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Or just print the negative so the image only shows up if you wash it in acids
...in what online shop???
Its called 'Amazing' I think ? !
To answer the mirror question Bill, it's related to a vampire, so no, it won't show up in a mirror.
Maybe with an old fashioned camera obscura, a pair of prisms and a pola filter, at 13:00hrs, on the 13th of Octember (given the sun is in the right position), and standing 1 degree east of north, exactly 3/4 of a mile from Stonehenge, at an elevation of two feet, we could figure this out.