THE  ETE  AS  A  SEmOBT  APPARATUS.  553

That  it  is  not  is  shown  by  cases  of  persons  born  blind,  who
have  had  sight  restored  later  in  life  by  surgical  operations.
Such  persons  have  at  first  no  visual  perceptions  of  distance:
all  objects  seem  spread  out  on  a  flat  surface  in  contact  with
the  eyes,  and  they  only  learn  gradually  to  interpret  their
sensations  so  as  to  form  judgments  about  distances,  as  the
rest  of  us  did  unconsciously  in  childhood  before  we  thought
about  such  things.
The  Visual  Perception  of  Size.  The  dimensions  of  the
retinal  image  determine  primarily  the  sensations  on  which
conclusions  as  to  size  are  based;  and  the  larger  the  visual
angle  the  larger  the  retinal  image :  since  the  visual  angle  de-
pends on  the  distance  of  an  object  the  correct  perception  of
size  depends  largely  upon  a  correct  perception  of  distance;
having  formed  a  judgment,  conscious  or  unconscious,  as  to
that,  we  conclude  as  to  size  from  the  extent  of  the  retinal
region  affected.  Most  people  have  been  surprised  now  and
then  to  find  that  what  appeared  a  large  bird  in  the  clouds
was  only  a  small  insect  close  to  the  eye;  the  large  apparent
size  being  due  to  the  previous  incorrect  judgment  as  to  the
distance  of  the  object.  The  presence  of  an  object  of  toler-
ably well-known  height,  as  a  man,  also  assists  in  forming
conceptions  (by  comparison)  as  to  size;  artists  for  this  pur-
pose frequently  introduce  human  figures  to  assist  in  giving
an  idea  of  the  size  of  other  objects  represented.
The  Visual  Perception  of  a  Third  Dimension  of  Space.
