﻿494 
  Mr. 
  L. 
  Wright 
  on 
  Microscopic 
  Images 
  and 
  Vision. 
  

  

  and 
  which 
  look 
  like 
  a 
  honey-comb 
  ; 
  " 
  and 
  he 
  adds 
  that 
  (i 
  this 
  

   is 
  the 
  first 
  and 
  only 
  step 
  we 
  can 
  take 
  towards 
  learning 
  what 
  

   the 
  actual 
  detail 
  is," 
  because 
  no 
  objective 
  will 
  embrace 
  the 
  

   other 
  orders. 
  Examining 
  these 
  several 
  statements, 
  there 
  is 
  

   every 
  reason 
  to 
  believe 
  that 
  a 
  dry 
  objective 
  with 
  a 
  wide 
  cone 
  

   of 
  light 
  gives 
  a 
  perfectly 
  truthful 
  image, 
  while 
  it 
  will 
  give 
  

   the 
  hexagons 
  quite 
  easily 
  if 
  that 
  figure 
  is 
  preferred 
  ; 
  Zeiss' 
  s 
  

   well-known 
  large-scale 
  photograph 
  is 
  of 
  a 
  valve 
  so 
  coarse 
  

   that 
  it 
  is 
  beyond 
  dispute 
  * 
  that 
  a 
  portion 
  of 
  the 
  second-order 
  

   spectra 
  were 
  included 
  by 
  the 
  lens 
  used, 
  with 
  the 
  result 
  of 
  

   introducing 
  a 
  false 
  doubled 
  resolution 
  impossible 
  with 
  first 
  

   orders 
  alone 
  ; 
  and 
  an 
  immense 
  further 
  step 
  can 
  be 
  taken 
  by 
  

   using 
  a 
  first-rate 
  immersion-lens 
  of 
  1*40 
  aperture, 
  with 
  a 
  wide 
  

   cone. 
  The 
  Zeiss 
  photograph 
  x 
  4900, 
  and 
  the 
  Van 
  Heurck 
  

   photograph, 
  are 
  confessedly 
  the 
  highest 
  triumphs 
  of 
  photo- 
  

   graphy 
  by 
  the 
  Abbe 
  method 
  : 
  one 
  has 
  only 
  to 
  compare 
  both 
  

   with 
  the 
  beautiful 
  photograph 
  x 
  4900 
  taken 
  in 
  this 
  other 
  way 
  

   by 
  Mr. 
  E. 
  M. 
  Nelson, 
  and 
  other 
  similar 
  ones 
  up 
  to 
  a 
  scale 
  

   of 
  x 
  6400, 
  to 
  see, 
  once 
  for 
  all, 
  which 
  is 
  the 
  truest 
  image 
  f 
  , 
  

   and 
  the 
  all-importance 
  of 
  a 
  sufficiency 
  of 
  heterogeneous 
  

   light. 
  

  

  17. 
  We 
  can 
  also, 
  however, 
  see 
  the 
  large 
  amount 
  of 
  truth 
  

   in 
  the 
  Abbe 
  theory, 
  and 
  its 
  important, 
  though 
  not 
  all- 
  

   important, 
  place 
  in 
  microscopic 
  vision, 
  especially 
  for 
  certain 
  

   classes 
  of 
  objects. 
  Wherever 
  we 
  have 
  a 
  known 
  periodic 
  

   structure 
  in 
  transparent 
  objects, 
  plane-wave 
  illumination 
  and 
  

   the 
  consequent 
  interference-lines 
  formed 
  by 
  the 
  beams 
  dif- 
  

   fracted 
  by 
  that 
  structure, 
  have 
  an 
  extraordinary 
  effect 
  in 
  

   intensifying 
  into 
  black 
  and 
  white 
  a 
  more 
  or 
  less 
  accurate 
  

   representation 
  of 
  the 
  periodic 
  detail. 
  How 
  this 
  occurs 
  can 
  

   be 
  easily 
  seen 
  from 
  two 
  examples, 
  macroscopic 
  and 
  micro- 
  

   scopic. 
  

  

  * 
  As 
  demonstrated 
  by 
  Mr. 
  E. 
  M. 
  Nelson, 
  Journal 
  of 
  the 
  Quekett 
  Micro- 
  

   scopical 
  Club, 
  July 
  1890. 
  

  

  t 
  The 
  minute 
  detail 
  in 
  some 
  of 
  these 
  photographs 
  could 
  not 
  possibly 
  

   be 
  shown 
  by 
  Dr. 
  Stoney's 
  method, 
  because, 
  minute 
  as 
  they 
  are, 
  they 
  are 
  

   unsijmmetrical 
  and 
  not 
  periodic. 
  In 
  regard 
  to 
  the 
  P. 
  angulatum, 
  both 
  

   circular 
  disks 
  and 
  hexagons 
  can 
  be 
  seen, 
  depending 
  upon 
  the 
  precise 
  

   focus 
  ; 
  the 
  sharpest 
  portions 
  show 
  the 
  circles, 
  which, 
  disposed 
  in 
  quin- 
  

   cunx 
  arrangement, 
  most 
  diatomists 
  who 
  have 
  worked 
  with 
  English 
  

   appliances 
  believe 
  to 
  be 
  the 
  true 
  figure. 
  Besides 
  the 
  sharpest 
  image, 
  

   we 
  have 
  the 
  phenomena 
  of 
  " 
  postage-stamp 
  fracture," 
  and 
  the 
  shape 
  of 
  

   far 
  coarser 
  markings 
  in 
  other 
  diatoms 
  to 
  guide 
  us. 
  Mr. 
  C. 
  Haughton 
  Gill 
  

   has 
  demonstrated 
  that 
  the 
  spots 
  are 
  either 
  apertures 
  or 
  depressions, 
  by 
  

   depositing 
  pigment 
  in 
  them 
  ; 
  and 
  the 
  various 
  images 
  can 
  be 
  imitated 
  

   with 
  perforated 
  zinc. 
  It 
  is 
  the 
  distinct 
  outlines 
  of 
  the 
  fractures, 
  and 
  

   broken- 
  through 
  apertures, 
  which 
  are 
  so 
  magnificently 
  shown 
  in 
  Mr. 
  Nelson's 
  

   photograph 
  with 
  a 
  wide 
  cone. 
  

  

  