Stare at the hat using only your right eye while holding
the image at about half an arm’s length. Move closer or
further until the rabbit vanishes.
Close your left eye and look at the hat in the figure using
only your right eye. Slowly move the page closer to or
further from your eye, at a distance somewhere in the
region of half an arm’s length, while you stare at the hat.
At some point you’ll notice that the rabbit vanishes, as
though by magic. This is because the part of your retina
that should be seeing the rabbit is occupied by your
blind spot.
Close your right eye. With your left eye, look at the +.
Slowly move the card closer towards you. The space in
the middle of the vertical lines will disappear. Measure
this distance.