What makes La Bella Principessa a genuine Leonardo Da Vinci portrait?

Martin Kemp discusses 'La Bella Principessa', a profile portrait of a Milanese lady, and the roles of himself, Pascal Cotte, and others including Paul Biro, in attributing this radiant and exciting picture to Leonardo da Vinci. Martin describes his story in coming across the picture, and moves on to the attribution process, first in terms of materials and technique, and then the new technology used in scientific investigation, and social and cultural aspects of the sitter. Martin...