ne-w  Cafe  in  Squinting.  gi 
The  facility  with  which  matter  sandford  received 
the  images  of  oblique  objects  on  the  infenfible  part  of 
the  retina  of  one  eye,  whilft  he  viewed  them  with  the 
other,  induced  me  to  obferve  the  fize  of  this  infenfible 
fpot,  and  to  endeavour  to  afcertain  the  caufe  of  it. 
There  was  formerly  a difpute  among  philofophers, 
whether  the  choroid  coat  of  the  eye  or  the  retina  was  the 
immediate  organ  of  vifion,  which  has  lately  been  re- 
vived in  fome  meafure  in  Dr.  Priestley’s  valuable  Hif- 
tory  of  Light  and  Colours ; and  it  was  then  thought  by 
one  party  in  this  difpute,  that  the  defedt  of  the  choroid 
coat,  where  the  optic  nerve  enters  the  eye,  was  the  caufe 
of  this  want  of  vifion  in  that  part. 
But  the  following  obfervation  fhews  beyond  a doubt 
the  fallacy  of  this  fuppofition : the  diameter  of  the  optic 
nerve,  at  its  entrance  into  the  eye,  is  about  one-fixth  of 
an  inch,  and  the  perforation  of  the  choroid  coat,  through 
which  it  pafles,  mutt  of  neceffity  be  of  the  fame  diame- 
ter : now  the  dark  fpot,  which  is  feen  in  objedts  oppofed 
to  the  center  of  the  optic  nerve,  if  it  was  occafioned  by 
the  deficiency  of  the  choroid  coat,  fhould,  at  nine  inches 
diftance  from  the  eye,  be  fifty-four  times  the  diameter 
of  this  aperture,  or  nine  inches  in  diameter;  whereas  I 
find,  by  experiment,  that  a paper  of  one  inch  in  diameter 
could  not  be  totally  concealed  at  nine  inches  diftance 
N a from 
