The tapetum behind the retinas is of such high optical quality that it focuses the light onto the retinas. The lens only appears to compensate for the fact that the mirror is spherical and not a parabola, as a perfect focusing mirror should be. This is one of only two cases in the animal kingdom where an animal has made a mirror of optical quality. Why the eye needs two retinas, why it uses a mirror, and why what is essentially a glorified clam needs fifty to one hundred good eyes are open questions that another of my former students, Dan Speiser, took on. My favorite experiment of his involved showing scallops movies of food (in the form of particles moving on a computer screen). The scallops, held in little seats, would open their shells to feed if the particles were big enough and not moving too fast, suggesting that at least one function of their eyes may be to assess conditions for filter feeding. It was a classic case of an experiment that nobody (including me) ever thought would work, but did anyway.
In case you're wondering, the other animal with an optical quality mirror in its eye is the deep-sea spookfish, Dolichopteryx longipes. Make sure to keep that one in your pocket as the follow-up bit of cocktail party chatter after you tell people about the scallops.
UPDATE!!! A reader sends in this captivating, creepy video of a Martha's Vineyard bay scallop. It was by technician Kathryn Markey of Roger Williams University.