“Finish your drink and come up to my sitting-room,” said Allerdyke.  “I’ll give you a cigar up there.  Yes,” he added, as they left the restaurant and went upstairs.  “I do want to see it again—­or, rather, the photograph.  You’re in no hurry?”

“A good hour to spare yet,” replied Chettle.

Allerdyke locked the door of the sitting-room when they were once inside it, and that done he placed a decanter, a syphon, and a glass on his table, and flanked them with a box of cigars.  He waved a hospitable hand towards these comforts.

“Sit down and help yourself, Chettle,” he said.  “A drop of my whisky’ll do you no harm—­that’s some I got down from home, and you’ll not find its like everywhere.  Light a cigar—­and put a couple in your pocket to smoke in the train.  Now then, let’s see that photograph once more.”

Chettle handed over the watch, and Allerdyke, opening the case, delicately removed the print.  He sat down at the table with his back to the light, and carefully examined the thing back and front, while the detective, glass in hand, cigar in lips, and thumb in the armhole of his waistcoat, watched him appreciatively and inquisitively.

“Make aught new out of it, sir?” he asked after a while.

Instead of answering, Allerdyke laid the photograph down, went across to another table, and took from it his album.  He turned its leaves over until he came to a few loose prints.  He picked them up one after another and examined them.  And suddenly he knew the secret.  There was no longer any problem, any difficulty about that photograph.  He knew—­now!  And with a sharp exclamation, he flung the album back to the side-table, and turned to the detective.

“Chettle!” he said.  “You know me well enough to know that I can make it well worth any man’s while to keep a secret until I tell him he can speak about it!  What!”

“I should think so, Mr. Allerdyke,” responded Chettle, readily enough.  “And if you want me to keep a secret—­”

“I do—­for the time being,” answered Allerdyke.  He sat down again and picked up the photograph which had exercised his thoughts so intensely.  “I’ve found out the truth concerning this,” he said, tapping it with his finger.  “Yes, I’ve hit it!  Listen, now—­I told you I’d only made four prints of this photo, and that I knew exactly where they all were—­one in my own album there, two given by James to friends in Bradford, one—­as we more recently found out—­given by James to Mrs. Marlow.  That one—­the Mrs. Marlow one—­we believed to be—­this—­this!”

“And isn’t it, Mr. Allerdyke?” asked Chettle wonderingly.

Allerdyke laughed—­a laugh of relief and satisfaction.

“Less than an hour ago,” he replied, “in fact, just before you came in, Mrs. Marlow showed me the photo which James gave her—­showed it to me, out below there in the hall.  No mistaking it!  And so—­when you came, I was racking my brains to rags trying to settle what this photo—­this!—­was.  And now I know what it is—­and damn me if I know whether the discovery makes things plainer or more mixed up!  But—­I know what this is, anyway.”