﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  3 
  

  

  eyes 
  there 
  is 
  a 
  lens 
  which 
  can, 
  and 
  in 
  fact 
  does, 
  project 
  

   an 
  image 
  of 
  external 
  objects 
  on 
  to 
  a 
  sensitive 
  layer, 
  the 
  

   retina, 
  in 
  the 
  same 
  way 
  as 
  the 
  lens 
  of 
  a 
  photographic 
  

   camera 
  throws 
  an 
  image 
  on 
  to 
  the 
  sensitive 
  plate 
  at 
  the 
  

   back. 
  There 
  are 
  minor 
  differences, 
  such 
  as 
  the 
  curved 
  

   surface 
  of 
  the 
  retina, 
  the 
  method 
  of 
  focal 
  adjustment, 
  etc., 
  

   but 
  the 
  general 
  principle 
  is 
  the 
  same. 
  The 
  image 
  is 
  sharp 
  

   and 
  complete, 
  and 
  as 
  in 
  the 
  camera 
  it 
  is 
  upside 
  down. 
  

   That 
  we 
  are 
  unconscious 
  of 
  the 
  latter 
  fact 
  is 
  merely 
  due 
  to 
  

   interpretation 
  based 
  on 
  experience. 
  Objects 
  viewed 
  in 
  a 
  

   flat 
  mirror 
  appear 
  to 
  us 
  to 
  be 
  laterally 
  reversed 
  though 
  not 
  

   inverted. 
  Such 
  operations 
  as 
  we 
  habitually 
  perform 
  with 
  

   the 
  aid 
  of 
  a 
  mirror 
  are 
  done 
  with 
  perfect 
  facility 
  and 
  proper 
  

   co-ordination 
  of 
  movement, 
  whilst 
  those 
  which 
  are 
  not 
  

   generally 
  done 
  by 
  reflection, 
  such 
  as 
  writing 
  or 
  drawing, 
  are 
  

   found, 
  when 
  attempted, 
  to 
  be 
  extremely 
  difficult. 
  Nor 
  are 
  

   we 
  normally 
  conscious 
  of 
  the 
  fact 
  that 
  owing 
  to 
  the 
  

   possession 
  of 
  two 
  eyes 
  we 
  see 
  two 
  images, 
  those 
  of 
  near 
  

   objects 
  differing 
  considerably, 
  whilst 
  those 
  of 
  more 
  distant 
  

   ones 
  are 
  less 
  diverse. 
  Our 
  perceptive 
  faculties 
  combine 
  

   the 
  two 
  images 
  into 
  one, 
  at 
  the 
  same 
  time 
  utihsing 
  their 
  

   differences 
  to 
  obtain 
  an 
  estimate 
  of 
  distance. 
  The 
  well- 
  

   known 
  stereoscopic 
  photograph 
  is 
  adapted 
  to 
  this 
  faculty 
  

   and 
  gives 
  an 
  apparently 
  solid 
  picture 
  from 
  two 
  rather 
  

   different 
  flat 
  ones. 
  Examination 
  of 
  other 
  vertebrate 
  eyes 
  

   shows 
  their 
  structure 
  to 
  be 
  so 
  similar 
  that 
  we 
  are 
  justified 
  

   in 
  assuming 
  that 
  their 
  action 
  is 
  the 
  same, 
  though 
  there 
  

   are 
  a 
  few 
  exceptions, 
  such 
  as 
  the 
  chamaeleon, 
  whose 
  eyes 
  

   can 
  be 
  moved 
  quite 
  independently, 
  and 
  cannot 
  be 
  supposed 
  

   to 
  give 
  a 
  stereoscopic 
  image 
  except 
  when 
  they 
  both 
  happen 
  

   to 
  be 
  looking 
  in 
  the 
  same 
  direction.* 
  Also 
  most 
  fishes' 
  

   eyes 
  would 
  seem 
  to 
  be 
  so 
  placed 
  that 
  both 
  can 
  hardly 
  see 
  

   the 
  same 
  object 
  at 
  once. 
  

  

  So 
  far, 
  then, 
  we 
  may 
  claim 
  to 
  know 
  something 
  of 
  the 
  

   nature 
  of 
  our 
  own 
  eyes 
  and 
  of 
  those 
  which 
  are 
  similarly 
  

   formed. 
  The 
  knowledge 
  does 
  not 
  carry 
  us 
  very 
  far. 
  An 
  

   image 
  optically 
  described 
  as 
  a 
  real 
  image, 
  as 
  opposed 
  to 
  

   what 
  is 
  known 
  as 
  a 
  virtual 
  image, 
  seen 
  in 
  certain 
  lens 
  com- 
  

   binations, 
  is 
  projected 
  on 
  to 
  a 
  highly 
  complicated 
  nervous 
  

   tissue, 
  the 
  retina. 
  How 
  that 
  image 
  in 
  all 
  its 
  intricacy 
  of 
  

   form 
  and 
  colour 
  is 
  conveyed 
  to 
  and 
  perceived 
  by 
  the 
  

   brain, 
  we 
  do 
  not 
  know. 
  

  

  * 
  As 
  Prof. 
  Poulton 
  reminds 
  me, 
  the 
  chamaeleon 
  'f^' 
  eyes 
  are 
  always 
  

   co-ordinated 
  at 
  the 
  moment 
  of 
  striking. 
  

  

  