The glass sphere that Christ holds in one hand may well prove the clinching evidence that a recently cleaned painting is a newly discovered work by none other than Leonardo da Vinci.

Leonardo had many imitators in 16th century Milan but their efforts are miserably below his standard. Looking at the newly cleaned Salvator Mundi, which will be exhibited at the National Gallery this autumn as a rediscovered work by Leonardo, the transparent orb is far too brilliantly painted to be the work of one of his disciples.

Leonardo was obsessed with problems like how to create translucent effects in painting, how to capture the mysteries of light – the very problems posed by painting a glass sphere.