﻿the 
  Echelon 
  Spectroscope. 
  

  

  389 
  

  

  If 
  the 
  definition 
  is 
  poor 
  in 
  one 
  of 
  the 
  two 
  systems, 
  a 
  mono- 
  

   chromatic 
  radiation 
  would 
  be 
  represented 
  by 
  dots 
  elongated 
  

   in 
  the 
  direction 
  of 
  the 
  spectrum 
  lines 
  of 
  the 
  other 
  system, 
  

   and 
  this 
  would 
  in 
  general 
  spoil 
  the 
  sharpness 
  of 
  the 
  lines 
  

   representing 
  in 
  the 
  joint 
  system 
  a 
  spectrum 
  continuous 
  

   between 
  narrow 
  limits 
  ; 
  but 
  if 
  these 
  lines 
  are 
  nearly 
  parallel 
  

   to 
  the 
  spectrum 
  lines 
  of 
  one 
  system, 
  the 
  want 
  of 
  definition 
  in 
  

   the 
  other 
  will 
  not 
  spoil 
  the 
  definition 
  of 
  the 
  lines, 
  as 
  each 
  dot 
  

   representing 
  a 
  single 
  wave-length 
  will 
  be 
  drawn 
  out 
  in 
  a 
  

   direction 
  nearly 
  parallel 
  to 
  the 
  length 
  of 
  the 
  joint 
  spectra. 
  

   This 
  special 
  case 
  is 
  realized 
  when 
  the 
  echelon 
  is 
  in 
  the 
  direct 
  

   or 
  reversed 
  normal 
  positions. 
  

  

  The 
  diagrams 
  A 
  and 
  B 
  in 
  fig. 
  7 
  represent 
  the 
  conditions 
  

  

  Fis. 
  7. 
  

  

  u 
  u 
  

  

  A 
  B 
  

  

  in 
  these 
  cases. 
  They 
  are 
  similar 
  to 
  fig. 
  6, 
  but 
  spectrum 
  lines 
  

   have 
  been 
  drawn 
  in 
  the 
  Fabry 
  and 
  Perot 
  system 
  for 
  a 
  series 
  

   of 
  five 
  wave-lengths 
  whose 
  values 
  increase 
  by 
  equal 
  increments 
  

   from 
  \i 
  to 
  Xs. 
  In 
  the 
  primary 
  echelon 
  system 
  the 
  lines 
  repre- 
  

   senting 
  alternate 
  wave-lengths 
  have 
  been 
  omitted. 
  B 
  represents 
  

   the 
  case 
  in 
  which 
  the 
  echelon 
  is 
  in 
  the 
  reversed 
  position, 
  the 
  

   radii 
  of 
  the 
  circles^ 
  representing 
  the 
  spectra 
  in 
  the 
  Fabry 
  and 
  

   Perot 
  system^ 
  being 
  chosen 
  so 
  that 
  their 
  squares 
  increase 
  by 
  

   equal 
  increments. 
  The 
  horizontal 
  lines 
  in 
  diagram 
  A 
  repre- 
  

   sent 
  the 
  same 
  spectra 
  when 
  the 
  echelon 
  is 
  turned 
  round 
  into 
  

   the 
  direct 
  normal 
  position. 
  

  

  For 
  convenience 
  in 
  drawing 
  the 
  diagrams, 
  the 
  dispersion 
  

   in 
  the 
  Fabry 
  and 
  Perot 
  system 
  has 
  been 
  represented 
  as 
  only 
  

   about 
  ten 
  times 
  as 
  great 
  as 
  that 
  in 
  the 
  primary 
  echelon 
  

  

  Phil. 
  Mag. 
  S. 
  (). 
  Vol. 
  18. 
  No. 
  105. 
  Sept. 
  1909. 
  2 
  D 
  

  

  