Kurt Andersen, writing for Vanity Fair:
But still, exactly how did Vermeer do it? One day, in the bathtub, Jenison had a eureka moment: a mirror. If the lens focused its image onto a small, angled mirror, and the mirror was placed just between the painter’s eye and the canvas, by glancing back and forth he could copy that bit of image until the color and tone precisely matched the reflected bit of reality. Five years ago, Jenison tried it out on the kitchen table. He took a black-and-white photograph and mounted it upside down, since a lens would project an image upside down. He put a round two-inch mirror on a stand between the photo and his painting surface. He immediately found that “when the color is the same, the mirror edge disappears,” and you’re through with that bit. Five hours later, he had painted a perfect duplicate of the photo, an astounding proof of concept by someone who can’t draw and had never painted a thing. Then he used his mirror trick to copy a color photo. Again, perfect. “I couldn’t believe my eyes,” he says.
The real story here is the lengths that Tim Jenison went to in order to test his theory. Insane. Aside from, oh, you know, just reproducing the room in the painting, there's also this:
For his experimental purposes—using a device that Vermeer himself could have made—Jenison decided that modern lenses are too fine. So he learned how to make lenses himself, to melt and polish glass using 17th-century techniques. Jenison painted only with pigments available in the late 1600s and learned to mix them himself, including grinding lapis lazuli stones (“they’re kind of poisonous,” he points out) to make ultramarine blue.