In 1977, David Hockney painted My Parents. In bright Crayola colors, it pictures his mother and father seated on each side of a green cabinet, on top of which stands a small mirror. Reflected in the glass is a fragment of Piero della Francesca’s The Baptism of Christ. Also in 1977, Hockney painted his friend, the curator Henry Geldzahler, looking at a series of pictures hanging on a screen. The same work by Piero is among them. A touchstone? A talisman? An omen? At the National Gallery, Hockney’s two paintings are on view alongside Piero’s The Baptism of Christ—a masterpiece that inspires him to this day. —Elena Clavarino