﻿28 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  the 
  ends 
  of 
  the 
  retinula 
  cells, 
  as, 
  for 
  instance, 
  in 
  the 
  glow- 
  

   worm's 
  eye, 
  but 
  along 
  the 
  inner 
  edge 
  of 
  each 
  cell 
  through- 
  

   out 
  its 
  whole 
  length. 
  

  

  Now 
  Grenacher 
  maintains 
  that 
  the 
  retinula 
  cells 
  of 
  each 
  

   ommatidium, 
  though 
  multiple, 
  can 
  only 
  be 
  regarded 
  as 
  a 
  

   physiological 
  entity, 
  and 
  if 
  we 
  are 
  to 
  compare 
  the 
  receptive 
  

   faculty 
  of 
  the 
  compound 
  with 
  that 
  of 
  the 
  vertebrate 
  eye, 
  

   we 
  must 
  compare 
  the 
  number 
  of 
  ommatidia 
  in 
  the 
  former 
  

   with 
  the 
  number 
  of 
  rods 
  and 
  cones 
  in 
  the 
  latter. 
  

  

  However 
  this 
  may 
  be 
  in 
  other 
  compound 
  eyes, 
  I 
  hold 
  it 
  

   to 
  be 
  an 
  error 
  for 
  that 
  of 
  a 
  butterfly. 
  If 
  there 
  are 
  eight 
  

   retinula 
  cells 
  in 
  each 
  ommatidium 
  of 
  the 
  eye 
  of 
  V. 
  urticae 
  

   I 
  regard 
  them 
  as 
  eight 
  separate 
  elements, 
  the 
  more 
  so 
  since 
  

   in 
  some 
  species, 
  as, 
  for 
  instance, 
  V. 
  io, 
  they 
  are 
  to 
  some 
  

   extent 
  separated 
  by 
  pigment. 
  Hence 
  the 
  little 
  pencil 
  of 
  

   light 
  projected 
  down 
  the 
  rhabdom 
  by 
  each 
  facet 
  unit, 
  and 
  

   having 
  its 
  rays 
  arranged 
  in 
  their 
  proper 
  order 
  and 
  position 
  

   by 
  the 
  re-erection 
  brought 
  about 
  by 
  the 
  cone, 
  is 
  analysed, 
  

   and 
  presumably 
  perceived 
  with 
  eight 
  times 
  the 
  accuracy 
  

   with 
  which 
  a 
  mere 
  amorphous 
  spot 
  of 
  Ught 
  would 
  be 
  per- 
  

   ceived, 
  and 
  similarly 
  one 
  tortoiseshell 
  butterfly 
  may 
  be 
  

   supposed 
  to 
  see 
  another, 
  at 
  a 
  distance 
  of 
  two 
  inches, 
  eight 
  

   times 
  more 
  distinctly 
  than 
  as 
  shown 
  at 
  PL 
  III, 
  fig. 
  2. 
  Two 
  

   factors 
  thus 
  contribute 
  to 
  the 
  better 
  perception 
  of 
  the 
  

   image, 
  viz. 
  the 
  proper 
  co-ordination 
  of 
  the 
  rays 
  and 
  the 
  

   multiplicity 
  of 
  the 
  perceptive 
  elements. 
  

  

  One 
  point 
  remains. 
  It 
  has 
  been 
  suggested 
  that 
  there 
  

   is 
  a 
  difficulty 
  in 
  understanding 
  how 
  light 
  passing 
  down 
  a 
  

   transparent 
  rod, 
  the 
  rhabdom, 
  can 
  effect 
  the 
  retinulae 
  

   adjacent 
  to 
  it, 
  since 
  light 
  entering 
  a 
  glass 
  rod 
  at 
  one 
  end 
  

   emerges 
  only 
  at 
  the 
  other 
  end 
  and 
  not 
  through 
  the 
  sides. 
  

  

  I 
  have 
  submitted 
  this 
  point 
  to 
  Lord 
  Rayleigh, 
  who 
  has 
  

   kindly 
  assisted 
  me 
  on 
  several 
  occasions. 
  In 
  his 
  reply 
  he 
  

   points 
  out 
  that 
  this 
  action 
  of 
  a 
  transparent 
  rod 
  is 
  true 
  

   for 
  the 
  rod 
  surrounded 
  by 
  air, 
  but 
  that 
  in 
  the 
  ommatidium 
  

   of 
  the 
  compound 
  eye 
  this 
  condition 
  does 
  not 
  obtain. 
  

  

  The 
  physiological 
  continuity 
  of 
  the 
  rhabdom 
  and 
  

   retinula 
  cells 
  doubtless 
  provides 
  for 
  the 
  due 
  action 
  on 
  

   the 
  latter 
  of 
  the 
  hght 
  stimulus. 
  

  

  Technique 
  

  

  Many 
  thousands 
  of 
  preparations 
  of 
  eyes 
  were 
  made 
  in 
  

   the 
  course 
  of 
  my 
  investigations. 
  Of 
  hardening 
  re-agents 
  

  

  