﻿tlie 
  Echelon 
  Spectroscope. 
  385 
  

  

  retardation 
  o£ 
  this 
  beam 
  be 
  A. 
  There 
  is 
  another 
  secondary 
  

   beam 
  whose 
  retardation 
  is 
  2 
  A, 
  made 
  up 
  of 
  n— 
  1 
  faint 
  beams 
  

   which 
  have 
  each 
  gone 
  back 
  through 
  two 
  phites, 
  and 
  so 
  on. 
  

   Hence 
  the 
  secondary 
  action 
  o£ 
  the 
  pile 
  of 
  plates 
  in 
  the 
  echelon 
  

   is 
  similar 
  to 
  that 
  of 
  a 
  Fabry 
  and 
  Perot 
  spectroscope, 
  and, 
  

   under 
  suitable 
  conditions 
  of 
  illumination, 
  the 
  secondary 
  light 
  

   by 
  this 
  action 
  would 
  be 
  thrown 
  into 
  a 
  ring 
  spectrum 
  of 
  a 
  

   high 
  order. 
  The 
  retardation 
  is 
  ^jjut 
  for 
  normal 
  incidence, 
  so 
  

  

  it 
  is 
  -^— 
  ^ 
  times 
  as 
  great, 
  in 
  this 
  case 
  about 
  five 
  and 
  a 
  half 
  

  

  times 
  as 
  great, 
  as 
  that 
  of 
  the 
  primary 
  spectra. 
  

  

  The 
  secondary 
  light 
  also 
  undergoes 
  the 
  ordinary 
  echelon 
  

   treatment 
  as 
  it 
  leaves 
  by 
  the 
  step-faces, 
  and 
  it 
  is 
  therefore 
  

   confined 
  to 
  the 
  primary 
  spectrum 
  lines. 
  

  

  The 
  photograph 
  No. 
  2, 
  Plate 
  XI., 
  which 
  shows 
  short 
  portions 
  

   of 
  the 
  rings 
  of 
  the 
  Fabry 
  and 
  Perot 
  spectrum 
  crossing 
  the 
  

   echelon 
  spectrum, 
  was 
  obtained 
  by 
  stopping 
  the 
  primary 
  

   light 
  in 
  the 
  way 
  described 
  above, 
  and 
  making 
  the 
  echelon 
  

   lines 
  broad 
  by 
  widening 
  the 
  slit. 
  The 
  spectrum 
  lines 
  pro- 
  

   duced 
  by 
  the 
  primary 
  action 
  of 
  the 
  echelon 
  upon 
  the 
  

   secondary 
  light 
  are 
  inclined 
  because 
  the 
  echelon 
  is 
  tilted 
  as 
  

   shown 
  in 
  fig. 
  5, 
  and 
  the 
  centre 
  of 
  the 
  ring 
  system 
  is 
  above 
  

   the 
  field 
  of 
  view 
  for 
  the 
  same 
  reason. 
  The 
  echelon 
  has 
  

   also 
  been 
  rotated 
  (in 
  the 
  positive 
  direction 
  of 
  6), 
  so 
  the 
  

   centre 
  of 
  the 
  ring 
  system 
  is 
  displaced 
  laterally 
  as 
  well 
  as 
  

   vertically. 
  

  

  The 
  Fabry 
  and 
  Perot 
  spectrum 
  lines 
  are 
  not 
  dependent 
  for 
  

   their 
  definition, 
  like 
  the 
  echelon 
  spectrum 
  lines, 
  on 
  the 
  

   narrowness 
  ot" 
  the 
  slit 
  ; 
  their 
  want 
  of 
  clearness 
  in 
  this 
  photo- 
  

   graph 
  is 
  partly 
  due 
  to 
  the 
  overlapping 
  of 
  lines 
  belonging 
  to 
  

   different 
  orders, 
  which 
  is 
  produced 
  by 
  the 
  orders 
  overlapping 
  

   four 
  deep. 
  

  

  The 
  Secondary 
  Point 
  Spectrum. 
  

  

  If 
  the 
  slit, 
  instead 
  of 
  being 
  opened 
  wide 
  to 
  show 
  portions 
  

   of 
  the 
  rings, 
  is 
  made 
  narrow 
  enough 
  to 
  give 
  good 
  definition 
  

   to 
  the 
  primary 
  echelon 
  action, 
  the 
  secondary 
  light 
  which 
  has 
  

   undergone 
  both 
  actions 
  is 
  confined 
  to 
  dots 
  indicating 
  the 
  

   position 
  of 
  the 
  points 
  of 
  intersection 
  of 
  the 
  spectrum 
  lines 
  in 
  

   one 
  system 
  with 
  corresponding 
  lines, 
  that 
  is 
  lines 
  representing 
  

   the 
  same 
  wave-length, 
  in 
  the 
  other 
  system. 
  

  

  The 
  horizontal 
  dispersion 
  of 
  the 
  dots, 
  given 
  by 
  the 
  ordinary 
  

   echelon 
  action, 
  now 
  prevents 
  overlapping, 
  as 
  the 
  wave-length 
  

   interval 
  between 
  the 
  orders 
  in 
  this 
  system 
  is 
  a 
  little 
  greater 
  

   than 
  the 
  length 
  of 
  the 
  spectrum, 
  and 
  the 
  secondary 
  light 
  

  

  