﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  19 
  

  

  due 
  to 
  the 
  fact 
  that 
  the 
  facets 
  and 
  cones 
  are 
  not 
  homo- 
  

   geneous 
  bodies, 
  but 
  have 
  a 
  refractive 
  index 
  which 
  con- 
  

   tinually 
  decreases 
  from 
  the 
  axis 
  to 
  the 
  outer 
  wall. 
  This 
  

   form 
  of 
  refractive 
  body 
  he 
  calls 
  a 
  lens 
  cyhnder. 
  

  

  In 
  order 
  more 
  easily 
  to 
  explain 
  the 
  action 
  of 
  such 
  an 
  

   apparatus 
  I 
  have 
  made 
  the 
  two 
  diagrams 
  on 
  PI. 
  II, 
  

   which 
  are 
  similar 
  to 
  those 
  given 
  in 
  Exner's 
  work. 
  Fig. 
  7 
  

   represents 
  a 
  lens 
  cyhnder, 
  the 
  actual 
  length 
  of 
  which 
  is 
  

   the 
  same 
  as 
  its 
  focal 
  length. 
  Now 
  the 
  rays 
  emanating 
  

   from 
  some 
  external 
  point, 
  and 
  represented 
  by 
  the 
  hnes 
  

   cd, 
  ef, 
  are 
  made 
  to 
  converge 
  by 
  the 
  refractive 
  action 
  of 
  

   the 
  medium 
  so 
  that 
  they 
  come 
  to 
  a 
  focus 
  at 
  the 
  base 
  of 
  

   the 
  cylinder. 
  Similarly 
  rays 
  from 
  some 
  other 
  point, 
  repre- 
  

   sented 
  by 
  c'd', 
  e'f, 
  come 
  to 
  a 
  focus 
  at 
  the 
  same 
  level, 
  

   and 
  an 
  inverted 
  image 
  is 
  formed 
  at 
  the 
  base 
  of 
  the 
  

   cylinder. 
  A 
  pecuhar 
  difference 
  between 
  this 
  action 
  and 
  

   that 
  of 
  a 
  lens 
  is, 
  that 
  whereas 
  in 
  the 
  latter 
  the 
  central 
  

   or 
  chief 
  rays, 
  ab, 
  ah', 
  emerge 
  at 
  an 
  angle, 
  in 
  the 
  lens 
  

   cyhnder 
  the 
  chief 
  rays 
  emerge 
  parallel. 
  Now 
  in 
  the 
  case 
  

   where 
  such 
  a 
  lens 
  cylinder 
  is 
  twice 
  as 
  long 
  as 
  its 
  own 
  focal 
  

   length 
  we 
  have 
  the 
  effect 
  diagrammatically 
  shown 
  in 
  

   fig. 
  8. 
  Here 
  the 
  inverted 
  image 
  occurs 
  in 
  the 
  middle 
  of 
  

   the 
  cylinder, 
  and 
  the 
  lower 
  half 
  of 
  the 
  cyhnder 
  acting 
  

   in 
  the 
  same 
  way, 
  rays 
  which 
  enter 
  the 
  cyhnder 
  on 
  the 
  

   right, 
  as 
  c'd', 
  e'f 
  , 
  emerge 
  again 
  on 
  the 
  right, 
  and 
  the 
  

   image 
  is 
  thus 
  erected. 
  In 
  insect 
  eyes 
  both 
  these 
  optical 
  

   effects 
  occur 
  with 
  apparently 
  varying 
  results. 
  According 
  

   to 
  Exner 
  two 
  kinds 
  of 
  image 
  are 
  formed 
  in 
  the 
  compound 
  

   eye. 
  The 
  first 
  is 
  the 
  superposition 
  image. 
  A 
  very 
  perfect 
  

   example 
  of 
  this 
  image 
  is 
  furnished 
  by 
  the 
  eye 
  of 
  the 
  male 
  

   glow-worm, 
  Lampyris 
  noctiluca. 
  In 
  this 
  insect 
  the 
  visual 
  

   rods 
  or 
  retinulae 
  do 
  not 
  extend 
  forward 
  so 
  as 
  to 
  touch 
  

   the 
  ends 
  of 
  the 
  cones, 
  but 
  are 
  separated 
  therefrom 
  by 
  a 
  

   small 
  space, 
  filled 
  with 
  what 
  in 
  hfe 
  is 
  a 
  doubtless 
  trans- 
  

   parent 
  tissue. 
  The 
  pigment 
  between 
  the 
  cones 
  is 
  not 
  

   stationary, 
  as 
  in 
  some 
  other 
  eyes 
  we 
  shall 
  examine, 
  but 
  

   is 
  capable 
  of 
  movement 
  backwards 
  or 
  forwards 
  according 
  

   to 
  the 
  amount 
  of 
  hght 
  entering 
  the 
  eye. 
  Thus 
  if 
  the 
  

   light 
  is 
  weak 
  the 
  pigment 
  moves 
  forward 
  towards 
  the 
  

   corneal 
  layer, 
  and 
  exposes 
  a 
  larger 
  portion 
  of 
  the 
  apex 
  

   of 
  each 
  cone. 
  Now, 
  as 
  Exner 
  has 
  shown, 
  the 
  corneal 
  facet 
  

   and 
  the 
  cone 
  in 
  this 
  beetle 
  act 
  together 
  as 
  a 
  lens 
  cyhnder 
  

   of 
  twice 
  its 
  own 
  focal 
  length. 
  Hence 
  the 
  rays 
  emerging 
  

   from 
  the 
  cone 
  apex 
  form 
  a 
  httle 
  erect 
  image 
  of 
  that 
  

  

  