The application of positive and negative spaces — that is, the contrast of light and dark — is a basic concept in visual art. One of these will appear solid; the other, empty. Think of black versus white, with no shades in between.
In another way, think of a doughnut or bagel, defining an empty hole.
In my novel What’s Left, she applies a similar strategy after her father vanishes in an avalanche when she’s 11. She yearns to know much more about who he was — in fact, intends to recover him in her own way — so she assembles everything she can find to create a positive impression and then dives into the remainder, the negative, to dig up the rest. Maybe you’d see this as trying to assemble a jigsaw puzzle.
Since he was a professional photographer, she carefully investigates another kind of negative for answers — the many strips of film stored in his studio. At first what she views is reversed from normal perception, where everything that’s light should be dark and everything that’s dark should be light, but then she learns to transform what’s there into contact sheets and glossy prints. Just like he did.

Thanks to digital photography, negatives are ancient history. Maybe that’s somehow appropriate, since Cassia’s life at that point would now be ancient history, too, even as she’s investigating what she would consider ancient history.
Have you ever handled photographic negatives? Is there some other way you’ve looked at things reversed from normal? How about funny mirrors? How did it change your perception?
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Remember, What’s Left is available at Smashwords.