﻿the 
  Echelon 
  Spectroscope. 
  

  

  373 
  

  

  Use 
  of 
  Auxiliary 
  Prism. 
  

  

  Auxiliary 
  spectroscopes 
  have 
  generally 
  been 
  employed 
  to 
  

   pick 
  out 
  the 
  particular 
  line 
  in 
  the 
  spectrum 
  to 
  be 
  examined 
  by 
  

   the 
  echelon, 
  the 
  slit 
  of 
  the 
  echelon 
  spectroscope 
  being 
  illumi- 
  

   nated 
  with 
  the 
  selected 
  line 
  ; 
  but 
  a 
  modification 
  in 
  the 
  

   design 
  o£ 
  this 
  instrument 
  was 
  made 
  for 
  Professor 
  Schuster 
  

   so 
  that 
  the 
  prism 
  from 
  the 
  auxiliary 
  spectroscope 
  could 
  be 
  

   mounted 
  next 
  to 
  the 
  echelon, 
  as 
  shown 
  in 
  fig. 
  1, 
  the 
  prism 
  

   being 
  made 
  larger 
  than 
  usual 
  in 
  order 
  to 
  take 
  the 
  full 
  width 
  

   and 
  height 
  of 
  the 
  echelon 
  beam. 
  This 
  arrangement, 
  which 
  

   appears 
  to 
  have 
  been 
  originally 
  contemplated 
  by 
  Professor 
  

   Michelson, 
  has 
  been 
  found 
  to 
  have 
  important 
  advantages. 
  

  

  The 
  dispersion 
  produced 
  by 
  the 
  prism, 
  which 
  is 
  2 
  per 
  cent, 
  

   of 
  that 
  given 
  by 
  the 
  echelon, 
  is 
  subtracted 
  from 
  the 
  echelon 
  

   dispersion 
  when 
  th« 
  echelon 
  is 
  in 
  the 
  usual 
  position 
  shown 
  by 
  

   full 
  lines, 
  and 
  is 
  added 
  to 
  it 
  when 
  the 
  echelon 
  is 
  in 
  the 
  reversed 
  

   position 
  shown 
  by 
  the 
  dotted 
  lines. 
  

  

  The 
  chfinge 
  of 
  4 
  per 
  cent, 
  in 
  the 
  dispersion 
  obtained 
  in 
  

   this 
  way 
  produces 
  the 
  alteration 
  in 
  the 
  spectrum 
  shown 
  in 
  

   fig. 
  1. 
  The 
  distance 
  apart 
  of 
  successive 
  orders 
  of 
  the 
  same 
  

   wave-length 
  is 
  not 
  altered 
  ; 
  but 
  when 
  the 
  dispersion 
  is 
  reduced 
  

   by 
  the 
  prism, 
  all 
  the 
  lines 
  belonging 
  to 
  the 
  same 
  order 
  

   approach 
  one 
  another 
  and 
  draw 
  away 
  from 
  the 
  neighbouring 
  

   orders. 
  It 
  is 
  evident 
  from 
  fig. 
  1 
  that 
  all 
  but 
  two 
  of 
  the 
  lines 
  

  

  Fig. 
  1. 
  

  

  DO 
  

   DD 
  

  

  DD 
  

   DD 
  

  

  1 
  u 
  

  

  2 
  5 
  ^5 
  

  

  1 
  i<L 
  

  

  represented 
  belong 
  to 
  the 
  same 
  order, 
  and 
  that 
  these 
  two 
  

   lines, 
  marked 
  1' 
  and 
  1^«, 
  belong 
  to 
  the 
  next 
  higher 
  order. 
  

   Fha. 
  May. 
  S. 
  6. 
  Vol. 
  18. 
  ^o, 
  105. 
  Sept. 
  1909. 
  2 
  C 
  

  

  