WhUe  leaning  on  a  large  table  fix  a  point  on  the  table,  and  then  try  to  arrange
three  small  pieces  of  coloured  paper  in  a  straight  line.  Invariably,  the  papers,
being  at  a  distance  from  the  fixation-point,  and  being  seen  by  indirect  vision,
are  arranged  not  in  a  straight  line,  but  in  the  arc  of  a  circle  with  a  long
radius*

8.  Perception  of  Size.
Fix  the  centre  of  fig.  270  at  a  distance  of  3  to  4  cm.  from


LXXIII.]


PERIMETRY,   IRRADIATION,  ETC.
349


the  eye,  when  by  indirect  vision  the  broad  white  and  black  areas
of  the  peripheral  parts,  bounded  by  hyperbolic  curves,  will  appear
as  small  aiid  the  lines  bounding  them  as  straight  as  the  smaller
areas  in  the  middle  zone.
9.  Convergence  of  the  Visual  Axes  Influences  one's  Concep-
tions of  Size  and  Distance.
(a.)  Place  a  blackened  paper  tube  before  each  eye,  look  at  a  fixed
object,  and  then  gradually  converge  the  tubes ;  the  object  appears
larger  and  nearer.
Jia.  ajo.
(5.)  Look  at  an  object  through  two  pieces  of  glass  (2^x2^x^  in.),  held

At  first  in  the  same  plane,  one  in  front  of  each  eye.  Let  the  adjoining  edges  of
the  two  plates  of  glass  be  moved  each  on  a  vertical  axis,  so  that  they  form  either
a  more  or  less  obtuse  angle  with  each  other.  In  order  to  see  the  object  dis-
tinctly the  axes  of  the  eyeballs  must  converge  to  a  greater  or  less  extent,  as
the  case  may  be,  with  the  result  that  the  object  appears  larger  or  smaller,  or
appears  to  approach  or  recede  as  the  plates  are  rotated.  Special  forms  of
a]»paratus  contrived  by  Rollett,  and  another  by  Landois,  are  used  for  this
purpose.
350


PRACTICAL  PHYSIOLOGY.
[LXXIV.
10.  Apparent  Movements.
