The show ahead consists of ever clearer evidence that Turner was a great and tremendous artist, Claude a charming one. At the heart of the display is the famous face-off between their two views of ancient Carthage; a face-off Turner engineered when he left his work to the nation, on the condition that these two works always hang together. Which they usually do, in the National’s octagonal gallery.

In Claude’s Seaport with the Embarkation of the Queen of Sheba, the weak beam from a low sun on the horizon seems barely to reach the shore, and even the figures humping stuff onto boats in the foreground appear utterly exhausted. But when Turner adopts more or less the same viewpoint, he flicks a switch and the electricity surges on. The sun is rebooted to its nuclear setting. A volcano erupts in the bay and the waters boil.

The usual interpretation of Turner’s insistence that his work hangs next to Claude’s is that it was intended as a homage: a pupil’s thanks to his master. But I no longer believe that. On this evidence, Turner’s great burst of atomic sunshine constitutes an effort to flatten Claude in battle. This wasn’t an homage. This was a beating-up.