WHEN TOURISTS return to Florence, some will wander into its cathedral and be struck by Andrea del Castagno’s “Equestrian Monument of Niccolò da Tolentino”.

Painted in 1456, the fresco of the mercenary commander and his horse is an optical illusion. The pair are depicted as a statue and the perspective scheme enables the painting to be viewed from different angles.

As they walk on, some of those tourists who admire it may speculate what more the artist might have achieved had not the plague, which returned to Florence in 1457, killed him in his mid-30s.