Many of the same themes and inquiries concerning vision continued to preoccupy Leonardo when he executed another, better-known painting of the Virgin and Child not long afterward, the so-called Benois Madonna (c. 1478–80), named for its last private owner (Leon Benois), and now in the State Hermitage Museum in St. Petersburg (fig. 28). In what can be regarded almost as a further elaboration on the actions of the Munich infant, Leonardo portrayed the older, and rather immense, child in the Benois Madonna as gamely trying to focus his eyes on a sprig of cruciform-shaped flowers. The painter would have had a largely erroneous idea about the nature of the child's struggle, because he did not know of the existence of lenses in the eyes.

Instead, following the eminent Florentine theorist Leon Battista Alberti, Leonardo, at least into the 1490s, ascribed to the fallacious, Platonic “theory of emission” in the operations of sight – that is, the belief that the eyes emit rays that extend to the object seen. In his widely read treatise On Painting of 1435, Alberti had applied the geometric elaboration of Euclid's Optics (c. 300 b.c.) to the visual rays (opseis) that Plato described in his fourth-century dialogue, the Timaeus. There, Plato spoke of such fictive rays as an ocular “fire” mixing in sympathy with external light. With great authority, Alberti explained how an infinite number of these rays issued from the eyes, the most powerful being the central one or “centric ray,” which, he believed, played “the largest part in the determination of sight.”