﻿the 
  Echelon 
  Spectroscope. 
  383 
  

  

  and 
  in 
  this 
  case 
  ^ 
  = 
  5, 
  so 
  that 
  

  

  t-to 
  = 
  Me-^>^ 
  (8) 
  

  

  HeDce, 
  for 
  small 
  values 
  of 
  i, 
  the 
  spectrum 
  lines 
  are 
  parabolas. 
  

   The 
  curvature 
  is 
  very 
  small. 
  If, 
  for 
  example, 
  the 
  extreme 
  

   values 
  of 
  i 
  are 
  ten 
  times 
  the 
  angular 
  separation 
  of 
  the 
  orders, 
  

   A-^lr, 
  then 
  the 
  top 
  and 
  bottom 
  of 
  the 
  curved 
  image 
  of 
  the 
  slit, 
  

   representing 
  one 
  order 
  of 
  the 
  spectrum, 
  will 
  be 
  2 
  per 
  cent, 
  

   of 
  A-v/r 
  to 
  the 
  violet 
  side 
  of 
  the 
  centre 
  of 
  the 
  image. 
  

  

  Effects 
  produced 
  hy 
  Rotating 
  the 
  Echelon 
  about 
  a 
  Horizontal 
  

   Axis 
  parallel 
  to 
  the 
  Plates. 
  

  

  The 
  echelon 
  can 
  readily 
  be 
  tilted 
  in 
  this 
  way 
  by 
  placing 
  a 
  

   block 
  under 
  the 
  foot 
  at 
  the 
  small 
  end. 
  The 
  block 
  which 
  I 
  

   employ 
  tilts 
  the 
  echelon 
  through 
  an 
  angle 
  of 
  nearly 
  3°, 
  and 
  

   the 
  photographs 
  Nos. 
  1 
  to 
  3 
  in 
  Plate 
  XI. 
  of 
  the 
  green 
  mercury 
  

   line 
  were 
  taken 
  with 
  this 
  angle 
  of 
  tilt. 
  The 
  horizontal 
  axis 
  

   of 
  the 
  parabolic 
  lines, 
  which 
  passes 
  through 
  the 
  normal 
  to 
  

   the 
  plates, 
  has 
  been 
  raised 
  much 
  above 
  the 
  field 
  of 
  view 
  of 
  

   these 
  photographs, 
  so 
  that 
  the 
  lines 
  where 
  they 
  cross 
  the 
  field 
  

   of 
  view 
  are 
  considerably 
  inclined 
  to 
  the 
  vertical. 
  The 
  

   reproductions 
  in 
  Plate 
  XI. 
  have 
  the 
  centre 
  of 
  the 
  curves 
  below 
  

   them, 
  as 
  thsy 
  have 
  been 
  turned 
  round 
  in 
  order 
  to 
  put 
  the 
  

   shorter 
  wave-leno^ths 
  on 
  the 
  left. 
  

  

  Part 
  II. 
  — 
  Secondary 
  Action 
  of 
  the 
  Echelon. 
  

  

  Secondary 
  Bands 
  in 
  the 
  Primary 
  Spectrum, 
  

  

  The 
  inclined 
  spectrum 
  lines 
  of 
  photograph 
  No. 
  1, 
  Plate 
  XI. 
  

   are 
  broken 
  up 
  and 
  have 
  a 
  ropy 
  or 
  screw-like 
  appearance 
  

   because 
  they 
  are 
  crossed 
  by 
  a 
  secondary 
  system 
  of 
  bands. 
  

   This 
  screw-like 
  structure 
  of 
  echelon 
  spectrum 
  lines 
  was 
  

   observed 
  by 
  Gehrcke 
  *. 
  He 
  does 
  not 
  explain 
  how 
  the 
  new 
  

   bands 
  are 
  produced, 
  but 
  shows 
  that 
  the 
  appearance 
  can 
  be 
  

   imitated 
  by 
  tilting 
  an 
  echelon 
  with 
  only 
  two 
  apertures, 
  so 
  

   that 
  the 
  echelon 
  bands 
  slope 
  across 
  the 
  central 
  vertical 
  

   diffraction 
  band. 
  

  

  When 
  the 
  echelon 
  is 
  in 
  the 
  ordinary 
  position 
  the 
  secondary 
  

   bands 
  are 
  parallel 
  to 
  the 
  spectrum 
  lines, 
  and 
  so 
  their 
  effects, 
  

   though 
  very 
  important, 
  are 
  not 
  so 
  easily 
  recognized 
  as 
  they 
  

   are 
  in 
  this 
  photograph 
  taken 
  with 
  the 
  echelon 
  tilted. 
  

  

  * 
  Aimalen 
  der 
  Pht/sik, 
  xviii. 
  p. 
  1074 
  (1905). 
  

  

  