﻿the 
  Echelon 
  Spectroscope, 
  

  

  379 
  

  

  the 
  normal 
  position. 
  B 
  shows 
  the 
  echelon 
  rotated 
  in 
  the 
  

   opposite 
  direction 
  so 
  that 
  e 
  is 
  increased 
  and 
  the 
  dispersion 
  

   decreased. 
  

  

  The 
  way 
  in 
  which 
  R, 
  e^ 
  and 
  / 
  change 
  with 
  is 
  represented 
  

   in 
  fig. 
  4. 
  The 
  scale 
  for 
  R 
  is 
  given 
  in 
  wave-lengths, 
  m 
  being 
  

  

  Fig. 
  3. 
  

  

  the 
  number 
  o£ 
  wave-lengths 
  in 
  the 
  retardation 
  o£ 
  the 
  order 
  

   which 
  is 
  nearest 
  to 
  the 
  position 
  o£ 
  greatest 
  brightness 
  on 
  the 
  

   lower 
  retardation 
  side 
  when 
  ^ 
  = 
  0. 
  The 
  positions 
  o£ 
  the 
  row 
  

   o£ 
  marks 
  on 
  the 
  ^ 
  = 
  line 
  represent, 
  by 
  their 
  distances 
  £rom 
  

   the 
  apex 
  o£ 
  the 
  curve, 
  the 
  distances 
  o£ 
  the 
  orders 
  near 
  the 
  

   m\h 
  from 
  the 
  position 
  o£ 
  greatest 
  brightness 
  when 
  ^ 
  = 
  0, 
  and 
  

   the 
  abscissae 
  o£ 
  the 
  curve 
  passing 
  through 
  the 
  marks 
  which 
  

   lie 
  within 
  it, 
  give 
  the 
  angle 
  through 
  which 
  the 
  echelon 
  must 
  

   be 
  turned 
  in 
  either 
  direction 
  in 
  order 
  to 
  bring 
  orders 
  higher 
  

   than 
  the 
  mth 
  up 
  to 
  the 
  position 
  o£ 
  greatest 
  brightness. 
  

  

  The 
  reciprocals 
  o£ 
  e 
  and 
  / 
  are 
  also 
  plotted 
  to 
  show 
  how 
  the 
  

   dispersion 
  is 
  changed 
  by 
  the 
  rotation 
  in 
  the 
  direct 
  and 
  reversed 
  

   cases. 
  These 
  changes 
  in 
  dispersion 
  are 
  simply 
  changes 
  in 
  

   magnification, 
  as 
  they 
  are 
  not 
  accompanied 
  by 
  any 
  sensible 
  

   change 
  in 
  the 
  wave-length 
  intervals 
  between 
  the 
  orders. 
  

  

  The 
  way 
  in 
  which 
  the 
  effective 
  width, 
  ly, 
  o£ 
  the 
  individual 
  

   step 
  apertures 
  is 
  reduced 
  when 
  the 
  echelon 
  is 
  rotated 
  so 
  as 
  

  

  