﻿Dr. 
  H. 
  Eltringham 
  on 
  Bntterfitj 
  Vision. 
  11 
  

  

  as 
  the 
  retina, 
  and 
  in 
  reply 
  to 
  the 
  difficulty 
  of 
  its 
  shape 
  for 
  

   receiving 
  a 
  picture 
  points 
  out 
  that 
  the 
  human 
  retina 
  is 
  not 
  

   truly 
  spherical. 
  

  

  Schliltze 
  (Untersuch. 
  uber 
  zusammenges. 
  Aug. 
  Bonn, 
  

   1868) 
  held 
  that 
  the 
  mosaic 
  theory 
  was 
  physically 
  unten- 
  

   able, 
  as 
  also 
  Leydig's 
  view 
  of 
  the 
  nervous 
  nature 
  of 
  the 
  

   cones, 
  these 
  latter 
  being 
  in 
  his 
  view 
  a 
  purely 
  dioptric 
  

   apparatus 
  for 
  the 
  production 
  of 
  the 
  image. 
  Schiiltze's 
  

   monograph 
  is 
  one 
  of 
  the 
  most 
  interesting 
  of 
  the 
  older 
  works 
  

   on 
  the 
  compound 
  eye, 
  since 
  it 
  enters 
  into 
  great 
  detail 
  on 
  

   the 
  question 
  of 
  the 
  nerve 
  distribution. 
  According 
  to 
  that 
  

   author 
  the 
  central 
  rod 
  or 
  rhabdom 
  of 
  the 
  ommatidium 
  is 
  

   the 
  nerve 
  rod, 
  formed 
  of 
  several 
  fibres 
  in 
  a 
  common 
  sheath. 
  

   He 
  claims 
  to 
  have 
  seen 
  these 
  fibres 
  very 
  distinctly. 
  They 
  

   are 
  said 
  by 
  him 
  to 
  have 
  a 
  laminated 
  structure 
  as 
  though 
  

   made 
  up 
  of 
  a 
  great 
  number 
  of 
  small 
  plates 
  transversely 
  

   superimposed. 
  At 
  the 
  outer 
  or 
  distal 
  end 
  they 
  divide 
  and 
  

   enter 
  the 
  cone, 
  to 
  the 
  number 
  in 
  some 
  cases 
  of 
  eight, 
  being 
  

   then 
  apparently 
  lost 
  in 
  the 
  substance 
  of 
  the 
  cone. 
  Schiiltze's 
  

   figures 
  and 
  descriptions 
  are 
  very 
  convincing, 
  and 
  one 
  feels 
  

   almost 
  inclined 
  to 
  believe 
  that 
  he 
  saw 
  the 
  structures 
  he 
  

   describes, 
  though 
  Grenacher, 
  who 
  spent 
  years 
  in 
  examining 
  

   these 
  eyes, 
  does 
  not 
  appear 
  to 
  have 
  found 
  the 
  nerve 
  fibres, 
  

   and 
  indeed 
  professes 
  to 
  doubt 
  their 
  existence. 
  He 
  states 
  

   that 
  Schiiltze 
  found 
  them 
  only 
  in 
  a 
  very 
  few 
  cases, 
  and 
  

   himself 
  confessed 
  that 
  he 
  was 
  unable 
  to 
  find 
  them 
  in 
  the 
  

   majority. 
  Boll 
  (1871) 
  pointed 
  out 
  the 
  unimportance 
  of 
  

   Leuwenhoek's 
  images, 
  reobserved 
  by 
  Gottsche, 
  these 
  being 
  

   an 
  incidental 
  consequence 
  of 
  the 
  lenses 
  of 
  the 
  facets. 
  He 
  

   also 
  pointed 
  out 
  the 
  lack 
  of 
  accommodation 
  in 
  the 
  insect 
  

   eye, 
  and 
  the 
  impossibihty 
  of 
  combining 
  an 
  endless 
  number 
  

   of 
  images 
  into 
  one 
  picture. 
  

  

  In 
  1879 
  appeared 
  Grenacher's 
  great 
  work 
  on 
  the 
  Arthro- 
  

   pod 
  eye 
  (Untersuch. 
  uber 
  der 
  Arthrop., 
  etc., 
  Gottingen, 
  

   1879). 
  For 
  beauty 
  of 
  illustration 
  this 
  work 
  has 
  probably 
  

   not 
  been 
  surpassed. 
  It 
  will 
  be 
  necessary 
  to 
  review 
  at 
  

   some 
  length 
  that 
  author's 
  conclusions. 
  In 
  the 
  course 
  of 
  

   his 
  introduction 
  , 
  from 
  which 
  I 
  have 
  extracted 
  part 
  of 
  the 
  

   foregoing 
  summary, 
  he 
  remarks 
  that 
  the 
  perception 
  of 
  a 
  

   number 
  of 
  inverted 
  images 
  need 
  not 
  at 
  once 
  be 
  dismissed 
  in 
  

   view 
  of 
  the 
  simple 
  Arthropod 
  and 
  the 
  Vertebrate 
  eye, 
  but 
  its 
  

   probability 
  will 
  depend 
  on 
  the 
  nature, 
  and 
  especially 
  on 
  the 
  

   number, 
  of 
  the 
  percipient 
  elements 
  belonging 
  to 
  each 
  corneal 
  

   facet. 
  If, 
  where 
  the 
  light 
  rays 
  fall, 
  there 
  be 
  only 
  a 
  single 
  

  

  