In the uppermost example, 
we have a square containing a 
small circle a little to the right 
of the center in one member 
and a little to the left in the 
other. If you view the figure 
with both eyes, using the 
stereoscope or partition 
method, you should see the 
circle no longer in the plane of 
the screen but standing out in 
front of it about an inch or so. 
Similarly, you should see the 
second figure as a circle behind 
the plane of the screen. You see 
the circle in front or behind 
the screen because your retinas 
are getting exactly the same 
information they would get if 
the circle were in front or 
behind.

    In 1960 Bela Julesz, at Bell 
Telephone Laboratories, 
invented an ingenious, highly 
useful method for 
demonstrating stereopsis: a 
figure will at first glance seem 
like a uniformly random mass 
of tiny triangles--and indeed it 
is except for a concealed larger 
triangle in the center part. If it 
is seen through pieces of 
colored cellophane, red over 
one eye and green over the 
other, the center-triangle 
region stands out in front, just 
as the circle in the 
illustrations here did. 
Reversing the cellophane 
windows would reverse the 
depth. The usefulness of these 
Julesz patterns is that the 
triangle standing out in front 
or receding can not possibly be 
seen without one's having 
intact stereopsis.

    To sum up, our ability to see 
depth depends on five 
principles:

1. We have many cues to depth, 
such as occlusion, parallax, 
rotation of objects, relative 
size, shadow casting, and 
perspective. Probably the most 
important cue is stereopsis.

2. If we fixate on, or look at, a 
point in space, the images of 
the point on our two retinas fall 
on the two foveas. Any point 
judged to be the same distance 
away as the point fixated casts 
its two images on corresponding 
retinal points.

3. Stereopsis depends on the 
simple geometric fact that as an 
object gets closer to us, the two 
images it casts on the two 
retinas become outwardly 
displaced, compared with 
corresponding points.

4. The central fact of 
stereopsis--a biological fact 
learned from testing people--is 
this: an object whose images 
fall on corresponding points in 
the two retinas is perceived as 
being the same distance away as 
the point fixated. When the 
images are outwardly displaced 
relative to corresponding 
points, the object is seen as 
nearer than the fixated point, 
and when the displacement is 
inward, the object is seen as 
farther away.

5. Horizontal displacements 
greater than about 2 degrees or 
vertical displacements of over a 
few minutes of arc lead to 
double vision.