When I passed through the doorway my eyes struggled to take in the crowded contents of the cabin. The obvious things caught my eye first: small squares of sunlight from the stern windows landing brightly on a simple cot and washstand. A box near my feet, open and containing the accoutrements of medicine: weights and splints, paper packets containing bandages, lengths of silk and fine instruments. A row of shelves against the starboard wall: each fitted with a lip to protect rows and rows of large specimen jars filled with clear fluid, awaiting occupants. A couple of them were already tenanted with small fish and crabs, drifting faintly with the little movements of the boat.

There was a desk against the opposite wall and there, relaxed in his seat, was the doctor. He was smiling, studying me. He stood, moving like a much younger man, and indicated I should take the chair; sat himself on the edge of the desk. Around him were scattered papers and curious instruments. By the faint light of a wax lamp, a chart of a spray of islands was pinned to the timber of the wall in front of him. A jar stood empty, its lid removed.

‘You wanted to see me?’

‘Welcome, welcome,’ he said, pushing the mess aside. ‘I wanted to make us tea. Will you have tea?’ Without waiting for an answer, he turned to a side table where a pot and teacups stood. He spoke over his shoulder as he poured. ‘There ought to be some sort of petit fours, I know. But we make do, don’t we?’ I was still finding new objects in the crowded room, more laboratory than living space.

‘I’m sure the convicts would agree with you. What does this do?’ I pointed at a tall wooden box with an open top.

He followed my gaze: ‘Ah! Viewing box. Look…’ He flipped it over and I saw that the end opposite the open end was made of glass: a small windowpane had been caulked into the timber. ‘One presses it into the surface of the sea and it affords a clear view of what goes on beneath. Water has the effect of distorting human vision, as you know. I am interested, among other things, in the difference between the human eye and those of the fishes. Consider: do they see clearly? It seems obvious to conclude that they do. But it is also possible that they see underwater the same way we do, but are more adept at interpreting that vision cerebrally. You follow? One is a function of the eye, the other of the brain. So we turn our attention to the anatomical structures of the eye and seek out the differences…’

He was determined to run on with this explanation. I was determined to test it. ‘Do you mean the human eye or the fish eye?’