﻿22 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  the 
  retinulae 
  adjacent 
  to 
  the 
  cones. 
  Having 
  obtained 
  such 
  

   a 
  section 
  as, 
  when 
  mounted 
  in 
  the 
  manner 
  already 
  described, 
  

   showed 
  a 
  series 
  of 
  small 
  dots 
  of 
  hght 
  corresponding 
  to 
  the 
  

   cone 
  apices, 
  he 
  proceeded 
  to 
  endeavour 
  to 
  study 
  the 
  image 
  

   there 
  formed. 
  

  

  In 
  an 
  ordinary 
  humble-bee 
  the 
  light 
  dots 
  were 
  so 
  small 
  

   that 
  when 
  he 
  used 
  two 
  light 
  spots 
  as 
  objects 
  the 
  brilliancy 
  

   of 
  the 
  hght 
  dots 
  was 
  increased, 
  but 
  the 
  two 
  hght 
  rays 
  

   could 
  not 
  be 
  separated. 
  It 
  was 
  thus 
  hopeless 
  to 
  expect 
  

   to 
  see 
  an 
  image. 
  On 
  the 
  other 
  hand, 
  he 
  succeeded 
  in 
  the 
  

   case 
  of 
  Calliphora 
  vomitoria 
  in 
  seeing 
  the 
  two 
  dots 
  sepa- 
  

   rately, 
  and 
  in 
  proving 
  that 
  the 
  image 
  at 
  the 
  cone 
  apex 
  

   is 
  an 
  inverted 
  one. 
  By 
  measuring 
  the 
  distance 
  between 
  

   the 
  two 
  hght 
  images, 
  and 
  then 
  wiping 
  away 
  the 
  cones, 
  

   and 
  measuring 
  again, 
  he 
  found 
  that 
  the 
  distance 
  was 
  

   practically 
  the 
  same, 
  from 
  which 
  he 
  concludes 
  that 
  the 
  

   cone 
  in 
  this 
  insect 
  has 
  an 
  inappreciable 
  optical 
  effect. 
  

   He 
  admits 
  his 
  surprise 
  at 
  this 
  result. 
  The 
  main 
  conclusion 
  

   is, 
  however, 
  that 
  the 
  images 
  produced 
  by 
  the 
  facet 
  units 
  

   in 
  diurnal 
  insects 
  having 
  apposition 
  images 
  are 
  inverted, 
  

   and 
  therefore 
  the 
  picture 
  presented 
  to 
  the 
  percipient 
  layer 
  

   of 
  the 
  insect's 
  eye 
  is 
  made 
  up 
  of 
  a 
  multitude 
  of 
  inverted 
  

   images, 
  or 
  alternatively 
  of 
  mere 
  light 
  spots. 
  One 
  must 
  

   conclude 
  from 
  this 
  that 
  the 
  elements 
  composing 
  the 
  whole 
  

   picture 
  are 
  not, 
  as 
  images, 
  of 
  any 
  value, 
  but 
  merely 
  act 
  

   as 
  light 
  spots 
  of 
  the 
  average 
  quality 
  of 
  that 
  emanating 
  

   from 
  the 
  portion 
  of 
  the 
  field 
  recorded 
  by 
  each 
  facet. 
  

  

  If 
  this 
  be 
  so, 
  then 
  the 
  apposition 
  image 
  is, 
  as 
  an 
  optical 
  

   performance, 
  very 
  inferior 
  to 
  the 
  superposition 
  image, 
  

   although 
  nevertheless 
  produced 
  by 
  a 
  much 
  more 
  highly 
  

   developed 
  and 
  intricate 
  optical 
  apparatus, 
  at 
  any 
  rate 
  in 
  

   the 
  case 
  of 
  eucone 
  eyes. 
  

  

  I 
  have 
  here 
  endeavoured 
  to 
  give 
  as 
  concise 
  an 
  account 
  

   as 
  possible 
  of 
  the 
  image 
  formation 
  in 
  the 
  compound 
  eye 
  

   as 
  set 
  forth 
  by 
  Exner. 
  There 
  is 
  much 
  more 
  in 
  his 
  book, 
  

   which 
  is 
  a 
  record 
  of 
  by 
  far 
  the 
  most 
  complete 
  research 
  

   yet 
  made 
  on 
  the 
  physiology 
  of 
  the 
  compound 
  eye, 
  and 
  is 
  

   of 
  a 
  value 
  which 
  even 
  the 
  occasional 
  profound 
  obscurity 
  

   of 
  expression 
  peculiar 
  to 
  the 
  German 
  language 
  fails 
  entirely 
  

   to 
  hide. 
  

  

  Author's 
  Experiments 
  

  

  It 
  remains 
  to 
  describe 
  some 
  of 
  my 
  own 
  experiments, 
  

   and 
  to 
  record 
  the 
  extent 
  to 
  which 
  they 
  confirm 
  or 
  refute 
  

  

  