﻿Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  491 
  

  

  replacing 
  the 
  eyepiece, 
  the 
  striae 
  were 
  just 
  seen. 
  The 
  diatom 
  

   was 
  probably 
  something 
  less 
  than 
  95,000 
  per 
  inch, 
  and 
  any 
  

   dry 
  lens 
  must 
  be 
  some 
  little 
  less 
  than 
  1*0 
  in 
  N.A. 
  Here 
  then, 
  

   with 
  very 
  intense 
  plane-wave 
  illumination 
  — 
  in 
  fact 
  as 
  nearly 
  

   as 
  possible 
  in 
  practice 
  Dr. 
  Stoney's 
  " 
  uniform 
  plane 
  waves 
  " 
  — 
  

   we 
  have 
  also 
  as 
  nearly 
  as 
  possible 
  the 
  theoretical 
  limit 
  attained, 
  

   or 
  closely 
  approached, 
  with 
  a 
  circular 
  aperture. 
  

  

  Turning 
  now 
  to 
  the 
  more 
  average 
  kind 
  of 
  microscopic 
  image, 
  

   the 
  extreme 
  closeness 
  with 
  which 
  Lord 
  Kayleiglr's 
  10 
  per 
  cent, 
  

   reduction 
  of 
  efficiency 
  in 
  circular 
  apertures 
  represents 
  the 
  

   facts 
  of 
  observation 
  as 
  found 
  by 
  the 
  most 
  competent 
  observers, 
  

   will 
  forcibly 
  strike 
  everyone 
  who 
  has 
  studied 
  microscopy 
  for 
  

   any 
  length 
  of 
  time. 
  But 
  Dr. 
  A. 
  Clifford 
  Mercer, 
  of 
  Syracuse 
  

   University, 
  U.S., 
  has 
  recently 
  tested 
  the 
  question 
  photo- 
  

   graphically. 
  It 
  is 
  comparatively 
  easy 
  to 
  prepare 
  circular 
  and 
  

   square 
  apertures 
  of 
  equal 
  dimensions. 
  He 
  also 
  ruled 
  upon 
  

   the 
  same 
  glass 
  plate 
  six 
  sets 
  of 
  lines 
  at 
  intervals 
  of 
  0*42, 
  

   0'46, 
  and 
  0*5 
  mm. 
  and 
  their 
  doubled 
  intervals 
  of 
  084, 
  0'92, 
  

   and 
  l'O 
  mm. 
  apart. 
  The 
  apertures 
  were 
  5*0, 
  5*5, 
  and 
  6*0 
  mm. 
  

   diameter. 
  It 
  will 
  be 
  seen 
  that 
  both 
  lines 
  and 
  apertures 
  give 
  

   excesses 
  of 
  about 
  10 
  and 
  20 
  per 
  cent., 
  representing 
  those 
  

   calculated 
  by 
  Lord 
  Rayleigh 
  and 
  Airy 
  respectively. 
  An 
  

   aerial 
  image 
  of 
  these 
  lines 
  focussed 
  by 
  the 
  condenser, 
  was 
  used 
  

   as 
  the 
  object, 
  and 
  successive 
  photographs 
  taken 
  with 
  all 
  the 
  

   square 
  and 
  circular 
  apertures. 
  Then 
  onl 
  y 
  similarity 
  of 
  resolu- 
  

   tion 
  had 
  to 
  be 
  compared, 
  which 
  can 
  be 
  done 
  within 
  very 
  small 
  

   limits 
  of 
  observational 
  error. 
  The 
  results 
  agreed 
  with 
  Lord 
  

   Eayleigh's 
  calculation 
  and 
  experiments, 
  not 
  with 
  the 
  Abbe 
  

   calculation 
  or 
  with 
  Airy's. 
  

  

  14. 
  Dr. 
  Stoney 
  himself, 
  moreover, 
  recognizes 
  essentially 
  

   what 
  is 
  here 
  mainlained 
  in 
  his 
  Proposition 
  5 
  (Phil. 
  Mag. 
  

   Oct. 
  189(j, 
  p. 
  346) 
  . 
  This 
  reads 
  : 
  — 
  " 
  The 
  standard 
  image 
  is 
  the 
  

   outcome, 
  partly 
  of 
  the 
  features 
  upon 
  the 
  object, 
  and 
  partly 
  of 
  

   the 
  state 
  of 
  the 
  light 
  by 
  which 
  the 
  object 
  is 
  illuminated. 
  

   It 
  may 
  be 
  improved 
  by 
  increasing 
  the 
  degree 
  in 
  which 
  the 
  first 
  

   of 
  these 
  factors, 
  and 
  by 
  decreasing 
  the 
  degree 
  in 
  which 
  the 
  second, 
  

   contributes 
  to 
  produce, 
  to 
  modify, 
  or 
  to 
  efface 
  detail 
  in 
  the 
  

   image" 
  So 
  closely 
  does 
  this 
  practically 
  coincide 
  with 
  my 
  

   proposition, 
  that 
  had 
  it 
  stood 
  alone 
  or 
  as 
  the 
  final 
  conclusion 
  

   of 
  his 
  exposition, 
  nothing 
  more 
  would 
  have 
  been 
  necessary 
  ; 
  

   and 
  it 
  has 
  the 
  further 
  merit 
  of 
  recognizing 
  the 
  fact 
  (which 
  

   constitutes 
  the 
  real 
  place 
  and 
  proportion 
  of 
  the 
  " 
  spectrum 
  " 
  

   theory 
  in 
  microscopy, 
  and 
  the 
  nexus 
  between 
  it 
  and 
  the 
  Airy 
  

   theory) 
  that 
  we 
  have 
  two 
  distinct 
  elements 
  to 
  deal 
  with 
  in 
  an 
  

   image, 
  whose 
  respective 
  preponderance 
  or 
  proportion 
  are 
  

   highly 
  variable. 
  The 
  present 
  attempt 
  at 
  further 
  treatment 
  is 
  

  

  