﻿Experimental 
  Study 
  of 
  Optical 
  Systems. 
  55 
  

  

  by 
  the 
  lens 
  and 
  mirror 
  mount 
  shown 
  in 
  fig. 
  9. 
  T 
  represents 
  

   the 
  lens 
  under 
  test, 
  U 
  a 
  convex 
  mirror 
  in 
  such 
  a 
  position 
  

   that 
  it 
  reflects 
  back 
  along 
  their 
  own 
  paths 
  the 
  rays 
  received 
  

   from 
  T. 
  The 
  mirror 
  U 
  can 
  be 
  moved 
  by 
  a 
  screw 
  motion 
  

  

  Fig. 
  9. 
  

  

  Lens 
  and 
  Mirror 
  Mount. 
  

  

  actuated 
  by 
  the 
  handle 
  V, 
  so 
  that 
  its 
  distance 
  from 
  T 
  can 
  be 
  

   varied 
  at 
  will. 
  It 
  will 
  be 
  seen 
  that 
  when 
  the 
  adjustment 
  of 
  

   this 
  part 
  o£ 
  the 
  apparatus 
  is 
  correct, 
  the 
  whole 
  lens 
  addition 
  

   will, 
  if 
  the 
  lens 
  T 
  be 
  perf 
  ect, 
  receive 
  the 
  beam 
  of 
  plane 
  wave- 
  

   front 
  and 
  deliver 
  it 
  back 
  again 
  with 
  a 
  plane 
  wave-front. 
  

   If 
  it 
  does 
  not 
  do 
  so 
  the 
  departures 
  from 
  planeness 
  of 
  the 
  so 
  

   delivered 
  wave-front 
  will 
  form 
  a 
  contour 
  map 
  of 
  the 
  cor- 
  

   rections 
  which 
  have 
  to 
  be 
  applied 
  to 
  the 
  lens 
  in 
  order 
  to 
  

   make 
  its 
  performance, 
  when 
  in 
  actual 
  use, 
  perfect. 
  

  

  The 
  general 
  procedure 
  is 
  the 
  same 
  as 
  in 
  the 
  case 
  of 
  prisms, 
  

   namely, 
  to 
  choose 
  such 
  an 
  adjustment 
  as 
  presents 
  a 
  favourable 
  

   aspect 
  for 
  working 
  and 
  to 
  polish 
  off 
  those 
  portions 
  of 
  the 
  

   surface 
  corresponding 
  with 
  the 
  parts 
  of 
  the 
  contour 
  map 
  

   which 
  represent 
  hills. 
  

  

  2. 
  Various 
  Uses 
  of 
  the 
  Interferometers. 
  

   (a) 
  Control 
  of 
  " 
  retouching." 
  

  

  The 
  instruments 
  were 
  primarily 
  designed 
  for 
  the 
  correction 
  

   by 
  retouching 
  of 
  object-glasses 
  and 
  prisms. 
  

  

  The 
  process 
  of 
  retouching 
  (viz. 
  local 
  polishing 
  away 
  of 
  the 
  

   glass) 
  appears 
  to 
  have 
  beenadoptedbyall 
  the 
  great 
  constructors 
  

  

  