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.