﻿56 
  Mr. 
  F. 
  Twyman 
  on 
  Interferometers 
  for 
  

  

  o£ 
  large 
  astronomical 
  lenses*. 
  Although 
  the 
  necessity 
  for 
  

   this 
  device 
  has 
  not 
  generally 
  been 
  referred 
  by 
  them 
  to 
  the 
  want 
  

   of 
  homogeneity 
  of 
  the 
  glass, 
  yet 
  the 
  almost 
  invariable 
  pre- 
  

   sence 
  of 
  such 
  heterogeneity 
  would 
  render 
  retouching 
  essential 
  

   even 
  were 
  there 
  no 
  other 
  causes 
  of 
  optical 
  imperfection. 
  

   The 
  method 
  of 
  carrying 
  out 
  the 
  process 
  with 
  the 
  aid 
  of 
  this 
  

   apparatus 
  has 
  been 
  sufficiently 
  described 
  above. 
  It 
  has 
  been 
  

   found 
  both 
  speedy 
  and 
  effective 
  in 
  the 
  manufacture 
  of 
  prisms 
  

   and 
  lenses 
  where 
  the 
  highest 
  accuracy 
  is 
  desired. 
  

  

  Many 
  other 
  methods 
  of 
  testing 
  telescope 
  or 
  camera 
  objec- 
  

   tives 
  have 
  been 
  devised 
  with 
  a 
  view 
  to 
  the 
  control 
  of 
  retouching. 
  

   With 
  each 
  of 
  the 
  well-known 
  methods 
  results 
  have 
  been 
  

   attained 
  of 
  an 
  excellence 
  commensurate 
  with 
  the 
  reputation 
  

   for 
  optical 
  work 
  of 
  high 
  quality 
  which 
  has 
  distinguished 
  the 
  

   individual 
  exponents; 
  but 
  none 
  of 
  the 
  methods 
  appear 
  to 
  give 
  

   indications 
  upon 
  which 
  the 
  optician 
  could 
  take 
  action 
  without 
  

   a 
  more 
  or 
  less 
  complex 
  process 
  of 
  reasoning. 
  

  

  The 
  apparatus 
  here 
  described, 
  on 
  the 
  other 
  hand, 
  produces, 
  

   as 
  has 
  been 
  shown 
  above, 
  a 
  system 
  of 
  interference-bands 
  

   which 
  may 
  be 
  regarded 
  as 
  a 
  " 
  contour 
  map 
  " 
  of 
  the 
  imper- 
  

   fections. 
  This 
  contour 
  map 
  can 
  for 
  practical 
  purposes 
  be 
  

   considered 
  as 
  located 
  at 
  any 
  of 
  the 
  optical 
  surfaces 
  involved 
  ; 
  

   and 
  in 
  the 
  case 
  of 
  the 
  control 
  of 
  retouching, 
  the 
  observer 
  

   may, 
  if 
  he 
  likes, 
  draw 
  this 
  map 
  upon 
  the 
  surface 
  under 
  

   treatmeat. 
  He 
  is 
  then 
  in 
  a 
  position 
  without 
  further 
  preli- 
  

   minary 
  to 
  remove 
  the 
  superfluous 
  material 
  from 
  the 
  pro- 
  

   minences 
  by 
  polishing 
  with 
  pads 
  of 
  suitable 
  size 
  and 
  shape, 
  

   the 
  " 
  contour 
  map 
  " 
  giving 
  all 
  that 
  is 
  necessary 
  for 
  him 
  to 
  

   know 
  both 
  as 
  to 
  the 
  location 
  and 
  the 
  magnitude 
  of 
  the 
  

   sources 
  of 
  the 
  imperfections. 
  

  

  (b) 
  1 
  he 
  testing 
  of 
  lens 
  systems. 
  

  

  The 
  apparatus 
  obviously 
  affords, 
  when 
  applied 
  to 
  any 
  

   optical 
  system, 
  all 
  the 
  data 
  necessary 
  for 
  a 
  complete 
  and 
  

   precise 
  statement 
  of 
  the 
  degree 
  of 
  optical 
  perfection 
  of 
  that 
  

   system. 
  As 
  regards 
  small 
  aberrations 
  it 
  is 
  sufficient 
  to 
  quote 
  

   the 
  well-known 
  and 
  valuable 
  generalization 
  : 
  — 
  

  

  "An 
  obvious 
  inference 
  from 
  the 
  necessary 
  imperfection 
  of 
  

   optical 
  images 
  is 
  the 
  uselessness 
  of 
  attempting 
  anything 
  like 
  

  

  * 
  The 
  opinions 
  of 
  Schroeder, 
  Grubb, 
  Czapski, 
  and 
  Alvan 
  Clark 
  on 
  

   this 
  subject 
  are 
  cited 
  in 
  a 
  summary 
  by 
  H. 
  Fassbender 
  of 
  the 
  then 
  known 
  

   methods 
  of 
  testing 
  object-glasses: 
  "Altere 
  und 
  neuere 
  Methoden 
  zur 
  

   Pruning 
  von 
  Objektiven," 
  Deutsche 
  Mechaniker 
  Zeitung, 
  July 
  1913, 
  

   pp. 
  133-138 
  & 
  149-155. 
  This 
  report 
  should 
  be 
  read 
  by 
  all 
  interested 
  

   in 
  the 
  subject. 
  It 
  omits, 
  however, 
  an 
  ingenious 
  method 
  due 
  to 
  

   Dr. 
  Chalmers, 
  see 
  Proc. 
  of 
  the 
  Optical 
  Convention, 
  vol. 
  ii. 
  p. 
  56 
  (1912). 
  

  

  