﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  9 
  

  

  most 
  generally 
  accepted 
  is 
  that 
  of 
  Mliller 
  (Zur 
  vergleichen- 
  

   den 
  Physiologic 
  des 
  Gesichtsinnes 
  der 
  Menschen 
  und 
  der 
  

   Thiere, 
  1826) 
  and 
  may 
  be 
  freely 
  translated 
  as 
  follows 
  — 
  

  

  " 
  If 
  hght 
  from 
  a 
  particular 
  part 
  of 
  the 
  object 
  can 
  only 
  

   reach 
  a 
  particular 
  part 
  of 
  the 
  retina, 
  all 
  other 
  parts 
  of 
  the 
  

   retina 
  will 
  be 
  excluded 
  from 
  this 
  particular 
  hght, 
  and 
  so 
  

   an 
  image 
  is 
  formed. 
  This 
  happens 
  in 
  the 
  composite 
  eyes 
  

   of 
  insects 
  and 
  Crustacea 
  by 
  means 
  of 
  the 
  cones 
  which 
  

   lie 
  between 
  the 
  corneal 
  facets 
  and 
  the 
  fibres 
  of 
  the 
  visual 
  

   nerves, 
  united 
  to 
  each 
  at 
  their 
  extremities 
  and 
  clothed 
  

   laterally 
  with 
  pigment. 
  Each 
  of 
  these 
  cones 
  peripherally 
  

   placed 
  round 
  a 
  convex 
  nerve 
  mass 
  passes, 
  to 
  the 
  individual 
  

   nerve 
  endings 
  with 
  which 
  it 
  is 
  united 
  at 
  its 
  apex, 
  only 
  

   that 
  light 
  which 
  falls 
  directly 
  through 
  the 
  axis 
  of 
  the 
  cone. 
  

   All 
  other 
  rays 
  emanating 
  from 
  the 
  same 
  point 
  and 
  falhng 
  

   obliquely 
  on 
  the 
  cornea 
  will 
  fail 
  to 
  reach 
  the 
  lower 
  extremity 
  

   of 
  the 
  cone 
  and 
  so 
  will 
  not 
  come 
  to 
  the 
  perception 
  of 
  the 
  

   other 
  fibres 
  of 
  the 
  visual 
  nerves. 
  They 
  will 
  be 
  absorbed, 
  

   if 
  they 
  fall 
  obhquely, 
  by 
  the 
  pigment-covered 
  walls 
  of 
  the 
  

   cones, 
  these 
  being 
  transparent 
  only 
  in 
  their 
  axes." 
  

  

  Further 
  : 
  — 
  " 
  The 
  convexity 
  of 
  the 
  corneal 
  facets 
  will 
  

   guide 
  the 
  hght 
  in 
  the 
  direction 
  of 
  the 
  axis, 
  and 
  cause 
  a 
  

   greater 
  concentration 
  in 
  the 
  depth 
  of 
  the 
  eye. 
  So 
  it 
  may 
  

   happen 
  that 
  the 
  whole 
  light 
  passing 
  through 
  the 
  cone 
  will 
  

   be 
  concentrated 
  at 
  the 
  apex 
  where 
  the 
  nerve 
  fibres 
  are 
  

   attached, 
  in 
  the 
  form 
  of 
  a 
  point, 
  whereby 
  the 
  brightness 
  

   of 
  the 
  image 
  must 
  be 
  much 
  increased. 
  The 
  refraction 
  of 
  

   the 
  outer 
  surface 
  of 
  the 
  cornea 
  is, 
  however, 
  not 
  so 
  great 
  

   that 
  it 
  can 
  form 
  a 
  special 
  small 
  image 
  for 
  each 
  facet. 
  If 
  

   it 
  did 
  no 
  image 
  could 
  be 
  produced, 
  for 
  if 
  one 
  were 
  formed 
  

   in 
  the 
  focal 
  distance 
  of 
  the 
  lens, 
  it 
  would 
  necessarily 
  be 
  

   reversed. 
  Not 
  the 
  whole 
  field 
  would 
  be 
  reversed, 
  but 
  the 
  

   picture 
  formed 
  by 
  each 
  of 
  the 
  facets 
  respectively 
  and 
  

   unnaturally. 
  As 
  also 
  the 
  facets 
  in 
  the 
  compound 
  eyes 
  of 
  

   insects 
  frequently 
  show 
  little 
  or 
  no 
  convexity, 
  we 
  can 
  

   attribute 
  to 
  the 
  peripheral 
  convex 
  surface 
  no 
  other 
  func- 
  

   tion 
  than 
  the 
  concentration 
  of 
  the 
  hght 
  towards 
  the 
  pointed 
  

   apices 
  of 
  the 
  cones." 
  

  

  This 
  is 
  what 
  is 
  known 
  as 
  the 
  " 
  mosaic 
  " 
  theory 
  of 
  the 
  

   sight 
  of 
  the 
  compound 
  eye, 
  and 
  may 
  shortly 
  be 
  enunciated 
  

   thus. 
  

  

  Each 
  facet 
  registers 
  on 
  a 
  sensitive 
  layer 
  a 
  spot 
  of 
  light 
  

   corresponding 
  in 
  quahty 
  to 
  the 
  average 
  of 
  the 
  hght 
  re- 
  

   flected, 
  normally 
  to 
  the 
  facet, 
  by 
  that 
  part 
  of 
  the 
  object 
  

  

  