﻿Explanation 
  of 
  Plate 
  III. 
  

  

  Fig. 
  1. 
  A 
  Tortoiseshell 
  butterfly 
  as 
  it 
  appears 
  to 
  the 
  human 
  eye. 
  

  

  2. 
  The 
  same 
  as 
  it 
  might 
  be 
  supposed 
  to 
  appear 
  to 
  another 
  

  

  of 
  the 
  same 
  species, 
  at 
  a 
  distance 
  of 
  two 
  inches, 
  as- 
  

   suming 
  the 
  correctness 
  of 
  the 
  original 
  mosaic 
  theory. 
  

  

  3. 
  The 
  same, 
  on 
  the 
  same 
  assumption, 
  at 
  a 
  distance 
  of 
  twelve 
  

  

  inches. 
  

  

  Note. 
  — 
  An 
  image 
  such 
  as 
  Fig. 
  3 
  may 
  fairly 
  be 
  regarded 
  as 
  about 
  

   the 
  limit 
  of 
  recognisable 
  visibility. 
  It 
  is 
  produced 
  by 
  approximately 
  

   72 
  facets. 
  Now 
  the 
  number 
  of 
  facets 
  engaged 
  varies 
  inversely 
  as 
  

   the 
  square 
  of 
  the 
  distance, 
  hence 
  at 
  three 
  feet 
  ^ 
  of 
  72, 
  or 
  eight 
  

   facets 
  only 
  would 
  be 
  engaged. 
  If, 
  however, 
  the 
  theory 
  here 
  ad- 
  

   vanced 
  be 
  correct 
  {vide 
  p. 
  28), 
  the 
  image 
  is 
  eight 
  times 
  more 
  distinct 
  

   than 
  was 
  previously 
  supposed, 
  and 
  hence 
  at 
  three 
  feet 
  the 
  image 
  

   would 
  be 
  equal 
  to 
  that 
  laroduced, 
  on 
  the 
  older 
  theory, 
  by 
  8 
  x 
  8, 
  

   = 
  64 
  facets. 
  

  

  In 
  other 
  words, 
  Fig. 
  3 
  would 
  represent 
  very 
  nearly 
  the 
  degree 
  

   of 
  definition 
  at 
  a 
  distance 
  of 
  three 
  feet, 
  and 
  this 
  distance 
  is 
  about 
  

   the 
  observed 
  limit 
  of 
  recognition 
  in 
  these 
  insects. 
  

  

  