﻿Interferometers 
  for 
  Study 
  of 
  Optical 
  Systems. 
  49 
  

  

  The 
  y8-curve 
  even 
  for 
  o-=10\ 
  will, 
  therefore, 
  deviate 
  but 
  

   very 
  little 
  from 
  the 
  central 
  or 
  Cornu-spiral. 
  In 
  other 
  words, 
  

   the 
  axial 
  intensity-gradient, 
  for 
  appreciable 
  v, 
  will 
  be 
  very 
  

   small 
  (as 
  compared 
  with 
  the 
  transversal 
  one, 
  dl/dp). 
  Both 
  

   curves 
  will 
  start 
  from 
  the 
  origin 
  horizontally. 
  The 
  Cornu 
  

   spiral 
  is 
  initially 
  flat 
  while 
  the 
  j8-curve 
  has 
  the 
  initial 
  

   curvature 
  f3 
  = 
  27ra/\, 
  and, 
  if 
  o-<0, 
  an 
  inflexion 
  point 
  for 
  

  

  v— 
  ' 
  , 
  i. 
  e., 
  even 
  for 
  \<r\ 
  = 
  50X, 
  very 
  near 
  the 
  origin. 
  

  

  With 
  the 
  exception 
  of 
  such 
  details 
  the 
  /3-curves 
  deviate 
  

   but 
  insignificantly 
  from 
  the 
  focal 
  Cornu 
  spiral, 
  and 
  it 
  

   would 
  therefore 
  be 
  hardly 
  worth 
  the 
  trouble 
  to 
  draw 
  them 
  

   accurately. 
  

  

  London, 
  October 
  1917, 
  

   Research 
  Dept., 
  Adam 
  Hilger, 
  Ltd. 
  

  

  VI. 
  Interferometers 
  for 
  the 
  Experimental 
  Study 
  of 
  Optical 
  

   Systems 
  from 
  the 
  point 
  of 
  view 
  of 
  the 
  Wave 
  Theory. 
  By 
  

   F. 
  TWYMAN*. 
  

  

  Contents. 
  

  

  1. 
  Description 
  of 
  the 
  Interferometers. 
  

  

  2. 
  Various 
  Uses. 
  

  

  1. 
  Description 
  of 
  the 
  Interferometers. 
  

  

  T11HESE 
  instruments 
  in 
  their 
  simplest 
  form 
  resemble 
  the 
  

  

  A 
  well-known 
  Michelson 
  interferometer, 
  the 
  main 
  essential 
  

  

  optical 
  difference 
  being 
  that 
  the 
  two 
  interfering 
  beams 
  of 
  

  

  light 
  are 
  brought 
  to 
  a 
  focus 
  at 
  the 
  eye 
  of 
  the 
  observer. 
  

  

  Optical 
  elements 
  or 
  combinations 
  suitable 
  for 
  examination 
  

   by 
  means 
  of 
  these 
  instruments 
  may 
  almost 
  all 
  be 
  classed 
  in 
  

   two 
  categories. 
  Into 
  the 
  one 
  category 
  fall 
  those 
  combinations 
  

   which 
  are 
  required 
  to 
  receive 
  a 
  beam 
  of 
  light 
  which 
  has 
  a 
  

   plane 
  wave-front 
  and 
  deliver 
  it 
  again 
  after 
  transmission 
  with 
  

   a 
  plane 
  wave-front 
  ; 
  and 
  into 
  the 
  other 
  fall 
  those 
  the 
  object 
  

   of 
  which 
  is 
  to 
  impart 
  spherical 
  wave-fronts 
  to 
  beams 
  which 
  

   are 
  incident 
  on 
  them 
  with 
  plane 
  wave-fronts. 
  The 
  two 
  corre- 
  

   sponding 
  arrangements 
  will 
  be 
  referred 
  to 
  as 
  the 
  prism 
  inter- 
  

   ferometer 
  and 
  the 
  lens 
  interferometer 
  respectively. 
  

  

  The 
  Prism 
  Interferometer. 
  

  

  The 
  prism 
  interferometer 
  is 
  shown 
  in 
  diagram 
  (fig. 
  1) 
  as 
  

   arranged 
  for 
  the 
  correction 
  of 
  a 
  60° 
  prism, 
  such 
  as 
  is 
  used 
  for 
  

   spectroscopy. 
  

  

  The 
  light 
  used 
  must 
  consist 
  of 
  monochromatic 
  rays. 
  Such 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

   Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  35. 
  No. 
  205. 
  Jan. 
  1918. 
  E 
  

  

  