﻿390 
  Dr. 
  H. 
  Stansfield 
  on 
  

  

  system. 
  It 
  should 
  be 
  several 
  hundred 
  times 
  as 
  great 
  even 
  

   for 
  those 
  parts 
  of 
  the 
  diagram 
  farthest 
  from 
  the 
  centre. 
  

  

  It 
  will 
  be 
  seen 
  that 
  in 
  both 
  diagrams 
  the 
  secondary 
  point 
  

   spectra, 
  drawn 
  through 
  the 
  points 
  of 
  intersection 
  of 
  lines 
  

   representing 
  the 
  same 
  wave-length 
  in 
  the 
  two 
  systems 
  which 
  

   are 
  combined, 
  are 
  curved 
  so 
  as 
  to 
  be 
  concave 
  towards 
  the 
  side 
  

   of 
  shorter 
  wave-length 
  in 
  the 
  primary 
  echelon 
  system 
  ; 
  the 
  

   curvature 
  has 
  been 
  much 
  exaggerated 
  by 
  taking 
  the 
  ratio 
  of 
  

   the 
  dispersions 
  so 
  much 
  smaller 
  than 
  it 
  actually 
  is. 
  

  

  Origin 
  of 
  the 
  Secondary 
  Bands. 
  

  

  There 
  is 
  no 
  doubt 
  that 
  the 
  secondary 
  bands 
  which 
  are 
  

   observed 
  when 
  the 
  echelon 
  is 
  employed 
  in 
  the 
  usual 
  way, 
  

   that 
  is, 
  with 
  the 
  secondary 
  light 
  superposed 
  on 
  the 
  primary, 
  

   are 
  very 
  closely 
  connected 
  with 
  the 
  point 
  spectra 
  produced 
  

   by 
  the 
  secondary 
  light. 
  All 
  the 
  characteristics 
  of 
  the 
  secon- 
  

   dary 
  bands 
  described 
  on 
  page 
  384 
  may 
  be 
  explained 
  by 
  

   supposing 
  that 
  they 
  are, 
  like 
  the 
  point 
  spectra 
  of 
  the 
  secondary 
  

   light, 
  the 
  loci 
  of 
  the 
  points 
  of 
  intersection 
  of 
  lines 
  repre- 
  

   senting 
  the 
  same 
  wave-length 
  in 
  the 
  primary 
  echelon 
  and 
  

   Pabry 
  and 
  Perot 
  spectra. 
  

  

  Another 
  characteristic 
  which 
  may 
  be 
  explained 
  in 
  the 
  

   .-same 
  way, 
  with 
  the 
  help 
  of 
  fig. 
  7, 
  is 
  that 
  when 
  the 
  echelon 
  

   is 
  in 
  the 
  direct 
  normal 
  position 
  the 
  secondary 
  bands 
  affect 
  

   ^each 
  order 
  of 
  the 
  spectrum 
  in 
  the 
  same 
  way, 
  while, 
  when 
  

   the 
  echelon 
  is 
  reversed, 
  there 
  are 
  marked 
  differences 
  between 
  

   the 
  different 
  orders. 
  

  

  The 
  secondary 
  bands 
  have 
  other 
  characteristics 
  which 
  may 
  

   find 
  their 
  explanation 
  in 
  interference 
  taking 
  place 
  between 
  

   the 
  secondary 
  light 
  and 
  the 
  primary. 
  The 
  dark 
  secondary 
  

   bands 
  appear 
  to 
  cut 
  through 
  the 
  bright 
  primary 
  spectra. 
  

   This 
  is 
  shown 
  to 
  some 
  extent 
  in 
  photograph 
  No. 
  1, 
  Plate 
  XI., 
  

   but 
  it 
  is 
  much 
  more 
  marked 
  when 
  the 
  secondary 
  light 
  is 
  

   made 
  relatively 
  stronger 
  by 
  covering 
  half 
  the 
  echelon 
  aper- 
  

   tures 
  so 
  as 
  to 
  stop 
  all 
  the 
  light 
  which 
  has 
  passed 
  through 
  

   less 
  than 
  half 
  the 
  plates 
  in 
  the 
  echelon. 
  The 
  brightness 
  of 
  

   the 
  primary 
  lines 
  is 
  no 
  doubt 
  increased 
  above 
  and 
  below 
  the 
  

   dark 
  bands, 
  making 
  them 
  appear 
  darker 
  by 
  contrast, 
  but 
  I 
  

   think 
  there 
  is 
  little 
  doubt 
  that 
  there 
  has 
  been 
  an 
  actual 
  

   reduction 
  in 
  the 
  brightness 
  of 
  the 
  primary 
  light 
  in 
  the 
  dark 
  

   secondary 
  bands. 
  

  

  Another 
  feature 
  of 
  the 
  secondary 
  bands, 
  which 
  may 
  be 
  

   seen 
  in 
  photograph 
  No. 
  1, 
  Plate 
  XI., 
  where 
  they 
  cross 
  the 
  

   line 
  1, 
  is 
  that 
  they 
  always 
  appear 
  in 
  pairs, 
  fainter 
  and 
  stronger 
  

   bands 
  alternating 
  with 
  one 
  another. 
  This 
  may 
  be 
  connected 
  

  

  