To make a photograph with this peculiarity, it is necessary to make
two exposures of a head in exactly the same position, one with the
eyes closed and the other with them open. Two positives are made from
the two negatives and bound in contact by means of lantern slide
binders, so that the outlines coincide. If they are now held in front
of a flickering lamp or match flame, the combined portrait will be
seen to rapidly open and close its eyes, giving a very weird effect.
This effect depends upon the fact that the human eye receives
impressions slowly and has a tendency to judge that a motion is
uniform, when rapidly varying phases of it are seen. The flickering
flame, moving sideways, shows first one and then the other of the two
images, which are separated by the thickness of the glass. The same
effect can be produced by sliding the pictures slightly sideways on
each other, but the perfection of the illusion will depend somewhat on
the regularity of the movement, and the flame method is better. If the
two pictures are printed on one piece of paper, the combined image may
show the same illusion.