﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  21 
  

  

  It 
  has, 
  of 
  course, 
  lost 
  much 
  of 
  its 
  sharpness. 
  The 
  prac- 
  

   tical 
  difficulties 
  in 
  making 
  such 
  a 
  photograph 
  are 
  con- 
  

   siderable. 
  The 
  exposure 
  is 
  prolonged, 
  and 
  whilst 
  it 
  is 
  

   taking 
  place 
  the 
  water 
  is 
  evaporating 
  from 
  the 
  glycerine, 
  

   altering 
  all 
  the 
  time 
  the 
  refractive 
  index, 
  and 
  thus 
  afTecting 
  

   the 
  clearness 
  of 
  the 
  image. 
  Moreover, 
  small 
  as 
  the 
  picture 
  

   is, 
  it 
  is 
  an 
  enormously 
  enlarged 
  view 
  of 
  the 
  optical 
  image 
  

   which 
  in 
  the 
  actual 
  eye 
  has 
  an 
  area 
  calculated 
  roughly 
  

   at 
  "154 
  sq. 
  mm. 
  Exner 
  has 
  given 
  an 
  elaborate 
  mathematical 
  

   proof 
  of 
  the 
  formation 
  of 
  this 
  image, 
  but 
  I 
  have 
  neither 
  

   the 
  space 
  nor 
  the 
  mathematical 
  knowledge 
  to 
  deal 
  with 
  

   that 
  aspect 
  of 
  the 
  matter. 
  

  

  It 
  should 
  be 
  noted 
  that 
  the 
  picture 
  is 
  not 
  a 
  mosaic 
  at 
  

   all, 
  but 
  a 
  continuous 
  image, 
  and 
  doubtless 
  perceived 
  as 
  

   such 
  by 
  the 
  insect. 
  Those 
  who 
  have 
  examined 
  the 
  beetle 
  

   will 
  have 
  noted 
  how 
  the 
  whole 
  head, 
  including 
  the 
  eyes, 
  

   is 
  completely 
  overshadowed 
  above 
  by 
  an 
  opaque 
  chitinous 
  

   projection. 
  Evidently 
  the 
  insect 
  can 
  only 
  see 
  forwards 
  

   and 
  downwards. 
  No 
  doubt 
  the 
  natural 
  eye 
  shade 
  it 
  

   possesses 
  helps 
  to 
  give 
  it 
  an 
  additionally 
  acute 
  vision 
  for 
  

   objects 
  on 
  the 
  ground, 
  amongst 
  which 
  is 
  to 
  be 
  found 
  its 
  

   main 
  object 
  in 
  life, 
  its 
  mate. 
  

  

  Exner 
  points 
  out 
  that 
  two 
  features 
  easily 
  recognisable 
  

   in 
  the 
  eyes 
  of 
  insects 
  are, 
  when 
  present, 
  especially 
  adapted 
  

   to 
  the 
  production 
  of 
  a 
  superposition 
  image. 
  These 
  are, 
  

   first, 
  the 
  movement 
  under 
  varying 
  hght 
  intensity 
  of 
  the 
  

   " 
  iris 
  " 
  pigment, 
  and 
  secondly, 
  the 
  existence 
  of 
  a 
  space 
  

   between 
  the 
  cone 
  apices 
  and 
  the 
  visual 
  rods, 
  or 
  the 
  thickest 
  

   part 
  of 
  them. 
  

  

  Superposition 
  images 
  are 
  found 
  in 
  all 
  those 
  insects 
  which 
  

   have 
  to 
  make 
  the 
  most 
  of 
  feeble 
  light, 
  and 
  hence 
  are 
  

   specially 
  characteristic 
  of 
  nocturnal 
  and 
  crepuscular 
  

   Lepidoptera. 
  The 
  exclusively 
  apposition 
  image, 
  on 
  the 
  

   other 
  hand, 
  is 
  found 
  only 
  in 
  diurnal 
  insects, 
  such 
  as 
  butter- 
  

   flies, 
  flies, 
  and 
  dragon-flies. 
  This 
  form 
  of 
  image 
  approxi- 
  

   mates 
  very 
  closely, 
  according 
  to 
  Exner, 
  to 
  that 
  of 
  the 
  

   original 
  mosaic 
  theory 
  of 
  Miiller, 
  in 
  that 
  the 
  individual 
  

   " 
  image 
  " 
  produced 
  by 
  each 
  facet 
  unit 
  is 
  of 
  less 
  importance 
  

   as 
  an 
  image 
  than 
  as 
  a 
  spot 
  of 
  light. 
  

  

  It 
  is 
  at 
  this 
  point 
  that 
  the 
  results 
  of 
  my 
  own 
  researches 
  

   in 
  the 
  case 
  of 
  butterflies, 
  at 
  any 
  rate, 
  lead 
  me 
  to 
  con- 
  

   clusions 
  markedly 
  different 
  from 
  those 
  of 
  Exner; 
  but 
  I 
  

   will 
  speak 
  of 
  these 
  later. 
  To 
  continue, 
  Exner 
  describes 
  how 
  

   he 
  cut 
  off 
  slices 
  of 
  the 
  eyes 
  of 
  various 
  insects 
  which 
  have 
  

  

  