﻿284 
  rfiOCEEDINGS 
  OF 
  THE 
  FOURTH 
  ENTOMOLOGICAL 
  MEETING 
  

  

  at 
  a 
  considerable 
  speed. 
  This 
  amount 
  of 
  movement 
  of 
  so 
  large 
  an 
  

   object 
  would 
  rarely 
  be 
  met 
  with 
  under 
  ordinary 
  conditions 
  but 
  although 
  

   the 
  numerical 
  relations 
  here 
  suggested 
  obviously 
  require 
  verification, 
  

   the 
  paragraph 
  has 
  been 
  included 
  because 
  it 
  may 
  give 
  us 
  a 
  rough 
  idea 
  

   of 
  the 
  actual 
  powers 
  of 
  vision 
  in 
  this 
  particular 
  fly. 
  It 
  is 
  reasonable 
  

   to 
  suppose 
  that 
  the 
  minimum 
  movement 
  perceptible 
  to 
  the 
  fly 
  is 
  at 
  

   least 
  considerably 
  less 
  than 
  the 
  minimum 
  which 
  is 
  sufficient 
  to 
  make 
  

   it 
  dodge. 
  If 
  we 
  are 
  justified 
  in 
  taking 
  the 
  latter 
  as 
  equivalent 
  to 
  the 
  

   stmiulation 
  of 
  16 
  facets, 
  let 
  us 
  say 
  as 
  motion 
  perceived 
  across 
  a 
  patch 
  

   of 
  eye 
  4 
  facets 
  square, 
  it 
  seems 
  not 
  unlikely 
  that 
  the 
  fly 
  might 
  be 
  able 
  

   to 
  perceive 
  motion 
  which 
  involved 
  only 
  two 
  facets. 
  

  

  In 
  the 
  upper 
  quadrant 
  of 
  the 
  circle 
  there 
  are 
  approximately 
  30 
  

   facets. 
  Each 
  facet 
  thus 
  covers 
  about 
  3° 
  of 
  arc. 
  In 
  the 
  lower 
  quadrant 
  

   the 
  facets 
  are 
  smaller 
  and 
  more 
  numerous, 
  and 
  each 
  one 
  commands 
  

   about 
  2° 
  of 
  arc. 
  In 
  the 
  above 
  calculations 
  it 
  has 
  been 
  assumed 
  that 
  

   we 
  are 
  dealing 
  only 
  with 
  the 
  upper 
  and 
  larger 
  facets. 
  

  

  If 
  the 
  fly 
  sees 
  things 
  as 
  a 
  mosaic 
  of 
  which 
  each 
  element 
  represents 
  

   the 
  amount 
  of 
  the 
  visual 
  field 
  covered 
  by 
  a 
  single 
  facet 
  (3°), 
  then 
  this 
  

   mosaic 
  would 
  have 
  the 
  same 
  definiteness 
  as 
  one 
  composed 
  of 
  one-centi- 
  

   metre 
  blocks, 
  when 
  seen 
  at 
  a 
  distance 
  of 
  eight 
  inches. 
  Put 
  in 
  another 
  

   way, 
  we 
  could 
  paint 
  a 
  picture 
  representing 
  what 
  we 
  may 
  suppose 
  the 
  

   fly 
  sees 
  by 
  fixing 
  a 
  glass, 
  ruled 
  in 
  centimetre 
  squares, 
  at 
  a 
  distance 
  of 
  

   8 
  inches 
  from 
  the 
  eye. 
  Looking 
  through 
  each 
  square 
  in 
  turn, 
  and 
  

   having 
  a 
  piece 
  of 
  centimetre-squared 
  paper 
  before 
  us, 
  we 
  should 
  cover 
  

   each 
  square. 
  on 
  the 
  paper 
  with 
  a 
  uniform 
  patch 
  of 
  paint 
  representing 
  

   in 
  tone 
  and 
  colour 
  the 
  average 
  of 
  all 
  the 
  tones 
  and 
  colours 
  which 
  we 
  

   could 
  see 
  through 
  that 
  particular 
  square 
  of 
  glass. 
  

  

  Similarly 
  the 
  fly's 
  vision 
  of 
  objects 
  at 
  a 
  distance 
  of 
  two 
  centimetres 
  

   from 
  its 
  eye 
  would 
  show 
  details 
  as 
  small 
  as 
  one 
  milHmetre-square, 
  while 
  

   in 
  objects 
  at 
  a 
  distance 
  of 
  two 
  millimetres 
  it 
  could 
  distinguish 
  details 
  

   down 
  to 
  Yoth. 
  of 
  a 
  millimetre 
  in 
  size. 
  

  

  Our 
  own 
  eyes 
  enable 
  us 
  to 
  distinguish 
  points 
  separated 
  by 
  only 
  

   2-Joth 
  of 
  an 
  inch 
  (|th 
  mm.) 
  at 
  a 
  distance 
  of 
  eight 
  inches. 
  This 
  corres- 
  

   ponds 
  to 
  an 
  angular 
  range 
  of 
  about 
  J^^th 
  of 
  a 
  degree, 
  as 
  compared 
  with 
  

   the 
  fly's 
  2-3. 
  The 
  meaning 
  of 
  this 
  curious 
  and 
  very 
  characteristic 
  

   habit 
  is 
  now, 
  I 
  think, 
  made 
  clear. 
  The 
  movements 
  are 
  executed 
  only 
  

   when 
  the 
  fly 
  is 
  disturbed 
  by 
  movement 
  of 
  some 
  object 
  in 
  its 
  vicinity, 
  

   and 
  are 
  in 
  all 
  probability 
  a 
  defensive 
  manoeuvre. 
  It 
  might 
  at 
  first 
  

   sight 
  appear 
  somewhat 
  improbable 
  that 
  these 
  flies 
  should 
  adopt 
  as 
  a 
  

   defensive 
  measure 
  a 
  series 
  of 
  movements 
  which 
  can 
  hardly 
  make 
  them 
  

   less 
  conspicuous 
  than 
  they 
  would 
  be 
  if 
  they 
  remained 
  motionless. 
  To 
  

   our 
  eyes, 
  indeed, 
  they 
  are 
  more 
  conspicuous 
  when 
  moving 
  than 
  when 
  

  

  