﻿384 
  

  

  Dr. 
  H. 
  Stansfield 
  

  

  on 
  

  

  Character 
  of 
  the 
  Secondary 
  Bands, 
  

  

  The 
  secondary 
  bands 
  are 
  superposed 
  on 
  the 
  echelon 
  lines 
  

   and 
  resemble 
  them 
  in 
  appearance; 
  they 
  are 
  also 
  affected 
  in 
  the 
  

   same 
  way, 
  but 
  to 
  a 
  greater 
  extent, 
  by 
  adjustments 
  o£ 
  the 
  

   echelon. 
  When 
  the 
  echelon 
  is 
  rotated, 
  for 
  example, 
  the 
  

   secondary 
  bands 
  move 
  faster 
  than 
  the 
  spectrum 
  lines 
  in 
  

   the 
  same 
  direction 
  and 
  so 
  move 
  across 
  them. 
  When 
  the 
  

   echelon 
  plates 
  are 
  vertical, 
  the 
  secondary 
  bands, 
  like 
  

   the 
  echelon 
  lines, 
  are 
  vertical 
  in 
  the 
  centre 
  o£ 
  the 
  field 
  ; 
  they 
  

   are 
  also 
  curved 
  in 
  the 
  same 
  direction 
  but 
  more 
  strongly. 
  

   Their 
  width 
  is 
  about 
  the 
  same 
  as 
  that 
  o£ 
  the 
  finer 
  spectrum 
  

   lines, 
  and 
  so, 
  in 
  the 
  ordinary 
  position 
  of 
  the 
  echelon, 
  they 
  

   are 
  not 
  easily 
  recognized 
  ; 
  but 
  when 
  the 
  echelon 
  is 
  tilted 
  they 
  

   become 
  more 
  inclined 
  than 
  the 
  spectrum 
  lines 
  in 
  the 
  same 
  

   direction 
  and 
  can 
  then 
  be 
  seen 
  as 
  in 
  photograph 
  No. 
  1, 
  

   Plate 
  XI., 
  intersecting 
  all 
  the 
  spectrum 
  lines. 
  

  

  The 
  behaviour 
  of 
  the 
  secondary 
  bands 
  suggested 
  the 
  idea 
  

   that 
  they 
  might 
  be 
  spectrum 
  lines 
  of 
  a 
  higher 
  order, 
  such 
  

   as 
  might 
  be 
  produced 
  by 
  the 
  reflexion 
  of 
  light 
  in 
  the 
  

   echelon. 
  

  

  Fabry 
  and 
  Perot 
  Spectrum 
  produced 
  hy 
  the 
  Secondary 
  Action. 
  

  

  The 
  secondary 
  light, 
  which 
  has 
  been 
  twice 
  reflected 
  in 
  the 
  

   echelon, 
  is 
  by 
  no 
  means 
  negligible. 
  The 
  echelon 
  was 
  tilted 
  

   as 
  shown 
  in 
  fig. 
  5, 
  screens 
  SS 
  being 
  arranged 
  to 
  cut 
  out 
  the 
  

  

  Fiff. 
  5. 
  

  

  primary 
  light, 
  and 
  it 
  was 
  found 
  that 
  the 
  echelon 
  was 
  bright 
  

   with 
  secondary 
  light 
  coming 
  out 
  below 
  the 
  second 
  screen, 
  

   the 
  brightness 
  extending 
  down 
  to 
  the 
  bottom 
  of 
  some 
  of 
  the 
  

   step 
  apertures. 
  

  

  Suppose 
  that 
  each 
  interface 
  reflects 
  a 
  very 
  small 
  proportion 
  

   of 
  the 
  light 
  falling 
  upon 
  it 
  and 
  leave 
  out 
  of 
  account 
  for 
  a 
  

   moment 
  the 
  step 
  structure 
  of 
  the 
  echelon. 
  There 
  will 
  be 
  a 
  

   secondary 
  beam 
  produced 
  by 
  the 
  combination 
  of 
  the 
  n 
  faint 
  

   secondary 
  beams 
  which 
  have 
  gone 
  back 
  through 
  one 
  plate, 
  n 
  

   being 
  the 
  number 
  of 
  plates 
  coming 
  into 
  action 
  ; 
  let 
  the 
  

  

  