﻿Experimental 
  Study 
  of 
  Optical 
  Systems. 
  51 
  

  

  When 
  the 
  mirrors 
  are 
  adjusted, 
  interference-bands 
  are 
  

   seen 
  which 
  form 
  a 
  contour 
  map 
  to 
  a 
  scale 
  of 
  wave-lengths 
  o£ 
  

   the 
  extent 
  to 
  which 
  passage 
  of 
  the 
  beam 
  twice 
  through 
  the 
  

   prism 
  has 
  distorted 
  the 
  wave-front. 
  This 
  distortion 
  can 
  be 
  

  

  Fig. 
  2. 
  

  

  Mirror 
  in 
  Mount. 
  

  

  corrected 
  by 
  removing 
  from 
  each 
  point 
  of 
  one 
  prism 
  face, 
  by 
  

   local 
  polishing, 
  an 
  amount 
  of 
  glass 
  proportional 
  to 
  the 
  distortion 
  

   of 
  the 
  wave-front 
  at 
  that 
  point; 
  hence 
  it 
  follows 
  that 
  the 
  bands 
  

   also 
  form 
  a 
  contour 
  map 
  of 
  the 
  glass 
  requiring 
  to 
  be 
  removed 
  

   in 
  order 
  to 
  make 
  the 
  optical 
  performance 
  of 
  the 
  prism 
  perfect. 
  

  

  Fig. 
  3. 
  

  

  Fig. 
  3 
  represents 
  in 
  diagram 
  a 
  typical 
  map, 
  where 
  Q 
  

   represents 
  the 
  highest 
  point 
  of 
  a 
  " 
  hill." 
  The 
  procedure 
  in 
  

   such 
  a 
  case 
  is 
  to 
  mark 
  out 
  the 
  contour 
  lines 
  on 
  the 
  surface 
  

   of 
  the 
  prism 
  with 
  a 
  paint-brush 
  dipped 
  in 
  rouge, 
  and 
  then 
  to 
  

  

  E2 
  

  

  