How to account for the bare knife found unstained beneath the body? And the hint that the red hand upon the wall must have been drawn by some one whose life was passed in darkness thrilled him with a suggestion of dim and infinite horror. Hence he was in truth not a little curious as to what was to come, and some ten days after he had returned the tablet he again visited the 'mystery-man', as he privately named his friend.

Arrived in the grave and airy chambers in Great Russell Street, he found the moral atmosphere of the place had been transformed. All Dyson's irritation had disappeared, his brow was smoothed with complacency, and he sat at a table by the window gazing out into the street with an expression of grim enjoyment, a pile of books and papers lying unheeded before him.

'My dear Phillipps, I am delighted to see you! Pray excuse my moving. Draw your chair up here to the table, and try this admirable shag tobacco.'

'Thank you,' said Phillipps, 'judging by the flavour of the smoke, I should think it is a little strong. But what on earth is all this? What are you looking at?'

'I am on my watch-tower. I assure you that the time seems short while I contemplate this agreeable street and the classic grace of the Museum portico.'

'Your capacity for nonsense is amazing,' replied Phillipps, 'but have you succeeded in deciphering the tablet? It interests me.'

'I have not paid much attention to the tablet recently,' said Dyson. 'I believe the spiral character may wait.'

'Really! And how about the Vivian murder?'

'Ah, you do take an interest in that case? Well, after all, we cannot deny that it was a queer business. But is not "murder" rather a coarse word? It smacks a little, surely, of the police poster. Perhaps I am a trifle decadent, but I cannot help believing in the splendid word; "sacrifice", for example, is surely far finer than "murder".'

'I am all in the dark,' said Phillipps. 'I cannot even imagine by what track you are moving in this labyrinth.'

'I think that before very long the whole matter will be a good deal clearer for us both, but I doubt whether you will like hearing the story.'

Dyson lit his pipe afresh and leant back, not relaxing, however, in his scrutiny of the street. After a somewhat lengthy pause, he startled Phillipps by a loud breath of relief as he rose from the chair by the window and began to pace the floor.

'It's over for the day,' he said, 'and, after all, one gets a little tired.'

Phillipps looked with inquiry into the street. The evening was darkening, and the pile of the Museum was beginning to loom indistinct before the lighting of the lamps, but the pavements were thronged and busy. The artist in chalks across the way was gathering together his materials, and blurring all the brilliance of his designs, and a little lower down there was the clang of shutters being placed in position. Phillipps could see nothing to justify Mr. Dyson's sudden abandonment of his attitude of surveillance, and grew a little irritated by all these thorny enigmas.

'Do you know, Phillipps,' said Dyson, as he strolled at ease up and down the room, 'I will tell you how I work. I go upon the theory of improbability. The theory is unknown to you? I will explain. Suppose I stand on the steps of St. Paul's and look out for a blind man lame of the left leg to pass me, it is evidently highly improbable that I shall see such a person by waiting for an hour. If I wait two hours the improbability is diminished, but is still enormous, and a watch of a whole day would give little expectation of success. But suppose I take up the same position day after day, and week after week, don't you perceive that the improbability is lessening constantly—growing smaller day after day. Don't you see that two lines which are not parallel are gradually approaching one another, drawing nearer and nearer to a point of meeting, till at last they do meet, and improbability has vanished altogether. That is how I found the black tablet: I acted on the theory of improbability. It is the only scientific principle I know of which can enable one to pick out an unknown man from amongst five million.'

'And you expect to find the interpreter of the black tablet by this method?'

'Certainly.'

'And the murderer of Sir Thomas Vivian also?'

'Yes, I expect to lay my hands on the person concerned in the death of Sir Thomas Vivian in exactly the same way.'

The rest of the evening after Phillipps had left was devoted by Dyson to sauntering in the streets, and afterwards, when the night grew late, to his literary labours, or the chase of the phrase, as he called it. The next morning the station by the window was again resumed. His meals were brought to him at the table, and he ate with his eyes on the street. With briefest intervals, snatched reluctantly from time to time, he persisted in his survey throughout the day, and only at dusk, when the shutters were put up and the 'screever' ruthlessly deleted all his labour of the day, just before the gas-lamps began to star the shadows, did he feel at liberty to quit his post. Day after day this ceaseless glance upon the street continued, till the landlady grew puzzled and aghast at such a profitless pertinacity.

But at last, one evening, when the play of lights and shadows was scarce beginning, and the clear cloudless air left all distinct and shining, there came the moment. A man of middle age, bearded and bowed, with a touch of grey about the ears, was strolling slowly along the northern pavement of Great Russell Street from the eastern end. He looked up at the Museum as he went by, and then glanced involuntarily at the art of the 'screever', and at the artist himself, who sat beside his pictures, hat in hand. The man with the beard stood still an instant, swaying slightly to and fro as if in thought, and Dyson saw his fists shut tight, and his back quivering, and the one side of his face in view twitched and grew contorted with the indescribable torment of approaching epilepsy. Dyson drew a soft hat from his pocket, and dashed the door open, taking the stair with a run.

When he reached the street, the person he had seen so agitated had turned about, and, regardless of observation, was racing wildly towards Bloomsbury Square, with his back to his former course. Mr. Dyson went up to the artist of the pavement and gave him some money, observing quietly, 'You needn't trouble to draw that thing again.'

Then he, too, turned about, and strolled idly down the street in the opposite direction to that taken by the fugitive.