The diagram above illustrates the cross-eyed method of 3-d viewing.  This is an
overhead diagram so imagine that it was drawn from a position directly over the top of
the user and their computer.

Humans have two eyes.  The image that each eye perceives is slightly different from the other.  
To prove this, hold your finger up in front of your nose at arms length.  Without moving your finger
close one eye and then alternately close the other eye.  You will notice that your finger appears to
'jump' in relation to objects in the background.  The mind uses the differences in the images from
each eye to judge distances and perceive depth.

Most humans are able to cross their eyes.  This allows both of their eyes to focus on an object
that comes close to them.  By using your finger as a focal point (see diagram above), you can
designate the spot where the line of sight from both or your eyes cross.

In this way you can direct your right eye to view the left most image and your left to view the right
most image.  By moving your finger closer and further from your eyes you can change the focal
point and find a spot where the two images (seen separately by each eye) overlap in the center.  

This central image (created by two overlapping images that are slightly different from each other)
gives the illusion of 3-dimensionality.


The other method of 3-d viewing is called stereo-viewing.  You have probably seen old-fashioned
cards like this one:

The right and left images on these cards are placed a distance apart that is equal to the distance
between the average human's eyes.  Generally these cards were viewed with a device that aided
the viewer in focusing their eyes straight forward.  That is, the line of sight for both eyes ran
parallel to each other.  Each eye would receive its own image that was directly ahead of it.  These
two images would be melded together in the viewer's mind and the slight differences in the two
images would, again, give the illusion of 3-dimensionality.

With some practice, it is possible to view these stereo cards without a viewer.  The method is not
the same as described for the cross-eyed method.  Using the cross-eyed method with these cards
will cause the 3-d effect to be reversed (meaning the parts of the image that should appear closer
would try to appear further away).   Instead of crossing your eyes, you must focus your eyes as if
looking to a distant object on the horizon.  As with the cross-eyed method this can be challenging
as your mind knows that the image is just a couple feet away and tries to make you focus there
instead of at infinity where the 3-d effect will work.

I prefer the cross-eyed method because it does not limit the size of the images.  Images for stereo
viewing can be no larger than the ones in the zeppelin picture above.  This distance is dictated by
the distance between human eyes.  The cross-eyed method allows for images of any size.  They
could be projected on a screen or even painted on a billboard and the 3-d effect would still work.  
Larger pictures allow for more detail and therefore richer images.