﻿496 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  

  

  departure 
  from 
  truth, 
  to 
  which 
  (when 
  we 
  can 
  get 
  resolution 
  

   at 
  alj) 
  the 
  more 
  indistinct 
  self-luminous 
  image 
  is 
  in 
  reality 
  a 
  

   far 
  nearer 
  approach. 
  It 
  tends 
  to 
  make 
  details 
  which 
  should 
  

   be 
  only 
  geometrically 
  symmetrical 
  to 
  a 
  limited 
  extent, 
  

   perfectly 
  so. 
  In 
  extreme 
  forms 
  it 
  makes 
  rows 
  of 
  spots 
  into 
  

   lines, 
  and 
  these 
  lines 
  straight 
  when 
  not 
  really 
  so. 
  It 
  is 
  

   always 
  liable 
  to 
  false 
  resolutions 
  of 
  double 
  fineness. 
  It 
  fails 
  

   (as 
  JDr. 
  Stoney 
  explains) 
  to 
  give 
  even 
  a 
  tolerable 
  image 
  of 
  

   the 
  larger 
  features 
  of 
  the 
  object, 
  thereby 
  showing 
  its 
  failure 
  

   to 
  be 
  a 
  real 
  "image" 
  at 
  all. 
  All 
  that 
  can 
  really 
  be 
  learnt 
  

   from 
  it, 
  is 
  that 
  there 
  is 
  probably 
  (for 
  this 
  is 
  subject 
  to 
  possible 
  

   delusion 
  from 
  the 
  false 
  intercostals 
  above 
  mentioned) 
  some 
  

   periodic 
  difference 
  of 
  structure 
  in 
  the 
  object 
  similar 
  in 
  

   dimensional 
  intervals 
  to 
  " 
  lines 
  " 
  shown 
  : 
  in 
  regard 
  to 
  " 
  spots 
  " 
  

   this 
  is 
  more 
  uncertain, 
  since 
  these 
  are 
  often 
  produced 
  by 
  

   false 
  diffraction-fringes 
  from 
  any 
  long 
  line 
  which 
  may 
  cross 
  

   the 
  true 
  ones. 
  That 
  the 
  lines 
  are 
  lines, 
  or 
  that 
  the 
  " 
  pattern 
  " 
  

   is 
  so 
  geometrical 
  as 
  appears, 
  is 
  in 
  the 
  highest 
  degree 
  impro- 
  

   bable. 
  That 
  the 
  " 
  spectrum 
  " 
  theory 
  and 
  method 
  so 
  long 
  

   retained 
  exclusive 
  predominance, 
  is 
  because 
  attention 
  has 
  been 
  

   so 
  concentrated 
  upon 
  either 
  gratings 
  or 
  diatoms 
  of 
  known 
  

   periodicity 
  in 
  structure, 
  but 
  which 
  only 
  represent 
  to 
  a 
  very 
  

   small 
  extent 
  indeed 
  any 
  serious 
  kind 
  of 
  investigation. 
  

  

  18. 
  We 
  are 
  evidently 
  reaching 
  a 
  very 
  practical 
  conclusion. 
  

   It 
  appears 
  that 
  in 
  microscopy 
  we 
  have 
  to 
  deal 
  with 
  two 
  

   characteristics 
  of 
  an 
  image, 
  which 
  often 
  are 
  only 
  to 
  a 
  limited 
  

   extent 
  compatible 
  ; 
  that 
  we 
  have 
  at 
  command 
  two 
  methods 
  

   of 
  illumination 
  which 
  respectively 
  promote 
  more 
  especially 
  

   each 
  of 
  such 
  characteristics 
  ; 
  and 
  that 
  in 
  most 
  cases 
  our 
  problem 
  

   is 
  so 
  to 
  combine 
  and 
  balance 
  these 
  two 
  methods 
  as 
  to 
  produce 
  

   the 
  best 
  result. 
  Fidelity 
  of 
  contour 
  will 
  be 
  secured 
  in 
  pro- 
  

   portion 
  as 
  we 
  are 
  able 
  to 
  obtain 
  our 
  image 
  by 
  heterogeneous 
  

   illumination, 
  approximating 
  the 
  object 
  to 
  a 
  self-luminous 
  

   condition. 
  But 
  this 
  method 
  may 
  prove 
  utterly 
  unable 
  to 
  

   give 
  us 
  contrast, 
  which 
  we 
  may 
  therefore 
  be 
  compelled 
  to 
  

   increase 
  by 
  using 
  to 
  a 
  greater 
  or 
  less 
  (even 
  to 
  a 
  very 
  large) 
  

   extent 
  plaice-wave 
  illumination, 
  at 
  the 
  expense, 
  however, 
  of 
  

   some 
  greater 
  or 
  less 
  degree 
  of 
  infidelity 
  in 
  contour. 
  Thus 
  

   an 
  opaque 
  subject, 
  even 
  of 
  much 
  minuteness, 
  may 
  be 
  best 
  

   shown 
  by 
  ground-glass 
  illumination, 
  or 
  a 
  very 
  wide 
  cone; 
  

   while 
  a 
  diatom, 
  unless 
  in 
  a 
  very 
  dense 
  medium, 
  or 
  dry 
  in 
  air, 
  

   may 
  require 
  narrow 
  pencils 
  of 
  approximately 
  plane 
  waves. 
  

   It 
  is 
  interesting 
  to 
  observe 
  that 
  there 
  is 
  thus 
  a 
  great 
  degree 
  

   ot 
  practical 
  truth 
  in 
  Prof. 
  Abbe's 
  early 
  contention 
  as 
  to 
  

   " 
  different 
  origins 
  " 
  of 
  different 
  parts 
  of 
  the 
  image. 
  Many 
  

   of 
  us 
  have 
  written 
  of 
  this 
  as 
  an 
  " 
  error," 
  now 
  " 
  recanted," 
  

  

  