your right pointer finger up to your face, about six inches
in front your nose. First focus on your finger by crossing
your eyes; then focus on the background. Move slowly from
one image to the other. Notice how this focusing involves
two different things: one is the merging or splitting of
the double image, and the other is the blurring or sharpening of the
Now try this: focus on the background, so that you see two blurry fingers, and then slowly bring the fingers into sharp focus but do not bring them together. See, if you can, a single blurry background, and a double sharp focused finger.