563.  The  stereoscope.   Binocular  yision.   When  an  object  is  seen  with
both  eyes  the  images  formed  on  the  two  retinas  differ  slightly,  because


Fia.  469.    Principle
of  the  stereoscope


OPTICAL  IN^STKUMENTS

of  the  fact  that  the  two  eyea,  on  account  of  their  lateral  Beparation,  are
Tiewing  the  ohject  from  sUghtly  different  angles.    It  ia  this  difference
In  the  two  images  which  gives  to  an  object  or  land-
scape  viewed   with    two    eyea    au   appearance    of
depth,  or  solidity,  which  is  wholly  wanting  when
s  closed.    The  stereoscope  ia  an  instru-
ment which  reproduces  in  photographs  this  effect
of  binocular  vision.    Two  photographs  of  the  saiiie
object  are  taken  from  sliglitly  different  puints  of
These  photographs  are  mounted  at  A  and  B
(Fig.  460),  where  they  are  simultaneously  viewed
by  the  two  eyes  through  the  two  prismatic  lenses  m
and  n.    These  two  lenses  superpose  the  two  images
at  C  because  of  their  action  as  prisitiS,  aiid  at  the
time  magnify  them  because  of  their  action
iple  magnifying  lenaes.    The  result  ia  that  the
observer  ia  consciona  of  viewing  but  one  photograph  ;  but  this  differs
Irom  ordinary  photographs  in  that,  instead  of  being  flat,  it  has  all  of
the  charaoteriatica  of  an  ohject  actually  seen  with  both  eyes.
The  opera  glass  has  the  advantage  over  the  terrestrial  telescope  of

affording  the  benefit  of  binocular  vision;  for  while  telescopes  are  uaually

constructed  with  but  one  tube,

operi  glasses  alwajs  have  two,

one  for  each  eye

564  The  Zeiss  binocular.
