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VARIETIES. 
fcarte  ties. 
Binocular  Vision. — Of  the  thousands  who  gaze  with  delight  upon  the 
magical  effects  produced  by  that  small  instrument  known  as  the  stereo- 
scope, how  few  there  are  who  comprehend,  or  attempt  to  assign  reasons 
for,  the  extraordinary  optical  illusions  experienced  through  its  instrumen- 
tality. 
It  is  with  the  view  of,  in  some  degree,  elucidating  the  principles  of  vi- 
sion upon  which  these  are  founded,  that  the  following  article  is  written. 
It  will  in  the  first  place  be  well  to  consider  the  difference  between  mo- 
nocular and  binocular  vision.  Nature  has  furnished  us  with  several  means 
of  determining  the  distance  of  objects  which  may  happen  to  come  within 
reach  of  our  visual  organs.  One  is  that  of  distinctness ;  a  greater  or  less 
degree  of  which — other  things  being  equal — gives  an  idea  of  greater  or 
lesser  distance  in  the  object  viewed.  The  second  is  through  the  change  of 
focus  required  in  the  lens  of  the  eye  in  refracting  to  a  point  on  the  retina, 
rays  of  light  entering  it  with  a  greater  or  lesser  degree  of  parallelism, 
thus  producing  in  the  brain  a  consciousness  of  unequal  distances  in  the 
objects  from  which  they  emanate. 
The  means  above  alluded  to,  it  is  evident,  are  enjoyed  in  almost  the 
same  degree,  when  viewing  with  one  eye  as  where  both  are  used. 
By  far,  however,  the  greatest  power  with  which  nature  has  endowed  us 
of  discriminating  distances,  is  through  the  agency  of  binocular  vision ;  or 
in  other  words,  in  the  sensation  produced  in  the  brain  by  the  different  de- 
grees of  convergency  of  the  optic  axes  required  in  obtaining  distinct  vi- 
sion of  the  differently  distant  points  of  objects  upon  which  they  are  di- 
rected. It  is  to  this  faculty  that  we  are  indebted  for  our  most  palpable 
evidence  of  differential  distances,  and  for  that  consciousness  of  solidity  and 
relief  so  remarkably  experienced  in  the  stereoscope. 
It  is  evident,  for  example,  when  we  are  looking  at  a  house  or  other  ob- 
ject that  has  depth  as  well  as  breadth,  from  such  a  point  of  view  as  to  ena- 
ble us  to  see  two  sides  of  it  at  once,  that  we  receive  a  differently  perspec- 
tive image  upon  the  retina  of  either  eye,  or  that  we  must  see  more  of  one 
side  and  less  of  the  other  with  the  right  eye  than  with  the  left,  or  vice 
versa.  Thus  accomplishing  with  one  view  what  a  person  with  but  one  eye 
would  require  two  views  at  positions  two  and  a  half  inches  apart — the  dis- 
ance  between  the  eyes — to  accomplish.  These  are  the  different  perspec- 
tive views  of  the  stereoscopic  cards,  and  it  is  the  effort  to  reconcile  these 
dissimilar  pictures  by  converging  the  optic  axes  at  points  differently  dis- 
tant from  the  eyes  which  produces  the  wonderful  effects  above  alluded  to, 
