National Gallery, LondonCUSTODIANS OF the national collections of paintings and works of art have a duty to those on whose behalf they care for the works – all of us, in other words – not only to make sure the collections are safe from rain and beetles and available fairly ­regularly for us to look at, but also to help us enjoy them by understanding them, if that is what we want to do. It is a duty London’s National Gallery seems to take particularly seriously. As well as being involved in large-scale international exhib­itions (with glamorous loans and pricey tickets), it has over the years spotlighted its own collection by organising free displays and sharp-focused shows to throw light on the history, context and creation of selected pictures that live permanen