﻿388 
  Dr. 
  H. 
  Stansfield 
  on 
  

  

  although 
  if 
  the 
  slit 
  had 
  been 
  opened 
  wide 
  so 
  as 
  to 
  show 
  

   portions 
  o£ 
  the 
  Fabry 
  and 
  Perot 
  rings, 
  they 
  would 
  have 
  been 
  

   inclined 
  about 
  45°, 
  as 
  in 
  photograph 
  No. 
  2. 
  

  

  When 
  the 
  echelon 
  is 
  reversed, 
  the 
  lateral 
  diffraction 
  takes 
  

   place 
  as 
  the 
  light 
  enters 
  the 
  echelon 
  and 
  the 
  lines 
  o£ 
  equal 
  

   retardation 
  in 
  the 
  Fabry 
  and 
  Perot 
  system 
  will 
  be 
  represented 
  

   by 
  circles 
  whose 
  centre 
  is 
  the 
  point 
  in 
  the 
  image 
  plane 
  corres- 
  

   ponding 
  to 
  the 
  direction 
  of 
  the 
  normal 
  to 
  the 
  plates. 
  

  

  The 
  four 
  long 
  inclined 
  lines 
  in 
  the 
  diagram 
  (fig. 
  6) 
  

   represent 
  spectra 
  in 
  the 
  primary 
  echelon 
  system 
  ; 
  they 
  are 
  

   inclined 
  to 
  the 
  vertical 
  because 
  the 
  echelon 
  is 
  tilted. 
  The 
  

   orders 
  of 
  a 
  wave-length 
  \i 
  are 
  represented 
  in 
  the 
  two 
  systems 
  

   by 
  full 
  lines, 
  the 
  dotted 
  lines 
  representing 
  similarly 
  the 
  

   spectra 
  of 
  a 
  second 
  wave-length 
  Xg 
  greater 
  than 
  Xj. 
  The 
  

   order 
  of 
  the 
  spectra 
  in 
  the 
  two 
  systems 
  is 
  indicated 
  at 
  the 
  

   ends 
  of 
  the 
  lines. 
  The 
  points 
  where 
  the 
  full 
  lines 
  of 
  the 
  two 
  

   systems 
  intersect 
  give 
  a 
  series 
  of 
  points 
  (marked 
  by 
  the 
  

   larger 
  circles 
  in 
  the 
  diagram 
  j 
  which 
  represent 
  Xi 
  in 
  the 
  joint 
  

   system, 
  each 
  point 
  being 
  defined 
  by 
  two 
  orders. 
  The 
  top 
  

   left-hand 
  point 
  in 
  fig. 
  6 
  may 
  be 
  described 
  for 
  example 
  as 
  the 
  

   np 
  order 
  of 
  wave-length 
  Xj. 
  In 
  the 
  same 
  way 
  the 
  intersections 
  

   of 
  the 
  dotted 
  lines 
  give 
  the 
  positions 
  where 
  X2 
  is 
  represented 
  in 
  

   the 
  joint 
  system, 
  and 
  the 
  shorter 
  inclined 
  lines 
  joining 
  the 
  Xj 
  

   and 
  X2 
  points 
  of 
  the 
  same 
  denomination 
  represent 
  the 
  appear- 
  

   ance 
  of 
  the 
  joint 
  spectra 
  corresponding 
  to 
  a 
  spectrum 
  

   continuous 
  between 
  these 
  limits. 
  

  

  It 
  will 
  be 
  seen 
  that 
  there 
  is 
  no 
  chance 
  of 
  spectra 
  of 
  

   different 
  denominations 
  overlapping 
  in 
  the 
  joint 
  system 
  as 
  

   long 
  as 
  there 
  is 
  no 
  overlapping 
  in 
  one 
  of 
  the 
  two 
  systems 
  

   which 
  are 
  combined. 
  

  

  The 
  spectra 
  in 
  the 
  two 
  systems 
  may 
  be 
  regarded 
  as 
  forming 
  

   an 
  oblique 
  system 
  of 
  coordinates 
  ; 
  the 
  echelon 
  system 
  giving 
  

   horizontal 
  dispersion 
  may 
  be 
  called 
  the 
  X 
  system 
  and 
  the 
  

   Fabry 
  and 
  Perot 
  system 
  the 
  Y 
  system. 
  Then 
  the 
  slope 
  of 
  

  

  the 
  spectra 
  in 
  the 
  joint 
  system, 
  ,-, 
  is 
  the 
  ratio 
  of 
  ^, 
  the 
  

  

  dispersion 
  in 
  the 
  X 
  system, 
  keeping 
  y 
  constant, 
  to 
  ^, 
  the 
  

  

  dispersion 
  in 
  the 
  Y 
  system, 
  keeping 
  x 
  constant. 
  

  

  One 
  important 
  feature 
  of 
  these 
  point 
  spectra 
  is 
  that 
  they 
  

   give 
  a 
  system 
  of 
  lines 
  whose 
  definition 
  depends 
  in 
  general 
  on 
  

   the 
  defining 
  power 
  of 
  the 
  two 
  systems 
  which 
  are 
  combined, 
  

   but 
  does 
  not 
  depend 
  oh 
  the 
  smallness 
  of 
  the 
  range 
  of 
  wave- 
  

   length 
  in 
  the 
  light 
  examined, 
  so 
  long 
  as 
  that 
  range 
  does 
  not 
  

   -exceed 
  a 
  certain 
  relatively 
  large 
  limit. 
  

  

  