Alternate Illusion

With a convex lens of about an inch focus, look attentively at a
silver seal, on which a cipher is engraved. It will at first appear
cut in, as to the naked eye; but if you continue to observe it some
time, without changing your situation, it will seem to be in relief,
and the lights and shades will appear the same as they did before. If
you regard it with the same attention still longer, it will again
appear to be engraved: and so on alternately.

If you look off the seal for a few moments, when you view it again,
instead of seeing it, as at first, engraved, it will appear in relief.

If, while you are turned towards the light, you suddenly incline the
seal, while you continue to regard it, those parts that seemed to be
engraved will immediately appear in relief: and if, when you are
regarding these seemingly prominent parts, you turn yourself so that
the light may fall on the right hand, you will see the shadows on the
same side from whence the light comes, which will appear not a little
extraordinary. In like manner the shadows will appear on the left, if
the light fall on that side. If instead of a seal you look at a piece
of money, these alterations will not be visible, in whatever situation
you place yourself.

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