﻿Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  489 
  

  

  rendering 
  the 
  very 
  finest 
  hairs 
  as 
  tapering 
  to 
  a 
  perfect 
  point, 
  

   with 
  entire 
  absence 
  of 
  the 
  diffraction-fringes 
  shown 
  round 
  such 
  

   details 
  with 
  a 
  narrow 
  pencil. 
  Where 
  and 
  why 
  " 
  resolution 
  " 
  

   often 
  fails 
  with 
  high 
  powers 
  as 
  regards 
  some 
  objects 
  so 
  illu- 
  

   minated, 
  belongs 
  to 
  the 
  question 
  before 
  us, 
  and 
  is 
  dealt 
  with 
  

   presently 
  ; 
  but 
  the 
  method 
  can 
  be 
  carried 
  much 
  farther 
  than 
  

   many 
  would 
  suppose. 
  The 
  diatom 
  P. 
  angulatum 
  (45,000 
  to 
  

   the 
  inch) 
  is 
  resolved 
  by 
  it 
  beautifully 
  with 
  a 
  dry 
  lens 
  ; 
  and 
  

   this 
  self-luminous 
  resolution 
  has 
  the 
  cardinal 
  superiority 
  over 
  

   Abbe's 
  with 
  a 
  narrow 
  pencil, 
  that 
  by 
  no 
  possibility 
  can 
  any 
  

   images 
  be 
  produced 
  by 
  it 
  other 
  than 
  the 
  small 
  white 
  disks 
  on 
  

   dark 
  ground, 
  or 
  black 
  spots 
  on 
  white 
  ground, 
  at 
  different 
  foci, 
  

   which 
  can 
  be 
  produced 
  in 
  the 
  same 
  way 
  from 
  a 
  sheet 
  of 
  

   perforated 
  zinc. 
  By 
  grinding 
  the 
  back 
  of 
  the 
  slide 
  itself, 
  

   even 
  an 
  immersion-lens 
  can 
  be 
  more 
  or 
  less 
  filled 
  with 
  direct 
  

   rays, 
  and 
  in 
  this 
  way 
  all 
  the 
  spots 
  can 
  be 
  seen 
  (as 
  spots, 
  and 
  

   not 
  falsely 
  as 
  spherules) 
  in 
  A. 
  Lindlieimerii 
  (69^000 
  to 
  the 
  

   inch) 
  . 
  With 
  a 
  first-rate 
  apochromatic 
  and 
  one 
  of 
  the 
  slides 
  

   mounted 
  by 
  Dr. 
  Van 
  Heurck 
  in 
  As 
  2 
  S 
  2 
  referred 
  to 
  by 
  

   Dr. 
  Stoney*, 
  I 
  have 
  seen 
  the 
  striae 
  in 
  A. 
  pe.lhici.da 
  ; 
  though 
  

   with 
  such 
  objects 
  as 
  these 
  the 
  method 
  comparatively 
  fails, 
  for 
  

   reasons 
  presently 
  to 
  be 
  seen. 
  

  

  13. 
  We 
  may 
  also 
  compare 
  the 
  results 
  of 
  mathematical 
  

   analysis 
  with 
  those 
  of 
  experiment. 
  We 
  have 
  two 
  kinds 
  of 
  

   possible 
  image, 
  for 
  the 
  Abbe 
  or 
  " 
  spectrum 
  " 
  image 
  is 
  a 
  real 
  

   fact 
  enough 
  under 
  the 
  necessary 
  conditions 
  ; 
  our 
  inquiry 
  here 
  is 
  

   simply 
  what 
  proportion 
  and 
  value 
  must 
  be 
  assigned 
  to 
  it 
  in 
  

   ordinary 
  research. 
  Lord 
  Rayleigh's 
  articles 
  here 
  and 
  else- 
  

   where 
  seem 
  to 
  supply 
  useful 
  criteria 
  as 
  regards 
  that 
  question.. 
  

   If 
  I 
  rightly 
  understand 
  him, 
  he 
  show 
  T 
  s 
  that 
  according 
  to 
  the 
  

   " 
  spectrum 
  " 
  theory 
  a 
  square 
  and 
  circular 
  aperture 
  of 
  the 
  same 
  

   width 
  give 
  the 
  same 
  resolution 
  for 
  points 
  or 
  short 
  lines. 
  On 
  

   the 
  other 
  hand, 
  respecting 
  the 
  resolution 
  of 
  self-luminous 
  lines 
  

   of 
  sensible 
  length, 
  another 
  analysis 
  of 
  his 
  f 
  led 
  to 
  the 
  conclusion 
  

   that 
  a 
  circular 
  aperture 
  must 
  exceed 
  a 
  square 
  aperture 
  by 
  say 
  

   10 
  per 
  cent, 
  to 
  give 
  equal 
  resolution. 
  Airy 
  in 
  a 
  slightly 
  dif- 
  

   ferent 
  manner 
  calculated 
  that 
  the 
  circular 
  aperture 
  must 
  exceed 
  

   by 
  about 
  20 
  per 
  cent. 
  Of 
  course 
  any 
  analysis 
  must 
  start 
  from 
  

   certain 
  assumptions, 
  but 
  Lord 
  Rayleigh's 
  appear 
  reasonable. 
  

  

  * 
  It 
  is 
  an 
  error 
  to 
  suppose 
  no 
  one 
  had 
  succeeded 
  in 
  mounting 
  objects 
  

   in 
  this 
  medium 
  except 
  or 
  until 
  Dr. 
  Van 
  Heurck. 
  Long 
  ago 
  it 
  was 
  used 
  

   (I 
  think 
  invented) 
  by 
  Prof. 
  Hamilton 
  Smith 
  in 
  America. 
  Father 
  Thomp- 
  

   son 
  was 
  also 
  fond 
  of 
  it, 
  and 
  published 
  a 
  description 
  of 
  his 
  methods 
  of 
  

   manipulation. 
  Unfortunately 
  the 
  medium 
  is 
  rather 
  apt 
  to 
  slowly 
  

   crystallize. 
  

  

  t 
  See 
  Enc. 
  Brit. 
  " 
  Wave 
  Theory," 
  and 
  Phil. 
  Mag. 
  1879, 
  1880, 
  for 
  

   details. 
  

  

  