﻿58 
  Interferometers 
  for 
  Study 
  of 
  Optical 
  Systems. 
  

  

  accuracy 
  suitable 
  for 
  the 
  purpose 
  in 
  the 
  case 
  of 
  a 
  few 
  ex- 
  

   ceptional 
  types 
  of 
  lenses, 
  for 
  instance 
  the 
  special 
  spectacle 
  

   lenses 
  designed 
  by 
  Von 
  Rohr 
  for 
  those 
  whose 
  eyes 
  have 
  

   been 
  operated 
  on 
  for 
  cataract* 
  ; 
  and 
  with 
  considerable 
  

   accuracy 
  (though 
  with 
  a 
  smaller 
  departure 
  from 
  true 
  sphe- 
  

   ricity) 
  in 
  the 
  " 
  figured 
  " 
  lenses 
  or 
  mirrors 
  for 
  large 
  telescopes 
  

   and 
  for 
  other 
  special 
  purposes. 
  

  

  Now, 
  experience 
  has 
  shown 
  that 
  with 
  the 
  aid 
  of 
  this 
  

   apparatus, 
  and 
  even 
  with 
  the 
  ordinary 
  means 
  of 
  retouching, 
  

   the 
  optician 
  can 
  face 
  without 
  dismay 
  the 
  task 
  of 
  making 
  with 
  

   precision 
  quite 
  considerable 
  departures 
  from 
  the 
  sphericity 
  

   of 
  his 
  surfaces. 
  It 
  appears 
  to 
  the 
  writer 
  certain 
  that 
  by 
  

   modifying 
  from 
  true 
  sphericity 
  the 
  surfaces 
  of 
  systems 
  which 
  

   have 
  been 
  suitably 
  computed 
  for 
  the 
  purpose, 
  results 
  will 
  be 
  

   attained 
  more 
  perfect 
  than 
  has 
  hitherto 
  been 
  possible 
  in, 
  for 
  

   instance, 
  camera 
  lenses. 
  It 
  is 
  even 
  likely 
  that 
  valuable 
  

   results 
  may 
  be 
  attained 
  at 
  some 
  future 
  time 
  by 
  using 
  

   apparatus 
  of 
  this 
  kind 
  to 
  assist 
  the 
  correction 
  to 
  a 
  pitch 
  of 
  

   high 
  accuracy 
  of 
  lens 
  systems 
  wherein 
  definitely 
  aspherical 
  

   surfaces 
  have 
  been 
  generated 
  by 
  processes 
  essentially 
  different 
  

   from 
  those 
  ordinarily 
  used 
  by 
  opticians. 
  

  

  (d) 
  The 
  experimental 
  study 
  of 
  lens 
  systems. 
  

  

  There 
  are 
  probably 
  few 
  interested 
  in 
  optical 
  systems 
  who 
  

   have 
  not 
  felt 
  disappointment 
  at 
  the 
  comparative 
  lack 
  of 
  

   success 
  which 
  has 
  attended 
  attempts 
  to 
  deal 
  with 
  the 
  passage 
  

   of 
  light 
  through 
  such 
  systems 
  by 
  means 
  of 
  the 
  Wave 
  Theory. 
  

   The 
  difficulties 
  are 
  mathematical, 
  and 
  not 
  of 
  course 
  to 
  be 
  

   directly 
  relieved 
  by 
  such 
  instrumental 
  aids 
  as 
  those 
  now 
  

   described. 
  

  

  But 
  it 
  is 
  perhaps 
  not 
  too 
  much 
  to 
  hope 
  that 
  the 
  existence 
  

   of 
  means 
  of 
  direct 
  demonstration 
  of 
  the 
  effect 
  on 
  wave 
  

   surfaces 
  of 
  passage 
  through 
  optical 
  systems 
  may 
  attract 
  the 
  

   attention 
  of 
  mathematicians 
  to 
  this 
  aspect 
  of 
  the 
  subject 
  of 
  

   dioptrics, 
  with 
  good 
  result. 
  

  

  Research 
  Department, 
  

   Adam 
  Hilgfer 
  Ltd. 
  

  

  See 
  Proc. 
  of 
  the 
  Optical 
  Convention, 
  1912, 
  p. 
  118. 
  

  

  