﻿498 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  

  

  III. 
  

  

  It 
  only 
  remains 
  to 
  show 
  how 
  directly 
  the 
  questions 
  here 
  

   discussed 
  affect 
  practical 
  microscopy 
  and 
  the 
  work 
  of 
  the 
  

   microscope 
  optician, 
  and 
  also 
  determine 
  the 
  prospect 
  of 
  

   further 
  advances 
  in 
  our 
  powers 
  of 
  microscopical 
  research. 
  

  

  20. 
  The 
  Abhe 
  or 
  " 
  spectrum 
  " 
  theory 
  has 
  in 
  its 
  time, 
  

   confessedly, 
  led 
  to 
  enormous 
  improvement 
  in 
  objectives. 
  

   Owing 
  to 
  that 
  specialization 
  and 
  ignorance 
  of 
  what 
  physicists 
  

   had 
  done, 
  which 
  Lord 
  Rayleigh 
  has 
  alluded 
  to, 
  there 
  was 
  

   amongst 
  microscopists 
  no 
  understanding 
  of 
  the 
  direct 
  function 
  

   of 
  aperture 
  in 
  resolution 
  ; 
  and 
  so 
  the 
  Abbe 
  theory 
  was 
  for 
  

   years 
  written 
  about, 
  and 
  advanced, 
  as 
  "the 
  first 
  explanation 
  

   ever 
  given." 
  It 
  thus 
  produced 
  a 
  vivid 
  consciousness 
  of 
  that 
  

   function 
  which 
  was 
  entirely 
  new, 
  to 
  which 
  we 
  owe 
  our 
  present 
  

   immersion 
  and 
  other 
  high-aperture 
  lenses. 
  But 
  it 
  is 
  as 
  

   easy 
  to 
  show 
  that, 
  this 
  work 
  being 
  done, 
  its 
  undue 
  prepon- 
  

   derance 
  and 
  acceptance 
  as 
  the 
  only 
  theory, 
  especially 
  on 
  the 
  

   Continent, 
  is 
  now 
  causing 
  distinct 
  prejudicial 
  results, 
  owing 
  

   chiefly 
  to 
  its 
  connexion 
  in 
  practice 
  with 
  a 
  narrow 
  pencil 
  or 
  

   cone. 
  Dr. 
  Stoney 
  disclaims 
  this 
  for 
  his 
  more 
  general 
  

   presentment, 
  as 
  he 
  has 
  of 
  course 
  a 
  right 
  to 
  do 
  ; 
  but 
  it 
  is 
  not 
  

   so 
  with 
  the 
  "spectrum" 
  theory. 
  Abbe 
  himself 
  throughout 
  

   insisted 
  upon 
  the 
  narrow 
  pencil. 
  Dr. 
  Van 
  Heurck 
  does 
  the 
  

   same 
  ; 
  Dr. 
  Peragallo 
  writes 
  that 
  a 
  cone 
  of 
  more 
  than 
  

   0*50 
  N 
  A. 
  is 
  of 
  no 
  use 
  ; 
  and 
  Dr. 
  Dallinger, 
  and 
  authorities 
  

   like 
  him, 
  who 
  in 
  a 
  general 
  way 
  accept 
  the 
  Abbe 
  theorv 
  as 
  

   the 
  " 
  theory/' 
  but 
  know 
  from 
  their 
  own 
  experience 
  the 
  vital 
  

   necessity 
  in 
  difficult 
  research 
  of 
  a 
  wide 
  cone, 
  write 
  expresslv 
  

   of 
  " 
  theory 
  and 
  practice 
  being 
  thus 
  at 
  variance/'' 
  in 
  some 
  way 
  

   or 
  other 
  which 
  had 
  to 
  be 
  explained. 
  

  

  It 
  is 
  difficult 
  to 
  estimate 
  the 
  prejudicial 
  effect 
  of 
  this 
  upon 
  

   microscopy 
  on 
  the 
  Continent. 
  As 
  a 
  quite 
  uncorrected 
  

   condenser 
  will 
  give 
  a 
  fair 
  cone 
  up 
  to 
  0*50 
  N.A., 
  and 
  also 
  by 
  

   immersion 
  extremply 
  oblique 
  rays 
  from 
  its 
  margin 
  (equivalent 
  

   to 
  annular 
  marginal 
  illumination), 
  for 
  years 
  no 
  better 
  Con- 
  

   tinental 
  condenser 
  was 
  made. 
  Prof. 
  Abbe 
  at 
  last 
  was 
  driven 
  

   to 
  compute 
  an 
  achromatic, 
  but 
  this 
  last 
  production 
  of 
  

   Continental 
  microscopy 
  only 
  gives 
  an 
  aplanatic 
  cone 
  of 
  065. 
  

   Except 
  those 
  few 
  who 
  know 
  of 
  English 
  condensers, 
  with 
  

   their 
  aplanatic 
  cones 
  of 
  1'10 
  for 
  immersion 
  and 
  0*90 
  for 
  dry 
  

   combinations, 
  Continental 
  workers 
  have 
  thus 
  been 
  condemned 
  

   to 
  the 
  errors 
  and 
  weaknesses 
  of 
  narrow 
  pencils, 
  which 
  have 
  

   thence 
  been 
  propagated 
  through 
  our 
  own 
  medical 
  schools 
  ; 
  

   and 
  the 
  results 
  are 
  sufficiently 
  striking. 
  Dr. 
  Koch 
  at 
  last 
  

   found 
  out 
  for 
  himself, 
  empirically, 
  that 
  wide 
  cones 
  gave 
  

  

  