﻿Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  501 
  

  

  lenged, 
  by 
  adherents 
  of 
  the 
  u 
  spectrum 
  " 
  theory 
  as 
  heretofore 
  

   understood. 
  

  

  When, 
  however, 
  we 
  do 
  employ 
  adequate 
  tests, 
  and 
  at 
  the 
  

   same 
  time 
  make 
  careful 
  comparisons 
  between 
  one 
  objective 
  

   and 
  another, 
  we 
  find 
  that 
  the 
  perfect 
  correction 
  of 
  spherical 
  

   aberrations 
  is 
  just 
  all-important 
  in 
  determining 
  how 
  far 
  we 
  

   can 
  go 
  in 
  using 
  with 
  that 
  lens 
  the 
  heterogeneous 
  illuminating 
  

   cone 
  which 
  is 
  so 
  important 
  for 
  depicting 
  true 
  contours 
  in 
  our 
  

   image, 
  still 
  preserving 
  sufficient 
  resolution 
  of 
  minute 
  structure. 
  

   (We 
  are 
  here 
  postulating 
  sufficient 
  opacity 
  in 
  the 
  details, 
  to 
  

   dispense 
  with 
  much 
  of 
  the 
  aid 
  we 
  have 
  seen 
  to 
  be 
  often 
  

   necessary 
  in 
  hyaline 
  subjects.) 
  High-class 
  moderate 
  powers 
  

   now 
  easily 
  utilize 
  their 
  full 
  aperture, 
  with 
  ground 
  -glass 
  

   illumination. 
  With 
  high 
  powers, 
  the 
  amount 
  of 
  this, 
  or 
  of 
  

   aplanatic 
  cone 
  possible, 
  is 
  in 
  almost 
  direct 
  proportion 
  to 
  the 
  

   perfection 
  of 
  spherical 
  correction. 
  Few 
  lenses 
  over 
  0*60N.A. 
  

   will, 
  however, 
  even 
  yet 
  bear 
  more 
  than 
  three-fourths 
  of 
  their 
  

   aperture 
  as 
  direct 
  light 
  ; 
  many 
  very 
  good 
  ones 
  only 
  two- 
  

   thirds. 
  And 
  objectives 
  differ 
  strangely. 
  In 
  Zeiss's 
  apochro- 
  

   matic 
  series, 
  the 
  half-inch 
  of 
  0*65 
  N.A. 
  and 
  the 
  §- 
  immersion 
  of 
  

   1*40 
  stand 
  out 
  from 
  the 
  rest 
  : 
  some 
  rare 
  specimens 
  of 
  the 
  

   former 
  will 
  bear 
  their 
  full 
  cone, 
  and 
  occasionally 
  an 
  \ 
  of 
  1*40 
  

   has 
  been 
  used 
  in 
  photography 
  with 
  a 
  cone 
  of 
  1*10. 
  Very 
  

   recently 
  I 
  had 
  sent 
  me 
  for 
  examination 
  by 
  Messrs. 
  Swift 
  a 
  

   new 
  English 
  ^ 
  apochromatic 
  of 
  1*40, 
  which 
  I 
  soon 
  found 
  

   was 
  remarkably 
  well 
  corrected 
  spherically*. 
  It 
  was 
  accord- 
  

   ingly 
  next 
  tested 
  upon 
  the 
  object 
  mentioned 
  by 
  Dr. 
  Stoney, 
  

   A. 
  pellucida 
  mounted 
  in 
  arsenic 
  by 
  Dr. 
  Van 
  Heurck. 
  All 
  the 
  

   transverse 
  strise 
  in 
  the 
  diatom 
  were 
  most 
  easily 
  resolved 
  with 
  

   a 
  central, 
  solid, 
  unstopped 
  full 
  aplanatic 
  cone 
  of 
  over 
  0*90 
  

   from 
  a 
  dry 
  condenser. 
  The 
  larger 
  features 
  were 
  of 
  course 
  

   also 
  quite 
  correctly 
  and 
  sharply 
  imaged, 
  as 
  Dr. 
  Stoney's 
  are 
  

   not. 
  Need 
  it 
  be 
  asked 
  which 
  is 
  the 
  truest 
  image 
  ? 
  

  

  But 
  this 
  is 
  not 
  nearly 
  the 
  limit 
  j*. 
  Since 
  that 
  experiment 
  

   Mr. 
  E. 
  M. 
  Nelson 
  has 
  shown 
  me 
  A. 
  pellucida 
  clearly 
  resolved 
  

  

  * 
  A 
  rough 
  but 
  very 
  fair 
  idea 
  of 
  the 
  spherical 
  correction 
  may 
  be 
  obtained 
  

   almost 
  immediately 
  by 
  focussing 
  a 
  Fudura 
  test-scale 
  with 
  small 
  cone, 
  

   and 
  then 
  ascertaining 
  how 
  far 
  the 
  iris 
  can 
  be 
  opened 
  without 
  altering 
  

   the 
  image 
  of 
  the 
  exclamation-marks. 
  Using 
  successively 
  larger 
  annuli 
  of 
  

   light, 
  this 
  test 
  becomes 
  far 
  more 
  efficient 
  and 
  severe. 
  

  

  t 
  Owing 
  to 
  some 
  astigmatism 
  and 
  other 
  defects, 
  my 
  own 
  vision 
  is 
  

   very 
  coarse 
  and 
  imperfect 
  in 
  these 
  matters, 
  and 
  for 
  me 
  to 
  see 
  the 
  striae 
  

   means 
  mush 
  more 
  for 
  many 
  other 
  observers. 
  The 
  first 
  valve 
  Mr. 
  E. 
  M. 
  

   Nelson 
  showed 
  me 
  in 
  balsam 
  as 
  "strongly" 
  resolved, 
  was 
  to 
  my 
  sight 
  

   quite 
  unresolvable, 
  and 
  he 
  had 
  to 
  search 
  for 
  another, 
  which 
  I 
  was 
  able 
  

   to 
  see. 
  This 
  diatom 
  is 
  one 
  of 
  the 
  most 
  variable 
  in 
  resolvability 
  of 
  the 
  

   whole 
  list, 
  quite 
  apart 
  from 
  the 
  mere 
  coarseness 
  of 
  striation. 
  That 
  is 
  

   no 
  difficulty 
  at 
  all. 
  

  

  Phil. 
  Mag. 
  S. 
  5. 
  Vol. 
  45. 
  No. 
  277. 
  June 
  1898. 
  ■ 
  2 
  M 
  

  

  