﻿486 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  

  

  throughout, 
  as 
  Lord 
  Rayleigh 
  remarks, 
  considers 
  the 
  object 
  

   to 
  be 
  illuminated 
  by 
  plane 
  waves. 
  In 
  this 
  limited 
  case, 
  what 
  

   Dr. 
  Stoney 
  advances 
  is 
  more 
  or 
  less 
  true 
  ; 
  but 
  Abbe 
  differs 
  

   from 
  the 
  latter 
  in 
  constantly 
  recognizing 
  that 
  condition 
  and 
  

   its 
  consequences. 
  Thus, 
  while 
  Dr. 
  Stoney 
  states 
  that 
  a 
  cone 
  

   of 
  rays 
  from 
  the 
  condenser, 
  as 
  wide 
  as 
  possible, 
  may 
  be 
  used 
  

   (as 
  in 
  practice 
  it 
  may, 
  for 
  reasons 
  to 
  be 
  seen), 
  Abbe 
  again 
  

   and 
  again 
  insists 
  that 
  such 
  is 
  not 
  the 
  case, 
  and 
  this 
  at 
  great 
  

   length*. 
  "Strictly 
  similar 
  images," 
  he 
  says, 
  u 
  cannot 
  be 
  

   expected 
  except 
  with 
  a 
  central 
  illumination 
  with 
  a 
  narrow 
  

   incident 
  pencil 
  7 
  '|. 
  This 
  is 
  the 
  condition 
  for 
  securing 
  an 
  

   approximation 
  to 
  plane-wave 
  illumination, 
  with 
  its 
  dif- 
  

   fraction 
  phenomena. 
  

  

  II. 
  

  

  9. 
  We 
  may 
  now 
  consider 
  how 
  far 
  the 
  Abbe 
  theory, 
  which 
  

   possesses 
  more 
  or 
  less 
  undoubted 
  truth, 
  is 
  an 
  adequate 
  repre- 
  

   sentation 
  of 
  microscopic 
  vision 
  ; 
  and 
  the 
  most 
  satisfactory 
  

   feature 
  about 
  the 
  lengthy 
  discussion 
  from 
  which 
  these 
  remarks 
  

   originate, 
  is 
  that 
  in 
  several 
  ways 
  additional 
  light 
  is 
  thrown 
  

   upon 
  that 
  question. 
  Considering 
  the 
  grounds 
  for 
  it 
  after- 
  

   wards, 
  let 
  me 
  first 
  state 
  the 
  general 
  conclusion 
  at 
  which 
  I 
  

   have 
  arrived 
  as 
  the 
  result 
  of 
  long 
  and 
  patient 
  experiment, 
  and 
  

   which 
  is 
  further 
  confirmed 
  by 
  this 
  discussion. 
  Stated 
  briefly 
  

   as 
  before, 
  it 
  is 
  that 
  the 
  trustivortldness 
  of 
  a 
  microscopic 
  image 
  

   is 
  in 
  proportion 
  as 
  the 
  object 
  approximates 
  to 
  a 
  self-luminous 
  

   condition, 
  and 
  diminishes 
  in 
  proportion 
  as 
  it 
  is 
  or 
  has 
  to 
  be 
  

   (for 
  it 
  may 
  have 
  to 
  be) 
  examined 
  by 
  plane-wave 
  illumination. 
  

   Whether 
  this 
  view, 
  or 
  the 
  Abbe 
  view, 
  be 
  true, 
  is 
  of 
  most 
  

   fundamental 
  and 
  practical 
  importance 
  to 
  microscopy 
  and 
  

   microscopic 
  optics. 
  Let 
  us 
  see 
  what 
  light 
  we 
  have 
  upon 
  it. 
  

  

  10. 
  Supposing 
  the 
  " 
  spectrum 
  " 
  theory 
  to 
  be 
  true, 
  as 
  a 
  full 
  

   representation, 
  it 
  was 
  demonstrated 
  that 
  " 
  microscopic 
  vision 
  

   is 
  sui 
  generis 
  " 
  %. 
  Such 
  a 
  statement 
  need 
  not 
  now 
  be 
  criti- 
  

   cized 
  ; 
  it 
  has 
  vanished 
  in 
  the 
  discussion, 
  and 
  Dr. 
  Stoney 
  

   expressly 
  dissociates 
  himself 
  from 
  it. 
  

  

  11. 
  Another 
  fundamental 
  objection 
  to 
  the 
  competence 
  of 
  

   the 
  theory 
  as 
  a 
  general 
  one, 
  is 
  found 
  in 
  the 
  fact 
  mentioned 
  

   by 
  Lord 
  Rayleigh, 
  that 
  the 
  character 
  of 
  a 
  grating 
  may 
  be 
  

   such, 
  that 
  its 
  spectra 
  cannot 
  give 
  a 
  proper 
  image. 
  

  

  12. 
  As 
  he 
  also 
  brings 
  out 
  in 
  his 
  article, 
  the 
  object 
  may 
  

   conceivably 
  be 
  self-luminous 
  ; 
  in 
  which 
  case 
  there 
  will 
  be 
  no 
  

   spectra, 
  and 
  the 
  waves 
  emitted 
  from 
  different 
  points 
  of 
  the 
  

  

  * 
  Journ. 
  E. 
  Micr. 
  Soc, 
  Dec. 
  1889. 
  

  

  f 
  Dallinger, 
  p. 
  75. 
  

  

  % 
  Dallin-ger, 
  p. 
  Q>2, 
  and 
  many 
  other 
  writers* 
  

  

  