﻿24 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  to 
  be 
  called 
  an 
  image, 
  and 
  this 
  shadow 
  moved, 
  as 
  one 
  

   would 
  have 
  expected, 
  in 
  the 
  same 
  direction 
  (allowing 
  for 
  

   the 
  effect 
  of 
  the 
  microscope) 
  as 
  the 
  movement 
  of 
  the 
  

   pencil. 
  A 
  similar 
  experiment 
  with 
  an 
  eye 
  of 
  Hemaris 
  

   fuciformis 
  gave 
  a 
  rather 
  clearer 
  shadow 
  or 
  image. 
  In 
  

   general 
  the 
  observations, 
  which 
  were 
  numerous, 
  merely 
  

   confirmed 
  the 
  results 
  obtained 
  by 
  Exner 
  with 
  a 
  large 
  

   dragon-fly. 
  The 
  apices 
  of 
  the 
  cones 
  are 
  so 
  small 
  that 
  by 
  

   the 
  means 
  at 
  our 
  disposal 
  it 
  is 
  very 
  difficult 
  to 
  see 
  the 
  

   image, 
  if 
  any, 
  which 
  occurs 
  there. 
  Nevertheless 
  on 
  another 
  

   occasion, 
  having 
  succeeded 
  in 
  obtaining 
  a 
  portion 
  of 
  the 
  

   cornea 
  of 
  a 
  Libellula 
  with 
  the 
  cones 
  still 
  attached, 
  I 
  could 
  

   see 
  first 
  of 
  all 
  the 
  usual 
  sharp 
  inverted 
  image 
  caused 
  by 
  

   the 
  corneal 
  facet, 
  and 
  focussing 
  backwards, 
  there 
  appeared 
  

   to 
  be, 
  in 
  the 
  neighbourhood 
  of 
  the 
  cone 
  apices, 
  a 
  much 
  

   smaller, 
  much 
  less 
  distinct 
  image, 
  still 
  inverted. 
  I 
  have 
  

   already 
  mentioned 
  that 
  in 
  the 
  blow-fly 
  Exner 
  claims 
  to 
  

   have 
  seen 
  at 
  the 
  cone 
  apices 
  the 
  two 
  hght 
  points 
  used 
  by 
  

   him 
  as 
  objects, 
  and 
  to 
  have 
  satisfied 
  himself 
  that 
  the 
  

   image 
  at 
  the 
  apex 
  is 
  an 
  inverted 
  one. 
  I 
  have 
  examined 
  

   flies' 
  eyes 
  in 
  the 
  same 
  way, 
  and 
  though 
  the 
  presence 
  of 
  

   an 
  image 
  at 
  all, 
  beyond 
  that 
  of 
  the 
  corneal 
  facet, 
  seems 
  

   rather 
  doubtful, 
  there 
  certainly 
  seems 
  no 
  evidence 
  that 
  

   the 
  cone 
  rein 
  verts 
  the 
  corneal 
  image. 
  

  

  We 
  must, 
  I 
  think, 
  conclude 
  that 
  in 
  flies 
  and 
  dragon- 
  

   flies 
  the 
  picture 
  presented 
  to 
  the 
  perceptive 
  elements 
  is 
  

   a 
  mosaic 
  of 
  light 
  spots 
  but 
  little 
  if 
  at 
  all 
  modified 
  from 
  

   that 
  supposed 
  by 
  Miiller. 
  This 
  may 
  seem 
  a 
  disappointing 
  

   performance 
  for 
  the 
  enormous 
  and 
  comphcated 
  eye 
  of 
  a 
  

   dragon-fly, 
  but 
  we 
  must 
  not 
  forget 
  the 
  relative 
  size 
  of 
  

   the 
  eye. 
  The 
  sharpness 
  of 
  the 
  view 
  obtained 
  with 
  a 
  

   mosaic 
  of 
  hght 
  dots 
  obviously 
  increases 
  in 
  proportion 
  to 
  

   the 
  number 
  of 
  the 
  elements 
  making 
  up 
  the 
  mosaic. 
  If 
  

   the 
  rods 
  and 
  cones 
  of 
  the 
  vertebrate 
  eye 
  are 
  the 
  separate 
  

   elements 
  of 
  the 
  visual 
  apparatus, 
  presumably 
  each 
  is 
  

   stimulated 
  by 
  a 
  minute 
  bundle 
  of 
  rays 
  which 
  is 
  in 
  itself 
  

   merely 
  a 
  light 
  stimulus 
  and 
  not 
  a 
  picture, 
  so 
  that 
  our 
  

   own 
  vision 
  may 
  be 
  said 
  to 
  be 
  a 
  mosaic 
  with 
  exceedingly 
  

   small 
  elements, 
  and 
  the 
  difference 
  between 
  this 
  and 
  the 
  

   image 
  of 
  the 
  facetted 
  eye 
  may 
  be 
  roughly 
  compared 
  with 
  

   the 
  difference 
  between 
  a 
  half-tone 
  block 
  made 
  with 
  the 
  

   very 
  finest 
  screen 
  and 
  the 
  corresponding 
  picture 
  as 
  repre- 
  

   sented 
  by 
  the 
  very 
  coarse 
  screening 
  used 
  in 
  the 
  common 
  

   newspaper 
  pictorial 
  reproduction. 
  

  

  