The impression of depth 
from the stereograms that 
appear on this CD-ROM may be 
achieved without the aid of a 
stereoscope by imagining that 
you are looking through the two 
pictures, at a point far beyond, 
while blocking the view of one 
of the two pictures from each 
eye. A good way of doing this is 
to start with your nose against 
the screen and then slowly pull 
away. This effect can also be 
achieved by using a sheet of 
cardboard or paper, held 
perpendicular to the screen, 
between the pair of images. Or it 
can be achieved by fixating an 
imaginary point in front of the 
two pictures by crossing your 
eyes. Still another method is to 
view a picture with 
superimposed red and green 
components through red and 
green filters. However the 
disparity is achieved, some 
people have only a weak 
impression of depth from 
stereopsis, while a few are 
stereo "blind."

    A recent discovery makes 
dramatically clear just how 
powerful a cue retinal disparity 
is by itself. In 1960, Bela Julesz 
at the Bell Telephone 
Laboratories developed two 
patterns of random dots that, 
when viewed as a stereogram, 
will fuse in such a way that a 
figure appears to be floating in 
the foreground or background. 
In the illustration to the left,
for instance, a triangle 
appears in front of the 
background. Where does the 
triangle come from? Directly 
from the binocular fusion 
going on inside the head. 
The effect is an interesting 
illustration of psychological 
construction, since the 
triangle perceived has no 
existence either in the two
figures or in their retinal 
images.