﻿18 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  by 
  a 
  thread, 
  and 
  he 
  states 
  that 
  he 
  has 
  no 
  explanation 
  to 
  

   offer 
  for 
  the 
  structure. 
  Why 
  he 
  should 
  have 
  seen 
  it 
  only 
  

   in 
  C. 
  pamphihis 
  is 
  difficult 
  to 
  understand, 
  as 
  a 
  homologous 
  

   structure 
  is 
  so 
  obvious 
  in 
  V. 
  vrticae, 
  V. 
  io, 
  and 
  other 
  

   diurnal 
  Lepidoptera 
  that 
  it 
  is 
  a 
  prominent 
  feature 
  in 
  any 
  

   good 
  section. 
  I 
  have 
  already 
  described 
  this 
  feature 
  and 
  

   indicated 
  its 
  true 
  nature 
  as 
  a 
  part 
  of 
  the 
  tracheal 
  system. 
  

   Kurt 
  Bedau 
  (Zeit. 
  f. 
  Wiss. 
  Zool., 
  xcvii, 
  1911 
  : 
  Facettenauge 
  

   der 
  Wasserwanzen) 
  states 
  that 
  his 
  researches 
  into 
  the 
  

   innervation 
  of 
  the 
  retinula 
  have 
  not 
  had 
  much 
  result. 
  

   Even 
  with 
  a 
  yV'i^ich 
  apochromatic 
  and 
  compensating 
  

   ocular 
  he 
  has 
  not 
  been 
  able 
  to 
  trace 
  in 
  the 
  ommatidia 
  the 
  

   nerves 
  which 
  pass 
  thence 
  through 
  the 
  basal 
  membrane. 
  

   He 
  has 
  only 
  been 
  able 
  to 
  see 
  with 
  certainty 
  that 
  the 
  

   number 
  of 
  nerve 
  fibres 
  corresponds 
  to 
  the 
  eight 
  visual 
  

   cells. 
  He 
  criticises 
  unfavourably 
  Patten's 
  work 
  and 
  ex- 
  

   presses 
  the 
  view 
  that 
  what 
  that 
  author 
  took 
  for 
  nerve 
  

   fibrillae 
  in 
  the 
  crystalline 
  cone 
  were 
  the 
  intensively 
  stained 
  

   plasma 
  edges 
  of 
  the 
  four 
  .cells 
  of 
  the 
  cone. 
  Many 
  other 
  

   authors 
  have 
  dealt 
  with 
  various 
  aspects 
  of 
  the 
  subject, 
  

   but 
  with 
  one 
  exception 
  I 
  have 
  probably 
  given 
  a 
  sufficient 
  

   resume 
  of 
  previous 
  work 
  to 
  illustrate 
  the 
  difficulty 
  and 
  

   complexity 
  of 
  the 
  insect 
  eye. 
  The 
  exception 
  referred 
  to 
  

   is 
  the 
  work 
  of 
  Prof. 
  S. 
  Exner 
  (" 
  Die 
  Physiologic 
  der 
  facet- 
  

   tirten 
  Augen 
  von 
  Krebsen 
  und 
  Insekten. 
  Leipsig 
  und 
  

   Wien, 
  1891). 
  To 
  Exner 
  falls 
  the 
  credit 
  of 
  having 
  demon- 
  

   strated 
  experimentally 
  the 
  existence 
  and 
  nature 
  of 
  the 
  

   image 
  in 
  the 
  facetted 
  eye, 
  or 
  at 
  least 
  in 
  certain 
  types 
  of 
  

   that 
  organ, 
  and 
  it 
  is 
  my 
  intention 
  here 
  to 
  set 
  forth 
  the 
  

   conclusions 
  arrived 
  at 
  in 
  that 
  admirable 
  piece 
  of 
  research. 
  

   It 
  is 
  the 
  custom 
  of 
  many 
  writers 
  to 
  quote 
  from 
  foreign 
  

   works 
  in 
  the 
  original, 
  but 
  while 
  the 
  practice 
  may 
  reheve 
  

   them 
  of 
  any 
  errors 
  of 
  interpretation, 
  it 
  is 
  of 
  little 
  assistance 
  

   to 
  those 
  readers 
  who 
  may 
  not 
  have 
  had 
  the 
  opportunity 
  

   of 
  acquiring 
  a 
  knowledge 
  of 
  the 
  languages 
  in 
  which 
  such 
  

   quotations 
  are 
  written. 
  Exner 
  's 
  work 
  is 
  of 
  such 
  import- 
  

   ance, 
  and 
  the 
  only 
  Enghsh 
  summary 
  I 
  have 
  seen 
  (" 
  Senses 
  

   of 
  Insects," 
  Forel; 
  Eng. 
  Trans., 
  Yearsley. 
  London, 
  1908) 
  

   so 
  inadequate, 
  that 
  I 
  feel 
  justified 
  in 
  endeavouring 
  to 
  

   give 
  a 
  more 
  complete 
  account 
  of 
  it 
  than 
  has 
  hitherto 
  

   been 
  attempted. 
  

  

  Exner 
  first 
  experimented 
  with 
  the 
  eye 
  of 
  Hydrophilus 
  

   piceus, 
  and 
  found 
  that 
  in 
  such 
  eyes 
  the 
  refraction 
  of 
  the 
  

   rays 
  by 
  the 
  dioptric 
  portion 
  is 
  of 
  a 
  complicated 
  character, 
  

  

  