﻿16 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  be 
  inverted. 
  He 
  compares 
  the 
  simple 
  and 
  compound 
  eye 
  

   as 
  starting 
  from 
  a 
  primitive 
  form, 
  which 
  developed 
  on 
  the 
  

   one 
  hand 
  into 
  an 
  eye 
  with 
  better 
  lens, 
  and 
  more 
  perceptive 
  

   elements 
  (as 
  in 
  a 
  spider's 
  eye), 
  on 
  the 
  other 
  hand 
  an 
  indi- 
  

   vidual 
  retrogressive 
  movement, 
  compensated 
  by 
  great 
  

   numbers 
  of 
  elements 
  in 
  definite 
  arrangement 
  and 
  direction. 
  

   By 
  variation 
  in 
  form 
  and 
  co-operation 
  of 
  the 
  pigment, 
  they 
  

   effect 
  not, 
  like 
  the 
  simple 
  eye, 
  the 
  collection, 
  but 
  rather 
  the 
  

   isolation, 
  of 
  the 
  separate 
  rays. 
  

  

  Grenacher 
  undoubtedly 
  makes 
  out 
  a 
  good 
  case 
  for 
  the 
  

   mosaic 
  theory 
  of 
  vision, 
  both 
  on 
  general 
  principles 
  and 
  in 
  

   view 
  of 
  the 
  structure 
  of 
  the 
  eye, 
  though 
  we 
  shall 
  see 
  that 
  

   his 
  theories 
  have 
  to 
  undergo 
  considerable 
  modification 
  in 
  

   view 
  of 
  more 
  recent 
  research. 
  

  

  Lowne's 
  view 
  (Trans. 
  Linn. 
  Soc. 
  Zool. 
  ii, 
  pt. 
  ii, 
  p. 
  389, 
  

   etc., 
  1884) 
  that 
  the 
  insect 
  retina 
  Hes 
  behind 
  the 
  basal 
  

   membrane 
  merits 
  httle 
  consideration 
  in 
  view 
  of 
  our 
  present 
  

   knowledge. 
  It 
  is 
  largely 
  based 
  on 
  the 
  assertion 
  that 
  there 
  

   is 
  no 
  evidence 
  that 
  the 
  nerves 
  pass 
  through 
  the 
  basal 
  

   membrane. 
  It 
  is 
  difficult 
  to 
  understand 
  how 
  the 
  author 
  

   could 
  have 
  made 
  such 
  a 
  statement 
  had 
  he 
  examined 
  even 
  

   a 
  moderate 
  number 
  of 
  sections. 
  Moreover, 
  as 
  will 
  be 
  

   described 
  later, 
  we 
  can 
  under 
  certain 
  circumstances 
  see 
  

   the 
  image 
  in 
  an 
  insect's 
  eye, 
  and 
  that 
  image 
  certainly 
  does 
  

   not 
  he 
  behind 
  or 
  even 
  near 
  the 
  basal 
  membrane. 
  Hickson 
  

   {I.e.) 
  treats 
  mainly 
  of 
  the 
  eye 
  of 
  the 
  Blow-fly, 
  and 
  more 
  

   particularly 
  with 
  the 
  nervous 
  structure 
  of 
  the 
  gangha. 
  

   He 
  considers 
  that 
  the 
  balance 
  of 
  opinion 
  is 
  in 
  favour 
  of 
  

   regarding 
  the 
  retinulae 
  as 
  the 
  true 
  nerve-end 
  cells. 
  He 
  

   states 
  that 
  the 
  end 
  elements 
  of 
  the 
  human 
  eye 
  are 
  only 
  

   •004 
  mm. 
  apart, 
  whilst 
  the 
  corresponding 
  distance 
  in 
  

   Musca 
  is 
  '01 
  mm. 
  

  

  Patten's 
  work 
  (Eyes 
  of 
  Molluscs 
  and 
  Arthropods, 
  Mit- 
  

   theil. 
  a.d. 
  Stat, 
  zu 
  Neapel. 
  V. 
  6, 
  p. 
  542, 
  etc.) 
  has 
  met 
  with 
  

   no 
  general 
  acceptance 
  so 
  far 
  as 
  it 
  concerns 
  the 
  physiology 
  

   of 
  the 
  compound 
  eye. 
  His 
  main 
  contention 
  is 
  that 
  the 
  

   cone 
  is 
  the 
  seat 
  of 
  perception. 
  He 
  claims 
  to 
  have 
  seen 
  by 
  

   his 
  histological 
  methods 
  the 
  nerve 
  fibrillae 
  which 
  pass 
  up 
  

   the 
  rhabdom, 
  and, 
  spreading 
  out 
  over 
  the 
  cone, 
  end 
  therein 
  

   in 
  minute 
  horizontal 
  branches. 
  He 
  is 
  emphatically 
  certain 
  

   on 
  this 
  point, 
  and 
  one 
  gains 
  the 
  impression 
  that 
  nothing 
  

   could 
  be 
  easier 
  than 
  to 
  proceed 
  by 
  his 
  methods 
  and 
  see 
  

   these 
  fibrillae 
  without 
  the 
  least 
  difficulty; 
  nevertheless 
  

   others, 
  including 
  the 
  present 
  writer, 
  have 
  not 
  succeeded 
  

  

  