﻿Mr. 
  L. 
  "Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  497 
  

  

  which 
  strictly 
  is 
  true; 
  but 
  there 
  is 
  this 
  broad 
  practical 
  sense 
  

   in 
  which 
  it 
  also 
  is 
  true. 
  

  

  And 
  we 
  are 
  unable 
  to 
  use 
  either 
  kind 
  of 
  image 
  or 
  of 
  

   illumination 
  absolutely 
  pure, 
  if 
  we 
  desired 
  to 
  do 
  so. 
  The 
  

   narrowest 
  pencil 
  we 
  can 
  practically 
  use 
  will 
  not 
  give 
  us 
  

   absolutely 
  plane 
  waves 
  alone 
  : 
  there 
  will 
  be 
  some 
  amount 
  of 
  

   heterogeneity 
  in 
  the 
  pencil, 
  which 
  in 
  some 
  little 
  degree 
  serves 
  

   to 
  correct 
  our 
  image. 
  And 
  the 
  widest 
  cones 
  we 
  can 
  use, 
  or 
  

   even 
  ground 
  glass, 
  do 
  not 
  prevent 
  greater 
  or 
  less 
  approach 
  

   to 
  the 
  character 
  of 
  plane 
  waves, 
  as 
  the 
  rays 
  travel 
  farther 
  

   from 
  the 
  lamp 
  ; 
  and 
  these 
  by 
  their 
  interference 
  tend 
  to 
  

   intensify 
  the 
  image. 
  We 
  have 
  to 
  play 
  off 
  and 
  adjust 
  one 
  

   against 
  the 
  other, 
  and 
  we 
  are 
  helped 
  by 
  a 
  fact 
  which 
  I 
  had 
  

   not 
  fully 
  seen, 
  till 
  pointed 
  out 
  by 
  Dr. 
  Stoney 
  in 
  our 
  previous 
  

   discussion 
  "*. 
  In 
  so 
  far 
  as 
  we 
  may 
  regard 
  every 
  elementary 
  

   or 
  excessively 
  small 
  cone 
  or 
  pencil 
  of 
  rays 
  from 
  the 
  condenser 
  

   as 
  an 
  individual 
  beam 
  of 
  plane 
  waves 
  (which 
  no 
  doubt 
  is 
  the 
  

   case 
  in 
  some 
  degree), 
  in 
  passing 
  through 
  the 
  object 
  it 
  

   originates 
  two 
  or 
  more 
  pencils 
  from 
  the 
  same 
  point. 
  These 
  

   being 
  necessarily 
  in 
  the 
  same 
  phase 
  or 
  phase-relation, 
  so 
  far 
  

   as 
  they 
  exist 
  must 
  interfere 
  at 
  the 
  focus, 
  and 
  thus 
  intensify 
  

   the 
  image. 
  On 
  the 
  other 
  hand, 
  the 
  numerous 
  such 
  elemen- 
  

   tary 
  pencils 
  comprising 
  a 
  wide 
  cone, 
  are 
  in 
  many 
  discordant 
  

   phases 
  and 
  transversals, 
  as 
  Lord 
  Rayleigh 
  has 
  shown 
  : 
  and 
  

   this 
  very 
  heterogeneity 
  tends 
  to 
  correct 
  the 
  contours 
  in 
  the 
  

   image, 
  as 
  above. 
  We 
  thus 
  understand 
  why, 
  in 
  really 
  critical 
  

   work, 
  a 
  large 
  cone 
  from 
  a 
  good 
  condenser 
  usually 
  gives 
  us 
  

   the 
  best 
  result 
  ; 
  but 
  why 
  it 
  may 
  be 
  impossible, 
  even 
  with 
  a 
  

   perfect 
  objective, 
  to 
  use 
  a 
  cone 
  of 
  light 
  which 
  will 
  fill 
  its 
  

   aperture 
  completely. 
  It 
  may 
  be 
  necessary, 
  to 
  intensify 
  the 
  

   image, 
  while 
  using 
  as 
  much 
  heterogeneous 
  light 
  as 
  we 
  can, 
  

   to 
  use 
  only 
  pencils 
  each 
  of 
  which 
  throws 
  out 
  another 
  diffracted 
  

   pencil 
  grasped 
  by 
  the 
  aperture, 
  so 
  as 
  to 
  intensify, 
  or 
  as 
  

   Dr. 
  Stoney 
  prefers 
  to 
  say, 
  to 
  " 
  correct 
  " 
  it. 
  But 
  this 
  

   necessity 
  depends 
  on 
  the 
  nature 
  of 
  the 
  object, 
  and 
  does 
  not 
  

   exist 
  in 
  all 
  cases. 
  

  

  19. 
  There 
  is 
  a 
  very 
  obvious 
  and 
  simple, 
  yet 
  decisive 
  test 
  

   as 
  to 
  the 
  correctness 
  of 
  this 
  view. 
  According 
  to 
  the 
  Abbe 
  

   or 
  spectrum 
  theory 
  (and 
  even 
  according 
  to 
  some 
  of 
  Dr. 
  

   Stoney's 
  reasoning, 
  I 
  think), 
  the 
  amount 
  of 
  cone 
  or 
  hetero- 
  

   geneous 
  light 
  which 
  can 
  be 
  used 
  will 
  depend 
  upon 
  the 
  

   minuteness 
  of 
  the 
  structure 
  alone. 
  According 
  to 
  the 
  view- 
  

   here 
  maintained 
  (which 
  recognizes 
  the 
  Airy 
  theory 
  as 
  also 
  

   concerned 
  in 
  the 
  image) 
  the 
  density 
  or 
  contrast 
  of 
  the 
  structure 
  

   is 
  the 
  chief 
  factor 
  in 
  this 
  question. 
  All 
  experience 
  proves 
  

   that 
  the 
  latter 
  is 
  the 
  case. 
  

  

  * 
  English 
  Mechanic, 
  Jan. 
  31, 
  1806. 
  

  

  