Deep in reflection, Stuart walked alone along the Embankment. The full facts contained in the report form Paris the Commissioner had not divulged, but Stuart concluded that this sudden activity was directly due, not to the death of M. Max, but to the fact that he (Max) had left behind him some more or less tangible clue. Stuart fully recognized that the Commissioner had accorded him an opportunity to establish his reputation—or to wreck it.  
Yet, upon closer consideration, it became apparent that it was to Fate and not to the Commissioner that he was indebted. Strictly speaking, his association with the matter dated from the night of his meeting with the mysterious cabman in West India Dock Road. Or had the curtain first been lifted upon this occult drama that evening, five years ago, as the setting sun reddened the waters of the Imperial Canal and a veiled figure passed him on the Wu-Men Bridge?
 
"Shut your eyes tightly, master—the Scorpion is coming!"
 
He seemed to hear the boy's words now, as he passed along the Embankment; he seemed to see again the tall figure. And suddenly he stopped, stood still and stared with unseeing eyes across the muddy waters of the Thames. He was thinking of the cowled man who had stood behind the curtains in his study—of that figure so wildly bizarre that even now he could scarcely believe that he had ever actually seen it. He walked on.
 
Automatically his reflections led him to Mlle. Dorian, and he remembered that even as he paced along there beside the river the wonderful mechanism of New Scotland Yard was in motion, its many tentacles seeking—seeking tirelessly—for the girl, whose dark eyes haunted his sleeping and waking hours. He was responsible, and if she were arrested he would be called upon to identify her. He condemned himself bitterly.
 
After all, what crime had she committed? She had tried to purloin a letter—which did not belong to Stuart in the first place. And she had failed. Now—the police were looking for her. His reflections took a new form.
 
What of Gaston Max, foremost criminologist in Europe, who now lay dead and mutilated in an East-End mortuary? The telephone message which had summoned Dunbar away had been too opportune to be regarded as a mere coincidence. Mlle. Dorian was, therefore, an accomplice of a murderer.
 
Stuart sighed. He would have given much—more than he was prepared to admit to himself—to have known her to be guiltless.
 
The identity of the missing cabman now engaged his mind. It was quite possible, of course, that the man had actually found the envelope in his cab and was in no other way concerned in the matter. But how had Mlle. Dorian, or the person instructing her, traced the envelope to his study? And why, if they could establish a claim to it, had they preferred to attempt to steal it? Finally, why all this disturbance about a blank pieced of cardboard?
 
A mental picture of the envelope arose before him, the number, 30, written upon it and the two black seals securing the lapels. He paused again in his walk. His reflections had led him to a second definite point and he fumbled in his waistcoat pocket for a time, seeking a certain brass coin about the size of a halfpenny, having a square hole in the middle and peculiar characters engraved around the square, one on each of the four sides.
 
He failed to find the coin in his pocket, however, but he walked briskly up a side street unti............