brain,  by  which  means  the  mind  perceives  every  thing  in
the  direction  of  those  rays  of  light. Hence   it  is,  that  aU
though  the  rays  of  light  that  pass  from  the  upper  part  of  an
object,  and  those  from  the  lower  part  cross   each  other  as
they  make  their  way  through  the  humours  and  lenses  of  the
eye,  and  fall  upon  opposite  parts  of  the  retina,  those  that
are  emitted  or  rather  reflected  from  the  upper  part  of  the
object  going  to  the  lowest  on  the  retina,   and  the  contrary;
yet,  in  as  much  as  the  effect  is  produced  upon  the  corres-
ponding parts  of  the  brain,  we  perceive  the  upper  part  of  the
object  by  means  of  the  rays  that  fall  upon  the  lower  part  of
the  retina,  and  the  lower  part  of  the  object  by  means  of  rays
that  fall  upon  the  upper  part  of  the  retina,  as  we  feel  the  up-
per and  lower  part  of  any  thing  by  means  of  two  sticks  that
cross  each  other,  and  are  held  in   both  hands  at  the  same
time;  the  hand  that  is    below  enabling  us  to  perceive  by

3  E


402  Of  our  Acquired  PereeptionSj

means  of  it  the  higher  part  of  the  object,  and  that  which  is
above  enabling  us  to  perceive  by  means  of  another  stick
crossing  the  first,  the  lower  part. This  account  may  serve
well  enough  as  an  illustration  of  the  opferations  of  one  sense,
by  tracing  an  analogy  between  them  and  the  operations  of
another,  but  furnishes  no  solution  of  the  phenomenon  of  our
seeing  erect  objects  by  inverted  images. We  should  think
it  no  satisfactory  explanation  of  the  manner  in  which,  by  the
sights  we  are  able  to  hold  converse  with  objects  at  a  distance,
to  say  that  by  means  of  rays  of  light  transmitted  in  straight
lines  to  the  eye,  we  perceive  them,  as  we  perceive  remote  ob-
jects^by  means  of  a  stick  held  in  our  hands  by  which  we
touch  them,  or  in  the  language  of  Mr. Addison  in  his  Spec-
tator, that  the  sight  is  only  a  more  delicate  and  diffusive
kind  of  touch,  presenting  to  us  at  a  single  glance  the  great-
est distances  and  magnitudes  in  the  universe. 