﻿500 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  

  

  of 
  spherical 
  aberration 
  has 
  been 
  considerably 
  over-rated 
  in 
  

   objectives 
  ! 
  " 
  The 
  most 
  eminent 
  firm 
  of 
  Continental 
  opticians 
  

   states 
  that 
  its 
  lenses, 
  owing 
  to 
  the 
  system 
  of 
  calculation 
  and 
  

   manufacture, 
  are 
  uniformly 
  free 
  from 
  spherical 
  aberration, 
  so 
  

   much 
  so 
  that 
  there 
  is 
  no 
  need 
  for 
  any 
  "empirical 
  tests/' 
  viz., 
  

   testing 
  upon 
  the 
  microscope 
  itself. 
  That 
  is 
  not 
  the 
  case 
  when 
  

   tested 
  by 
  the 
  more 
  perfect 
  English 
  appliances*. 
  Not 
  long 
  ago, 
  

   having 
  the 
  opportunity 
  of 
  testing 
  and 
  comparing 
  three 
  

   similar 
  objectives 
  together, 
  I 
  was 
  enabled 
  to 
  see 
  the 
  difference. 
  

   With 
  the 
  Abbe 
  condenser 
  there 
  was 
  no 
  very 
  obvious 
  dis- 
  

   tinction 
  ; 
  but 
  tested 
  by 
  English 
  condensers 
  it 
  was 
  quite 
  

   otherwise. 
  The 
  great 
  firm 
  had 
  no 
  cause 
  to 
  blush 
  for 
  any 
  one 
  

   of 
  them 
  ; 
  all 
  were 
  good 
  lenses 
  ; 
  but 
  they 
  now 
  revealed 
  as 
  distinct 
  

   characteristic 
  features 
  as 
  one 
  sees 
  in 
  individual 
  faces. 
  On 
  a 
  

   graduated 
  series 
  of 
  Poduras, 
  one 
  of 
  them 
  now 
  gave 
  most 
  

   unusually 
  good 
  definition 
  with 
  rather 
  a 
  small 
  cone 
  under 
  the 
  

   highest 
  ( 
  X 
  27) 
  eyepiece 
  ; 
  while 
  a 
  second, 
  scarcely 
  equal 
  in 
  

   this 
  point, 
  excelled 
  the 
  others 
  in 
  the 
  wide 
  cone 
  it 
  was 
  able 
  to 
  

   use 
  on 
  this 
  object. 
  I 
  had 
  the 
  curiosity 
  to 
  ask 
  another 
  operator 
  

   to 
  make 
  the 
  same 
  test. 
  He 
  is 
  probably 
  more 
  skilful 
  than 
  

   myself, 
  and 
  certainly 
  has 
  keener 
  vision 
  : 
  quite 
  independently 
  

   he 
  reached 
  identical 
  conclusions. 
  Slight 
  variations 
  of 
  pressure 
  

   in 
  the 
  final 
  polishing 
  of 
  the 
  glasses 
  are 
  quite 
  sufficient 
  to 
  

   produce 
  such 
  differences 
  as 
  these, 
  in 
  such 
  small 
  lenses 
  as 
  are 
  

   here 
  in 
  question. 
  

  

  Whether 
  this 
  latter 
  be 
  the 
  cause, 
  or 
  some 
  other, 
  nearly 
  all 
  

   high-power 
  objectives 
  even 
  of 
  the 
  present 
  day, 
  and 
  of 
  the 
  

   very 
  best 
  makers, 
  show 
  a 
  very 
  sensible 
  amount 
  of 
  aberration. 
  

   Drawing 
  a 
  circle 
  to 
  represent 
  the 
  whole 
  aperture, 
  and 
  smaller 
  

   concentric 
  circles 
  to 
  define 
  zones 
  of 
  its 
  surface, 
  many 
  of 
  the 
  

   zones 
  have 
  slightly 
  different 
  foci. 
  This 
  fact 
  plays 
  all 
  sorts 
  of 
  

   insidious 
  hanky-panky 
  tricks 
  with 
  small-cone 
  interference 
  

   images 
  of 
  the 
  Abbe 
  kind 
  ; 
  giving 
  more 
  force 
  to 
  such 
  of 
  the 
  

   spectra 
  as 
  are 
  correctly 
  focussed 
  than 
  to 
  the 
  others. 
  But 
  in 
  

   other 
  respects, 
  with 
  small 
  cones, 
  these 
  zonal 
  differences 
  are 
  

   not 
  obvious, 
  and 
  often 
  escape 
  detection, 
  many 
  portions 
  of 
  the 
  

   aperture 
  not 
  being 
  utilized 
  at 
  all. 
  There 
  are 
  refined 
  tests 
  

   familiar 
  to 
  opticians, 
  and 
  some 
  others 
  employed 
  by 
  highly- 
  

   skilled 
  microscopists 
  ; 
  but 
  not 
  only 
  are 
  these 
  too 
  seldom 
  

   employed 
  by 
  even 
  the 
  best 
  makers 
  before 
  the 
  lens 
  is 
  sent 
  

   forth, 
  but 
  we 
  have 
  seen 
  that 
  even 
  their 
  necessity 
  is 
  disputed, 
  

   and 
  the 
  importance 
  of 
  spherical 
  aberration 
  itself 
  actually 
  chal- 
  

  

  * 
  The 
  condenser 
  itself 
  is 
  an 
  English 
  appliance. 
  Ten 
  years 
  ago 
  only 
  

   one 
  house, 
  I 
  think, 
  made 
  one 
  with 
  wide 
  aplanatic 
  cone. 
  To-day 
  every 
  

   English 
  house 
  of 
  any 
  standing 
  constructs 
  achromatic 
  combinations 
  with 
  

   O'UO 
  of 
  aplanatic 
  cone, 
  and 
  two, 
  I 
  think, 
  construct 
  ape 
  chromatics. 
  

  

  