  of 
  nitrogen), 
  the 
  line 
  

   constituents 
  of 
  which 
  are 
  highly 
  monochromatic, 
  and 
  hence 
  

   capable 
  of 
  interfering 
  regardless 
  of 
  which 
  half 
  of 
  the 
  beam 
  

   is 
  retarded, 
  an 
  infinitely 
  narrow 
  linear 
  (vertical) 
  element 
  of 
  

   the 
  spectrum 
  is 
  made 
  up 
  of 
  more 
  than 
  one 
  wave-length, 
  on 
  

   account 
  of 
  the 
  limited 
  resolving 
  power. 
  I 
  have 
  not, 
  however, 
  

   been 
  able 
  to 
  see 
  just 
  how 
  this 
  will 
  affect 
  the 
  case 
  just 
  

   considered. 
  

  

  Very 
  likely 
  Professor 
  Schuster 
  will 
  be 
  able 
  to 
  defend 
  his 
  

   position, 
  and 
  I 
  am 
  sure 
  that 
  I 
  hope 
  he 
  will, 
  for 
  his 
  explana- 
  

   tion 
  is 
  a 
  very 
  neat 
  one, 
  and 
  I 
  dislike 
  to 
  feel 
  that 
  we 
  must 
  

   abandon 
  it. 
  

  

  Talhofs 
  Fringes 
  produced 
  by 
  an 
  Echelon. 
  

  

  If, 
  instead 
  of 
  covering 
  the 
  aperture 
  with 
  a 
  single 
  thin 
  

   plate, 
  we 
  cover 
  it 
  with 
  a 
  pile 
  of 
  thin 
  plates 
  arranged 
  in 
  

   steps, 
  we 
  find 
  that 
  Talbot's 
  fringes 
  appear 
  as 
  before. 
  Their 
  

   appearance 
  is 
  however 
  quite 
  different, 
  for 
  the 
  minima 
  are 
  

   now 
  very 
  broad 
  and 
  the 
  maxima 
  very 
  narrow 
  and 
  bright. 
  

  

  