﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  5 
  

  

  layer 
  of 
  transparent 
  material 
  which 
  has 
  been 
  called 
  the 
  

   processus 
  corneae. 
  Beneath 
  this 
  layer 
  and 
  corresponding 
  

   to 
  the 
  number 
  and 
  position 
  of 
  the 
  corneal 
  facets 
  are 
  found 
  

   the 
  structures 
  known 
  as 
  " 
  ommatidia." 
  The 
  form 
  of 
  the 
  

   ommatidium 
  varies 
  within 
  rather 
  wide 
  limits. 
  In 
  V. 
  urticae 
  

   it 
  is 
  as 
  follows 
  — 
  - 
  

  

  Adjacent 
  to 
  the 
  inner 
  surface 
  of 
  the 
  processvs 
  corneae 
  is 
  

   a 
  highly 
  refractive 
  body 
  of 
  a 
  chitinous 
  nature 
  and 
  having 
  

   a 
  conical 
  shape. 
  It 
  is 
  known 
  as 
  the 
  crystalhne 
  cone, 
  or 
  

   shortly, 
  the 
  cone. 
  In 
  life 
  it 
  is 
  entirely 
  transparent 
  and 
  

   apparently 
  structureless, 
  though 
  in 
  prepared 
  microscopic 
  

   specimens 
  it 
  is 
  seen 
  to 
  consist 
  of 
  four 
  cells 
  whose 
  contact 
  

   surfaces 
  he 
  parallel 
  to 
  the 
  long 
  axis 
  cf 
  the 
  cone. 
  At 
  the 
  

   outer 
  end 
  or 
  base 
  of 
  each 
  cell 
  is 
  a 
  nucleus, 
  the 
  so-called 
  

   Semper's 
  nucleus. 
  The 
  whole 
  conical 
  body 
  appears 
  in 
  

   stained 
  preparations 
  as 
  a 
  cone 
  enclosed 
  in 
  a 
  sheath 
  like 
  

   a 
  nut 
  in 
  its 
  shell, 
  and 
  in 
  differential 
  staining 
  the 
  sheath, 
  

   and 
  sometimes 
  the 
  nuclei, 
  take 
  that 
  stain 
  which 
  indicates 
  

   a 
  lesser 
  degree 
  of 
  chitinisation, 
  the 
  cone 
  itself 
  usually 
  

   taking 
  the 
  same 
  stain 
  as 
  the 
  corneal 
  facets. 
  At 
  the 
  inner 
  

   or 
  apical 
  end 
  the 
  cone 
  sheath 
  is 
  continued 
  as 
  a 
  long 
  trans- 
  

   parent 
  rod, 
  forming 
  the 
  central 
  part 
  of 
  the 
  ommatidium. 
  

   This 
  rod 
  is 
  known 
  as 
  the 
  "rhabdom" 
  (PI. 
  II, 
  fig. 
  2, 
  r.). 
  

   Surrounding 
  it 
  but 
  not, 
  in 
  V. 
  urticae, 
  reaching 
  quite 
  to 
  the 
  

   cone 
  apex, 
  there 
  are 
  several 
  (eight 
  in 
  V. 
  urticae) 
  elongated 
  

   cells 
  known 
  as 
  the 
  retinulae, 
  or 
  visual 
  cells 
  (PI. 
  II, 
  fig. 
  1 
  , 
  ret.). 
  

   These 
  extend 
  backwards 
  from 
  near 
  the 
  cone 
  apices 
  to 
  a 
  

   transverse 
  septum 
  in 
  the 
  depth 
  of 
  the 
  eye, 
  called 
  the 
  

   " 
  basal 
  membrane." 
  In 
  butterflies, 
  or 
  at 
  least 
  in 
  those 
  I 
  

   have 
  examined, 
  the 
  rhabdom 
  does 
  not 
  reach 
  the 
  basal 
  

   membrane, 
  but 
  stopping 
  abruptly 
  just 
  short 
  of 
  it, 
  leaves 
  an 
  

   interval 
  which 
  is 
  partly 
  occupied 
  by 
  a 
  highly 
  refractive 
  

   chitinous 
  body 
  which 
  is 
  a 
  development 
  of 
  the 
  tracheal 
  

   system. 
  Of 
  this 
  structure, 
  whicJi 
  has 
  been 
  observed 
  but 
  

   hitherto 
  incorrectly 
  interpreted, 
  I 
  shall 
  have 
  more 
  to 
  say 
  

   later. 
  As 
  regards 
  the 
  relationship 
  of 
  the 
  rhabdom 
  to 
  the 
  

   retinulae, 
  most 
  observers 
  are 
  of 
  the 
  opinion 
  that 
  the 
  former 
  

   is 
  really 
  a 
  complex 
  structure 
  formed 
  from 
  the 
  secretion 
  by 
  

   each 
  retinula 
  cell 
  of 
  a 
  chitinous 
  rod 
  along 
  its 
  inner 
  surface, 
  

   and 
  I 
  am 
  inchned 
  to 
  support 
  this 
  view. 
  In 
  more 
  primitive 
  

   eyes 
  the 
  rhabdom 
  is 
  incomplete 
  and 
  irregular 
  and 
  seems 
  to 
  

   be 
  httle 
  more 
  than 
  a 
  supporting 
  structure. 
  In 
  more 
  highly 
  

   developed 
  eyes 
  it 
  would 
  appear 
  to 
  be 
  of 
  optical 
  importance. 
  

   In 
  addition 
  to 
  the 
  structures 
  already 
  mentioned 
  we 
  find 
  

  

  