﻿372 
  Dr. 
  H. 
  Stansfield 
  on 
  

  

  plan, 
  and 
  two 
  end 
  elevations 
  o£ 
  the 
  echelon 
  are 
  reproduced 
  

   on 
  Plate 
  X., 
  the 
  plan 
  being 
  drawn 
  with 
  the 
  cover 
  removed. 
  

  

  There 
  are 
  33 
  glass 
  plates. 
  The 
  smallest 
  plate 
  is 
  13 
  mm. 
  

   wide, 
  and 
  they 
  increase 
  in 
  width 
  by 
  steps 
  o£ 
  1 
  mm. 
  until 
  the 
  

   last 
  plate 
  is 
  reached, 
  which 
  is 
  12 
  mm. 
  wider 
  than 
  the 
  last 
  

   but 
  one, 
  the 
  aperture 
  being 
  reduced 
  to 
  1 
  mm. 
  by 
  the 
  screen 
  S. 
  

   The 
  effective 
  aperture 
  o£ 
  the 
  first 
  plate 
  is 
  similarly 
  reduced 
  in 
  

   width 
  to 
  1 
  mm. 
  by 
  the 
  block 
  B. 
  All 
  the 
  plates 
  are 
  40 
  mm. 
  

   high, 
  and 
  the 
  common 
  thickness 
  is 
  9'4:8 
  mm. 
  The 
  plates 
  

   are 
  pressed 
  together 
  by 
  the 
  two 
  nickel 
  steel 
  rods 
  marked 
  T, 
  

   which 
  are 
  intended 
  to 
  have 
  the 
  same 
  coefficient 
  o£ 
  expansion 
  

   as 
  the 
  glass. 
  The 
  plates 
  are 
  in 
  very 
  close 
  contact, 
  in 
  most 
  

   cases 
  the 
  greater 
  part 
  o£ 
  an 
  inter£ace 
  being 
  taken 
  up 
  by 
  a 
  

   patch 
  o£ 
  definite 
  but 
  irregular 
  outline 
  that 
  appears 
  " 
  black 
  '^ 
  

   by 
  reflected 
  light. 
  The 
  remaining 
  part 
  of 
  the 
  interface 
  

   generally 
  shows 
  white 
  o£ 
  the 
  first 
  order, 
  but 
  here 
  and 
  there 
  

   the 
  film 
  o£ 
  air 
  may 
  be 
  thick 
  enough 
  to 
  show 
  the 
  yellow 
  or 
  

   even 
  the 
  red 
  o£ 
  the 
  first 
  order. 
  

  

  The 
  common 
  thickness 
  o£ 
  the 
  glass 
  plates 
  is 
  9*48 
  mm. 
  ; 
  

   their 
  refractive 
  index, 
  deduced 
  by 
  a 
  Hartmann 
  formula 
  from 
  

   the 
  values 
  given 
  by 
  the 
  makers 
  of 
  the 
  glass, 
  is 
  1*5802 
  for 
  the 
  

  

  green 
  line 
  5461 
  ; 
  and 
  the 
  dispersive 
  power, 
  -rr^ 
  is 
  —918 
  

   per 
  cm. 
  for 
  this 
  wave-length. 
  

  

  Optical 
  Effects 
  due 
  to 
  clamping 
  the 
  Plates, 
  

  

  The 
  echelon 
  produces 
  a 
  slight 
  cylindrical 
  convergence 
  in 
  a 
  

   beam 
  of 
  light, 
  altering 
  the 
  focus 
  of 
  the 
  observing 
  telescope 
  

   by 
  1 
  mm. 
  (the 
  focal 
  length 
  of 
  the 
  object-glass 
  being 
  53 
  cms.). 
  

   This 
  effect 
  is 
  probably 
  due 
  to 
  the 
  clamping, 
  as 
  Twyman 
  * 
  

   states 
  that 
  when 
  the 
  clamping 
  pressure 
  is 
  applied 
  a 
  change 
  

   of 
  focus 
  is 
  produced. 
  He 
  attributes 
  the 
  effect 
  to 
  a 
  uniform 
  

   increase 
  in 
  the 
  compression 
  of 
  the 
  plates 
  from 
  the 
  largest 
  to 
  

   the 
  smallest 
  : 
  but 
  there 
  is 
  direct 
  evidence 
  (see 
  p. 
  378) 
  that 
  

   the 
  echelon 
  plates 
  become 
  slightly 
  prismatic, 
  and 
  this 
  effect, 
  

   increasing 
  towards 
  the 
  smaller 
  end, 
  would 
  also 
  produce 
  

   convergence. 
  

  

  The 
  focus 
  is 
  altered 
  by 
  rotating 
  the 
  echelon 
  about 
  a 
  vertical 
  

   axis. 
  The 
  convergence 
  is 
  increased, 
  as 
  would 
  be 
  expected, 
  

   by 
  turning 
  the 
  echelon 
  so 
  as 
  to 
  reduce 
  the 
  width 
  of 
  the 
  

   emergent 
  beam 
  (see 
  fig. 
  3 
  A), 
  and 
  diminished 
  when 
  it 
  is 
  

   turned 
  so 
  as 
  to 
  make 
  the 
  beam 
  broader 
  (fig. 
  3 
  B). 
  Changes 
  

   in 
  the 
  focus 
  are 
  also 
  produced 
  by 
  covering 
  some 
  of 
  the 
  step- 
  

   faces 
  at 
  either 
  end 
  of 
  the 
  echelon. 
  

  

  * 
  Twyman, 
  Proceedings 
  of 
  the 
  Optical 
  Convention, 
  1905, 
  p. 
  53. 
  

  

  