﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  13 
  

  

  tive 
  power 
  might 
  act 
  in 
  a 
  selective 
  manner. 
  We 
  shall 
  see 
  

   later, 
  however, 
  that 
  probably 
  neither 
  action 
  is 
  called 
  upon 
  

   in 
  the 
  perception 
  of 
  the 
  image. 
  Grenacher 
  further 
  points 
  

   out 
  that 
  the 
  image 
  projection 
  must 
  take 
  place 
  in 
  any 
  case 
  

   only 
  a 
  short 
  distance 
  behind 
  the 
  distal 
  ends 
  of 
  the 
  rods, 
  

   for 
  it 
  is 
  here 
  that 
  they 
  are 
  surrounded 
  by 
  pigment. 
  He 
  

   considers 
  that 
  it 
  is 
  for 
  obvious 
  reasons 
  established 
  that 
  the 
  

   physiological 
  region 
  of 
  the 
  area 
  of 
  hght 
  perception 
  lies 
  

   between 
  the 
  distal 
  and 
  proximal 
  ends 
  of 
  the 
  rods. 
  The 
  

   same 
  applies 
  to 
  the 
  compound 
  eye. 
  In 
  " 
  acone 
  " 
  eyes 
  * 
  

   of 
  Tipula 
  and 
  Ctenophora 
  the 
  hght 
  perception 
  is 
  associated 
  

   with 
  the 
  seven 
  rods, 
  of 
  which, 
  especially 
  in 
  the 
  latter, 
  the 
  

   central 
  is 
  much 
  longer 
  than 
  the 
  peripheral, 
  so 
  that 
  the 
  

   central 
  must 
  have 
  a 
  deeper 
  zone 
  of 
  perception. 
  He 
  then 
  

   considers 
  those 
  facetted 
  eyes 
  in 
  which 
  the 
  central 
  rod 
  is 
  

   drawn 
  out 
  into 
  an 
  axial 
  rhabdom, 
  as 
  in 
  Orthoptera, 
  Hymen- 
  

   optera, 
  Lepidoptera, 
  etc., 
  where 
  the 
  rhabdom 
  is 
  extended 
  

   into 
  the 
  whole 
  length 
  of 
  the. 
  retinula. 
  In 
  these 
  the 
  sec- 
  

   tional 
  area 
  is 
  reduced 
  to 
  a 
  minimum, 
  namely 
  the 
  cross 
  

   section 
  of 
  the 
  rhabdom. 
  Hence 
  it 
  reaches 
  a 
  very 
  consider- 
  

   able 
  depth, 
  and 
  there 
  is 
  nothing 
  to 
  prevent 
  the 
  nervous 
  

   stimulus 
  acting 
  throughout 
  its 
  entire 
  length. 
  The 
  visual 
  

   area 
  of 
  the 
  single 
  facet 
  will 
  be 
  reduced 
  the 
  more 
  it 
  departs 
  

   from 
  the 
  structure 
  of 
  the 
  simple 
  eye. 
  As 
  to 
  the 
  acuity 
  

   of 
  vision 
  we 
  may 
  come 
  to 
  some 
  conclusion, 
  in 
  for 
  example 
  

   the 
  spider's 
  eye, 
  by 
  taking 
  into 
  account 
  the 
  distance 
  apart 
  

   of 
  the 
  percipient 
  elements 
  in 
  different 
  forms. 
  An 
  eye 
  with 
  

   a 
  great 
  number 
  of 
  small 
  rods 
  on 
  the 
  retinal 
  surface 
  will 
  see 
  

   more 
  clearly 
  than 
  one 
  sparsely 
  provided. 
  Thus 
  the 
  front 
  

   eye 
  of 
  Epeira 
  will 
  surpass 
  the 
  hinder 
  in 
  this 
  respect, 
  for 
  

   the 
  former 
  has 
  an 
  incomparably 
  greater 
  number 
  of 
  rods 
  

   than 
  the 
  latter. 
  On 
  the 
  contrary, 
  the 
  posterior 
  eye 
  has 
  a 
  

   larger 
  visual 
  field 
  than 
  the 
  anterior. 
  In 
  the 
  compound 
  

   eye 
  we 
  may 
  compare 
  the 
  number 
  of 
  elements 
  in 
  the 
  

   retinulae. 
  Dytiscus 
  has 
  only 
  four, 
  Melolontha 
  seven. 
  

   Whether 
  or 
  not 
  the 
  former 
  is 
  compensated 
  by 
  a 
  smaller 
  

   field, 
  we 
  know 
  that 
  the 
  water-beetle 
  is 
  actively 
  voracious 
  

   and 
  gives 
  the 
  impression 
  of 
  having 
  a 
  greater 
  visual 
  power 
  

   than 
  the 
  cockchafer. 
  The 
  discovery 
  of 
  a 
  single 
  perceptive 
  

  

  * 
  Grenacher 
  divides 
  Arthropod 
  eyes 
  into 
  " 
  acone," 
  those 
  which 
  

   have 
  no 
  crystalline 
  cones 
  or 
  any 
  substitute 
  for 
  them 
  ; 
  " 
  pseudocone 
  " 
  

   eyes, 
  those 
  which 
  have 
  in 
  place 
  of 
  the 
  cone 
  a 
  more 
  or 
  less 
  cylindro- 
  

   conical 
  membrane 
  with 
  fluid 
  or 
  semifluid 
  contents; 
  and 
  " 
  eucone 
  " 
  

   eyes, 
  those 
  which 
  possess 
  a 
  true 
  crystaUine 
  cone. 
  

  

  