﻿52 
  

  

  Mr. 
  F. 
  Twyman 
  on 
  Interferometers 
  for 
  

  

  polish 
  first 
  on 
  the 
  region 
  Q, 
  subsequently 
  extending 
  the 
  area 
  

   of 
  polishing 
  first 
  partly, 
  then 
  wholly, 
  to 
  the 
  next 
  contour 
  line 
  ; 
  

   and 
  so 
  on. 
  The 
  marking 
  out 
  of 
  the 
  prism 
  surface 
  can 
  be 
  

   done 
  while 
  observing 
  the 
  bands. 
  

  

  Fig. 
  4. 
  

  

  ZZL 
  

  

  2! 
  

  

  S 
  

  

  s: 
  

  

  It 
  should 
  be 
  noted 
  that 
  variations 
  in 
  the 
  contour 
  lines 
  are 
  

   obtained 
  by 
  a 
  tilt 
  o£ 
  the 
  plane 
  of 
  reference. 
  Thus 
  a 
  slight 
  

   adjustment 
  of 
  mirror 
  F 
  (fig. 
  1) 
  might 
  change 
  a 
  contour 
  map 
  

  

  