Disapperances à la Sam Loyd

There are two halves in the pictures below: upper and lower. Slide either left or right. Do that slowly. When the eyes open, it's the sign that some counting is in order. In the first puzzle, a face may appear or disappear. The puzzle is due to Marting Gardner [Mathematics Magic and Mystery, p. 120]. This an artistic elaboration on a simplified version where a rectangle disappears only to pop up again.

In the second one count the number of humans, or are they leprechauns as Sam Loyd would have them. Sometimes there are 12 and sometimes 13 of them. I found the underlying picture at the site of David Balaban.

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References

M. Gardner, Mathematics Magic and Mystery, Dover, 1956, p. 120

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