Dr.  darwin  on  a 
92 
from  my  eye;  and  M.  le  cat  by  accurate  obfervations 
found,  that  the  infenfible  part  of  his  eye  was  but  be- 
tween the  thirtieth  and  fortieth  part  of  an  inch  in  dia- 
meter. This  experiment  is  fo  eafily  made,  that  it  can 
be  attended  with  no  fallacy ; and  at  the  fame  time  that 
it  fliews  that  the  infenfible  fpot,  where  the  optic  nerve 
enters  the  eye,  is  not  owing  to  the  deficiency  of  the  cho- 
roid coat,  intirely  fubverts  the  opinion  of  the  choroid 
coat  being  the  organ  of  vifion;  for  vifion  exifts  where 
the  choroid  coat  is  not. 
Nor  is  the  infenfibility  of  the  center  of  the  optic 
nerve  owing  to  the  ingrefs  of  the  arteries  along  with  it 
into  the  eye ; for  a large  branch  of  this  artery  runs  along 
the  bottom  of  the  eye,  where  vifion  is  moft  diftindt,  and 
becaufe  all  this  artery  is  covered  with  the  expanfe  of  the 
retina  on  the  external  fide  of  it.  Mr.  savage  made  an 
experiment  for  another  purpofe,  which  however  fhews, 
that  the  optic  artery,  where  it  is  branched  under  or 
through  the  retina,  does  not  much  difturb  the  power  of 
vifion.  It  is  this:  if  you  look  on  a white  wall  on  a 
luminous  day,  with  the  Sun  fhining  on  the  wall 
only  by  its  reflected  light,  you  will  difcern  the  parts  of 
the  wall  become  darker  and  lighter  at  every  pulfation  of 
the  optic  artery.  This  darker  and  lighter  appearance  is 
like  net- work,  and  not  uniform  like  the  wall  itfelf;  but 
the 
