﻿and 
  the 
  Echelon 
  Grating. 
  

  

  761 
  

  

  bv 
  removing 
  the 
  prism 
  and 
  viewing 
  the 
  slit 
  directly 
  with 
  

   the 
  telescope. 
  The 
  slit 
  can 
  be 
  illuminated 
  with 
  a 
  monochro- 
  

   matic 
  illuminator 
  (spectroscope 
  with 
  narrow 
  slit 
  in 
  place 
  of 
  

   the 
  eyepiece), 
  the 
  slit 
  o£ 
  the 
  instrmnent 
  being 
  substituted 
  for 
  

   the 
  slit 
  of 
  the 
  first 
  spectroscope. 
  

  

  Fio. 
  1, 
  

  

  As 
  we 
  gradually 
  decrease 
  the 
  wave-length 
  by 
  turning 
  the 
  

   prisms 
  of 
  the 
  illuminator 
  we 
  shall 
  see 
  the 
  line 
  become 
  double 
  

   and 
  single 
  in 
  succession, 
  the 
  doubling 
  being 
  accomplished 
  by 
  

   the 
  march 
  of 
  the 
  fringes 
  to 
  the 
  right 
  and 
  their 
  fluctuating 
  

   intensity 
  in 
  the 
  manner 
  already 
  described. 
  It 
  is 
  a 
  little 
  

   difficult 
  to 
  put 
  into 
  words 
  the 
  changes 
  which 
  accompany 
  the 
  

   alteration 
  of 
  wave-length. 
  The 
  appearance 
  is 
  much 
  like 
  that 
  

   presented 
  by 
  a 
  picket 
  fence 
  in 
  motion 
  viewed 
  through 
  a 
  

   narrow 
  vertical 
  aperture, 
  i. 
  e. 
  the 
  pickets 
  are 
  only 
  visible 
  as 
  

   they 
  pass 
  across 
  a 
  narrow 
  vertical 
  region. 
  If 
  we 
  imagine 
  

   the 
  visibility 
  of 
  the 
  pickets 
  to 
  be 
  a 
  maximum 
  as 
  they 
  cross 
  

   the 
  centre 
  of 
  the 
  aperture 
  and 
  least 
  when 
  at 
  the 
  edges, 
  we 
  

   shall 
  have 
  a 
  fairly 
  accurate 
  conception 
  of 
  the 
  appearance 
  of 
  

   the 
  moving 
  maxima. 
  Suppose 
  that 
  a 
  change 
  of 
  100 
  Angstrom 
  

   units 
  in 
  the 
  wave-length 
  is 
  sufficient 
  to 
  change 
  the 
  single 
  

   line 
  to 
  the 
  double 
  line. 
  The 
  changes 
  which 
  take 
  place 
  in 
  

   the 
  focal 
  plane 
  of 
  the 
  telescope 
  (prism 
  removed) 
  as 
  we 
  

   decrease 
  the 
  wave-length 
  over 
  this 
  range 
  are 
  indicated 
  in 
  

   fig. 
  1 
  a. 
  To 
  avoid 
  confusion 
  I 
  have 
  represented 
  each 
  suc- 
  

   cessive 
  appearance 
  a 
  little 
  lower 
  down 
  in 
  the 
  figure. 
  

  

  Now 
  consider 
  the 
  prism 
  in 
  place, 
  turning 
  the 
  telescope 
  so 
  

   as 
  to 
  view 
  the 
  deviated 
  image. 
  As 
  we 
  decrease 
  the 
  wave- 
  

   length 
  there 
  will 
  be 
  a 
  shift 
  to 
  the 
  right 
  as 
  before 
  resulting 
  

   from 
  the 
  retardation 
  of 
  the 
  plate, 
  and 
  a 
  shift 
  to 
  the 
  left 
  due 
  

  

  Phil 
  May. 
  S. 
  6. 
  \^ol. 
  18. 
  No. 
  107. 
  Nov. 
  J.909. 
  3 
  E 
  

  

  