One subject, six movements. Follow the dogs in famous paintings from Van Eyck to Seurat and you learn to spot Northern Renaissance, Baroque, Rococo, Romanticism, Impressionism and Pointillism at a glance.

On the back wall of Las Meninas there is a small mirror, and inside the mirror are two heads — the king and queen of Spain. They should not be there. The painting has been arguing about why they are for three hundred and seventy years.

A young princess, her maids, a dog, two dwarfs, and a painter in the corner with his back half-turned. Reading Velázquez's most quietly radical canvas — the figures, the mirror, and the question of who is actually being painted.