﻿482 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  

  

  the 
  machinery 
  by 
  which 
  good 
  definition 
  is 
  brought 
  about." 
  

   That 
  is, 
  of 
  course, 
  getting 
  back 
  to 
  the 
  ;; 
  spectrum 
  " 
  theory 
  ; 
  

   and 
  I 
  hope 
  to 
  show, 
  largely 
  from 
  material 
  the 
  discussion 
  has 
  

   provided, 
  this 
  theory 
  is 
  only 
  true 
  in 
  a 
  conditional 
  and 
  limited 
  

   sense, 
  while 
  its 
  acceptance 
  in 
  a 
  universal 
  sense 
  is 
  a 
  present 
  

   cause 
  of 
  positive 
  mischief 
  in 
  microscopy. 
  

  

  3. 
  There 
  appear 
  to 
  me, 
  then, 
  some 
  fundamental 
  physical 
  

   objections 
  to 
  Dr. 
  Stoney's 
  method 
  of 
  representing 
  what 
  takes 
  

   place. 
  Putting 
  that 
  most 
  briefly, 
  and 
  merely 
  for 
  the 
  purpose 
  

   of 
  recollection, 
  it 
  seems 
  to 
  be 
  as 
  follows 
  : 
  — 
  (A) 
  All 
  light 
  

   emitted 
  by 
  an 
  object 
  may 
  be 
  resolved 
  into 
  undulations 
  con- 
  

   sisting 
  of 
  uniform 
  plane 
  waves. 
  (B) 
  We 
  may 
  conceive 
  these 
  

   reversed 
  in 
  direction 
  (since 
  any 
  dynamical 
  system 
  may 
  be 
  

   reversed) 
  ; 
  and 
  when 
  they 
  thus 
  arrive 
  back 
  at 
  the 
  position 
  

   occupied 
  by 
  the 
  original 
  object, 
  they 
  will 
  there 
  "produce 
  an 
  

   image 
  the 
  most 
  perfect 
  that 
  the 
  light 
  emitted 
  is 
  capable 
  of 
  

   producing." 
  This 
  is 
  held 
  to 
  follow 
  because 
  (§ 
  8, 
  Phil. 
  Mag. 
  

   Oct. 
  181)6) 
  the 
  plane 
  waves 
  there, 
  as 
  at 
  each 
  step, 
  " 
  repro- 
  

   duce 
  exactly 
  the 
  same 
  state 
  of 
  the 
  aether 
  as 
  had 
  prevailed 
  at 
  

   the 
  same 
  stations 
  on 
  the 
  outward 
  journey. 
  - 
  " 
  Hence 
  in 
  general 
  

   (we 
  need 
  not 
  at 
  present 
  discuss 
  Dr. 
  Stoney's 
  other 
  images, 
  

   and 
  modifications) 
  " 
  plane 
  waves 
  converging 
  inwards 
  " 
  are 
  

   capable 
  of 
  producing 
  the 
  most 
  perfect 
  attainable 
  image 
  pro- 
  

   ducible 
  from 
  the 
  rays 
  grasped 
  by 
  the 
  objective. 
  Stating 
  

   objections 
  to 
  this 
  with 
  similar 
  generality 
  and 
  brevity, 
  it 
  

   appears 
  to 
  me 
  that 
  such 
  a 
  presentment 
  of 
  the 
  matter 
  must 
  

   break 
  down 
  as 
  a 
  full 
  and 
  complete 
  explanation, 
  however 
  true 
  

   in 
  a 
  limited 
  sense, 
  on 
  the 
  ground 
  that 
  " 
  uniform 
  plane 
  waves 
  " 
  

   such 
  as 
  are 
  spoken 
  of, 
  are 
  not 
  in 
  trustworthy 
  microscopy 
  the 
  

   actual 
  or 
  veritable 
  dynamical 
  system; 
  and 
  therefore 
  cannot, 
  

   as 
  it 
  will 
  be 
  shown 
  they 
  do 
  not, 
  produce 
  the 
  supposed 
  most 
  

   perfect 
  attainable 
  image 
  by 
  reversal. 
  

  

  4. 
  More 
  specifically, 
  it 
  seems 
  evident 
  that 
  we 
  are, 
  ah 
  initio, 
  

   debarred 
  from 
  considering 
  the 
  light 
  from 
  a 
  microscopic 
  object 
  

   as 
  consisting 
  of 
  uniform 
  plane 
  waves, 
  except 
  on 
  the 
  condition 
  

   of 
  plane-iuave 
  illumination 
  of 
  the 
  object. 
  (Here, 
  indeed, 
  we 
  

   have 
  the 
  secret 
  of 
  Abbe's 
  consistent 
  enforcement 
  of 
  illumina- 
  

   tion 
  by 
  a 
  small 
  luminous 
  cone 
  or 
  pencil, 
  which 
  gives 
  approxi- 
  

   mately 
  such 
  illumination.) 
  For 
  what 
  are 
  uniform 
  plane 
  

   waves 
  ? 
  A 
  wave-system 
  is 
  normal 
  to 
  the 
  surface 
  called 
  the 
  

   wave-surface, 
  over 
  which 
  undulations 
  from 
  the 
  same 
  disturb- 
  

   ance 
  are 
  in 
  the 
  same 
  phase. 
  Hence 
  the 
  plane 
  wave 
  arises 
  

   from 
  the 
  Huygenian 
  spherical 
  wave, 
  as 
  a 
  limiting 
  case, 
  in 
  

   the 
  manner 
  pointed 
  out 
  by 
  all 
  the 
  standard 
  authorities. 
  Thus 
  

   Lord 
  Eayleigh 
  says*: 
  — 
  " 
  So 
  long 
  as 
  the 
  radius 
  of 
  curvature 
  

  

  * 
  Encycl. 
  Brit., 
  " 
  Wave-Theory," 
  xxiv. 
  p. 
  424. 
  

  

  