When I was a child, I had an activity book called "The Anti-Coloring Book." It featured puzzles, games and activities designed to engage children in multiple ways beyond a simple coloring book. One of the "puzzle tricks" I remember was based on the (perhaps incorrectly named?) "persistence of vision" phenomenon. I'm not a retina specialist; I'll continue my story and leave the debate over the proper terminology to others.
The way this trick worked was you were to stare at, for example, a square with thick, black lines on an otherwise empty white page for 60 seconds, and then look away to another white surface, such as a wall. You would see an "after image" of the square when you looked away from the page. Bear in mind, I was a child with next to no understanding of vision and the human eye at the time, so this sounded nothing short of magical to me-- with something from a book jumping off onto the wall next to me!
The only problem was, I could never make it work. I'd stare at the page "forever", only to look away and see nothing. I actually wondered at one point if my eyes might be "broken" or "different" because I couldn't "see" the trick. (I still feel that way about those infuriating Magic Eye puzzles, but that's a story for another day!) I even remember talking to my parents to see if I was somehow doing it wrong.
I mention this childhood memory now, because as I have gotten older my eyes/retinas seem to have the afterimage phenomenon in excess. When I wake in the morning, if I look over at my dresser and mirror for only a few seconds, and then look away from it, I can see the "afterimage" of the mirror's thick black frame on the ceiling for a duration of several seconds. When this happens, I can't help but recall "The Anti-Coloring Book" and how I used to worry a little bit that I wasn't able to make that afterimage trick work.
Maybe I'll give those Magic Eye puzzles another go . . . after I turn 60. ;)
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