﻿14 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  element 
  behind 
  each 
  facet 
  would 
  have 
  been 
  overwhelmingly 
  

   in 
  favour 
  of 
  Miiller's 
  theory, 
  but 
  since 
  we 
  know 
  there 
  are 
  

   more, 
  it 
  remains 
  to 
  ask 
  whether 
  this 
  fact 
  invahdates 
  

   Miiller's 
  theory. 
  

  

  If 
  Gottsche's 
  image 
  theory 
  is 
  to 
  be 
  supported 
  it 
  must 
  be 
  

   true 
  for 
  all 
  facetted 
  eyes 
  ; 
  furthermore 
  the 
  image 
  must 
  be 
  

   projected 
  in 
  the 
  plane 
  where 
  the 
  percipient 
  elements 
  he, 
  and 
  

   there 
  must 
  be 
  a 
  retinal 
  layer 
  with 
  many 
  percipient 
  elements 
  

   to 
  receive 
  it. 
  The 
  formation 
  of 
  a 
  reversed 
  image 
  will 
  depend 
  

   on 
  the 
  spherical 
  surfaces 
  of 
  the 
  refractive 
  media 
  acting 
  as 
  

   lenses. 
  This 
  condition 
  is 
  in 
  most 
  cases 
  supplied. 
  It 
  is 
  

   not 
  to 
  be 
  excluded 
  that 
  the 
  cones 
  may 
  serve 
  this 
  purpose 
  

   through 
  their 
  convex 
  anterior 
  surfaces. 
  The 
  second 
  con- 
  

   dition 
  demands 
  that 
  the 
  image 
  shall 
  be 
  projected 
  where 
  

   we 
  know 
  the 
  percipient 
  elements 
  to 
  he, 
  and 
  behind 
  the 
  

   cones. 
  The 
  Gottsche 
  experiments 
  demanded 
  that 
  the 
  

   image 
  should 
  pass 
  unaltered 
  through 
  the 
  cone 
  to 
  where 
  

   the 
  sensitive 
  layer 
  must 
  he. 
  The 
  material 
  used 
  by 
  Gottsche 
  

   was 
  very 
  ill 
  adapted 
  to 
  illustrate 
  the 
  effect 
  of 
  the 
  cones, 
  

   since 
  his 
  flies 
  were 
  insects 
  with 
  pseudocone 
  eyes, 
  the 
  cones 
  

   of 
  which 
  must 
  have 
  been 
  inevitably 
  destroyed, 
  and, 
  

   shortly, 
  the 
  experiments 
  do 
  nothing 
  more 
  than 
  show 
  that 
  

   the 
  corneal 
  lenses 
  act 
  in 
  the 
  same 
  way 
  as 
  would 
  lenses 
  

   of 
  glass. 
  Grenacher 
  proceeds 
  to 
  recount 
  how 
  he 
  has 
  tried 
  

   similar 
  experiments 
  with 
  the 
  eyes 
  of 
  nocturnal 
  moths, 
  in 
  

   spirit 
  specimens 
  of 
  which 
  the 
  internal 
  parts 
  of 
  the 
  eye 
  can 
  

   be 
  removed 
  and 
  the 
  pigment 
  destroyed 
  by 
  nitric 
  acid. 
  

   Such 
  an 
  eye 
  so 
  placed 
  as 
  to 
  make 
  it 
  possible 
  to 
  look 
  through 
  

   it 
  from 
  the 
  back, 
  shows 
  no 
  images 
  at 
  the 
  bases 
  of 
  the 
  cones, 
  

   but 
  does 
  show 
  sharp 
  images 
  just 
  behind 
  the 
  lenses, 
  where, 
  

   however, 
  no 
  sensitive 
  elements 
  lie. 
  

  

  The 
  third 
  point 
  which 
  Grenacher 
  brings 
  forward 
  is 
  the 
  

   question 
  whether 
  the 
  image, 
  if 
  formed, 
  has 
  the 
  necessary 
  

   area. 
  In 
  general 
  the 
  cone 
  is 
  pointed 
  posteriorly, 
  and 
  

   therefore 
  we 
  look 
  in 
  vain 
  for 
  a 
  projection 
  which 
  has 
  any 
  

   size 
  at 
  all. 
  If 
  an 
  image 
  be 
  possible 
  where 
  the 
  rods 
  do 
  reach 
  

   the 
  hinder 
  end 
  of 
  the 
  cone, 
  it 
  is 
  clearly 
  impossible 
  in 
  those 
  

   cases 
  where 
  the 
  rods 
  are 
  drawn 
  out 
  to 
  a 
  thread-hke 
  struc- 
  

   ture, 
  often 
  of 
  considerable 
  length. 
  It 
  is 
  further 
  impossible 
  

   to 
  suppose 
  that 
  an 
  image 
  produced 
  at 
  the 
  forward 
  part 
  

   could 
  be 
  carried 
  inwards 
  like 
  sound 
  on 
  a 
  string. 
  Grenacher 
  

   then 
  discusses 
  the 
  question 
  of 
  the 
  existence 
  of 
  a 
  retina, 
  

   remarking 
  that 
  the 
  reply 
  depends 
  very 
  much 
  on 
  the 
  number 
  

   of 
  elements 
  necessary 
  to 
  form 
  a 
  tissue 
  which 
  can 
  be 
  so 
  

  

  