Another piece of science that augments the Mona Lisa’s smile comes from Leonardo’s research on optics: He realized that light rays do not come to a single point in the eye, but instead hit the whole area of the retina. The central area of the retina, known as the fovea, has closely packed cones and is best at seeing small details; the area surrounding the fovea is best at picking up shadows and shadings of black and white. When we look at an object straight on, it appears sharper. When we look at it peripherally, glimpsing it with the corner of our eye, it is a bit blurrier, as if it were farther away.

With this knowledge, Leonardo was able to create an interactive smile, one that is elusive if we are too intent on seeing it. The fine lines at the corners of Lisa’s mouth show a small downturn—just like the mouth floating atop the anatomy sheet. If you stare directly at the mouth, the retina catches these tiny details and delineations, making her appear not to be smiling. But if you move your gaze slightly away, to look at her eyes or cheeks or some other part of the painting, you will catch sight of her mouth only peripherally. It will be a bit blurrier. The tiny delineations at the corners of the mouth become indistinct, but you will still see the shadows at her mouth’s edge. These shadows and the soft sfumato at the edge of her mouth make her lips seem to turn upward into a subtle smile. The result is a smile that twinkles brighter the less you search for it.