Holmes nodded. “I doubt that he originally intended to strike the professor quite so violently. Having done so, he would probably wait until he was sure his victim was dead by hanging, and then contrive to make it look as if the noose had slipped from the tree, and the professor had fallen and struck his head on a stone.”

“Had you any suspicions of Martin before we arrived here and found them both dead?” I asked my friend.

“I had noted that Professor Palfreyman’s really tangible troubles – the arrival of the anonymous, blank letters, and the tile – only began after Martin started calling at the house. Of course, that might have been mere chance – after all, the same observation could be applied to Miss Calloway herself. But it was also notable from Miss Calloway’s account that Martin seemed to take every opportunity he could to try to persuade her that Professor Palfreyman was dangerously insane, and that she should leave Bluebell Cottage. There was, moreover, one particular incident that I thought especially odd: when Miss Calloway mentioned her intention to consult me, Martin at first dismissed the idea as pointless, but when he changed his mind he said that perhaps I could somehow discover what lay behind the professor’s troubles. But there was no mystery there to be uncovered, nor had there ever been. Apart from the professor’s private feelings of guilt, what had happened in Macedonia all those years ago was a matter of general knowledge to all of Palfreyman’s colleagues, and thus, probably, to Martin, too.

‘‘No one, as far as we know, seemed to attach any blame to Palfreyman over the matter: the guilt he felt about it was simply his conscience prodding him with the thought that he could perhaps have acted differently. This feeling of personal guilt was, of course, exacerbated by the fact that – as he willingly admitted himself – he disliked Strange intensely. In other words, the trouble, up until this year, was really all in Professor Palfrey-man’s head, and there was nothing there that a detective could ‘discover’. The arrival of the letter and the tile, however, were quite different. They were not simply in the professor’s head, but definite, provocative acts, which any detective worth his salt would see as the starting point of his investigation.”

“I quite agree,” said Gregson. “When you were describing the matter to me earlier, I at once thought that those things were the most important part of the case.”

Holmes nodded. “And yet, Martin, a supposedly highly intelligent, and certainly highly educated young man, did not mention them to Miss Calloway at all, but referred simply to what had occurred many years ago. It seemed to me almost as if he was deliberately trying to deflect her attention from what was obviously the central part of the whole case – as indeed he probably was. It seems certain now that it was Martin himself who sent those things to Professor Palfreyman.’’

“But why?” I asked. “What could his aim have been?”

“He knew, as did everyone, that the professor was troubled in his mind – that he suffered from nightmares and so on – and considered, I imagine, that by persecuting him he might be able to drive him mad and suicidal. And if he couldn’t succeed in making the professor kill himself, then he would contrive to make it appear that he had – which is what this hideous and evil tableau was about. In fact, of course, the professor was nowhere near as unhinged as Martin seemed to think: guilt-ridden certainly, a little unbalanced perhaps, but otherwise, he was, for most of the time, as sane as anyone else.

‘‘When he dug out his little pistol from the old tin trunk in which it had lain for twenty-odd years it was not to use it on himself, but to protect himself against the threat he recognized was closing in upon him. I even wonder if he suspected that the threat might come from Martin. His questions to Miss Calloway concerning her future, and her feelings for Martin, were somewhat ambiguous, and it may be that he was in fact ‘fishing’ for information as to what she really thought about the young man.”

“What of the person loitering in the woods?” asked Gregson. “Do you reckon that was Martin, too?”

“It must have been. He would know when Miss Calloway was likely to catch her train home, and it would have been easy for him to take an earlier train and get down here before she did. His intention was no doubt to frighten her into leaving Bluebell Cottage altogether, as this would make it easier for him to pursue his scheme against the professor, and would allow him to portray himself as Miss Calloway’s ‘protector’, and thus advance his matrimonial prospects. In addition, if Miss Calloway was unsure of Professor Palfreyman’s whereabouts at the time of these frightening episodes, she might begin to suspect that the professor himself was responsible, which thought, to judge from her account, had already crossed her mind, and which was all to the good for Martin’s evil scheme to make the professor appear insane.’’

“But why?” I repeated. “I understand all that you are saying about Martin trying to make Professor Palfreyman appear insane, but what could he possibly hope to gain by murdering the man or driving him to suicide? Was it simply some form of vengeance for the death of Strange?”

Holmes shook his head. “I very much doubt it,” said he. “According to Professor Palfreyman’s account, no one really mourned Strange’s passing, let alone harboured any grievance about it. The professor wrote his account for Miss Calloway about thirty years after the events he described in it, and it is evident that in those thirty years he had not encountered any ill feeling over the matter, so I think we may take it that there is none. It seems apparent, then, that Martin was using the Macedonian business – ‘the Smiling Face’ – simply as a means of achieving his aim, and that his true motives lay elsewhere.”

“But what on earth could those motives be?” I asked in some puzzlement.

“There are certain facts you may be overlooking,” responded Holmes after a moment. “In the first place, Georgina Calloway is the professor’s only known relative, and as such would, upon his death, inherit anything he possessed.”

“I admit that that hadn’t occurred to me,” I said.

“In the second place,” Holmes continued, “you must remember that although Martin was working on his thesis under the guidance of the archaeology department, that had not previously been his principal field of study. He had studied art and history of art, and was no doubt something of an expert in that field. Perhaps, I speculated, while helping Miss Calloway to sort and catalogue the professor’s random heaps of paintings, sculptures and drawings, as she described to us, Martin had come across something which he, and he alone, recognized as being of immense value. If so, that might have provided the motive for his wicked plan. He would have realized that he could not hope to get away with simply stealing what he wanted, but if he could get rid of the older man and persuade Miss Calloway to marry him, then he could get his hands on a possible fortune.”

“It is certainly an interesting notion, and it would make sense,” said Gregson. “There is usually avarice at the bottom of this sort of crime. Do you have any evidence for it, Mr Holmes, or is it just speculation?”

“I had a quick look through some of the things in one of the professor’s old tin trunks when I was examining the study earlier,” replied Holmes. “I am no expert on art, but there were a large number of sheets there which looked to me suspiciously like pages from a notebook of Leonardo da Vinci’s. He generally wrote backwards, you know, Gregson. I think he referred to it as ‘mirror writing’. It is quite distinctive. On another large, folded sheet there is what looks to me like a series of preliminary sketches for his celebrated painting, The Virgin of the Rocks.”