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VARIETIES. 
83 
and  which  enables  us  to  experience  all  the  sensations  of  delight  which 
would  be  produced  by  the  contemplation  of  the  landscape  itself. 
The  stereoscopic  pictures  will  of  course  never  quite  correspond.  They 
are  taken  simultaneously  with  a  camera  constructed  with  two  lenses,  or 
consecutively  with  a  camera  with  one  movable  lens. 
The  lenses  of  the  stereoscope,  besides  magnifying  the  pictures,  are  so 
placed  as  to  unite  certain  similar  points  of  them,  thus  relieving  the  eyes  of 
too  great  effort  by  uniting  them  entirely  by  convergency  of  the  axes. 
The  means  above  alluded  to,  by  which  we  are  enabled  to  judge  of  differ- 
ential distances,  are  of  course  much  diminished  by  the  distance  that  the 
objects  viewed  are  removed  from  us. 
Our  consciousness  of  different  distances  by  distinctness  is  diminished 
through  decrease  of  light. 
Our  judgment,  through  change  of  focus,  is  diminished  in  consequence  of 
the  parallelism  of  rays  from  distant  objects  being  so  nearly  the  same  as  to 
require  but  little  change  in  refracting  them  to  a  point  on  the  retina.  And 
lastly,  the  binocular  effect  is  in  a  great  degree  impaired  through  the  iden- 
tity of  distant  views  when  seen  from  positions  only  separated  by  a  base  of 
two  and  a  half  inches. 
Nature  has  thus  observed  her  usual  economy  in  providing  for  our  neces- 
sities alone.  It  being  of  little  comparative  importance  to  us  generally,  to 
be  acquainted  with  the  relative  positions  of  distant  objects,  whereas  our 
personal  convenience  and  even  safety  depend  greatly  on  our  knowledge  of 
those  near  at  hand.  We  are  therefore  provided  with  much  more  ample 
means  of  determining  the  latter  than  the  former. 
It  may  not  be  out  of  place  here  to  allude  to  another  subject  immediately 
connected  with  vision. 
Sir  David  Brewster  says,  in  allusion  to  the  cause  of  erect  vision  from  an 
inverted  image,  "  That  it  has  long  been  a  problem  among  the  learned." 
And  further  remarks,  "  That  it  is  perfectly  explained  by  the  law  of  visible 
direction."  Now,  although  it  is  evidently  a  consequence  of  that  law,  the 
question  arises,  why  should  we  see  objects  in  a  direction  perpendicular  to 
the  retina  at  the  point  where  their  image  meets  it? 
In  truth,  it  is  one  of  those  facts  that  , requires  no  explanation,  as  there 
is  nothing  inconsistent  or  irrational  in  it. 
There  is  no  more  reason  that  an  inverted  image  should  give  us  the  idea 
of  an  inverted  object,  than  that  the  reverse  should  be  the  case,  cr  that  it 
should  give  the  idea  of  a  horizontal  one.  There  is  no  unity  necessary  be- 
tween the  direction  of  an  object  and  the  direction  of  its  image  upon  the 
retina.  Nature  has  so  associated  impressions  upon  the  retina  with  im- 
pressions on  the  brain,  that  an  inverted  image  on  the  former  is  evidence 
of  an  erect  object  to  the  latter.  And  we  have  no  consciousness  of  anything 
to  the  contrary, — Jour.  Frank.  Ins.  Dec.  1858.  W. 
