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The visual trick may have been created by the artist’s use of a common optical viewing aid called a camera lucida.

Some think he employed a camera lucida’s lens to render sitters as accurately as possible, which makes two details surprising.

Unprompted, she tells me she’s been meaning to read “Camera Lucida,” a book on photography by the French writer Roland Barthes that I mentioned to her in passing.

At some point in the past year, I bought a used copy of “Camera Lucida.”

Reading back through “Camera Lucida” for this essay, I was not always sure who made which mark, which of us left which note.