﻿tlie 
  Echelon 
  Spectroscope. 
  387 
  

  

  Character 
  of 
  the 
  Point 
  Spectra 
  produced 
  hy 
  the 
  

   Secondary 
  Light. 
  

  

  Fig. 
  6 
  is 
  a 
  diasram 
  representing 
  the 
  type 
  of 
  spectrum 
  

   produced 
  by 
  the 
  secondary 
  light 
  when 
  the 
  echelon 
  is 
  in 
  the 
  

   direct 
  position 
  and 
  tilted 
  about 
  a 
  horizontal 
  axis, 
  that 
  is, 
  the 
  

  

  Fi-. 
  6. 
  

  

  \ 
  

   > 
  TJ 
  n-i 
  n*z 
  n-t-3 
  

  

  f-^i 
  — 
  6 
  \ 
  \, 
  Q 
  \ 
  V 
  6 
  \ 
  V 
  \ 
  — 
  

  

  ^.1 
  

  

  type 
  represented 
  in 
  the 
  photograph 
  of 
  the 
  secondary 
  point 
  

   spectrum 
  Xo. 
  3, 
  Plate 
  XI. 
  : 
  but 
  in 
  order 
  to 
  make 
  the 
  diagram 
  

   clearer, 
  the 
  ratio 
  o£ 
  the 
  dispersion 
  in 
  the 
  Fabry 
  and 
  Perot 
  

   system 
  to 
  the 
  dispersion 
  in 
  the 
  primary 
  echelon 
  system 
  has 
  

   oeen 
  made 
  much 
  smaller 
  than 
  it 
  is 
  in 
  the 
  photograph. 
  

  

  In 
  this 
  case 
  all 
  the 
  rays 
  of 
  light 
  from 
  a 
  given 
  point 
  of 
  the 
  

   slit 
  are 
  parallel 
  to 
  one 
  another 
  during 
  their 
  passage 
  through 
  

   the 
  plates, 
  as 
  the 
  lateral 
  diffraction 
  does 
  not 
  take 
  place 
  until 
  

   the 
  light 
  leaves 
  the 
  echelon 
  by 
  the 
  step-faces. 
  The 
  spectra, 
  

   or 
  lines 
  o£ 
  constant 
  retardation, 
  in 
  the 
  Fabry 
  and 
  Perot 
  

   system 
  are 
  therefore 
  drawn 
  in 
  the 
  diagram 
  as 
  horizontal 
  lines. 
  

   When 
  the 
  photograph 
  was 
  taken 
  the 
  echelon 
  had 
  been 
  rotated 
  

   (from 
  the 
  normal 
  position 
  about 
  a 
  vertical 
  axis) 
  as 
  well 
  as 
  

   tilted, 
  so 
  the 
  plane 
  parallel 
  to 
  the 
  axis 
  of 
  the 
  collimator 
  and 
  

   the 
  diffracting 
  apertures 
  would 
  not 
  be 
  quite 
  vertical, 
  but 
  the 
  

   deviation 
  from 
  the 
  vertical 
  is 
  a 
  small 
  angle 
  of 
  the 
  second 
  

   order 
  (equal 
  to 
  the 
  product 
  of 
  the 
  small 
  angles 
  of 
  tilt 
  and 
  

   rotation), 
  and 
  it 
  wiU 
  be 
  seen 
  that 
  lines 
  joining 
  two 
  dots 
  

   representing 
  the 
  same 
  wave-length 
  and 
  of 
  the 
  same 
  order 
  in 
  

   the 
  Fabry 
  and 
  Perot 
  system, 
  would 
  be 
  sensibly 
  horizontal, 
  

  

  