﻿26 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  perpendicular 
  to 
  an 
  imaginary 
  facet, 
  and 
  these 
  lines 
  formed 
  

   a 
  great 
  sheaf 
  radiating 
  from 
  the 
  eye, 
  the 
  space 
  between 
  

   the 
  lines 
  of 
  course 
  increasing 
  as 
  the 
  distance 
  from 
  the 
  

   eye 
  increased. 
  Then 
  a 
  line 
  two 
  inches 
  long, 
  drawn 
  two 
  

   inches 
  away 
  from 
  the 
  eye, 
  and 
  at 
  right 
  angles 
  to 
  the 
  centre 
  

   line 
  of 
  the 
  sheaf, 
  would 
  represent 
  an 
  object 
  two 
  inches 
  

   from 
  the 
  insect's 
  eye. 
  Such 
  a 
  hne 
  cuts 
  across 
  a 
  number 
  

   of 
  the 
  other 
  lines, 
  and 
  this 
  number 
  represents 
  in 
  hnear 
  

   fashion 
  the 
  number 
  of 
  facets 
  which 
  would 
  be 
  engaged 
  in 
  

   viewing 
  an 
  object 
  two 
  inches 
  long, 
  two 
  inches 
  from 
  the 
  

   eye. 
  Another 
  line 
  of 
  the 
  same 
  length 
  but 
  twelve 
  inches 
  

   from 
  the 
  eye 
  cuts 
  across 
  a 
  much 
  smaller 
  number 
  of 
  the 
  

   lines, 
  since 
  here 
  they 
  are 
  wider 
  apart. 
  From 
  this 
  diagram 
  

   I 
  obtained 
  a 
  number 
  for 
  the 
  facets 
  engaged 
  in 
  viewing 
  an 
  

   object 
  two 
  inches 
  long 
  but 
  twelve 
  inches 
  away 
  from 
  the 
  eye. 
  

   I 
  was 
  thus 
  able 
  to 
  calculate 
  approximately 
  the 
  number 
  

   of 
  facets 
  of 
  a 
  tortoiseshell 
  butterfly's 
  eye 
  which 
  would 
  be 
  

   engaged 
  in 
  viewing 
  another 
  of 
  the 
  same 
  species 
  at 
  distances 
  

   of 
  two 
  and 
  twelve 
  inches 
  respectively. 
  I 
  then 
  made 
  a 
  

   drawing 
  of 
  the 
  butterfly 
  and 
  divided 
  it 
  off 
  into 
  small 
  

   areas 
  equal 
  to 
  the 
  number 
  of 
  facets 
  at 
  two 
  and 
  at 
  twelve 
  

   inches, 
  and 
  assuming 
  that 
  each 
  facet 
  unit 
  conveys, 
  not 
  

   an 
  image, 
  but 
  only 
  a 
  spot 
  of 
  light 
  of 
  the 
  average 
  value 
  of 
  

   that 
  part 
  of 
  the 
  field 
  which 
  it 
  covers, 
  I 
  built 
  up 
  a 
  drawing 
  

   of 
  the 
  butterfly 
  \vith 
  spots 
  of 
  colour 
  in 
  number 
  corresponding 
  

   to 
  the 
  small 
  areas 
  into 
  which 
  my 
  first 
  drawing 
  had 
  been 
  

   divided. 
  I 
  did 
  this 
  for 
  both 
  cases, 
  the 
  two-inch 
  and 
  the 
  

   twelve-inch, 
  and 
  the 
  result 
  is 
  shown 
  on 
  PI. 
  III. 
  Fig. 
  1 
  

   shows 
  the 
  tortoiseshell 
  butterfly 
  as 
  it 
  appears 
  to 
  our 
  eyes, 
  

   fig. 
  2 
  the 
  same 
  as 
  it 
  may 
  be 
  "supposed 
  to 
  appear 
  if 
  made 
  

   up 
  of 
  little 
  dots 
  of 
  light 
  corresponding 
  to 
  the 
  number 
  of 
  

   facets 
  engaged 
  at 
  a 
  distance 
  of 
  two 
  inches, 
  and 
  fig. 
  3 
  the 
  

   same 
  at 
  a 
  distance 
  of 
  twelve 
  inches. 
  I 
  have 
  furthermore 
  

   endeavoured 
  to 
  represent 
  the 
  appearance 
  of 
  the 
  same 
  

   butterfly 
  at 
  twelve 
  inches, 
  assuming 
  that 
  each 
  facet 
  unit 
  

   records 
  a 
  sharp 
  but 
  inverted 
  image 
  of 
  its 
  own 
  field 
  of 
  

   view. 
  For 
  this 
  purpose 
  I 
  cut 
  a 
  drawing 
  of 
  the 
  butterfly 
  

   into 
  hexagonal 
  pieces 
  each 
  representing 
  a 
  facet 
  view, 
  and 
  

   then 
  turned 
  them 
  all 
  upside 
  down. 
  The 
  result 
  is 
  shown 
  

   at 
  PI. 
  IV, 
  fig. 
  1. 
  The 
  effect 
  is 
  pecuhar, 
  and 
  the 
  reader 
  may 
  

   derive 
  some 
  amusement 
  from 
  trying 
  to 
  decide 
  whether 
  

   PI. 
  Ill, 
  fig. 
  3, 
  or 
  PI. 
  IV, 
  fig. 
  1 
  looks 
  most 
  like 
  the 
  original 
  

   butterfly, 
  for 
  if 
  we 
  accept 
  the 
  mosaic 
  theory, 
  either 
  as 
  

   spots 
  of 
  Ught 
  or 
  as 
  clear 
  inverted 
  images, 
  one 
  of 
  these 
  

  

  