﻿502 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision, 
  

  

  into 
  stripe, 
  mounted 
  in 
  balsam, 
  as 
  well 
  as 
  " 
  dry," 
  with 
  a 
  similar 
  

   cone 
  of 
  over 
  0*90 
  from 
  Powell's 
  apochromatic 
  condenser, 
  and 
  

   a 
  Zeiss 
  g- 
  apochromatic 
  of 
  1*40. 
  This 
  latter 
  lens 
  was 
  probably 
  

   one 
  of 
  the 
  finest 
  ever 
  made 
  — 
  I 
  at 
  least 
  never 
  saw 
  its 
  equal 
  — 
  

   and 
  the 
  mere 
  stria? 
  were 
  not 
  all 
  it 
  had 
  to 
  tell 
  us, 
  using 
  no 
  

   arrangements 
  beyond 
  the 
  0'90 
  full 
  cone, 
  and 
  Giffard's 
  green 
  

   light-filter. 
  On 
  a 
  dry 
  valve, 
  it 
  clearly 
  displayed 
  where 
  bits 
  

   of 
  coarser 
  upper 
  membrane 
  with 
  their 
  blacker 
  lines 
  were 
  

   overlying 
  the 
  lower, 
  as 
  is 
  more 
  often 
  seen 
  in 
  A. 
  Lindheimerii. 
  

   And 
  on 
  a 
  strong 
  valve 
  in 
  quinidine, 
  carefully 
  adjusting 
  for 
  

   what 
  may 
  be 
  termed 
  the 
  " 
  white 
  w 
  focus, 
  each 
  of 
  the 
  striae 
  

   could 
  be 
  seen 
  clearly 
  outlined 
  at 
  both 
  edges, 
  the 
  outlines 
  

   being 
  a 
  series 
  of 
  small 
  convex 
  curves, 
  scalloping 
  out 
  the 
  stria 
  

   into 
  partly-defined 
  oval 
  beads. 
  The 
  divisions 
  or 
  narrower 
  

   necks 
  between 
  these 
  partly-defined 
  ovals 
  did 
  not 
  lie 
  in 
  longi- 
  

   tudinal 
  rows, 
  but 
  occurred 
  with 
  a 
  considerable 
  degree 
  of 
  

   irregularity. 
  I 
  think 
  it 
  probable 
  that 
  such 
  resolution, 
  which 
  

   most 
  closely 
  parallels 
  the 
  coarser 
  Lindheimerii 
  valve, 
  may 
  be 
  

   the 
  truest 
  resolution 
  yet 
  attained. 
  

  

  No 
  doubt 
  the 
  above 
  lens 
  was 
  (for 
  the 
  present) 
  an 
  almost 
  

   phenomenal 
  one. 
  Every 
  practical 
  microscopist 
  knows 
  that 
  

   the 
  " 
  similar 
  " 
  objectives, 
  by 
  even 
  the 
  very 
  best 
  makers, 
  are 
  

   not 
  " 
  all 
  alike," 
  whatever 
  the 
  makers 
  may 
  affirm. 
  They 
  differ 
  

   in 
  features 
  as 
  in 
  a 
  case 
  above 
  mentioned 
  ; 
  most 
  of 
  all 
  in 
  the 
  

   cone 
  they 
  can 
  employ 
  in 
  critical 
  work, 
  and 
  in 
  what 
  such 
  a 
  

   cone 
  will 
  reveal. 
  Eveiyone 
  engaged 
  in 
  difficult 
  research 
  has 
  

   some 
  favourite 
  objective, 
  treasured 
  and 
  spared 
  in 
  work 
  as 
  

   much 
  as 
  possible 
  ; 
  because 
  he 
  knows 
  full 
  well 
  that 
  if 
  parted 
  

   with 
  or 
  injured, 
  though 
  he 
  can 
  buy 
  a 
  "similar." 
  one 
  at 
  the 
  

   list 
  price, 
  it 
  may 
  be 
  long 
  ere 
  he 
  finds 
  such 
  another! 
  

  

  22. 
  And 
  this 
  brings 
  us 
  to 
  the 
  last 
  point. 
  The 
  question 
  of 
  

   how 
  far 
  we 
  may 
  still 
  expect 
  advances 
  in 
  our 
  optical 
  powers 
  

   of 
  research 
  is 
  important 
  ; 
  and 
  it 
  is 
  answered 
  very 
  differently 
  

   according 
  to 
  the 
  " 
  spectrum 
  " 
  theory, 
  or 
  the 
  qualified 
  views 
  

   here 
  maintained. 
  It 
  not 
  only 
  follows 
  from 
  the 
  foregoing, 
  

   but 
  has 
  been 
  over 
  and 
  over 
  again 
  stated 
  expressly 
  by 
  the 
  Abbe 
  

   school, 
  that 
  we 
  have 
  no 
  hope 
  of 
  further 
  advance, 
  except 
  

   through 
  increase 
  of 
  aperture 
  ; 
  and 
  on 
  that 
  ground 
  was 
  con- 
  

   structed 
  the 
  lens 
  of 
  1*63 
  N. 
  A. 
  to 
  be 
  used 
  with 
  flint-glass 
  

   mounts 
  and 
  dense 
  fluid 
  media 
  — 
  conditions 
  under 
  which 
  it 
  is 
  

   practically 
  useless. 
  So 
  little 
  are 
  other 
  conditions 
  recognized, 
  

   that 
  Dr. 
  Van 
  Heurck 
  has 
  only 
  used 
  the 
  chromatic 
  condenser 
  

   in 
  his 
  skilful 
  published 
  diatom 
  photographs 
  ; 
  and 
  those 
  results 
  

   are 
  simply 
  nil, 
  not 
  one 
  of 
  them 
  surpassing, 
  or 
  in 
  some 
  

   respects 
  even 
  equalling, 
  what 
  has 
  been 
  done- 
  in 
  England 
  

   with 
  1*40 
  lenses. 
  

  

  