﻿490 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  

  

  Experimental 
  test 
  was 
  made 
  in 
  conjunction 
  with 
  Mr. 
  R. 
  T 
  

   Grlazebrook, 
  using 
  a 
  50 
  to 
  the 
  inch 
  wire 
  grating 
  in 
  front 
  of 
  a 
  

   sodium 
  flame, 
  and 
  two 
  different 
  rectangular 
  apertures 
  (with 
  

   sides 
  parallel 
  to 
  the 
  wires) 
  on 
  the 
  object-glass 
  of 
  a 
  telescope, 
  

   measuring 
  the 
  distance 
  at 
  which 
  the 
  object-glass 
  (with 
  aper- 
  

   ture) 
  resolved 
  the 
  grating. 
  Of 
  circular 
  apertures, 
  four 
  were 
  

   employed 
  in 
  the 
  same 
  way. 
  The 
  two 
  observers 
  differed 
  very 
  

   slightly, 
  and 
  the 
  mean 
  for 
  the 
  four 
  circular 
  apertures 
  worked 
  

   out 
  in 
  the 
  proportions 
  of 
  1*13, 
  1*09, 
  1*09, 
  and 
  T09 
  to 
  1*0 
  of 
  

   rectangular 
  aperture. 
  Here 
  the 
  grating 
  in 
  front 
  of 
  the 
  flame 
  

   is 
  regarded 
  as 
  self-luminous, 
  just 
  as 
  in 
  the 
  experiment 
  with 
  

   the 
  microscope 
  above 
  described. 
  

  

  Thus 
  far 
  experiment 
  confirms 
  the 
  analysis 
  ; 
  but 
  Dr. 
  Stoney 
  

   considers 
  (in 
  the 
  previous 
  discussion 
  with 
  me 
  which 
  Lord 
  

   Ravleigh 
  alludes 
  to) 
  that 
  the 
  same 
  methods 
  cannot 
  be 
  applied 
  to 
  

   microscopical 
  resolution, 
  on 
  account 
  of 
  the 
  wider 
  angle 
  of 
  the 
  

   cones 
  of 
  rays 
  concerned, 
  and 
  the 
  physical 
  consequences 
  of 
  that 
  

   difference. 
  I 
  think 
  he 
  has 
  over-rated 
  such 
  distinction, 
  and 
  it 
  

   is 
  rather 
  hard 
  to 
  see 
  just 
  where 
  it 
  begins 
  or 
  how 
  far 
  it 
  operates; 
  

   at 
  all 
  events, 
  the 
  agreement 
  of 
  experiment 
  with 
  analysis 
  as 
  

   regards 
  both 
  kinds 
  of 
  image 
  *, 
  in 
  the 
  microscope 
  also, 
  is 
  

   remarkable. 
  

  

  Calculating 
  (as 
  usually 
  done) 
  by 
  the 
  E 
  line 
  for 
  white 
  light, 
  

   the 
  ultimate 
  limit 
  of 
  resolution 
  for 
  a 
  dry 
  objective 
  of 
  utmost 
  

   aperture 
  (N".A. 
  TO) 
  is 
  96,410 
  lines 
  per 
  inch, 
  which 
  we 
  suppose 
  

   to 
  be 
  attainable 
  according 
  to 
  the 
  " 
  spectrum 
  " 
  theory, 
  although 
  

   the 
  aperture 
  is 
  circular. 
  In 
  1888 
  Mr. 
  E. 
  M. 
  Nelson, 
  whose 
  

   microscopic 
  vision 
  is 
  phenomenally 
  keen, 
  just 
  " 
  glimpsed 
  " 
  

   the 
  striae 
  of 
  A. 
  pellucida, 
  mounted 
  in 
  the 
  arsenic 
  medium 
  

   described 
  by 
  Dr. 
  Stoney, 
  but 
  prepared 
  by 
  Prof. 
  H. 
  L. 
  Smith. 
  

   Including 
  the 
  double 
  system, 
  or 
  all 
  across 
  the 
  valve, 
  these 
  

   striae 
  are 
  about 
  ysq6 
  °^ 
  an 
  ^ 
  ncn 
  m 
  length. 
  He 
  used 
  an 
  oil- 
  

   immersion 
  condenser 
  of 
  much 
  greater 
  aperture 
  than 
  1*0, 
  with 
  

   a 
  single-notched 
  stop, 
  through 
  which 
  sun-rays 
  were 
  sent 
  by 
  a 
  

   heliostat. 
  The 
  beam 
  through 
  the 
  notch 
  being 
  first 
  so 
  oblique 
  

   as 
  to 
  be 
  outside 
  or 
  excluded 
  by 
  the 
  1*0 
  dry 
  aperture 
  of 
  the 
  

   objective, 
  a 
  strong 
  green 
  spectrum 
  alone 
  appeared 
  at 
  one 
  side 
  of 
  

   that 
  aperture, 
  at 
  back 
  of 
  the 
  lens 
  [x 
  in 
  fig. 
  1). 
  The 
  notch 
  was 
  

   then 
  gradually 
  deepened 
  until 
  a 
  very 
  small 
  direct 
  or 
  dioptric 
  

   pencil 
  was 
  just 
  seen 
  on 
  the 
  opposite 
  side 
  of 
  the 
  aperture 
  — 
  

  

  * 
  As 
  Lord 
  Rayleigh 
  pointed 
  out 
  to 
  me, 
  and 
  as 
  will 
  be 
  understood, 
  the 
  

   absolute 
  theoretical 
  limit 
  of 
  resolution 
  must 
  be 
  considered 
  the 
  same 
  in 
  

   both 
  apertures 
  in 
  each 
  case. 
  But 
  resolution 
  also 
  requires 
  a 
  certain 
  

   luminous 
  intensity 
  compared 
  with 
  the 
  held 
  of 
  view 
  ; 
  and 
  in 
  this 
  matter 
  

   the 
  circular 
  aperture 
  may 
  be 
  more 
  or 
  less 
  at 
  disadvantage, 
  depending 
  

   either 
  on 
  the 
  nature 
  of 
  the 
  luminosity 
  or 
  partly 
  on 
  the 
  length 
  of 
  the 
  lines 
  

   compared 
  with 
  the 
  aperture. 
  

  

  