﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  27 
  

  

  two 
  pictures 
  must 
  represent 
  the 
  sort 
  of 
  view 
  from 
  which 
  the 
  

   sensitive 
  layers 
  of 
  the 
  insect's 
  eye 
  obtain 
  their 
  impression. 
  

  

  That 
  these 
  insects 
  are 
  extremely 
  short-sighted 
  every 
  

   one 
  who 
  has 
  observed 
  them 
  will 
  agree. 
  We 
  know 
  what 
  

   a 
  conspicuous 
  object 
  a 
  white 
  butterfly 
  is. 
  We 
  can 
  see 
  it 
  

   at 
  great 
  distances, 
  and 
  yet 
  two 
  white 
  butterflies 
  will 
  often 
  

   pass 
  within 
  a 
  few 
  feet 
  of 
  each 
  other 
  without 
  either 
  being 
  

   apparently 
  conscious 
  of 
  the 
  other's 
  presence. 
  We 
  know 
  

   how 
  readily 
  one 
  white 
  butterfly 
  will 
  pursue 
  and 
  investigate 
  

   another 
  to 
  see 
  if 
  it 
  is 
  a 
  suitable 
  mate, 
  but 
  I 
  have 
  never 
  

   seen 
  this 
  kind 
  of 
  flirtation 
  begin 
  from 
  a 
  distance 
  of 
  more 
  

   than 
  a 
  few 
  feet. 
  Nevertheless, 
  in 
  my 
  view 
  a 
  butterfly's 
  

   sight 
  is 
  much 
  more 
  acute 
  than 
  the 
  figures 
  on 
  Pis. 
  Ill 
  and 
  

   IV 
  would 
  suggest, 
  and 
  for 
  this 
  reason 
  : 
  I 
  believe 
  that 
  the 
  

   rays 
  of 
  Hght 
  entering 
  the 
  facet 
  unit 
  are 
  not 
  recorded 
  as 
  a 
  

   light 
  spot, 
  or 
  as 
  an 
  inverted 
  image, 
  but 
  as 
  an 
  erect 
  image, 
  

   the 
  whole 
  field 
  of 
  view 
  being 
  represented 
  by 
  a 
  mosaic 
  of 
  

   little 
  erect 
  images, 
  thus 
  forming 
  a 
  continuous 
  picture. 
  

   I 
  have 
  already 
  explained 
  how 
  I 
  have 
  actually 
  seen 
  these 
  

   erect 
  images, 
  albeit 
  with 
  difficulty 
  and 
  not 
  very 
  clearly. 
  

  

  There 
  is 
  one 
  feature 
  of 
  the 
  butterfly's 
  eye 
  which 
  does 
  

   not 
  appear 
  to 
  be 
  present 
  in 
  dipterous 
  and 
  other 
  pseudocone 
  

   eyes 
  — 
  viz. 
  the 
  already 
  described 
  processus 
  corneae. 
  Now 
  

   Exner 
  regards 
  the 
  lens 
  and 
  cone 
  together, 
  in 
  an 
  apposition 
  

   eye, 
  as 
  forming 
  a 
  lens 
  cylinder 
  of 
  which 
  the 
  actual 
  length 
  and 
  

   focal 
  length 
  are 
  the 
  same, 
  hence 
  giving 
  an 
  inverted 
  image. 
  

  

  This 
  condition 
  may, 
  and 
  apparently 
  does, 
  obtain 
  in 
  some 
  

   eyes 
  in 
  which 
  the 
  short 
  cone 
  is 
  closely 
  adjacent 
  to 
  the 
  lens, 
  

   forming 
  what 
  may 
  be 
  regarded 
  as 
  a 
  physical 
  entity, 
  or 
  

   where, 
  as 
  appears 
  to 
  be 
  the 
  case 
  in 
  a 
  fly's 
  eye, 
  the 
  pseudo- 
  

   cone 
  is 
  of 
  such 
  feeble 
  refractive 
  power 
  as 
  to 
  have 
  little 
  

   effect 
  on 
  the 
  image 
  already 
  produced 
  by 
  the 
  facet 
  lens. 
  

  

  Now 
  in 
  the 
  butterfly 
  the 
  'processus 
  corneae 
  lies 
  between 
  

   the 
  lens 
  and 
  the 
  cone, 
  so 
  that 
  these 
  latter 
  structures 
  pre- 
  

   sumably 
  do 
  not 
  act 
  together 
  as 
  one 
  lens 
  cylinder 
  of 
  its 
  own 
  

   focal 
  length. 
  What 
  I 
  conceive 
  takes 
  place 
  is 
  that 
  the 
  

   lens 
  produces 
  an 
  inverted 
  image 
  (this 
  can, 
  of 
  course, 
  be 
  

   very 
  clearly 
  seen), 
  the 
  rays 
  from 
  it 
  are 
  collected 
  by 
  the 
  

   cone, 
  and 
  that 
  is 
  a 
  lens 
  cylinder 
  of 
  its 
  own 
  focal 
  length. 
  

   Acting 
  on 
  the 
  principle 
  already 
  explained, 
  the 
  cone 
  reinverts 
  

   the 
  image, 
  passing 
  the 
  rays 
  out 
  at 
  its 
  apex 
  or 
  inner 
  end 
  

   practically 
  parallel. 
  

  

  From 
  here 
  they 
  pass 
  down 
  the 
  rhabdom 
  and 
  stimulate 
  

   the 
  retinula 
  cells. 
  The 
  stimulation 
  does 
  not 
  take 
  place 
  on 
  

  

  