Dr.  darwin  on  a 
94 
ferve  them ; but  I fufped  this  veffel  may,  after  the  growth 
of  the  animal,  be  totally  obliterated;  and  that,  in  fome  few 
inftances,  the  optic  nerve  may  even  in  this  part  become 
ienfible  to  light.  One  inftance  I am  certain  I have 
feen,  as  it  was  in  a man  capable  of  the  moft  patient 
and  accurate  obfervation,  who  on  numberlefs  repeated 
trials,  at  different  times,  in  my  prefence,  could  never 
lofe  fight  of  the  fmallell  object  with  either  of  his  eyes. 
Supplement  to  the  cafe  in  Squinting. 
IT  fince  occurred  to  me,  that  the  unufual  mode  of 
fquinting  defcribed  in  the  above  paper  muft  have  arifen 
from  fome  original  difference  in  the  fenfibility  of  fome 
parts  of  the  eye,  which  might  have  rendered  it  more 
eafy  for  mafter  sandford,  when  a child,  to  obferve  ob- 
jects with  one  eye  only,  and  that  with  the  eye  moft 
diftant  from  objects  prefented  obliquely  to  him. 
Two  circular  papers,  each  of  four  inches  diameter, 
were  ftuck  againft  the  wall,  their  centers  being  exactly 
at  eight  inches  diftance  from  each  other.  On  doling 
one  eye,  and  viewing  the  central  fpot  of  one  of  thefe 
papers  with  the  eye  furtheft  from  it,  and  then  retreating 
twenty- 
