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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 eng
aved: 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.