What was striking for me was the orb, and I've subsequently researched it quite heavily. The "Salvator Mundi" obviously holds the mundus, the world which he's saving, and it was absolutely unlike anything I've seen before. The orbs in other Salvator Mundis, often they're of a kind of brass or solid. Sometimes they're terrestrial globes, sometimes they're translucent glass, and one or two even have little landscapes in them. What this one had was an amazing series of glistening little apertures — they're like bubbles, but they're not round — painted very delicately, with just a touch of impasto, a touch of dark, and these little sort of glistening things, particularly around the part where you get the back reflections. And that said to me: rock crystal. Because rock crystal gets what are called inclusions, and to get clear rock crystal is very difficult, particularly big bits. So there are these little gaps, which are slightly irregular in shape, and I thought, well, that's pretty fancy. And Leonardo was a bit of an expert on rock crystal. He was asked to judge vases that Isabella d'Este was thinking of buying, and he loved those materials. 

So when I was back in Oxford, I went to the geology department, and I said, "Let's have a look at some rock crystal." And in the Ashmolean Museum, in a wunderkammer of curiosities, there is a big rock crystal ball, and that has inclusions, so we photographed it under comparable lighting conditions I also began to look at the heel of the hand underneath the globe in the "Salvator Mundi"; there are two heels. The restorer thought it was a pentimento, but I wondered if he was recording a double refraction of the kind you get with a calcite sphere. If this proves to be right, it would be absolutely Leonardesque. I like these things when they're not just connoisseurship. None of the copyists knew that. They just transcribed it. Some of them do better than others, but none of them got this crystal with its possible double refraction. And one of the points of the crystal sphere is that it relates iconographically to the crystalline sphere of the heavens, because in Ptolemaic cosmology the stars were in the fixed crystalline sphere, and so they were embedded. So what you've got in the "Salvator Mundi" is really a "a savior of the cosmos", and this is a very Leonardesque transformation.