Pity the poor art historian writing a survey of painters from Giotto to, say, Poussin. In order to produce a history that is as gender-balanced as possible it is absolutely necessary to include every woman painter ever heard of, some perfectly worthy of their place, some less so. How the art historian might envy the novelist who at the stroke of a pen can change the sex of a little-known 15th-century painter — Francesco del Cossa, say — from male to female.