Above us was the white brilliance of the lantern. The light of each flame at its centre was intensified by being sealed in a glass funnel with a mirror behind it. This was surrounded in turn by square banks of parabolic reflectors in silvered glass like rows of shallow cups standing on their edges, turning slowly. A beam shone brilliantly out to sea, day and night, but was seldom visible during daylight, except in fog or adverse weather.

Black iron shutters in their runners were adjusted across many sections of the glass window panes. By this means, the revolving beam flashed out to sea intermittently, at one angle only. This corresponded with the markings of the Old Light on the charts of the Boston Deeps. After the silting up of the little ports and the building of the bridges at Sutton Cross, no shipping used this estuary. The corresponding light on the opposite bank of the estuary, which had once shown red and so measured the river mouth, was no longer in use.

Holmes finished his inspection of the gleaming apparatus.

“This, Watson, is the dioptric system of Augustin Fresnel. In other words, a stationary chandelier of white light surrounded by banks of Bodier Mercet’s silvered reflectors. The device is no longer modern, of course, but it is sufficient. Our French colleagues were pioneers in such matters. I daresay this one will be replaced before long or may be taken out of service altogether.”

Even if antiquated, it was a magnificent creation-a wonderland as Holmes had called it. The effect was almost hypnotic as the rows of reflectors rotated slowly, endlessly, and almost silently in the warm air, except for an occasional creak of wood at their axle. In the lower level of the lantern-room, standing before the tall wooden case with its clock dial and governor, Holmes opened the pendulum door. The mechanism of its smaller descending weights also controlled the flow of paraffin oil to the lamps. I noticed that a measuring scale of some kind had been carved on the interior of the case to indicate the progress of the weights. The length of time before the device must be wound again was clearly indicated.

To prevent the reflectors from coming to a halt, a metal strike-bell was attached a little above the base of this scale. Like the incessant striking of a clock, this alarm system would summon those in the barrack-room or nearby before the weight was fully unwound. According to Inspector Wainwright, “our man from Freiston Beach ” had attended to the winding early that morning and again while we were with Dr Rixon.

I turned to the little wooden table and opened the log-book. It was divided into columns for the date and time of winding, another for the point on the scale which the weights had reached before the winding took place, and a fourth for any comments to be read by the next keeper on duty. Underneath was the name of the keeper on watch.

These columns indicated that Abraham Chastelnau had wound up the weights for the night watch just before 8pm on Sunday evening. The time must have been almost immediately before Roland’s warning gunshot had lured him to their fatal confrontation on the flooded sands. An entry by the Freiston Shore relief on the following day confirmed that the Old Light had ceased flashing by 5.20 on Monday morning, according to the collier’s signal.

Holmes glanced at the entries and closed the book.

“The log and the pebbles can wait their turn. We have all night for them. Before it is too late in the day, it would pay us to take a stroll along the beach and visit the scene of the brothers’ last encounter.”

I looked out of the window across the ribbon of sand darkening to grey, and a tide that had already turned to the flood.

“I should have thought, Holmes, that it was already rather late. The sun has almost gone and the twilight will be upon us very soon.”

“All the better. It will be just as it was at the time of the quarrel. In our case, however, we are not going to quarrel and we shall have the strong beam of the Old Light to lead us back. Keep that in view and we shall not go far wrong.”

I noticed, however, that he had brought his redoubtable walking-stick, that “Penang Lawyer” which settled all arguments. We lowered ourselves down the iron ladder and stood upon the wet sands. The filling estuary was immediately to our right and the incoming tide lay ahead. I feared I might have better reason for my caution than I guessed. The pale yellow sun of that October evening had disappeared half an hour ago behind a silhouette of black Scotch firs which marked the flat inland horizon of Sutton Cross. Darkness was gathering across the wide sands with their low dunes and deserted shore. This impression of twilight was intensified by a contrast between the beam of white brilliance flashing out to sea, followed by sudden darkness as the silvered reflectors of the Old Light turned away from us behind their black shutters. They then shone only upon the interior of the iron, which covered the panes of the lantern on the far sides.

Holmes, swinging the stick in his hand, set off across the dark mud towards the point where Roderick Gilmore and his sexton had seen two diminutive figures struggling on Sunday evening. We were about half a mile seaward from St Clement’s church tower. Now, as then, a descending night mist was rolling in with the tide, progressively veiling the spot from onlookers ashore.

We had turned our backs on the Old Light and were walking away from its intermittent beam which shot across the waves. No ship’s light marked the horizon anchorage of the Boston Deeps. By comparison with the brilliance of the Old Light, even the beacon on the church tower was a mere glimmer confirming our position on the treacherous sands.

As soon as we had completed our reconnaissance, we need only walk back in a straight line until the Old Light shone straight in our faces. Then we should turn right at forty-five degrees and make our way steadily towards its iron ladder, along the raised path of the river bank. Or so it seemed. I could already feel the sand yielding more easily underfoot, as the tide seeped beneath us. By the yellow oil-light of my lantern, I could also see that each of my footprints now flooded progressively deeper and more quickly as my boot was lifted from the mud.

Holmes was at his most dogmatic.

“I think we need another half mile to reach the scene of their encounter. I should like to determine a direct line from St Clement’s beacon to the incoming surf.”

As he said this, I noticed that the blurred but luminous line of surf was now the only thing visible to our right between the darkness of sky and sea. We walked for about fifteen minutes more, scarcely exchanging a word. I felt the coldness of the October night coming in with the mist. Even the lemon afterglow of sunset had vanished from the inland horizon beyond the village. The rest of the shoreline was obscure and the descending mist which the rector had described now hung between us and St Clement’s beacon, condensing slowly into fog. I tried a cheerful note.

“The ground seems a little higher here, Holmes, a rib of firm sand. If we follow it back, when we turn, and keep Mr Gilmore’s beacon in view to the right, we should soon have the beam of the Old Light full in our faces.”

“I daresay,” he said impatiently but with no sign of turning back, “If I had been the survivor of that fight between the two brothers on Sunday night, I should have assumed much the same.”

“I thought we had agreed that there was no survivor of the encounter.”

“On the contrary. That is a matter which we are about to put to the proof.”

By this time, I had very little wish to put it to the proof. Indeed, with the light gone, I began not to like the whole business. We were not more than eight feet apart but it had become increasingly difficult to make out the gaunt purposeful stride of my companion or anything but the hazy flicker of his lamp. As if reading my thoughts, he added,