﻿20 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  part 
  of 
  the 
  visual 
  field 
  opposite 
  its 
  own 
  particular 
  facet. 
  

   If 
  the 
  corresponding 
  \nsual 
  rod 
  were 
  adjacent 
  to 
  the 
  cone 
  

   apex 
  and 
  separated 
  from 
  other 
  visual 
  rods 
  by 
  a 
  sleeve 
  

   of 
  pigment, 
  such 
  rod 
  would 
  receive 
  only 
  the 
  stimulus 
  

   proceeding 
  from 
  its 
  own 
  facet 
  unit 
  (lens 
  and 
  cone). 
  But 
  

   we 
  have 
  seen 
  that 
  not 
  only 
  are 
  the 
  visual 
  rods 
  at 
  some 
  

   distance 
  behind 
  the 
  cones, 
  but 
  also 
  the 
  pigment 
  is, 
  under 
  

   weak 
  light 
  conditions, 
  drawn 
  forward 
  so 
  as 
  to 
  expose 
  a 
  

   considerable 
  part 
  of 
  the 
  cone 
  apex. 
  The 
  result 
  is 
  that 
  

   each 
  visual 
  element 
  receives 
  not 
  only 
  the 
  stimulus 
  from 
  

   its 
  own 
  facet 
  unit, 
  but 
  also 
  that 
  of 
  the 
  peripheral 
  rays 
  

   from 
  neighbouring 
  facet 
  units. 
  The 
  images 
  it 
  receives 
  

   from 
  such 
  rays 
  are 
  superimposed 
  upon 
  those 
  thrown 
  from 
  

   its 
  own 
  cone 
  and 
  lens, 
  and 
  thus 
  the 
  image 
  is 
  made 
  brighter 
  

   and 
  more 
  of 
  the 
  light 
  entering 
  the 
  eye 
  is 
  utihsed. 
  This 
  is 
  

   the 
  true 
  superposition 
  image. 
  If 
  the 
  external 
  hght 
  be 
  

   strong, 
  the 
  pigment 
  moves 
  backwards 
  hke 
  a 
  dark 
  sleeve, 
  

   cutting 
  off 
  more 
  and 
  more 
  of 
  the 
  peripheral 
  rays, 
  thus 
  

   decreasing 
  the 
  luminosity 
  of 
  the 
  image, 
  without 
  interfering 
  

   with 
  its 
  clearness. 
  The 
  pigment, 
  in 
  fact, 
  has 
  a 
  similar 
  

   function 
  to 
  that 
  of 
  the 
  iris 
  of 
  the 
  vertebrate 
  eye. 
  

  

  Owing 
  to 
  a 
  peculiarity 
  in 
  the 
  morphology 
  of 
  the 
  glow- 
  

   worm's 
  eye 
  this 
  image 
  can 
  be 
  easily 
  seen. 
  The 
  cones 
  are 
  

   so 
  firmly 
  attached 
  to 
  the 
  facet 
  lenses 
  that 
  in 
  a 
  fresh 
  eye 
  

   all 
  the 
  nervous 
  apparatus 
  can 
  be 
  brushed 
  away 
  from 
  the 
  

   inside 
  of 
  the 
  eye, 
  leaving 
  a 
  little 
  hemispherical 
  shell 
  

   consisting 
  of 
  the 
  cornea 
  with 
  its 
  cones 
  in 
  position. 
  

  

  If 
  this 
  little 
  shell 
  be 
  mounted 
  on 
  a 
  small 
  drop 
  of 
  dilute 
  

   glycerine 
  (to 
  reproduce 
  the 
  refractive 
  effect 
  of 
  beetle 
  

   blood) 
  on 
  a 
  glass 
  slide, 
  and 
  then 
  viewed 
  from 
  the 
  back 
  

   through 
  a 
  medium-power 
  objective, 
  and 
  using 
  the 
  plane 
  

   mirror, 
  any 
  object 
  placed 
  between 
  the 
  corneal 
  surface 
  and 
  

   the 
  mirror 
  can 
  be 
  distinctly 
  seen 
  as 
  an 
  erect 
  image, 
  allowing 
  

   of 
  course 
  for 
  the 
  fact 
  that 
  it 
  is 
  inverted 
  again 
  by 
  the 
  

   microscope 
  itself. 
  

  

  The 
  beauty 
  and 
  sharpness 
  of 
  this 
  image 
  are 
  remarkable. 
  

   If 
  the 
  microscope 
  be 
  tilted 
  up, 
  the 
  mirror 
  removed, 
  and 
  

   the 
  whole 
  apparatus 
  pointed 
  towards 
  a 
  distant 
  landscape, 
  

   trees 
  and 
  any 
  other 
  objects 
  in 
  the 
  field 
  of 
  view 
  are 
  all 
  

   seen 
  with 
  surprising 
  precision. 
  

  

  Exner 
  succeeded 
  in 
  photographing 
  such 
  an 
  image 
  in 
  an 
  

   allied 
  species, 
  and 
  I 
  have 
  repeated 
  the 
  experiment 
  with 
  

   the 
  result 
  shown 
  on 
  PI. 
  IV, 
  fig. 
  2, 
  which 
  is 
  a 
  portrait 
  of 
  

   my 
  friend 
  Prof. 
  Poulton 
  taken 
  through 
  a 
  glow-worm's 
  eye. 
  

  

  