Summary. It was Cousin R, and for no gain nor grievance at all, but for the making of a picture; here is how I came to it.

The Reckoning. The Colonel died of his own sleeping draught, taken up in the port he kept for himself and Dr Quill, and there is no mystery in the poisoning once the tableau about him is read rightly — for the poison was only the means, and the room was the object. Whoever dosed that glass had also to be the one left alone with the body afterward, at leisure, to draw the chair to the centre of the rug, square it to the fire, unhood the portrait, straighten collar and cuff and boot, and lay the wife's glove across the folded hands, building up the fire the while so the work should not go cold before it was finished. That takes time, and it takes a room emptied of witnesses. Only one guest in the house had both.

Nan and Lavinia had each other the whole evening, from the closing of the sitting to the finding of the body, in the morning-room, with Mrs Gow's word for it; and the will, in any case, lay unsigned, so neither had anything to gain by hurrying matters that night that would not have served her better waiting. Quill was in the hall and then at the front door being handed his coat, with Mrs Gow again to youch him — he named the dose but did not place it, which is not the same office. Wat had been shown out at the side door before the sitting even began and never returned; he was simply not in the house. Aziz and Xerxes sat the whole evening together in the library over the venture papers, each the other's witness, and neither came near the drawing-room decanter. Effie was settled unwell in the small parlour and looked in on twice, found there both times — no more free to dose a glass in the drawing-room than to arrange a body in it. Every one of these people, besides being elsewhere, lacked the plainer thing: a reason. The will unsigned meant nothing had yet shifted for Nan or Effie; Lavinia was paid better by a living patron than a dead one; Quill and Wat had no interest in the estate at all.

Cousin R alone remained in the drawing-room as the others dispersed, ostensibly setting the chairs to rights, and Mrs Gow, moving through the hall, saw him there and nowhere else. He had dosed the decanter earlier in the sitting, when it stood within easy reach of the circle, and the fortified port did the rest of that work for him. What he did with his solitary half hour was not concealment — a man hiding a crime does not draw a hooded portrait into the light, nor centre a chair on a rug where any caller must see it, nor build the fire up so as to be found working by it. He composed. The Colonel had gone the better part of two decades declining to grieve his wife above stairs or below, and R, who has sat at that table longer than anyone troubles to reckon and has watched the man's rigidity all the while, gave him at last the shape his grief had never been let take in life — enthroned in her chair, facing her unveiled portrait, her glove laid across his stilled hands. It is not a poisoner's tidiness. It is a collector's satisfaction. He wanted nothing from the will, owed nothing to a grievance, and feared no threat from the Colonel; he wanted only the picture, and he made it, and he stayed to see it finished before he let the house wake to it. I have the phial, I have Mrs Gow's placing of every soul in this house but his, and I have his own hand, by his own admission, tidying chairs no one asked him to tidy. That will do. I am charging him.

When Cousin R lingered to tidy the night,
The Colonel sat heavy, his vigil made light,
The port had been gentle, the draught smooth and deep,
And grief that lay hidden was dressed up in sleep,
While boots stood composed and the collar sat true,
A tableau of mourning that only R knew.