﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  15 
  

  

  called. 
  He 
  suggests 
  that 
  what 
  is 
  known 
  as 
  the 
  retinula 
  

   may 
  be 
  regarded 
  as 
  a 
  much-reduced 
  retina 
  having 
  seven 
  

   percipient 
  elements. 
  In 
  certain 
  acone 
  and 
  pseudocone 
  

   eyes 
  the 
  perceptive 
  elements 
  are 
  isolated, 
  but 
  it 
  is 
  to 
  be 
  

   noted 
  that 
  even 
  if 
  each 
  were 
  stimulated, 
  seven 
  elements 
  

   could 
  not 
  impart 
  to 
  the 
  sensorium 
  an 
  impression 
  of 
  

   every 
  single 
  object 
  of 
  the 
  picture, 
  nor 
  could 
  a 
  number 
  of 
  

   pictures 
  projected 
  by 
  neighbouring 
  facets 
  compensate 
  for 
  

   this 
  deficiency. 
  The 
  difficulty 
  is 
  greater 
  in 
  those 
  more 
  

   developed 
  eyes 
  in 
  which 
  the 
  rods 
  are 
  fused 
  into 
  a 
  rhabdom, 
  

   since 
  the 
  picture 
  elements 
  would 
  be 
  greatly 
  reduced 
  in 
  size. 
  

   From 
  the 
  close 
  approach 
  of 
  the 
  rods 
  it 
  must 
  be 
  doubtful 
  

   if 
  a 
  single 
  rod 
  responds 
  individually 
  to 
  any 
  one 
  stimulus 
  

   without 
  the 
  co-operation 
  of 
  its 
  neighbours. 
  

  

  Summing 
  up, 
  he 
  states 
  that 
  in 
  isolated 
  cases 
  there 
  is 
  a 
  

   lack 
  of 
  the 
  necessary 
  curved 
  refractive 
  medium. 
  In 
  other 
  

   cases 
  there 
  may 
  be 
  an 
  image, 
  but 
  far 
  behind 
  the 
  eye. 
  In 
  

   still 
  other 
  cases 
  an 
  image 
  has 
  been 
  seen, 
  but 
  far 
  from 
  where 
  

   it 
  could 
  be 
  effective. 
  The 
  more 
  proximal 
  projection 
  of 
  

   this 
  image 
  is 
  precluded 
  by 
  the 
  position 
  of 
  the 
  pigment, 
  

   added 
  to 
  which 
  there 
  is 
  in 
  most 
  cases 
  the 
  proximal 
  with- 
  

   drawal 
  of 
  the 
  percipient 
  from 
  the 
  refractive 
  media. 
  In 
  all 
  

   cases, 
  without 
  exception, 
  the 
  sharpest 
  projection 
  of 
  an 
  

   image 
  would 
  be 
  without 
  effect 
  owing 
  to 
  the 
  inadequate 
  

   number 
  of 
  the 
  perceptive 
  elements 
  in 
  each 
  ommatidium, 
  

   and 
  there 
  is 
  not 
  in 
  any 
  case, 
  he 
  holds, 
  much 
  ground 
  for 
  

   regarding 
  them 
  as 
  more 
  than 
  a 
  single 
  perceptive 
  entity. 
  

   The 
  picture 
  theory 
  is 
  therefore 
  untenable 
  . 
  He 
  then 
  proceeds 
  

   to 
  discuss 
  the 
  path 
  of 
  the 
  light 
  rays 
  entering 
  each 
  facet. 
  

   The 
  angle 
  at 
  which 
  these 
  fall 
  will 
  determine 
  whether 
  they 
  

   are 
  to 
  be 
  more 
  or 
  less 
  totally 
  absorbed 
  or 
  reflected. 
  The 
  

   axial 
  rays 
  will 
  have 
  a 
  simple 
  and 
  direct 
  path 
  to 
  the 
  rhabdom, 
  

   and 
  these 
  will 
  be 
  the 
  principal 
  ones 
  to 
  affect 
  the 
  percipient 
  

   elements. 
  He 
  reasserts 
  his 
  opinion 
  that 
  though 
  the 
  rod 
  

   elements 
  are 
  multiple, 
  they 
  can 
  only 
  be 
  regarded 
  physio- 
  

   logically 
  as 
  an 
  entity. 
  " 
  Each 
  set 
  of 
  rays 
  will 
  come 
  rela- 
  

   tively 
  to 
  the 
  position 
  they 
  occupied 
  in 
  the 
  outer 
  world, 
  

   and 
  this 
  constitutes 
  the 
  erect 
  image 
  of 
  the 
  compound 
  eye." 
  

   He 
  points 
  out 
  the 
  insufficiency 
  of 
  the 
  facet 
  unit 
  for 
  the 
  

   reception 
  of 
  the 
  many 
  elements 
  which 
  must 
  be 
  distin- 
  

   guished 
  to 
  form 
  a 
  true 
  image, 
  the 
  strong 
  support 
  which 
  

   anatomical 
  research 
  gives 
  to 
  the 
  mosaic 
  theory, 
  and 
  the 
  

   impossibility 
  of 
  understanding 
  an 
  aggregation 
  of 
  thousands 
  

   of 
  complete 
  eyes, 
  all 
  perceiving 
  images, 
  the 
  more 
  so 
  if 
  each 
  

  

  