﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  25 
  

  

  Both 
  flies 
  and 
  dragon-flies 
  have 
  pseudocone 
  eyes, 
  i. 
  e. 
  

   the 
  cone 
  is 
  merely 
  a 
  fluid 
  body, 
  and 
  not, 
  as 
  in 
  butterflies, 
  

   a 
  true 
  " 
  crystalhne 
  cone." 
  Presumably 
  the 
  latter 
  is 
  a 
  

   higher 
  development, 
  whilst 
  the 
  former 
  has 
  persisted, 
  at 
  

   least 
  in 
  the 
  dragon-flies, 
  from 
  great 
  antiquity. 
  It 
  is 
  

   when 
  we 
  turn 
  to 
  the 
  butterfly 
  eye 
  that 
  my 
  observations 
  

   do 
  not 
  confirm 
  Exner's 
  conclusions 
  as 
  to 
  the 
  manner 
  

   of 
  formation 
  of 
  an 
  apposition 
  image. 
  Here 
  we 
  have 
  to 
  

   do 
  with 
  a 
  eucone 
  eye, 
  and 
  I 
  am 
  convinced 
  that 
  in 
  butter- 
  

   flies 
  at 
  least 
  there 
  is 
  at 
  the 
  apex 
  of 
  the 
  cone 
  a 
  tiny 
  erect 
  

   image 
  of 
  that 
  part 
  of 
  the 
  field 
  appertaining 
  to 
  each 
  facet 
  

   unit. 
  I 
  first 
  saw 
  this 
  image 
  in 
  an 
  eye 
  of 
  Gonepteryx 
  rhamni 
  

   which 
  had 
  been 
  hardened 
  in 
  strong 
  formol 
  for 
  twenty-four 
  

   hours. 
  A 
  very 
  thin 
  slice 
  was 
  cut 
  off 
  and 
  mounted 
  on 
  a 
  

   drop 
  of 
  dilute 
  glycerine 
  on 
  a 
  cover 
  slip 
  and 
  the 
  whole 
  set 
  

   up 
  on 
  the 
  microscope 
  so 
  that 
  the 
  objective 
  was 
  applied 
  

   to 
  the 
  back 
  of 
  the 
  section. 
  Focussing 
  down, 
  in 
  spite 
  of 
  

   the 
  very 
  small 
  openings 
  corresponding 
  to 
  the 
  cone 
  apices, 
  

   the 
  usual 
  inverted 
  image 
  due 
  to 
  the 
  corneal 
  facets 
  could 
  

   be 
  seen. 
  Focussing 
  up 
  this 
  image 
  gradually 
  disappeared 
  

   and 
  was 
  replaced 
  by 
  a 
  much 
  less 
  distinct 
  but 
  nevertheless 
  

   erect 
  image, 
  i. 
  e. 
  an 
  image 
  of 
  a 
  pencil 
  or 
  similar 
  object 
  

   moves 
  the 
  same 
  way 
  as 
  the 
  object 
  itself 
  is 
  moved, 
  making 
  

   due 
  allowance 
  for 
  the 
  reversal 
  due 
  to 
  the 
  microscope. 
  

  

  This 
  peculiar 
  effect 
  is 
  by 
  no 
  means 
  easy 
  to 
  see. 
  It 
  is 
  

   not 
  due 
  to 
  the 
  combined 
  action 
  of 
  a 
  large 
  number 
  of 
  

   facet 
  units, 
  but 
  is 
  peculiar 
  to 
  each 
  facet. 
  One 
  must 
  

   have 
  a 
  very 
  favourable 
  section, 
  and 
  frequently 
  before 
  

   the 
  necessary 
  dehcate 
  adjustments 
  are 
  made 
  the 
  minute 
  

   drop 
  of 
  dilute 
  glycerine 
  has 
  suffered 
  from 
  the 
  evapora- 
  

   tion 
  of 
  the 
  water, 
  and 
  all 
  has 
  to 
  be 
  done 
  over 
  again. 
  

   For 
  some 
  time 
  after 
  I 
  first 
  saw 
  it 
  I 
  was 
  unable 
  to 
  get 
  

   it 
  again, 
  and 
  began 
  to 
  think 
  I 
  had 
  been 
  deceived. 
  Since 
  

   then, 
  however, 
  I 
  have 
  repeatedly 
  observed 
  it 
  in 
  the 
  

   eye 
  of 
  F. 
  urticae, 
  and 
  I 
  am 
  convinced 
  that 
  the 
  rays 
  of 
  

   light 
  which 
  reach 
  the 
  sensitive 
  layers 
  of 
  the 
  butterfly's 
  

   eye 
  do 
  so 
  in 
  their 
  proper 
  relations 
  corresponding 
  to 
  their 
  

   respective 
  positions 
  in 
  the 
  outer 
  world. 
  Let 
  us 
  now 
  

   consider 
  for 
  a 
  moment 
  the 
  difference 
  which 
  this 
  fact 
  may 
  

   make 
  in 
  the 
  acuity 
  of 
  the 
  insect's 
  vision. 
  I 
  have 
  drawn 
  

   on 
  a 
  large 
  sheet 
  of 
  paper 
  a 
  curve 
  representing 
  a 
  section 
  

   of 
  the 
  corneal 
  layer 
  of 
  the 
  eye 
  of 
  V. 
  urticae, 
  and 
  marked 
  

   it 
  ofi 
  into 
  as 
  many 
  divisions 
  as 
  I 
  found 
  facets 
  in 
  an 
  actual 
  

   section. 
  Then 
  I 
  drew 
  across 
  the 
  paper 
  straight 
  fines 
  each 
  

  

  