If Francesco de Zurbaran’s ‘Christ on the Cross’ were the only work in the National Gallery’s powerful exhibition of Spanish 17th century painting and sculpture you should still move heaven and earth to see it. The 9ft 6in-high masterpiece was painted in 1627 for a Dominican friary in Seville, where it hung in a shallow niche behind a grille, illuminated by natural light from two windows just to the right of the viewer’s field of vision. On loan from the Art Institute...