﻿Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  487 
  

  

  object 
  will 
  be 
  quite 
  heterogeneous, 
  and 
  in 
  no 
  permanent 
  phase-, 
  

   relations. 
  Yet 
  an 
  image 
  must 
  be 
  possible, 
  and 
  can 
  in 
  that 
  

   case 
  be 
  only 
  analysed 
  according 
  to 
  the 
  Airy 
  method. 
  We 
  can 
  

   only 
  employ 
  a 
  really 
  self-luminous 
  object 
  in 
  experiments 
  with 
  

   low 
  powers 
  of 
  the 
  microscope 
  — 
  perhaps 
  up 
  to 
  an 
  inch 
  *. 
  But 
  

   even 
  the 
  results 
  with 
  such 
  a 
  power 
  are 
  decisive 
  of 
  the 
  real 
  

   question 
  ; 
  and 
  with 
  high 
  powers 
  we 
  can 
  more 
  or 
  less 
  approxi- 
  

   mate 
  to 
  this 
  kind 
  of 
  luminosity 
  in 
  several 
  ways. 
  

  

  Thus, 
  even 
  a 
  wide 
  cone 
  from 
  the 
  condenser 
  approximates 
  to 
  

   it. 
  Lord 
  Rayleigh 
  has 
  shown 
  (Phil. 
  Mag. 
  Aug. 
  1896, 
  p. 
  175) 
  

   how 
  and 
  why 
  this 
  kind 
  of 
  illumination 
  must 
  introduce 
  a 
  large 
  

   amount 
  of 
  heterogeneity 
  into 
  the 
  rays 
  proceeding 
  from 
  the 
  

   object, 
  and 
  concludes 
  " 
  that 
  the 
  function 
  of 
  the 
  condenser 
  in 
  

   microscopic 
  practice 
  is 
  to 
  cause 
  the 
  object 
  to 
  behave, 
  at 
  any 
  

   rate 
  in 
  some 
  degree, 
  as 
  if 
  it 
  were 
  self-luminous, 
  and 
  thus 
  to 
  

   obviate 
  the 
  sharply-marked 
  interference-bands 
  which 
  arise 
  

   when 
  permanent 
  and 
  definite 
  phase-relations 
  are 
  permitted 
  to 
  

   exist 
  between 
  the 
  radiations 
  which 
  issue 
  from 
  various 
  points 
  

   of 
  the 
  object." 
  Since 
  Dr. 
  Stoney, 
  however, 
  seems 
  rather 
  to 
  

   regard 
  the 
  function 
  of 
  the 
  condenser 
  as 
  being 
  that 
  of 
  providing 
  

   illumination 
  by 
  plane 
  waves, 
  we 
  had 
  better 
  resort 
  to 
  other 
  

   methods, 
  which 
  may 
  help 
  us 
  to 
  decide 
  what 
  is 
  a. 
  very 
  important 
  

   practical 
  question. 
  For 
  while, 
  according 
  to 
  Dr. 
  Stoney, 
  the 
  

   ideal 
  is 
  to 
  get 
  absolutely 
  aplanatic 
  systems 
  of 
  plane 
  waves 
  

   transmitted 
  through 
  the 
  object, 
  and 
  all 
  conditions 
  short 
  of 
  this 
  

   (caused 
  by 
  imperfections 
  in 
  the 
  slide 
  or 
  various 
  other 
  details) 
  

   impair 
  the 
  image 
  (as 
  in 
  one 
  special 
  sense 
  they 
  do 
  impair 
  it, 
  

   with 
  some 
  objects); 
  according 
  to 
  the 
  view 
  expressed 
  above, 
  

   irregularities 
  of 
  phase 
  thus 
  produced 
  may 
  add 
  to 
  the 
  trust- 
  

   worthiness 
  of 
  the 
  image, 
  though 
  it 
  may 
  impair 
  it 
  in 
  some 
  

   other 
  features. 
  

  

  Let 
  us 
  take 
  therefore 
  as 
  an 
  object 
  on 
  the 
  stage, 
  a 
  grating 
  of 
  

   3000 
  or 
  6000 
  lines 
  to 
  an 
  inch, 
  illuminated 
  by 
  a 
  narrow 
  cone 
  

   from 
  the 
  condenser, 
  focussing 
  the 
  flat 
  of 
  a 
  rather 
  distant 
  

   lamp-flame. 
  Place 
  immediately 
  in 
  front 
  of 
  this 
  flame 
  a 
  coarse 
  

   grating, 
  50 
  to 
  100 
  lines 
  per 
  inch, 
  either 
  photographed 
  or 
  of 
  

   wire. 
  The 
  several 
  points 
  of 
  these 
  luminous 
  lines 
  emit 
  light- 
  

   waves 
  chiefly 
  in 
  the 
  self-luminous 
  manner, 
  indiscriminate 
  in 
  

   phases 
  and 
  transversals 
  as 
  the 
  points 
  of 
  the 
  flame 
  itself 
  t« 
  

   Arranging 
  the 
  stage 
  grating 
  so 
  as 
  to 
  cover 
  only 
  half 
  the 
  

  

  * 
  Thus, 
  we 
  may 
  employ 
  a 
  grating 
  of 
  platinum 
  wire 
  made 
  incandescent 
  

   by 
  an 
  electric 
  current. 
  

  

  t 
  If 
  this 
  be 
  questioned, 
  let 
  it 
  be 
  remembered 
  that 
  we 
  might 
  get 
  pure 
  

   self-luminosity 
  by 
  employing 
  in 
  the 
  dark 
  a 
  platinum 
  grating 
  made 
  incan- 
  

   descent 
  as 
  before. 
  Probably 
  no 
  physicist 
  believes 
  that 
  there 
  would 
  be 
  

   any 
  observable 
  difference 
  in 
  the 
  phenomena, 
  or 
  would 
  even 
  deem 
  it 
  

   worth 
  while 
  to 
  make 
  the 
  experiment 
  in 
  order 
  to 
  see 
  if 
  there 
  were 
  an} 
  r 
  . 
  

  

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