﻿Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  481 
  

  

  practical 
  question 
  not 
  solved 
  by 
  any 
  mere 
  mathematical 
  

  

  analysis, 
  and 
  scarcely 
  yet, 
  I 
  think, 
  made 
  clear 
  to 
  him. 
  This 
  

  

  question 
  is 
  at 
  the 
  bottom 
  of 
  the 
  term 
  u 
  spectrum 
  theory/' 
  

  

  happily 
  applied 
  by 
  Lord 
  Rayleigh 
  to 
  Prof. 
  Abbe's 
  view 
  of 
  

  

  the 
  matter, 
  to 
  which 
  Dr. 
  Stoney 
  objects 
  as 
  being 
  too 
  limited, 
  

  

  but 
  which 
  is 
  strictly 
  correct. 
  Upon 
  whatever 
  general 
  method 
  

  

  of 
  mathematical 
  resolution 
  the 
  Abbe 
  theory 
  of 
  microscopic 
  

  

  vision 
  ultimately 
  rested, 
  it 
  was 
  itself 
  expounded 
  to 
  micro- 
  

  

  scopists 
  and 
  discussed 
  by 
  them 
  for 
  many 
  years 
  as 
  a 
  matter 
  of 
  

  

  fact. 
  It 
  was 
  thus 
  and 
  then 
  confined 
  to 
  the 
  statement 
  that 
  

  

  microscopical 
  " 
  resolution/' 
  or 
  delineation 
  of 
  detail, 
  was 
  due 
  

  

  to 
  the 
  union 
  and 
  interference 
  (in 
  the 
  Fresnel 
  manner) 
  at 
  the 
  

  

  focal 
  plane, 
  of 
  the 
  direct 
  dioptric 
  beam 
  and 
  of 
  at 
  least 
  one 
  of 
  

  

  the 
  beams 
  "diffracted" 
  by 
  minute 
  periodic 
  structure, 
  in 
  the 
  

  

  manner 
  of 
  a 
  grating 
  illuminated 
  by 
  light 
  approximating 
  to 
  

  

  the 
  character 
  of 
  plane 
  waves 
  : 
  such 
  diffracted 
  beams 
  with 
  

  

  white 
  light 
  becoming 
  spectra. 
  The 
  Abbe 
  theory 
  further 
  

  

  affirmed 
  that 
  the 
  trustworthiness 
  of 
  the 
  microscopical 
  image 
  

  

  solely 
  depended 
  upon, 
  and 
  was 
  in 
  direct 
  proportion 
  to, 
  the 
  

  

  number 
  of 
  orders 
  of 
  these 
  spectra 
  which 
  were 
  grasped 
  by 
  the 
  

  

  aperture 
  of 
  the 
  lens 
  ; 
  and 
  it 
  explained 
  all 
  the 
  advantages 
  of 
  

  

  greater 
  aperture 
  in 
  greater 
  resolution, 
  upon 
  this 
  basis 
  alone. 
  

  

  This 
  was 
  a 
  definite, 
  limited, 
  and 
  practical 
  theory, 
  easily 
  

   grasped 
  ; 
  and 
  this 
  alone 
  was 
  what 
  came 
  to 
  be 
  known 
  as 
  the 
  

   Diffraction 
  Theory 
  or 
  Abbe 
  Theory. 
  Since, 
  therefore, 
  Dr. 
  

   Stoney 
  now 
  desires 
  to 
  apply 
  that 
  term 
  to 
  the 
  wider 
  manner 
  

   of 
  regarding 
  microscopic 
  vision 
  which 
  he 
  has 
  set 
  forth, 
  in 
  

   order 
  to 
  keep 
  things 
  clear 
  or 
  even 
  intelligible 
  to 
  any 
  micro- 
  

   scopist 
  who 
  has 
  followed 
  the 
  past 
  discussion, 
  there 
  is 
  really 
  

   no 
  other 
  course 
  than 
  to 
  find 
  a 
  new 
  name 
  for 
  the 
  more 
  limited 
  

   and 
  already 
  well 
  known 
  Abbe 
  theory, 
  as 
  Lord 
  Rayleigh 
  has 
  

   so 
  happily 
  done. 
  The 
  truth 
  or 
  error 
  of 
  this 
  "spectrum" 
  

   theory, 
  or 
  the 
  respective 
  measure 
  of 
  each 
  in 
  it, 
  is 
  a 
  matter 
  of 
  

   very 
  great 
  practical 
  importance, 
  as 
  will 
  appear. 
  The 
  interest 
  

   in 
  Dr. 
  Stoney's 
  wider 
  theory 
  is 
  largely 
  speculative 
  ; 
  but 
  there 
  

   are 
  obvious 
  points 
  of 
  contact 
  between 
  it 
  and 
  the 
  other, 
  which 
  

   also 
  have 
  to 
  be 
  considered, 
  and 
  which 
  throw 
  much 
  light 
  

   upon 
  it. 
  

  

  2. 
  With 
  the 
  purely 
  theoretical 
  bearings 
  of 
  Dr. 
  Stonev's 
  

   presentment 
  of 
  the 
  case 
  it 
  is 
  not 
  necessary 
  to 
  deal 
  at 
  length; 
  

   nor, 
  indeed, 
  am 
  I 
  qualified 
  to 
  do 
  so. 
  Yet 
  it 
  seems 
  desirable 
  

   to 
  mention 
  very 
  briefly 
  some 
  objections 
  which 
  suggest 
  them- 
  

   selves, 
  and 
  which, 
  if 
  valid, 
  have 
  much 
  bearing 
  on 
  the 
  con- 
  

   clusion 
  he 
  reaches 
  in 
  common 
  with 
  Prof. 
  Abbe, 
  that 
  " 
  dif- 
  

   fracted 
  light 
  [defined 
  as 
  " 
  light 
  which 
  advances 
  in 
  other 
  

   directions 
  than 
  those 
  prescribed 
  by 
  geometrical 
  optics 
  "] 
  is 
  

  

  