﻿12 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  percipient 
  organ, 
  it 
  must 
  either 
  perceive 
  only 
  a 
  small 
  

   portion 
  of 
  such 
  picture, 
  or 
  else, 
  if 
  large 
  enough 
  to 
  take 
  

   in 
  the 
  whole, 
  it 
  will 
  only 
  record 
  a 
  mixture 
  having 
  the 
  

   general 
  luminosity 
  of 
  the 
  picture. 
  The 
  difficulty 
  of 
  the 
  

   reversed 
  image 
  is 
  not 
  in 
  his 
  view 
  a 
  good 
  reason 
  for 
  rejecting 
  

   it, 
  since 
  spiders 
  with 
  six 
  to 
  eight 
  simple 
  eyes, 
  certain 
  water- 
  

   beetle 
  larvae 
  with 
  twelve, 
  and 
  other 
  creatures 
  must 
  have 
  

   inverted 
  images. 
  

  

  He 
  proceeds 
  to 
  classify 
  the 
  two 
  main 
  theories 
  as 
  first, 
  

   Miiller's 
  theory 
  of 
  mosaic 
  vision, 
  and 
  second, 
  " 
  the 
  little 
  

   picture 
  theory." 
  Miiller's 
  theory 
  provides 
  that 
  only 
  a 
  

   single 
  perceptive 
  element 
  occurs 
  behind 
  each 
  facet, 
  one 
  

   nerve 
  element 
  being 
  joined 
  to 
  each 
  cone. 
  In 
  opposition 
  

   thereto 
  the 
  picture 
  theory 
  will 
  be 
  upheld 
  when 
  a 
  multi- 
  

   phcity 
  of 
  such 
  elements, 
  serving 
  as 
  a 
  retina, 
  can 
  be 
  demon- 
  

   strated. 
  In 
  all 
  fairly 
  well-developed 
  visual 
  organs 
  in 
  the 
  

   animal 
  kingdom 
  are 
  found 
  structures 
  of 
  a 
  quite 
  specific 
  

   kind, 
  the 
  rods, 
  in 
  a 
  wide 
  sense, 
  which 
  are 
  to 
  be 
  regarded 
  

   as 
  the 
  percipient 
  end 
  organs. 
  In 
  the 
  simple 
  Arthropod 
  

   eye, 
  and 
  especially 
  in 
  the 
  stemma 
  of 
  insects 
  and 
  spiders, 
  

   these 
  rods 
  have 
  been 
  recognised. 
  In 
  their 
  main 
  agreement 
  

   with 
  those 
  of 
  other 
  animals 
  we 
  must 
  claim 
  for 
  them 
  the 
  

   same 
  signification, 
  the 
  more 
  so 
  since 
  other 
  retinal 
  elements 
  

   which 
  could 
  be 
  substituted 
  for 
  them 
  are 
  sought 
  in 
  vain. 
  

   If 
  we 
  accept 
  the 
  projection 
  on 
  the 
  retina 
  of, 
  for 
  example, 
  

   a 
  spider's 
  eye, 
  of 
  an 
  inverted 
  image, 
  we 
  must 
  also 
  agree 
  

   that 
  the 
  external 
  object 
  can 
  only 
  be 
  distinctly 
  seen 
  when 
  

   its 
  rays 
  come 
  to 
  a 
  focus 
  on 
  the 
  rod-bearing 
  region 
  of 
  the 
  

   retina. 
  For 
  the 
  sight 
  with 
  equal 
  distinctness 
  of 
  objects 
  

   at 
  varying 
  distances, 
  the 
  vertebrate 
  eye 
  possesses 
  an 
  

   accommodation 
  or 
  focussing 
  apparatus 
  of 
  which 
  there 
  is 
  

   no 
  trace 
  in 
  the 
  Arthropod 
  eye. 
  It 
  may 
  perhaps 
  find 
  a 
  

   partial 
  substitute 
  in 
  the 
  relatively 
  great 
  length 
  of 
  the 
  

   rods, 
  so 
  that 
  distant 
  objects 
  whose 
  images 
  fall 
  more 
  on 
  

   the 
  distal 
  ends 
  of 
  the 
  rods, 
  act 
  more 
  on 
  those 
  ends, 
  while 
  

   nearer 
  objects 
  focussed 
  more 
  in 
  the 
  depth 
  of 
  the 
  retina 
  

   may 
  specially 
  stimulate 
  the 
  hinder 
  end. 
  I 
  may 
  here 
  remark 
  

   in 
  parenthesis 
  that 
  this 
  remark 
  has 
  been 
  made 
  by 
  other 
  

   investigators, 
  as 
  for 
  instance 
  Patten 
  {vide 
  infra), 
  and 
  

   Avebury 
  has 
  raised 
  the 
  objection 
  that 
  the 
  nerves 
  which 
  

   are 
  first 
  reached 
  by 
  the 
  light 
  would 
  surely 
  be 
  affected 
  by 
  

   it. 
  As 
  against 
  this 
  objection, 
  it 
  may 
  be 
  urged 
  that 
  in 
  our 
  

   own 
  experience 
  the 
  attention 
  is 
  more 
  easily 
  focussed 
  on 
  a 
  

   defined 
  image 
  than 
  on 
  a 
  blurred 
  one, 
  and 
  thus 
  the 
  percep- 
  

  

  