﻿Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  483 
  

  

  [of 
  the 
  spherical 
  wave] 
  is 
  very 
  long 
  in 
  comparison 
  with 
  X,, 
  each 
  

   small 
  part 
  of 
  a 
  wave-surface 
  propagates 
  itself 
  just 
  as 
  an 
  in- 
  

   finite 
  plane 
  wave 
  coincident 
  with 
  the 
  tangent-plane." 
  Bassett 
  

   puts 
  it 
  similarly* 
  — 
  " 
  Spherical 
  waves 
  concentric 
  with 
  the 
  

   source 
  are 
  propagated 
  throughout 
  the 
  medium 
  ; 
  and 
  if 
  the 
  

   effect 
  which 
  these 
  waves 
  produce 
  at 
  some 
  portion 
  of 
  space 
  

   whose 
  greatest 
  linear 
  dimension 
  is 
  small 
  in 
  comparison 
  ivith 
  

   its 
  distance 
  from 
  the 
  source, 
  be 
  observed, 
  the 
  wave 
  may 
  be 
  

   regarded 
  as 
  approximately 
  plane. 
  We 
  arc 
  thus 
  led 
  to 
  study 
  

   in 
  the 
  first 
  instance 
  plane 
  waves." 
  

  

  The 
  student 
  of 
  physical 
  optics 
  knows 
  that 
  this 
  is 
  so 
  in 
  

   actual 
  fact. 
  To 
  study 
  plane-wave 
  phenomena, 
  or 
  to 
  verify 
  

   plane-wave 
  dimensional 
  calculations, 
  he 
  must 
  remove 
  his 
  

   source 
  of 
  light, 
  itself 
  relatively 
  small, 
  to 
  a 
  considerable 
  distance 
  

   from 
  his 
  grating 
  or 
  other 
  apparatus 
  ; 
  he 
  must 
  get 
  his 
  beams 
  

   of 
  rays 
  approximately 
  parallel, 
  that 
  the 
  normal 
  wave-surface 
  

   may 
  be 
  approximately 
  plane. 
  This 
  necessity 
  belongs 
  to 
  the 
  

   nature 
  of 
  plane 
  waves, 
  

  

  5. 
  But 
  considering 
  now 
  microscopic 
  objectives, 
  many 
  such 
  

   have 
  been 
  made 
  as 
  short 
  in 
  focus 
  as 
  -^q 
  of 
  an 
  inch. 
  It 
  is 
  

   impossible 
  to 
  regard 
  light 
  emitted 
  from 
  an 
  object, 
  as 
  consisting 
  

   of 
  uniform 
  plane 
  waves 
  on 
  arriving 
  at 
  the 
  surface 
  of 
  such 
  a 
  

   lens, 
  after 
  a 
  path 
  of, 
  perhaps, 
  gj^ 
  of 
  an 
  inch 
  ; 
  except 
  in 
  the 
  

   case 
  of 
  plane-ivave 
  illumination 
  of 
  the 
  object, 
  as 
  in 
  the 
  Abbe 
  

   theory. 
  Even 
  in 
  that 
  case 
  it 
  would 
  seem 
  that 
  considerable 
  

   modification 
  must 
  be 
  made 
  in 
  Dr. 
  Stoney's 
  presentment 
  of 
  

   events 
  in 
  his 
  fig. 
  1 
  (Nov. 
  1896, 
  p. 
  433) 
  here 
  reproduced. 
  His 
  

  

  Tier. 
  l. 
  

  

  objective 
  

  

  <% 
  1— 
  

  

  Zya 
  

  

  beam 
  Cb 
  of 
  uniform 
  plane 
  waves 
  is 
  represented 
  as 
  proceeding 
  

   from 
  the 
  object 
  C 
  ; 
  and 
  supposing 
  only 
  plane 
  waves 
  to 
  

  

  * 
  Treatise 
  on 
  Physical 
  Optics, 
  p. 
  56 
  (1892). 
  

  

  