﻿Mr. 
  L. 
  Wright 
  on 
  Microscopie 
  Images 
  and 
  Vision. 
  495 
  

  

  Take 
  first 
  quite 
  a 
  coarse 
  striation 
  of 
  50 
  to 
  the 
  inch, 
  visible 
  

   to 
  the 
  naked 
  eye, 
  represented 
  by 
  a 
  grating 
  of 
  platinum 
  wire 
  

   and 
  by 
  a 
  piece 
  of 
  platinum 
  foil 
  corrugated 
  to 
  the 
  same 
  gauge. 
  

   Make 
  the 
  wires 
  incandescent, 
  and 
  (checking 
  irradiation 
  by 
  a 
  

   smoked 
  glass) 
  the 
  striation 
  is 
  easily 
  seen. 
  Make 
  the 
  corru- 
  

   gated 
  foil 
  incandescent 
  (these 
  observations 
  are 
  supposed 
  to 
  be 
  

   in 
  the 
  dark) 
  and 
  probably 
  the 
  detail 
  will 
  be 
  quite 
  invisible. 
  

   The 
  eye 
  was 
  quite 
  competent 
  to 
  see 
  structure 
  of 
  this 
  

   fineness 
  by 
  the 
  Airy 
  self-luminous 
  method, 
  if 
  the 
  detail 
  was 
  

   in 
  contrast 
  ; 
  but 
  there 
  is 
  now 
  no 
  contrast, 
  and 
  the 
  detail 
  is 
  

   more 
  or 
  less 
  invisible. 
  Then 
  let 
  the 
  corrugated 
  foil 
  be 
  cold 
  

   and 
  illuminated 
  bv 
  extraneous 
  lioht, 
  and 
  the 
  detail 
  is 
  seen 
  

   again. 
  There 
  is 
  both 
  shadow 
  to 
  assist 
  the 
  contrasts, 
  and 
  also 
  

   there 
  are 
  phase-relations 
  between 
  the 
  tops 
  and 
  bottoms 
  of 
  the 
  

   striations 
  which 
  come 
  into 
  play. 
  

  

  Let 
  us 
  further 
  imagine 
  a 
  perfectly 
  transparent 
  structure 
  

   with 
  uniform 
  periodic 
  detail, 
  but 
  the 
  elements 
  of 
  that 
  detail 
  

   differing 
  in 
  thickness 
  only 
  ; 
  and 
  let 
  it 
  be 
  mounted 
  in 
  a 
  medium 
  

   of 
  nearly 
  the 
  same 
  refractive 
  index. 
  A 
  diatom 
  in 
  balsam 
  

   nearly 
  represents 
  such 
  a 
  case. 
  It 
  is 
  quite 
  evident 
  that 
  by 
  

   heterogeneous 
  illumination 
  at 
  all 
  approaching 
  the 
  self-luminous 
  

   character, 
  it 
  will 
  be 
  difficult 
  to 
  find 
  anything 
  sufficiently 
  

   contrasted 
  in 
  detail 
  to 
  see 
  at 
  all, 
  though 
  the 
  very 
  same 
  

   illumination 
  of 
  a 
  black-and-iuhite 
  photograph 
  of 
  same 
  scale, 
  

   or 
  of 
  the 
  same 
  diatom 
  in 
  a 
  medium 
  of 
  2*4 
  index, 
  might 
  show 
  

   it 
  easily. 
  But 
  plane-wave 
  illumination 
  might 
  very 
  easily 
  

   bring 
  about 
  phase-relations 
  more 
  or 
  less 
  approximating 
  to 
  

   half-wave 
  discordance, 
  which 
  we 
  know 
  well 
  would 
  be 
  more 
  

   effective 
  than 
  black-and-white 
  itself 
  by 
  direct 
  light 
  ; 
  in 
  any 
  

   case 
  these 
  phase-relations 
  will 
  produce 
  conspicuous 
  effect 
  in 
  

   a 
  Fresnel-fringe 
  image. 
  Thus 
  the 
  Abbe 
  method 
  has 
  a 
  most 
  

   important 
  function 
  in 
  enabling 
  us 
  to 
  see 
  contrast 
  in 
  the 
  

   details 
  of 
  a 
  large 
  class 
  of 
  objects 
  — 
  especially 
  hyaline 
  or 
  

   transparent 
  objects 
  — 
  which 
  do 
  not 
  present 
  contrast 
  or 
  opacity 
  

   sufficient 
  to 
  be 
  seen 
  in 
  any 
  other 
  way. 
  The 
  error 
  has 
  been 
  

   in 
  giving 
  to 
  it 
  the 
  sole 
  or 
  all-important 
  place, 
  not 
  recognizing 
  

   that 
  there 
  is 
  quite 
  another 
  kind 
  of 
  image 
  also 
  available, 
  

   depending 
  upon 
  Airy's 
  theory 
  ; 
  and 
  that 
  this 
  latter, 
  while 
  

   in 
  the 
  ease 
  of 
  transparent 
  details 
  often 
  giving 
  images 
  

   insufficient, 
  or 
  at 
  least 
  far 
  inferior, 
  in 
  black-and-white 
  contrast 
  

   (what 
  microscopists 
  call 
  " 
  resolution 
  ") 
  s 
  is 
  free 
  from 
  the 
  

   contour 
  errors 
  of 
  the 
  Abbe 
  image, 
  and 
  must 
  be 
  used 
  to 
  

   correct 
  it 
  so 
  far 
  as 
  is 
  possible 
  in 
  the 
  individual 
  cases. 
  

  

  The 
  errors 
  of 
  the 
  " 
  spectrum 
  " 
  image 
  are 
  well 
  known 
  : 
  

   Prof. 
  Abbe 
  himself 
  has 
  sufficiently 
  insisted 
  upon 
  them. 
  Its 
  

   very 
  contrast, 
  or 
  " 
  resolution," 
  is 
  in 
  most 
  cases 
  a 
  glaring 
  

  

  